ECMA-262, 14th edition, June 2023
ECMAScript® 2023 Language Specification

About this Specification

The document at https://tc39.es/ecma262/ is the most accurate and up-to-date ECMAScript specification. It contains the content of the most recent yearly snapshot plus any finished proposals (those that have reached Stage 4 in the proposal process and thus are implemented in several implementations and will be in the next practical revision) since that snapshot was taken.

This document is available as a single page and as multiple pages.

Contributing to this Specification

This specification is developed on GitHub with the help of the ECMAScript community. There are a number of ways to contribute to the development of this specification:

Refer to the colophon for more information on how this document is created.

Introduction

This Ecma Standard defines the ECMAScript 2023 Language. It is the fourteenth edition of the ECMAScript Language Specification. Since publication of the first edition in 1997, ECMAScript has grown to be one of the world's most widely used general-purpose programming languages. It is best known as the language embedded in web browsers but has also been widely adopted for server and embedded applications.

ECMAScript is based on several originating technologies, the most well-known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company's Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoft starting with Internet Explorer 3.0.

The development of the ECMAScript Language Specification started in November 1996. The first edition of this Ecma Standard was adopted by the Ecma General Assembly of June 1997.

That Ecma Standard was submitted to ISO/IEC JTC 1 for adoption under the fast-track procedure, and approved as international standard ISO/IEC 16262, in April 1998. The Ecma General Assembly of June 1998 approved the second edition of ECMA-262 to keep it fully aligned with ISO/IEC 16262. Changes between the first and the second edition are editorial in nature.

The third edition of the Standard introduced powerful regular expressions, better string handling, new control statements, try/catch exception handling, tighter definition of errors, formatting for numeric output and minor changes in anticipation of future language growth. The third edition of the ECMAScript standard was adopted by the Ecma General Assembly of December 1999 and published as ISO/IEC 16262:2002 in June 2002.

After publication of the third edition, ECMAScript achieved massive adoption in conjunction with the World Wide Web where it has become the programming language that is supported by essentially all web browsers. Significant work was done to develop a fourth edition of ECMAScript. However, that work was not completed and not published as the fourth edition of ECMAScript but some of it was incorporated into the development of the sixth edition.

The fifth edition of ECMAScript (published as ECMA-262 5th edition) codified de facto interpretations of the language specification that have become common among browser implementations and added support for new features that had emerged since the publication of the third edition. Such features include accessor properties, reflective creation and inspection of objects, program control of property attributes, additional array manipulation functions, support for the JSON object encoding format, and a strict mode that provides enhanced error checking and program security. The fifth edition was adopted by the Ecma General Assembly of December 2009.

The fifth edition was submitted to ISO/IEC JTC 1 for adoption under the fast-track procedure, and approved as international standard ISO/IEC 16262:2011. Edition 5.1 of the ECMAScript Standard incorporated minor corrections and is the same text as ISO/IEC 16262:2011. The 5.1 Edition was adopted by the Ecma General Assembly of June 2011.

Focused development of the sixth edition started in 2009, as the fifth edition was being prepared for publication. However, this was preceded by significant experimentation and language enhancement design efforts dating to the publication of the third edition in 1999. In a very real sense, the completion of the sixth edition is the culmination of a fifteen year effort. The goals for this edition included providing better support for large applications, library creation, and for use of ECMAScript as a compilation target for other languages. Some of its major enhancements included modules, class declarations, lexical block scoping, iterators and generators, promises for asynchronous programming, destructuring patterns, and proper tail calls. The ECMAScript library of built-ins was expanded to support additional data abstractions including maps, sets, and arrays of binary numeric values as well as additional support for Unicode supplementary characters in strings and regular expressions. The built-ins were also made extensible via subclassing. The sixth edition provides the foundation for regular, incremental language and library enhancements. The sixth edition was adopted by the General Assembly of June 2015.

ECMAScript 2016 was the first ECMAScript edition released under Ecma TC39's new yearly release cadence and open development process. A plain-text source document was built from the ECMAScript 2015 source document to serve as the base for further development entirely on GitHub. Over the year of this standard's development, hundreds of pull requests and issues were filed representing thousands of bug fixes, editorial fixes and other improvements. Additionally, numerous software tools were developed to aid in this effort including Ecmarkup, Ecmarkdown, and Grammarkdown. ES2016 also included support for a new exponentiation operator and adds a new method to Array.prototype called includes.

ECMAScript 2017 introduced Async Functions, Shared Memory, and Atomics along with smaller language and library enhancements, bug fixes, and editorial updates. Async functions improve the asynchronous programming experience by providing syntax for promise-returning functions. Shared Memory and Atomics introduce a new memory model that allows multi-agent programs to communicate using atomic operations that ensure a well-defined execution order even on parallel CPUs. It also included new static methods on Object: Object.values, Object.entries, and Object.getOwnPropertyDescriptors.

ECMAScript 2018 introduced support for asynchronous iteration via the AsyncIterator protocol and async generators. It also included four new regular expression features: the dotAll flag, named capture groups, Unicode property escapes, and look-behind assertions. Lastly it included object rest and spread properties.

ECMAScript 2019 introduced a few new built-in functions: flat and flatMap on Array.prototype for flattening arrays, Object.fromEntries for directly turning the return value of Object.entries into a new Object, and trimStart and trimEnd on String.prototype as better-named alternatives to the widely implemented but non-standard String.prototype.trimLeft and trimRight built-ins. In addition, it included a few minor updates to syntax and semantics. Updated syntax included optional catch binding parameters and allowing U 2028 (LINE SEPARATOR) and U 2029 (PARAGRAPH SEPARATOR) in string literals to align with JSON. Other updates included requiring that Array.prototype.sort be a stable sort, requiring that JSON.stringify return well-formed UTF-8 regardless of input, and clarifying Function.prototype.toString by requiring that it either return the corresponding original source text or a standard placeholder.

ECMAScript 2020, the 11th edition, introduced the matchAll method for Strings, to produce an iterator for all match objects generated by a global regular expression; import(), a syntax to asynchronously import Modules with a dynamic specifier; BigInt, a new number primitive for working with arbitrary precision integers; Promise.allSettled, a new Promise combinator that does not short-circuit; globalThis, a universal way to access the global this value; dedicated export * as ns from 'module' syntax for use within modules; increased standardization of for-in enumeration order; import.meta, a host-populated object available in Modules that may contain contextual information about the Module; as well as adding two new syntax features to improve working with “nullish” values (null or undefined): nullish coalescing, a value selection operator; and optional chaining, a property access and function invocation operator that short-circuits if the value to access/invoke is nullish.

ECMAScript 2021, the 12th edition, introduced the replaceAll method for Strings; Promise.any, a Promise combinator that short-circuits when an input value is fulfilled; AggregateError, a new Error type to represent multiple errors at once; logical assignment operators (??=, &&=, ||=); WeakRef, for referring to a target object without preserving it from garbage collection, and FinalizationRegistry, to manage registration and unregistration of cleanup operations performed when target objects are garbage collected; separators for numeric literals (1_000); and Array.prototype.sort was made more precise, reducing the amount of cases that result in an implementation-defined sort order.

ECMAScript 2022, the 13th edition, introduced top-level await, allowing the keyword to be used at the top level of modules; new class elements: public and private instance fields, public and private static fields, private instance methods and accessors, and private static methods and accessors; static blocks inside classes, to perform per-class evaluation initialization; the #x in obj syntax, to test for presence of private fields on objects; regular expression match indices via the /d flag, which provides start and end indices for matched substrings; the cause property on Error objects, which can be used to record a causation chain in errors; the at method for Strings, Arrays, and TypedArrays, which allows relative indexing; and Object.hasOwn, a convenient alternative to Object.prototype.hasOwnProperty.

ECMAScript 2023, the 14th edition, introduced the toSorted, toReversed, with, findLast, and findLastIndex methods on Array.prototype and TypedArray.prototype, as well as the toSpliced method on Array.prototype; added support for #! comments at the beginning of files to better facilitate executable ECMAScript files; and allowed the use of most Symbols as keys in weak collections.

Dozens of individuals representing many organizations have made very significant contributions within Ecma TC39 to the development of this edition and to the prior editions. In addition, a vibrant community has emerged supporting TC39's ECMAScript efforts. This community has reviewed numerous drafts, filed thousands of bug reports, performed implementation experiments, contributed test suites, and educated the world-wide developer community about ECMAScript. Unfortunately, it is impossible to identify and acknowledge every person and organization who has contributed to this effort.

Allen Wirfs-Brock
ECMA-262, Project Editor, 6th Edition

Brian Terlson
ECMA-262, Project Editor, 7th through 10th Editions

Jordan Harband
ECMA-262, Project Editor, 10th through 12th Editions

Shu-yu Guo
ECMA-262, Project Editor, 12th through 14th Editions

Michael Ficarra
ECMA-262, Project Editor, 12th through 14th Editions

Kevin Gibbons
ECMA-262, Project Editor, 12th through 14th Editions

1 Scope

This Standard defines the ECMAScript 2023 general-purpose programming language.

2 Conformance

A conforming implementation of ECMAScript must provide and support all the types, values, objects, properties, functions, and program syntax and semantics described in this specification.

A conforming implementation of ECMAScript must interpret source text input in conformance with the latest version of the Unicode Standard and ISO/IEC 10646.

A conforming implementation of ECMAScript that provides an application programming interface (API) that supports programs that need to adapt to the linguistic and cultural conventions used by different human languages and countries must implement the interface defined by the most recent edition of ECMA-402 that is compatible with this specification.

A conforming implementation of ECMAScript may provide additional types, values, objects, properties, and functions beyond those described in this specification. In particular, a conforming implementation of ECMAScript may provide properties not described in this specification, and values for those properties, for objects that are described in this specification.

A conforming implementation of ECMAScript may support program and regular expression syntax not described in this specification. In particular, a conforming implementation of ECMAScript may support program syntax that makes use of any “future reserved words” noted in subclause 12.7.2 of this specification.

A conforming implementation of ECMAScript must not implement any extension that is listed as a Forbidden Extension in subclause 17.1.

A conforming implementation of ECMAScript must not redefine any facilities that are not implementation-defined, implementation-approximated, or host-defined.

A conforming implementation of ECMAScript may choose to implement or not implement Normative Optional subclauses. If any Normative Optional behaviour is implemented, all of the behaviour in the containing Normative Optional clause must be implemented. A Normative Optional clause is denoted in this specification with the words "Normative Optional" in a coloured box, as shown below.

Normative Optional

2.1 Example Normative Optional Clause Heading

Example clause contents.

A conforming implementation of ECMAScript must implement Legacy subclauses, unless they are also marked as Normative Optional. All of the language features and behaviours specified within Legacy subclauses have one or more undesirable characteristics. However, their continued usage in existing applications prevents their removal from this specification. These features are not considered part of the core ECMAScript language. Programmers should not use or assume the existence of these features and behaviours when writing new ECMAScript code.

Legacy

2.2 Example Legacy Clause Heading

Example clause contents.
Normative Optional, Legacy

2.3 Example Legacy Normative Optional Clause Heading

Example clause contents.

3 Normative References

The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

ISO/IEC 10646 Information Technology — Universal Multiple-Octet Coded Character Set (UCS) plus Amendment 1:2005, Amendment 2:2006, Amendment 3:2008, and Amendment 4:2008, plus additional amendments and corrigenda, or successor

ECMA-402, ECMAScript 2015 Internationalization API Specification.
https://www.ecma-international.org/publications-and-standards/standards/ecma-402/

ECMA-404, The JSON Data Interchange Format.
https://www.ecma-international.org/publications-and-standards/standards/ecma-404/

4 Overview

This section contains a non-normative overview of the ECMAScript language.

ECMAScript is an object-oriented programming language for performing computations and manipulating computational objects within a host environment. ECMAScript as defined here is not intended to be computationally self-sufficient; indeed, there are no provisions in this specification for input of external data or output of computed results. Instead, it is expected that the computational environment of an ECMAScript program will provide not only the objects and other facilities described in this specification but also certain environment-specific objects, whose description and behaviour are beyond the scope of this specification except to indicate that they may provide certain properties that can be accessed and certain functions that can be called from an ECMAScript program.

ECMAScript was originally designed to be used as a scripting language, but has become widely used as a general-purpose programming language. A scripting language is a programming language that is used to manipulate, customize, and automate the facilities of an existing system. In such systems, useful functionality is already available through a user interface, and the scripting language is a mechanism for exposing that functionality to program control. In this way, the existing system is said to provide a host environment of objects and facilities, which completes the capabilities of the scripting language. A scripting language is intended for use by both professional and non-professional programmers.

ECMAScript was originally designed to be a Web scripting language, providing a mechanism to enliven Web pages in browsers and to perform server computation as part of a Web-based client-server architecture. ECMAScript is now used to provide core scripting capabilities for a variety of host environments. Therefore the core language is specified in this document apart from any particular host environment.

ECMAScript usage has moved beyond simple scripting and it is now used for the full spectrum of programming tasks in many different environments and scales. As the usage of ECMAScript has expanded, so have the features and facilities it provides. ECMAScript is now a fully featured general-purpose programming language.

4.1 Web Scripting

A web browser provides an ECMAScript host environment for client-side computation including, for instance, objects that represent windows, menus, pop-ups, dialog boxes, text areas, anchors, frames, history, cookies, and input/output. Further, the host environment provides a means to attach scripting code to events such as change of focus, page and image loading, unloading, error and abort, selection, form submission, and mouse actions. Scripting code appears within the HTML and the displayed page is a combination of user interface elements and fixed and computed text and images. The scripting code is reactive to user interaction, and there is no need for a main program.

A web server provides a different host environment for server-side computation including objects representing requests, clients, and files; and mechanisms to lock and share data. By using browser-side and server-side scripting together, it is possible to distribute computation between the client and server while providing a customized user interface for a Web-based application.

Each Web browser and server that supports ECMAScript supplies its own host environment, completing the ECMAScript execution environment.

4.2 Hosts and Implementations

To aid integrating ECMAScript into host environments, this specification defers the definition of certain facilities (e.g., abstract operations), either in whole or in part, to a source outside of this specification. Editorially, this specification distinguishes the following kinds of deferrals.

An implementation is an external source that further defines facilities enumerated in Annex D or those that are marked as implementation-defined or implementation-approximated. In informal use, an implementation refers to a concrete artefact, such as a particular web browser.

An implementation-defined facility is one that defers its definition to an external source without further qualification. This specification does not make any recommendations for particular behaviours, and conforming implementations are free to choose any behaviour within the constraints put forth by this specification.

An implementation-approximated facility is one that defers its definition to an external source while recommending an ideal behaviour. While conforming implementations are free to choose any behaviour within the constraints put forth by this specification, they are encouraged to strive to approximate the ideal. Some mathematical operations, such as Math.exp, are implementation-approximated.

A host is an external source that further defines facilities listed in Annex D but does not further define other implementation-defined or implementation-approximated facilities. In informal use, a host refers to the set of all implementations, such as the set of all web browsers, that interface with this specification in the same way via Annex D. A host is often an external specification, such as WHATWG HTML (https://html.spec.whatwg.org/). In other words, facilities that are host-defined are often further defined in external specifications.

A host hook is an abstract operation that is defined in whole or in part by an external source. All host hooks must be listed in Annex D. A host hook must conform to at least the following requirements:

A host-defined facility is one that defers its definition to an external source without further qualification and is listed in Annex D. Implementations that are not hosts may also provide definitions for host-defined facilities.

A host environment is a particular choice of definition for all host-defined facilities. A host environment typically includes objects or functions which allow obtaining input and providing output as host-defined properties of the global object.

This specification follows the editorial convention of always using the most specific term. For example, if a facility is host-defined, it should not be referred to as implementation-defined.

Both hosts and implementations may interface with this specification via the language types, specification types, abstract operations, grammar productions, intrinsic objects, and intrinsic symbols defined herein.

4.3 ECMAScript Overview

The following is an informal overview of ECMAScript—not all parts of the language are described. This overview is not part of the standard proper.

ECMAScript is object-based: basic language and host facilities are provided by objects, and an ECMAScript program is a cluster of communicating objects. In ECMAScript, an object is a collection of zero or more properties each with attributes that determine how each property can be used—for example, when the Writable attribute for a property is set to false, any attempt by executed ECMAScript code to assign a different value to the property fails. Properties are containers that hold other objects, primitive values, or functions. A primitive value is a member of one of the following built-in types: Undefined, Null, Boolean, Number, BigInt, String, and Symbol; an object is a member of the built-in type Object; and a function is a callable object. A function that is associated with an object via a property is called a method.

ECMAScript defines a collection of built-in objects that round out the definition of ECMAScript entities. These built-in objects include the global object; objects that are fundamental to the runtime semantics of the language including Object, Function, Boolean, Symbol, and various Error objects; objects that represent and manipulate numeric values including Math, Number, and Date; the text processing objects String and RegExp; objects that are indexed collections of values including Array and nine different kinds of Typed Arrays whose elements all have a specific numeric data representation; keyed collections including Map and Set objects; objects supporting structured data including the JSON object, ArrayBuffer, SharedArrayBuffer, and DataView; objects supporting control abstractions including generator functions and Promise objects; and reflection objects including Proxy and Reflect.

ECMAScript also defines a set of built-in operators. ECMAScript operators include various unary operations, multiplicative operators, additive operators, bitwise shift operators, relational operators, equality operators, binary bitwise operators, binary logical operators, assignment operators, and the comma operator.

Large ECMAScript programs are supported by modules which allow a program to be divided into multiple sequences of statements and declarations. Each module explicitly identifies declarations it uses that need to be provided by other modules and which of its declarations are available for use by other modules.

ECMAScript syntax intentionally resembles Java syntax. ECMAScript syntax is relaxed to enable it to serve as an easy-to-use scripting language. For example, a variable is not required to have its type declared nor are types associated with properties, and defined functions are not required to have their declarations appear textually before calls to them.

4.3.1 Objects

Even though ECMAScript includes syntax for class definitions, ECMAScript objects are not fundamentally class-based such as those in C , Smalltalk, or Java. Instead objects may be created in various ways including via a literal notation or via constructors which create objects and then execute code that initializes all or part of them by assigning initial values to their properties. Each constructor is a function that has a property named "prototype" that is used to implement prototype-based inheritance and shared properties. Objects are created by using constructors in new expressions; for example, new Date(2009, 11) creates a new Date object. Invoking a constructor without using new has consequences that depend on the constructor. For example, Date() produces a string representation of the current date and time rather than an object.

Every object created by a constructor has an implicit reference (called the object's prototype) to the value of its constructor's "prototype" property. Furthermore, a prototype may have a non-null implicit reference to its prototype, and so on; this is called the prototype chain. When a reference is made to a property in an object, that reference is to the property of that name in the first object in the prototype chain that contains a property of that name. In other words, first the object mentioned directly is examined for such a property; if that object contains the named property, that is the property to which the reference refers; if that object does not contain the named property, the prototype for that object is examined next; and so on.

Figure 1: Object/Prototype Relationships
An image of lots of boxes and arrows.

In a class-based object-oriented language, in general, state is carried by instances, methods are carried by classes, and inheritance is only of structure and behaviour. In ECMAScript, the state and methods are carried by objects, while structure, behaviour, and state are all inherited.

All objects that do not directly contain a particular property that their prototype contains share that property and its value. Figure 1 illustrates this:

CF is a constructor (and also an object). Five objects have been created by using new expressions: cf1, cf2, cf3, cf4, and cf5. Each of these objects contains properties named "q1" and "q2". The dashed lines represent the implicit prototype relationship; so, for example, cf3's prototype is CFp. The constructor, CF, has two properties itself, named "P1" and "P2", which are not visible to CFp, cf1, cf2, cf3, cf4, or cf5. The property named "CFP1" in CFp is shared by cf1, cf2, cf3, cf4, and cf5 (but not by CF), as are any properties found in CFp's implicit prototype chain that are not named "q1", "q2", or "CFP1". Notice that there is no implicit prototype link between CF and CFp.

Unlike most class-based object languages, properties can be added to objects dynamically by assigning values to them. That is, constructors are not required to name or assign values to all or any of the constructed object's properties. In the above diagram, one could add a new shared property for cf1, cf2, cf3, cf4, and cf5 by assigning a new value to the property in CFp.

Although ECMAScript objects are not inherently class-based, it is often convenient to define class-like abstractions based upon a common pattern of constructor functions, prototype objects, and methods. The ECMAScript built-in objects themselves follow such a class-like pattern. Beginning with ECMAScript 2015, the ECMAScript language includes syntactic class definitions that permit programmers to concisely define objects that conform to the same class-like abstraction pattern used by the built-in objects.

4.3.2 The Strict Variant of ECMAScript

The ECMAScript Language recognizes the possibility that some users of the language may wish to restrict their usage of some features available in the language. They might do so in the interests of security, to avoid what they consider to be error-prone features, to get enhanced error checking, or for other reasons of their choosing. In support of this possibility, ECMAScript defines a strict variant of the language. The strict variant of the language excludes some specific syntactic and semantic features of the regular ECMAScript language and modifies the detailed semantics of some features. The strict variant also specifies additional error conditions that must be reported by throwing error exceptions in situations that are not specified as errors by the non-strict form of the language.

The strict variant of ECMAScript is commonly referred to as the strict mode of the language. Strict mode selection and use of the strict mode syntax and semantics of ECMAScript is explicitly made at the level of individual ECMAScript source text units as described in 11.2.2. Because strict mode is selected at the level of a syntactic source text unit, strict mode only imposes restrictions that have local effect within such a source text unit. Strict mode does not restrict or modify any aspect of the ECMAScript semantics that must operate consistently across multiple source text units. A complete ECMAScript program may be composed of both strict mode and non-strict mode ECMAScript source text units. In this case, strict mode only applies when actually executing code that is defined within a strict mode source text unit.

In order to conform to this specification, an ECMAScript implementation must implement both the full unrestricted ECMAScript language and the strict variant of the ECMAScript language as defined by this specification. In addition, an implementation must support the combination of unrestricted and strict mode source text units into a single composite program.

4.4 Terms and Definitions

For the purposes of this document, the following terms and definitions apply.

4.4.1 implementation-approximated

an implementation-approximated facility is defined in whole or in part by an external source but has a recommended, ideal behaviour in this specification

4.4.2 implementation-defined

an implementation-defined facility is defined in whole or in part by an external source to this specification

4.4.3 host-defined

same as implementation-defined

Note

Editorially, see clause 4.2.

4.4.4 type

set of data values as defined in clause 6

4.4.5 primitive value

member of one of the types Undefined, Null, Boolean, Number, BigInt, Symbol, or String as defined in clause 6

Note

A primitive value is a datum that is represented directly at the lowest level of the language implementation.

4.4.6 object

member of the type Object

Note

An object is a collection of properties and has a single prototype object. The prototype may be null.

4.4.7 constructor

function object that creates and initializes objects

Note

The value of a constructor's "prototype" property is a prototype object that is used to implement inheritance and shared properties.

4.4.8 prototype

object that provides shared properties for other objects

Note

When a constructor creates an object, that object implicitly references the constructor's "prototype" property for the purpose of resolving property references. The constructor's "prototype" property can be referenced by the program expression constructor.prototype, and properties added to an object's prototype are shared, through inheritance, by all objects sharing the prototype. Alternatively, a new object may be created with an explicitly specified prototype by using the Object.create built-in function.

4.4.9 ordinary object

object that has the default behaviour for the essential internal methods that must be supported by all objects

4.4.10 exotic object

object that does not have the default behaviour for one or more of the essential internal methods

Note

Any object that is not an ordinary object is an exotic object.

4.4.11 standard object

object whose semantics are defined by this specification

4.4.12 built-in object

object specified and supplied by an ECMAScript implementation

Note

Standard built-in objects are defined in this specification. An ECMAScript implementation may specify and supply additional kinds of built-in objects. A built-in constructor is a built-in object that is also a constructor.

4.4.13 undefined value

primitive value used when a variable has not been assigned a value

4.4.14 Undefined type

type whose sole value is the undefined value

4.4.15 null value

primitive value that represents the intentional absence of any object value

4.4.16 Null type

type whose sole value is the null value

4.4.17 Boolean value

member of the Boolean type

Note

There are only two Boolean values, true and false.

4.4.18 Boolean type

type consisting of the primitive values true and false

4.4.19 Boolean object

member of the Object type that is an instance of the standard built-in Boolean constructor

Note

A Boolean object is created by using the Boolean constructor in a new expression, supplying a Boolean value as an argument. The resulting object has an internal slot whose value is the Boolean value. A Boolean object can be coerced to a Boolean value.

4.4.20 String value

primitive value that is a finite ordered sequence of zero or more 16-bit unsigned integer values

Note

A String value is a member of the String type. Each integer value in the sequence usually represents a single 16-bit unit of UTF-16 text. However, ECMAScript does not place any restrictions or requirements on the values except that they must be 16-bit unsigned integers.

4.4.21 String type

set of all possible String values

4.4.22 String object

member of the Object type that is an instance of the standard built-in String constructor

Note

A String object is created by using the String constructor in a new expression, supplying a String value as an argument. The resulting object has an internal slot whose value is the String value. A String object can be coerced to a String value by calling the String constructor as a function (22.1.1.1).

4.4.23 Number value

primitive value corresponding to a double-precision 64-bit binary format IEEE 754-2019 value

Note

A Number value is a member of the Number type and is a direct representation of a number.

4.4.24 Number type

set of all possible Number values including the special “Not-a-Number” (NaN) value, positive infinity, and negative infinity

4.4.25 Number object

member of the Object type that is an instance of the standard built-in Number constructor

Note

A Number object is created by using the Number constructor in a new expression, supplying a Number value as an argument. The resulting object has an internal slot whose value is the Number value. A Number object can be coerced to a Number value by calling the Number constructor as a function (21.1.1.1).

4.4.26 Infinity

Number value that is the positive infinite Number value

4.4.27 NaN

Number value that is an IEEE 754-2019 “Not-a-Number” value

4.4.28 BigInt value

primitive value corresponding to an arbitrary-precision integer value

4.4.29 BigInt type

set of all possible BigInt values

4.4.30 BigInt object

member of the Object type that is an instance of the standard built-in BigInt constructor

4.4.31 Symbol value

primitive value that represents a unique, non-String Object property key

4.4.32 Symbol type

set of all possible Symbol values

4.4.33 Symbol object

member of the Object type that is an instance of the standard built-in Symbol constructor

4.4.34 function

member of the Object type that may be invoked as a subroutine

Note

In addition to its properties, a function contains executable code and state that determine how it behaves when invoked. A function's code may or may not be written in ECMAScript.

4.4.35 built-in function

built-in object that is a function

Note

Examples of built-in functions include parseInt and Math.exp. A host or implementation may provide additional built-in functions that are not described in this specification.

4.4.36 property

part of an object that associates a key (either a String value or a Symbol value) and a value

Note

Depending upon the form of the property the value may be represented either directly as a data value (a primitive value, an object, or a function object) or indirectly by a pair of accessor functions.

4.4.37 method

function that is the value of a property

Note

When a function is called as a method of an object, the object is passed to the function as its this value.

4.4.38 built-in method

method that is a built-in function

Note

Standard built-in methods are defined in this specification. A host or implementation may provide additional built-in methods that are not described in this specification.

4.4.39 attribute

internal value that defines some characteristic of a property

4.4.40 own property

property that is directly contained by its object

4.4.41 inherited property

property of an object that is not an own property but is a property (either own or inherited) of the object's prototype

4.5 Organization of This Specification

The remainder of this specification is organized as follows:

Clause 5 defines the notational conventions used throughout the specification.

Clauses 6 through 10 define the execution environment within which ECMAScript programs operate.

Clauses 11 through 17 define the actual ECMAScript programming language including its syntactic encoding and the execution semantics of all language features.

Clauses 18 through 28 define the ECMAScript standard library. They include the definitions of all of the standard objects that are available for use by ECMAScript programs as they execute.

Clause 29 describes the memory consistency model of accesses on SharedArrayBuffer-backed memory and methods of the Atomics object.

5 Notational Conventions

5.1 Syntactic and Lexical Grammars

5.1.1 Context-Free Grammars

A context-free grammar consists of a number of productions. Each production has an abstract symbol called a nonterminal as its left-hand side, and a sequence of zero or more nonterminal and terminal symbols as its right-hand side. For each grammar, the terminal symbols are drawn from a specified alphabet.

A chain production is a production that has exactly one nonterminal symbol on its right-hand side along with zero or more terminal symbols.

Starting from a sentence consisting of a single distinguished nonterminal, called the goal symbol, a given context-free grammar specifies a language, namely, the (perhaps infinite) set of possible sequences of terminal symbols that can result from repeatedly replacing any nonterminal in the sequence with a right-hand side of a production for which the nonterminal is the left-hand side.

5.1.2 The Lexical and RegExp Grammars

A lexical grammar for ECMAScript is given in clause 12. This grammar has as its terminal symbols Unicode code points that conform to the rules for SourceCharacter defined in 11.1. It defines a set of productions, starting from the goal symbol InputElementDiv, InputElementTemplateTail, InputElementRegExp, InputElementRegExpOrTemplateTail, or InputElementHashbangOrRegExp, that describe how sequences of such code points are translated into a sequence of input elements.

Input elements other than white space and comments form the terminal symbols for the syntactic grammar for ECMAScript and are called ECMAScript tokens. These tokens are the reserved words, identifiers, literals, and punctuators of the ECMAScript language. Moreover, line terminators, although not considered to be tokens, also become part of the stream of input elements and guide the process of automatic semicolon insertion (12.10). Simple white space and single-line comments are discarded and do not appear in the stream of input elements for the syntactic grammar. A MultiLineComment (that is, a comment of the form /*…*/ regardless of whether it spans more than one line) is likewise simply discarded if it contains no line terminator; but if a MultiLineComment contains one or more line terminators, then it is replaced by a single line terminator, which becomes part of the stream of input elements for the syntactic grammar.

A RegExp grammar for ECMAScript is given in 22.2.1. This grammar also has as its terminal symbols the code points as defined by SourceCharacter. It defines a set of productions, starting from the goal symbol Pattern, that describe how sequences of code points are translated into regular expression patterns.

Productions of the lexical and RegExp grammars are distinguished by having two colons “::” as separating punctuation. The lexical and RegExp grammars share some productions.

5.1.3 The Numeric String Grammar

A numeric string grammar appears in 7.1.4.1. It has as its terminal symbols SourceCharacter, and is used for translating Strings into numeric values starting from the goal symbol StringNumericLiteral (which is similar to but distinct from the lexical grammar for numeric literals).

Productions of the numeric string grammar are distinguished by having three colons “:::” as punctuation, and are never used for parsing source text.

5.1.4 The Syntactic Grammar

The syntactic grammar for ECMAScript is given in clauses 13 through 16. This grammar has ECMAScript tokens defined by the lexical grammar as its terminal symbols (5.1.2). It defines a set of productions, starting from two alternative goal symbols Script and Module, that describe how sequences of tokens form syntactically correct independent components of ECMAScript programs.

When a stream of code points is to be parsed as an ECMAScript Script or Module, it is first converted to a stream of input elements by repeated application of the lexical grammar; this stream of input elements is then parsed by a single application of the syntactic grammar. The input stream is syntactically in error if the tokens in the stream of input elements cannot be parsed as a single instance of the goal nonterminal (Script or Module), with no tokens left over.

When a parse is successful, it constructs a parse tree, a rooted tree structure in which each node is a Parse Node. Each Parse Node is an instance of a symbol in the grammar; it represents a span of the source text that can be derived from that symbol. The root node of the parse tree, representing the whole of the source text, is an instance of the parse's goal symbol. When a Parse Node is an instance of a nonterminal, it is also an instance of some production that has that nonterminal as its left-hand side. Moreover, it has zero or more children, one for each symbol on the production's right-hand side: each child is a Parse Node that is an instance of the corresponding symbol.

New Parse Nodes are instantiated for each invocation of the parser and never reused between parses even of identical source text. Parse Nodes are considered the same Parse Node if and only if they represent the same span of source text, are instances of the same grammar symbol, and resulted from the same parser invocation.

Note 1

Parsing the same String multiple times will lead to different Parse Nodes. For example, consider:

let str = "1   1;";
eval(str);
eval(str);

Each call to eval converts the value of str into ECMAScript source text and performs an independent parse that creates its own separate tree of Parse Nodes. The trees are distinct even though each parse operates upon a source text that was derived from the same String value.

 Note 2
Parse Nodes are specification artefacts, and implementations are not required to use an analogous data structure.

Productions of the syntactic grammar are distinguished by having just one colon “:” as punctuation.

The syntactic grammar as presented in clauses 13 through 16 is not a complete account of which token sequences are accepted as a correct ECMAScript Script or Module. Certain additional token sequences are also accepted, namely, those that would be described by the grammar if only semicolons were added to the sequence in certain places (such as before line terminator characters). Furthermore, certain token sequences that are described by the grammar are not considered acceptable if a line terminator character appears in certain “awkward” places.

In certain cases, in order to avoid ambiguities, the syntactic grammar uses generalized productions that permit token sequences that do not form a valid ECMAScript Script or Module. For example, this technique is used for object literals and object destructuring patterns. In such cases a more restrictive supplemental grammar is provided that further restricts the acceptable token sequences. Typically, an early error rule will then state that, in certain contexts, "P must cover an N", where P is a Parse Node (an instance of the generalized production) and N is a nonterminal from the supplemental grammar. This means:

  1. The sequence of tokens originally matched by P is parsed again using N as the goal symbol. If N takes grammatical parameters, then they are set to the same values used when P was originally parsed.
  2. If the sequence of tokens can be parsed as a single instance of N, with no tokens left over, then:
    1. We refer to that instance of N (a Parse Node, unique for a given P) as "the N that is covered by P".
    2. All Early Error rules for N and its derived productions also apply to the N that is covered by P.
  3. Otherwise (if the parse fails), it is an early Syntax Error.

5.1.5 Grammar Notation

5.1.5.1 Terminal Symbols

In the ECMAScript grammars, some terminal symbols are shown in fixed-width font. These are to appear in a source text exactly as written. All terminal symbol code points specified in this way are to be understood as the appropriate Unicode code points from the Basic Latin block, as opposed to any similar-looking code points from other Unicode ranges. A code point in a terminal symbol cannot be expressed by a \ UnicodeEscapeSequence.

In grammars whose terminal symbols are individual Unicode code points (i.e., the lexical, RegExp, and numeric string grammars), a contiguous run of multiple fixed-width code points appearing in a production is a simple shorthand for the same sequence of code points, written as standalone terminal symbols.

For example, the production:

HexIntegerLiteral :: 0x HexDigits

is a shorthand for:

HexIntegerLiteral :: 0 x HexDigits

In contrast, in the syntactic grammar, a contiguous run of fixed-width code points is a single terminal symbol.

Terminal symbols come in two other forms:

  • In the lexical and RegExp grammars, Unicode code points without a conventional printed representation are instead shown in the form "<ABBREV>" where "ABBREV" is a mnemonic for the code point or set of code points. These forms are defined in Unicode Format-Control Characters, White Space, and Line Terminators.
  • In the syntactic grammar, certain terminal symbols (e.g. IdentifierName and RegularExpressionLiteral) are shown in italics, as they refer to the nonterminals of the same name in the lexical grammar.

5.1.5.2 Nonterminal Symbols and Productions

Nonterminal symbols are shown in italic type. The definition of a nonterminal (also called a “production”) is introduced by the name of the nonterminal being defined followed by one or more colons. (The number of colons indicates to which grammar the production belongs.) One or more alternative right-hand sides for the nonterminal then follow on succeeding lines. For example, the syntactic definition:

WhileStatement : while ( Expression ) Statement

states that the nonterminal WhileStatement represents the token while, followed by a left parenthesis token, followed by an Expression, followed by a right parenthesis token, followed by a Statement. The occurrences of Expression and Statement are themselves nonterminals. As another example, the syntactic definition:

ArgumentList : AssignmentExpression ArgumentList , AssignmentExpression

states that an ArgumentList may represent either a single AssignmentExpression or an ArgumentList, followed by a comma, followed by an AssignmentExpression. This definition of ArgumentList is recursive, that is, it is defined in terms of itself. The result is that an ArgumentList may contain any positive number of arguments, separated by commas, where each argument expression is an AssignmentExpression. Such recursive definitions of nonterminals are common.

5.1.5.3 Optional Symbols

The subscripted suffix “opt”, which may appear after a terminal or nonterminal, indicates an optional symbol. The alternative containing the optional symbol actually specifies two right-hand sides, one that omits the optional element and one that includes it. This means that:

VariableDeclaration : BindingIdentifier Initializeropt

is a convenient abbreviation for:

VariableDeclaration : BindingIdentifier BindingIdentifier Initializer

and that:

ForStatement : for ( LexicalDeclaration Expressionopt ; Expressionopt ) Statement

is a convenient abbreviation for:

ForStatement : for ( LexicalDeclaration ; Expressionopt ) Statement for ( LexicalDeclaration Expression ; Expressionopt ) Statement

which in turn is an abbreviation for:

ForStatement : for ( LexicalDeclaration ; ) Statement for ( LexicalDeclaration ; Expression ) Statement for ( LexicalDeclaration Expression ; ) Statement for ( LexicalDeclaration Expression ; Expression ) Statement

so, in this example, the nonterminal ForStatement actually has four alternative right-hand sides.

5.1.5.4 Grammatical Parameters

A production may be parameterized by a subscripted annotation of the form “[parameters]”, which may appear as a suffix to the nonterminal symbol defined by the production. “parameters” may be either a single name or a comma separated list of names. A parameterized production is shorthand for a set of productions defining all combinations of the parameter names, preceded by an underscore, appended to the parameterized nonterminal symbol. This means that:

StatementList[Return] : ReturnStatement ExpressionStatement

is a convenient abbreviation for:

StatementList : ReturnStatement ExpressionStatement StatementList_Return : ReturnStatement ExpressionStatement

and that:

StatementList[Return, In] : ReturnStatement ExpressionStatement

is an abbreviation for:

StatementList : ReturnStatement ExpressionStatement StatementList_Return : ReturnStatement ExpressionStatement StatementList_In : ReturnStatement ExpressionStatement StatementList_Return_In : ReturnStatement ExpressionStatement

Multiple parameters produce a combinatory number of productions, not all of which are necessarily referenced in a complete grammar.

References to nonterminals on the right-hand side of a production can also be parameterized. For example:

StatementList : ReturnStatement ExpressionStatement[ In]

is equivalent to saying:

StatementList : ReturnStatement ExpressionStatement_In

and:

StatementList : ReturnStatement ExpressionStatement[~In]

is equivalent to:

StatementList : ReturnStatement ExpressionStatement

A nonterminal reference may have both a parameter list and an “opt” suffix. For example:

VariableDeclaration : BindingIdentifier Initializer[ In]opt

is an abbreviation for:

VariableDeclaration : BindingIdentifier BindingIdentifier Initializer_In

Prefixing a parameter name with “?” on a right-hand side nonterminal reference makes that parameter value dependent upon the occurrence of the parameter name on the reference to the current production's left-hand side symbol. For example:

VariableDeclaration[In] : BindingIdentifier Initializer[?In]

is an abbreviation for:

VariableDeclaration : BindingIdentifier Initializer VariableDeclaration_In : BindingIdentifier Initializer_In

If a right-hand side alternative is prefixed with “[ parameter]” that alternative is only available if the named parameter was used in referencing the production's nonterminal symbol. If a right-hand side alternative is prefixed with “[~parameter]” that alternative is only available if the named parameter was not used in referencing the production's nonterminal symbol. This means that:

StatementList[Return] : [ Return] ReturnStatement ExpressionStatement

is an abbreviation for:

StatementList : ExpressionStatement StatementList_Return : ReturnStatement ExpressionStatement

and that:

StatementList[Return] : [~Return] ReturnStatement ExpressionStatement

is an abbreviation for:

StatementList : ReturnStatement ExpressionStatement StatementList_Return : ExpressionStatement

5.1.5.5 one of

When the words “one of” follow the colon(s) in a grammar definition, they signify that each of the terminal symbols on the following line or lines is an alternative definition. For example, the lexical grammar for ECMAScript contains the production:

NonZeroDigit :: one of 1 2 3 4 5 6 7 8 9

which is merely a convenient abbreviation for:

NonZeroDigit :: 1 2 3 4 5 6 7 8 9

5.1.5.6 [empty]

If the phrase “[empty]” appears as the right-hand side of a production, it indicates that the production's right-hand side contains no terminals or nonterminals.

5.1.5.7 Lookahead Restrictions

If the phrase “[lookahead = seq]” appears in the right-hand side of a production, it indicates that the production may only be used if the token sequence seq is a prefix of the immediately following input token sequence. Similarly, “[lookahead ∈ set]”, where set is a finite non-empty set of token sequences, indicates that the production may only be used if some element of set is a prefix of the immediately following token sequence. For convenience, the set can also be written as a nonterminal, in which case it represents the set of all token sequences to which that nonterminal could expand. It is considered an editorial error if the nonterminal could expand to infinitely many distinct token sequences.

These conditions may be negated. “[lookahead ≠ seq]” indicates that the containing production may only be used if seq is not a prefix of the immediately following input token sequence, and “[lookahead ∉ set]” indicates that the production may only be used if no element of set is a prefix of the immediately following token sequence.

As an example, given the definitions:

DecimalDigit :: one of 0 1 2 3 4 5 6 7 8 9 DecimalDigits :: DecimalDigit DecimalDigits DecimalDigit

the definition:

LookaheadExample :: n [lookahead ∉ { 1, 3, 5, 7, 9 }] DecimalDigits DecimalDigit [lookahead ∉ DecimalDigit]

matches either the letter n followed by one or more decimal digits the first of which is even, or a decimal digit not followed by another decimal digit.

Note that when these phrases are used in the syntactic grammar, it may not be possible to unambiguously identify the immediately following token sequence because determining later tokens requires knowing which lexical goal symbol to use at later positions. As such, when these are used in the syntactic grammar, it is considered an editorial error for a token sequence seq to appear in a lookahead restriction (including as part of a set of sequences) if the choices of lexical goal symbols to use could change whether or not seq would be a prefix of the resulting token sequence.

5.1.5.8 [no LineTerminator here]

If the phrase “[no LineTerminator here]” appears in the right-hand side of a production of the syntactic grammar, it indicates that the production is a restricted production: it may not be used if a LineTerminator occurs in the input stream at the indicated position. For example, the production:

ThrowStatement : throw [no LineTerminator here] Expression ;

indicates that the production may not be used if a LineTerminator occurs in the script between the throw token and the Expression.

Unless the presence of a LineTerminator is forbidden by a restricted production, any number of occurrences of LineTerminator may appear between any two consecutive tokens in the stream of input elements without affecting the syntactic acceptability of the script.

5.1.5.9 but not

The right-hand side of a production may specify that certain expansions are not permitted by using the phrase “but not” and then indicating the expansions to be excluded. For example, the production:

Identifier :: IdentifierName but not ReservedWord

means that the nonterminal Identifier may be replaced by any sequence of code points that could replace IdentifierName provided that the same sequence of code points could not replace ReservedWord.

5.1.5.10 Descriptive Phrases

Finally, a few nonterminal symbols are described by a descriptive phrase in sans-serif type in cases where it would be impractical to list all the alternatives:

SourceCharacter :: any Unicode code point

5.2 Algorithm Conventions

The specification often uses a numbered list to specify steps in an algorithm. These algorithms are used to precisely specify the required semantics of ECMAScript language constructs. The algorithms are not intended to imply the use of any specific implementation technique. In practice, there may be more efficient algorithms available to implement a given feature.

Algorithms may be explicitly parameterized with an ordered, comma-separated sequence of alias names which may be used within the algorithm steps to reference the argument passed in that position. Optional parameters are denoted with surrounding brackets ([ , name ]) and are no different from required parameters within algorithm steps. A rest parameter may appear at the end of a parameter list, denoted with leading ellipsis (, ...name). The rest parameter captures all of the arguments provided following the required and optional parameters into a List. If there are no such additional arguments, that List is empty.

Algorithm steps may be subdivided into sequential substeps. Substeps are indented and may themselves be further divided into indented substeps. Outline numbering conventions are used to identify substeps with the first level of substeps labelled with lowercase alphabetic characters and the second level of substeps labelled with lowercase roman numerals. If more than three levels are required these rules repeat with the fourth level using numeric labels. For example:

  1. Top-level step
    1. Substep.
    2. Substep.
      1. Subsubstep.
        1. Subsubsubstep
          1. Subsubsubsubstep
            1. Subsubsubsubsubstep

A step or substep may be written as an “if” predicate that conditions its substeps. In this case, the substeps are only applied if the predicate is true. If a step or substep begins with the word “else”, it is a predicate that is the negation of the preceding “if” predicate step at the same level.

A step may specify the iterative application of its substeps.

A step that begins with “Assert:” asserts an invariant condition of its algorithm. Such assertions are used to make explicit algorithmic invariants that would otherwise be implicit. Such assertions add no additional semantic requirements and hence need not be checked by an implementation. They are used simply to clarify algorithms.

Algorithm steps may declare named aliases for any value using the form “Let x be someValue”. These aliases are reference-like in that both x and someValue refer to the same underlying data and modifications to either are visible to both. Algorithm steps that want to avoid this reference-like behaviour should explicitly make a copy of the right-hand side: “Let x be a copy of someValue” creates a shallow copy of someValue.

Once declared, an alias may be referenced in any subsequent steps and must not be referenced from steps prior to the alias's declaration. Aliases may be modified using the form “Set x to someOtherValue”.

5.2.1 Abstract Operations

In order to facilitate their use in multiple parts of this specification, some algorithms, called abstract operations, are named and written in parameterized functional form so that they may be referenced by name from within other algorithms. Abstract operations are typically referenced using a functional application style such as OperationName(arg1, arg2). Some abstract operations are treated as polymorphically dispatched methods of class-like specification abstractions. Such method-like abstract operations are typically referenced using a method application style such as someValue.OperationName(arg1, arg2).

5.2.2 Syntax-Directed Operations

A syntax-directed operation is a named operation whose definition consists of algorithms, each of which is associated with one or more productions from one of the ECMAScript grammars. A production that has multiple alternative definitions will typically have a distinct algorithm for each alternative. When an algorithm is associated with a grammar production, it may reference the terminal and nonterminal symbols of the production alternative as if they were parameters of the algorithm. When used in this manner, nonterminal symbols refer to the actual alternative definition that is matched when parsing the source text. The source text matched by a grammar production or Parse Node derived from it is the portion of the source text that starts at the beginning of the first terminal that participated in the match and ends at the end of the last terminal that participated in the match.

When an algorithm is associated with a production alternative, the alternative is typically shown without any “[ ]” grammar annotations. Such annotations should only affect the syntactic recognition of the alternative and have no effect on the associated semantics for the alternative.

Syntax-directed operations are invoked with a parse node and, optionally, other parameters by using the conventions on steps 1, 3, and 4 in the following algorithm:

  1. Let status be SyntaxDirectedOperation of SomeNonTerminal.
  2. Let someParseNode be the parse of some source text.
  3. Perform SyntaxDirectedOperation of someParseNode.
  4. Perform SyntaxDirectedOperation of someParseNode with argument "value".

Unless explicitly specified otherwise, all chain productions have an implicit definition for every operation that might be applied to that production's left-hand side nonterminal. The implicit definition simply reapplies the same operation with the same parameters, if any, to the chain production's sole right-hand side nonterminal and then returns the result. For example, assume that some algorithm has a step of the form: “Return Evaluation of Block” and that there is a production:

Block : { StatementList }

but the Evaluation operation does not associate an algorithm with that production. In that case, the Evaluation operation implicitly includes an association of the form:

Runtime Semantics: Evaluation

Block : { StatementList }
  1. Return Evaluation of StatementList.

5.2.3 Runtime Semantics

Algorithms which specify semantics that must be called at runtime are called runtime semantics. Runtime semantics are defined by abstract operations or syntax-directed operations.

5.2.3.1 Completion ( completionRecord )

The abstract operation Completion takes argument completionRecord (a Completion Record) and returns a Completion Record. It is used to emphasize that a Completion Record is being returned. It performs the following steps when called:

  1. Assert: completionRecord is a Completion Record.
  2. Return completionRecord.

5.2.3.2 Throw an Exception

Algorithms steps that say to throw an exception, such as

  1. Throw a TypeError exception.

mean the same things as:

  1. Return ThrowCompletion(a newly created TypeError object).

5.2.3.3 ReturnIfAbrupt

Algorithms steps that say or are otherwise equivalent to:

  1. ReturnIfAbrupt(argument).

mean the same thing as:

  1. Assert: argument is a Completion Record.
  2. If argument is an abrupt completion, return Completion(argument).
  3. Else, set argument to argument.[[Value]].

Algorithms steps that say or are otherwise equivalent to:

  1. ReturnIfAbrupt(AbstractOperation()).

mean the same thing as:

  1. Let hygienicTemp be AbstractOperation().
  2. Assert: hygienicTemp is a Completion Record.
  3. If hygienicTemp is an abrupt completion, return Completion(hygienicTemp).
  4. Else, set hygienicTemp to hygienicTemp.[[Value]].

Where hygienicTemp is ephemeral and visible only in the steps pertaining to ReturnIfAbrupt.

Algorithms steps that say or are otherwise equivalent to:

  1. Let result be AbstractOperation(ReturnIfAbrupt(argument)).

mean the same thing as:

  1. Assert: argument is a Completion Record.
  2. If argument is an abrupt completion, return Completion(argument).
  3. Else, set argument to argument.[[Value]].
  4. Let result be AbstractOperation(argument).

5.2.3.4 ReturnIfAbrupt Shorthands

Invocations of abstract operations and syntax-directed operations that are prefixed by ? indicate that ReturnIfAbrupt should be applied to the resulting Completion Record. For example, the step:

  1. ? OperationName().

is equivalent to the following step:

  1. ReturnIfAbrupt(OperationName()).

Similarly, for method application style, the step:

  1. ? someValue.OperationName().

is equivalent to:

  1. ReturnIfAbrupt(someValue.OperationName()).

Similarly, prefix ! is used to indicate that the following invocation of an abstract or syntax-directed operation will never return an abrupt completion and that the resulting Completion Record's [[Value]] field should be used in place of the return value of the operation. For example, the step:

  1. Let val be ! OperationName().

is equivalent to the following steps:

  1. Let val be OperationName().
  2. Assert: val is a normal completion.
  3. Set val to val.[[Value]].

Syntax-directed operations for runtime semantics make use of this shorthand by placing ! or ? before the invocation of the operation:

  1. Perform ! SyntaxDirectedOperation of NonTerminal.

5.2.3.5 Implicit Normal Completion

In algorithms within abstract operations which are declared to return a Completion Record, and within all built-in functions, the returned value is first passed to NormalCompletion, and the result is used instead. This rule does not apply within the Completion algorithm or when the value being returned is clearly marked as a Completion Record in that step; these cases are:

It is an editorial error if a Completion Record is returned from such an abstract operation through any other means. For example, within these abstract operations,

  1. Return true.

means the same things as any of

  1. Return NormalCompletion(true).

or

  1. Let completion be NormalCompletion(true).
  2. Return Completion(completion).

or

  1. Return Completion Record { [[Type]]: normal, [[Value]]: true, [[Target]]: empty }.

Note that, through the ReturnIfAbrupt expansion, the following example is allowed, as within the expanded steps, the result of applying Completion is returned directly in the abrupt case and the implicit NormalCompletion application occurs after unwrapping in the normal case.

  1. Return ? completion.

The following example would be an editorial error because a Completion Record is being returned without being annotated in that step.

  1. Let completion be NormalCompletion(true).
  2. Return completion.

5.2.4 Static Semantics

Context-free grammars are not sufficiently powerful to express all the rules that define whether a stream of input elements form a valid ECMAScript Script or Module that may be evaluated. In some situations additional rules are needed that may be expressed using either ECMAScript algorithm conventions or prose requirements. Such rules are always associated with a production of a grammar and are called the static semantics of the production.

Static Semantic Rules have names and typically are defined using an algorithm. Named Static Semantic Rules are associated with grammar productions and a production that has multiple alternative definitions will typically have for each alternative a distinct algorithm for each applicable named static semantic rule.

A special kind of static semantic rule is an Early Error Rule. Early error rules define early error conditions (see clause 17) that are associated with specific grammar productions. Evaluation of most early error rules are not explicitly invoked within the algorithms of this specification. A conforming implementation must, prior to the first evaluation of a Script or Module, validate all of the early error rules of the productions used to parse that Script or Module. If any of the early error rules are violated the Script or Module is invalid and cannot be evaluated.

5.2.5 Mathematical Operations

This specification makes reference to these kinds of numeric values:

In the language of this specification, numerical values are distinguished among different numeric kinds using subscript suffixes. The subscript 𝔽 refers to Numbers, and the subscript ℤ refers to BigInts. Numeric values without a subscript suffix refer to mathematical values.

Numeric operators such as , ×, =, and ≥ refer to those operations as determined by the type of the operands. When applied to mathematical values, the operators refer to the usual mathematical operations. When applied to extended mathematical values, the operators refer to the usual mathematical operations over the extended real numbers; indeterminate forms are not defined and their use in this specification should be considered an editorial error. When applied to Numbers, the operators refer to the relevant operations within IEEE 754-2019. When applied to BigInts, the operators refer to the usual mathematical operations applied to the mathematical value of the BigInt.

In general, when this specification refers to a numerical value, such as in the phrase, "the length of y" or "the integer represented by the four hexadecimal digits ...", without explicitly specifying a numeric kind, the phrase refers to a mathematical value. Phrases which refer to a Number or a BigInt value are explicitly annotated as such; for example, "the Number value for the number of code points in …" or "the BigInt value for …".

Numeric operators applied to mixed-type operands (such as a Number and a mathematical value) are not defined and should be considered an editorial error in this specification.

This specification denotes most numeric values in base 10; it also uses numeric values of the form 0x followed by digits 0-9 or A-F as base-16 values.

When the term integer is used in this specification, it refers to a mathematical value which is in the set of integers, unless otherwise stated. When the term integral Number is used in this specification, it refers to a Number value whose mathematical value is in the set of integers.

Conversions between mathematical values and Numbers or BigInts are always explicit in this document. A conversion from a mathematical value or extended mathematical value x to a Number is denoted as "the Number value for x" or 𝔽(x), and is defined in 6.1.6.1. A conversion from an integer x to a BigInt is denoted as "the BigInt value for x" or ℤ(x). A conversion from a Number or BigInt x to a mathematical value is denoted as "the mathematical value of x", or ℝ(x). The mathematical value of 0𝔽 and -0𝔽 is the mathematical value 0. The mathematical value of non-finite values is not defined. The extended mathematical value of x is the mathematical value of x for finite values, and is ∞ and -∞ for ∞𝔽 and -∞𝔽 respectively; it is not defined for NaN.

The mathematical function abs(x) produces the absolute value of x, which is -x if x < 0 and otherwise is x itself.

The mathematical function min(x1, x2, … , xN) produces the mathematically smallest of x1 through xN. The mathematical function max(x1, x2, ..., xN) produces the mathematically largest of x1 through xN. The domain and range of these mathematical functions are the extended mathematical values.

The notation “x modulo y” (y must be finite and non-zero) computes a value k of the same sign as y (or zero) such that abs(k) < abs(y) and x - k = q × y for some integer q.

The phrase "the result of clamping x between lower and upper" (where x is an extended mathematical value and lower and upper are mathematical values such that lower ≤ upper) produces lower if x < lower, produces upper if x > upper, and otherwise produces x.

The mathematical function floor(x) produces the largest integer (closest to ∞) that is not larger than x.

The mathematical function truncate(x) removes the fractional part of x by rounding towards zero, producing -floor(-x) if x < 0 and otherwise producing floor(x).

Mathematical functions min, max, abs, floor, and truncate are not defined for Numbers and BigInts, and any usage of those methods that have non-mathematical value arguments would be an editorial error in this specification.

Note

floor(x) = x - (x modulo 1).

An interval from lower bound a to upper bound b is a possibly-infinite, possibly-empty set of numeric values of the same numeric type. Each bound will be described as either inclusive or exclusive, but not both. There are four kinds of intervals, as follows:

  • An interval from a (inclusive) to b (inclusive), also called an inclusive interval from a to b, includes all values x of the same numeric type such that a ≤ x ≤ b, and no others.
  • An interval from a (inclusive) to b (exclusive) includes all values x of the same numeric type such that a ≤ x < b, and no others.
  • An interval from a (exclusive) to b (inclusive) includes all values x of the same numeric type such that a < x ≤ b, and no others.
  • An interval from a (exclusive) to b (exclusive) includes all values x of the same numeric type such that a < x < b, and no others.

For example, the interval from 1 (inclusive) to 2 (exclusive) consists of all mathematical values between 1 and 2, including 1 and not including 2. For the purpose of defining intervals, -0𝔽 < 0𝔽, so, for example, an inclusive interval with a lower bound of 0𝔽 includes 0𝔽 but not -0𝔽. NaN is never included in an interval.

5.2.6 Value Notation

In this specification, ECMAScript language values are displayed in bold. Examples include null, true, or "hello". These are distinguished from ECMAScript source text such as Function.prototype.apply or let n = 42;.

5.2.7 Identity

In this specification, both specification values and ECMAScript language values are compared for equality. When comparing for equality, values fall into one of two categories. Values without identity are equal to other values without identity if all of their innate characteristics are the same — characteristics such as the magnitude of an integer or the length of a sequence. Values without identity may be manifest without prior reference by fully describing their characteristics. In contrast, each value with identity is unique and therefore only equal to itself. Values with identity are like values without identity but with an additional unguessable, unchangeable, universally-unique characteristic called identity. References to existing values with identity cannot be manifest simply by describing them, as the identity itself is indescribable; instead, references to these values must be explicitly passed from one place to another. Some values with identity are mutable and therefore can have their characteristics (except their identity) changed in-place, causing all holders of the value to observe the new characteristics. A value without identity is never equal to a value with identity.

From the perspective of this specification, the word “is” is used to compare two values for equality, as in “If bool is true, then ...”, and the word “contains” is used to search for a value inside lists using equality comparisons, as in "If list contains a Record r such that r.[[Foo]] is true, then ...". The specification identity of values determines the result of these comparisons and is axiomatic in this specification.

From the perspective of the ECMAScript language, language values are compared for equality using the SameValue abstract operation and the abstract operations it transitively calls. The algorithms of these comparison abstract operations determine language identity of ECMAScript language values.

For specification values, examples of values without specification identity include, but are not limited to: mathematical values and extended mathematical values; ECMAScript source text, surrogate pairs, Directive Prologues, etc; UTF-16 code units; Unicode code points; enums; abstract operations, including syntax-directed operations, host hooks, etc; and ordered pairs. Examples of specification values with specification identity include, but are not limited to: any kind of Records, including Property Descriptors, PrivateElements, etc; Parse Nodes; Lists; Sets and Relations; Abstract Closures; Data Blocks; Private Names; execution contexts and execution context stacks; agent signifiers; and WaiterLists.

Specification identity agrees with language identity for all ECMAScript language values except Symbol values produced by Symbol.for. The ECMAScript language values without specification identity and without language identity are undefined, null, Booleans, Strings, Numbers, and BigInts. The ECMAScript language values with specification identity and language identity are Symbols not produced by Symbol.for and Objects. Symbol values produced by Symbol.for have specification identity, but not language identity.

6 ECMAScript Data Types and Values

Algorithms within this specification manipulate values each of which has an associated type. The possible value types are exactly those defined in this clause. Types are further subclassified into ECMAScript language types and specification types.

Within this specification, the notation “Type(x)” is used as shorthand for “the type of x” where “type” refers to the ECMAScript language and specification types defined in this clause.

6.1 ECMAScript Language Types

An ECMAScript language type corresponds to values that are directly manipulated by an ECMAScript programmer using the ECMAScript language. The ECMAScript language types are Undefined, Null, Boolean, String, Symbol, Number, BigInt, and Object. An ECMAScript language value is a value that is characterized by an ECMAScript language type.

6.1.1 The Undefined Type

The Undefined type has exactly one value, called undefined. Any variable that has not been assigned a value has the value undefined.

6.1.2 The Null Type

The Null type has exactly one value, called null.

6.1.3 The Boolean Type

The Boolean type represents a logical entity having two values, called true and false.

6.1.4 The String Type

The String type is the set of all ordered sequences of zero or more 16-bit unsigned integer values (“elements”) up to a maximum length of 253 - 1 elements. The String type is generally used to represent textual data in a running ECMAScript program, in which case each element in the String is treated as a UTF-16 code unit value. Each element is regarded as occupying a position within the sequence. These positions are indexed with non-negative integers. The first element (if any) is at index 0, the next element (if any) at index 1, and so on. The length of a String is the number of elements (i.e., 16-bit values) within it. The empty String has length zero and therefore contains no elements.

ECMAScript operations that do not interpret String contents apply no further semantics. Operations that do interpret String values treat each element as a single UTF-16 code unit. However, ECMAScript does not restrict the value of or relationships between these code units, so operations that further interpret String contents as sequences of Unicode code points encoded in UTF-16 must account for ill-formed subsequences. Such operations apply special treatment to every code unit with a numeric value in the inclusive interval from 0xD800 to 0xDBFF (defined by the Unicode Standard as a leading surrogate, or more formally as a high-surrogate code unit) and every code unit with a numeric value in the inclusive interval from 0xDC00 to 0xDFFF (defined as a trailing surrogate, or more formally as a low-surrogate code unit) using the following rules:

The function String.prototype.normalize (see 22.1.3.14) can be used to explicitly normalize a String value. String.prototype.localeCompare (see 22.1.3.11) internally normalizes String values, but no other operations implicitly normalize the strings upon which they operate. Operation results are not language- and/or locale-sensitive unless stated otherwise.

Note

The rationale behind this design was to keep the implementation of Strings as simple and high-performing as possible. If ECMAScript source text is in Normalized Form C, string literals are guaranteed to also be normalized, as long as they do not contain any Unicode escape sequences.

In this specification, the phrase "the string-concatenation of A, B, ..." (where each argument is a String value, a code unit, or a sequence of code units) denotes the String value whose sequence of code units is the concatenation of the code units (in order) of each of the arguments (in order).

The phrase "the substring of S from inclusiveStart to exclusiveEnd" (where S is a String value or a sequence of code units and inclusiveStart and exclusiveEnd are integers) denotes the String value consisting of the consecutive code units of S beginning at index inclusiveStart and ending immediately before index exclusiveEnd (which is the empty String when inclusiveStart = exclusiveEnd). If the "to" suffix is omitted, the length of S is used as the value of exclusiveEnd.

The phrase "the ASCII word characters" denotes the following String value, which consists solely of every letter and number in the Unicode Basic Latin block along with U 005F (LOW LINE):
"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_".
For historical reasons, it has significance to various algorithms.

6.1.4.1 StringIndexOf ( string, searchValue, fromIndex )

The abstract operation StringIndexOf takes arguments string (a String), searchValue (a String), and fromIndex (a non-negative integer) and returns an integer. It performs the following steps when called:

  1. Let len be the length of string.
  2. If searchValue is the empty String and fromIndex ≤ len, return fromIndex.
  3. Let searchLen be the length of searchValue.
  4. For each integer i such that fromIndex ≤ i ≤ len - searchLen, in ascending order, do
    1. Let candidate be the substring of string from i to i searchLen.
    2. If candidate is searchValue, return i.
  5. Return -1.
 Note 1

If searchValue is the empty String and fromIndex ≤ the length of string, this algorithm returns fromIndex. The empty String is effectively found at every position within a string, including after the last code unit.

 Note 2

This algorithm always returns -1 if fromIndex the length of searchValue > the length of string.

6.1.5 The Symbol Type

The Symbol type is the set of all non-String values that may be used as the key of an Object property (6.1.7).

Each possible Symbol value is unique and immutable.

Each Symbol value immutably holds an associated value called [[Description]] that is either undefined or a String value.

6.1.5.1 Well-Known Symbols

Well-known symbols are built-in Symbol values that are explicitly referenced by algorithms of this specification. They are typically used as the keys of properties whose values serve as extension points of a specification algorithm. Unless otherwise specified, well-known symbols values are shared by all realms (9.3).

Within this specification a well-known symbol is referred to by using a notation of the form @@name, where “name” is one of the values listed in Table 1.

Table 1: Well-known Symbols
Specification Name [[Description]] Value and Purpose
@@asyncIterator "Symbol.asyncIterator" A method that returns the default AsyncIterator for an object. Called by the semantics of the for-await-of statement.
@@hasInstance "Symbol.hasInstance" A method that determines if a constructor object recognizes an object as one of the constructor's instances. Called by the semantics of the instanceof operator.
@@isConcatSpreadable "Symbol.isConcatSpreadable" A Boolean valued property that if true indicates that an object should be flattened to its array elements by Array.prototype.concat.
@@iterator "Symbol.iterator" A method that returns the default Iterator for an object. Called by the semantics of the for-of statement.
@@match "Symbol.match" A regular expression method that matches the regular expression against a string. Called by the String.prototype.match method.
@@matchAll "Symbol.matchAll" A regular expression method that returns an iterator, that yields matches of the regular expression against a string. Called by the String.prototype.matchAll method.
@@replace "Symbol.replace" A regular expression method that replaces matched substrings of a string. Called by the String.prototype.replace method.
@@search "Symbol.search" A regular expression method that returns the index within a string that matches the regular expression. Called by the String.prototype.search method.
@@species "Symbol.species" A function valued property that is the constructor function that is used to create derived objects.
@@split "Symbol.split" A regular expression method that splits a string at the indices that match the regular expression. Called by the String.prototype.split method.
@@toPrimitive "Symbol.toPrimitive" A method that converts an object to a corresponding primitive value. Called by the ToPrimitive abstract operation.
@@toStringTag "Symbol.toStringTag" A String valued property that is used in the creation of the default string description of an object. Accessed by the built-in method Object.prototype.toString.
@@unscopables "Symbol.unscopables" An object valued property whose own and inherited property names are property names that are excluded from the with environment bindings of the associated object.

6.1.6 Numeric Types

ECMAScript has two built-in numeric types: Number and BigInt. The following abstract operations are defined over these numeric types. The "Result" column shows the return type, along with an indication if it is possible for some invocations of the operation to return an abrupt completion.

Table 2: Numeric Type Operations
Operation Example source Invoked by the Evaluation semantics of ... Result
Number::unaryMinus -x Unary - Operator Number
BigInt::unaryMinus BigInt
Number::bitwiseNOT ~x Bitwise NOT Operator ( ~ ) Number
BigInt::bitwiseNOT BigInt
Number::exponentiate x ** y Exponentiation Operator and Math.pow ( base, exponent ) Number
BigInt::exponentiate either a normal completion containing a BigInt or a throw completion
Number::multiply x * y Multiplicative Operators Number
BigInt::multiply BigInt
Number::divide x / y Multiplicative Operators Number
BigInt::divide either a normal completion containing a BigInt or a throw completion
Number::remainder x % y Multiplicative Operators Number
BigInt::remainder either a normal completion containing a BigInt or a throw completion
Number::add x
x
x   y 		Postfix Increment Operator, Prefix Increment Operator, and The Addition Operator ( ) Number
BigInt::add BigInt
Number::subtract x --
-- x
x - y 		Postfix Decrement Operator, Prefix Decrement Operator, and The Subtraction Operator ( - ) Number
BigInt::subtract BigInt
Number::leftShift x << y The Left Shift Operator ( << ) Number
BigInt::leftShift BigInt
Number::signedRightShift x >> y The Signed Right Shift Operator ( >> ) Number
BigInt::signedRightShift BigInt
Number::unsignedRightShift x >>> y The Unsigned Right Shift Operator ( >>> ) Number
BigInt::unsignedRightShift a throw completion
Number::lessThan x < y
x > y
x <= y
x >= y 		Relational Operators, via IsLessThan ( x, y, LeftFirst ) Boolean or undefined (for unordered inputs)
BigInt::lessThan Boolean
Number::equal x == y
x != y
x === y
x !== y 		Equality Operators, via IsStrictlyEqual ( x, y ) Boolean
BigInt::equal
Number::sameValue Object.is(x, y) Object internal methods, via SameValue ( x, y ), to test exact value equality Boolean
Number::sameValueZero [x].includes(y) Array, Map, and Set methods, via SameValueZero ( x, y ), to test value equality, ignoring the difference between 0𝔽 and -0𝔽 Boolean
Number::bitwiseAND x & y Binary Bitwise Operators Number
BigInt::bitwiseAND BigInt
Number::bitwiseXOR x ^ y Number
BigInt::bitwiseXOR BigInt
Number::bitwiseOR x | y Number
BigInt::bitwiseOR BigInt
Number::toString String(x) Many expressions and built-in functions, via ToString ( argument ) String
BigInt::toString

Because the numeric types are in general not convertible without loss of precision or truncation, the ECMAScript language provides no implicit conversion among these types. Programmers must explicitly call Number and BigInt functions to convert among types when calling a function which requires another type.

Note

The first and subsequent editions of ECMAScript have provided, for certain operators, implicit numeric conversions that could lose precision or truncate. These legacy implicit conversions are maintained for backward compatibility, but not provided for BigInt in order to minimize opportunity for programmer error, and to leave open the option of generalized value types in a future edition.

6.1.6.1 The Number Type

The Number type has exactly 18,437,736,874,454,810,627 (that is, 264 - 253 3) values, representing the double-precision 64-bit format IEEE 754-2019 values as specified in the IEEE Standard for Binary Floating-Point Arithmetic, except that the 9,007,199,254,740,990 (that is, 253 - 2) distinct “Not-a-Number” values of the IEEE Standard are represented in ECMAScript as a single special NaN value. (Note that the NaN value is produced by the program expression NaN.) In some implementations, external code might be able to detect a difference between various Not-a-Number values, but such behaviour is implementation-defined; to ECMAScript code, all NaN values are indistinguishable from each other.

Note

The bit pattern that might be observed in an ArrayBuffer (see 25.1) or a SharedArrayBuffer (see 25.2) after a Number value has been stored into it is not necessarily the same as the internal representation of that Number value used by the ECMAScript implementation.

There are two other special values, called positive Infinity and negative Infinity. For brevity, these values are also referred to for expository purposes by the symbols ∞𝔽 and -∞𝔽, respectively. (Note that these two infinite Number values are produced by the program expressions Infinity (or simply Infinity) and -Infinity.)

The other 18,437,736,874,454,810,624 (that is, 264 - 253) values are called the finite numbers. Half of these are positive numbers and half are negative numbers; for every finite positive Number value there is a corresponding negative value having the same magnitude.

Note that there is both a positive zero and a negative zero. For brevity, these values are also referred to for expository purposes by the symbols 0𝔽 and -0𝔽, respectively. (Note that these two different zero Number values are produced by the program expressions 0 (or simply 0) and -0.)

The 18,437,736,874,454,810,622 (that is, 264 - 253 - 2) finite non-zero values are of two kinds:

18,428,729,675,200,069,632 (that is, 264 - 254) of them are normalized, having the form

s × m × 2e

where s is 1 or -1, m is an integer in the interval from 252 (inclusive) to 253 (exclusive), and e is an integer in the inclusive interval from -1074 to 971.

The remaining 9,007,199,254,740,990 (that is, 253 - 2) values are denormalized, having the form

s × m × 2e

where s is 1 or -1, m is an integer in the interval from 0 (exclusive) to 252 (exclusive), and e is -1074.

Note that all the positive and negative integers whose magnitude is no greater than 253 are representable in the Number type. The integer 0 has two representations in the Number type: 0𝔽 and -0𝔽.

A finite number has an odd significand if it is non-zero and the integer m used to express it (in one of the two forms shown above) is odd. Otherwise, it has an even significand.

In this specification, the phrase “the Number value for x” where x represents an exact real mathematical quantity (which might even be an irrational number such as π) means a Number value chosen in the following manner. Consider the set of all finite values of the Number type, with -0𝔽 removed and with two additional values added to it that are not representable in the Number type, namely 21024 (which is 1 × 253 × 2971) and -21024 (which is -1 × 253 × 2971). Choose the member of this set that is closest in value to x. If two values of the set are equally close, then the one with an even significand is chosen; for this purpose, the two extra values 21024 and -21024 are considered to have even significands. Finally, if 21024 was chosen, replace it with ∞𝔽; if -21024 was chosen, replace it with -∞𝔽; if 0𝔽 was chosen, replace it with -0𝔽 if and only if x < 0; any other chosen value is used unchanged. The result is the Number value for x. (This procedure corresponds exactly to the behaviour of the IEEE 754-2019 roundTiesToEven mode.)

The Number value for ∞ is ∞𝔽, and the Number value for -∞ is -∞𝔽.

Some ECMAScript operators deal only with integers in specific ranges such as the inclusive interval from -231 to 231 - 1 or the inclusive interval from 0 to 216 - 1. These operators accept any value of the Number type but first convert each such value to an integer value in the expected range. See the descriptions of the numeric conversion operations in 7.1.

6.1.6.1.1 Number::unaryMinus ( x )

The abstract operation Number::unaryMinus takes argument x (a Number) and returns a Number. It performs the following steps when called:

  1. If x is NaN, return NaN.
  2. Return the result of negating x; that is, compute a Number with the same magnitude but opposite sign.

6.1.6.1.2 Number::bitwiseNOT ( x )

The abstract operation Number::bitwiseNOT takes argument x (a Number) and returns an integral Number. It performs the following steps when called:

  1. Let oldValue be ! ToInt32(x).
  2. Return the result of applying bitwise complement to oldValue. The mathematical value of the result is exactly representable as a 32-bit two's complement bit string.

6.1.6.1.3 Number::exponentiate ( base, exponent )

The abstract operation Number::exponentiate takes arguments base (a Number) and exponent (a Number) and returns a Number. It returns an implementation-approximated value representing the result of raising base to the exponent power. It performs the following steps when called:

  1. If exponent is NaN, return NaN.
  2. If exponent is either 0𝔽 or -0𝔽, return 1𝔽.
  3. If base is NaN, return NaN.
  4. If base is ∞𝔽, then
    1. If exponent > 0𝔽, return ∞𝔽. Otherwise, return 0𝔽.
  5. If base is -∞𝔽, then
    1. If exponent > 0𝔽, then
      1. If exponent is an odd integral Number, return -∞𝔽. Otherwise, return ∞𝔽.
    2. Else,
      1. If exponent is an odd integral Number, return -0𝔽. Otherwise, return 0𝔽.
  6. If base is 0𝔽, then
    1. If exponent > 0𝔽, return 0𝔽. Otherwise, return ∞𝔽.
  7. If base is -0𝔽, then
    1. If exponent > 0𝔽, then
      1. If exponent is an odd integral Number, return -0𝔽. Otherwise, return 0𝔽.
    2. Else,
      1. If exponent is an odd integral Number, return -∞𝔽. Otherwise, return ∞𝔽.
  8. Assert: base is finite and is neither 0𝔽 nor -0𝔽.
  9. If exponent is ∞𝔽, then
    1. If abs(ℝ(base)) > 1, return ∞𝔽.
    2. If abs(ℝ(base)) = 1, return NaN.
    3. If abs(ℝ(base)) < 1, return 0𝔽.
  10. If exponent is -∞𝔽, then
    1. If abs(ℝ(base)) > 1, return 0𝔽.
    2. If abs(ℝ(base)) = 1, return NaN.
    3. If abs(ℝ(base)) < 1, return ∞𝔽.
  11. Assert: exponent is finite and is neither 0𝔽 nor -0𝔽.
  12. If base < -0𝔽 and exponent is not an integral Number, return NaN.
  13. Return an implementation-approximated Number value representing the result of raising ℝ(base) to the ℝ(exponent) power.
 Note

The result of base ** exponent when base is 1𝔽 or -1𝔽 and exponent is ∞𝔽 or -∞𝔽, or when base is 1𝔽 and exponent is NaN, differs from IEEE 754-2019. The first edition of ECMAScript specified a result of NaN for this operation, whereas later revisions of IEEE 754 specified 1𝔽. The historical ECMAScript behaviour is preserved for compatibility reasons.

6.1.6.1.4 Number::multiply ( x, y )

The abstract operation Number::multiply takes arguments x (a Number) and y (a Number) and returns a Number. It performs multiplication according to the rules of IEEE 754-2019 binary double-precision arithmetic, producing the product of x and y. It performs the following steps when called:

  1. If x is NaN or y is NaN, return NaN.
  2. If x is either ∞𝔽 or -∞𝔽, then
    1. If y is either 0𝔽 or -0𝔽, return NaN.
    2. If y > 0𝔽, return x.
    3. Return -x.
  3. If y is either ∞𝔽 or -∞𝔽, then
    1. If x is either 0𝔽 or -0𝔽, return NaN.
    2. If x > 0𝔽, return y.
    3. Return -y.
  4. If x is -0𝔽, then
    1. If y is -0𝔽 or y < -0𝔽, return 0𝔽.
    2. Else, return -0𝔽.
  5. If y is -0𝔽, then
    1. If x < -0𝔽, return 0𝔽.
    2. Else, return -0𝔽.
  6. Return 𝔽(ℝ(x) × ℝ(y)).
 Note

Finite-precision multiplication is commutative, but not always associative.

6.1.6.1.5 Number::divide ( x, y )

The abstract operation Number::divide takes arguments x (a Number) and y (a Number) and returns a Number. It performs division according to the rules of IEEE 754-2019 binary double-precision arithmetic, producing the quotient of x and y where x is the dividend and y is the divisor. It performs the following steps when called:

  1. If x is NaN or y is NaN, return NaN.
  2. If x is either ∞𝔽 or -∞𝔽, then
    1. If y is either ∞𝔽 or -∞𝔽, return NaN.
    2. If y is 0𝔽 or y > 0𝔽, return x.
    3. Return -x.
  3. If y is ∞𝔽, then
    1. If x is 0𝔽 or x > 0𝔽, return 0𝔽. Otherwise, return -0𝔽.
  4. If y is -∞𝔽, then
    1. If x is 0𝔽 or x > 0𝔽, return -0𝔽. Otherwise, return 0𝔽.
  5. If x is either 0𝔽 or -0𝔽, then
    1. If y is either 0𝔽 or -0𝔽, return NaN.
    2. If y > 0𝔽, return x.
    3. Return -x.
  6. If y is 0𝔽, then
    1. If x > 0𝔽, return ∞𝔽. Otherwise, return -∞𝔽.
  7. If y is -0𝔽, then
    1. If x > 0𝔽, return -∞𝔽. Otherwise, return ∞𝔽.
  8. Return 𝔽(ℝ(x) / ℝ(y)).

6.1.6.1.6 Number::remainder ( n, d )

The abstract operation Number::remainder takes arguments n (a Number) and d (a Number) and returns a Number. It yields the remainder from an implied division of its operands where n is the dividend and d is the divisor. It performs the following steps when called:

  1. If n is NaN or d is NaN, return NaN.
  2. If n is either ∞𝔽 or -∞𝔽, return NaN.
  3. If d is either ∞𝔽 or -∞𝔽, return n.
  4. If d is either 0𝔽 or -0𝔽, return NaN.
  5. If n is either 0𝔽 or -0𝔽, return n.
  6. Assert: n and d are finite and non-zero.
  7. Let quotient be ℝ(n) / ℝ(d).
  8. Let q be truncate(quotient).
  9. Let r be ℝ(n) - (ℝ(d) × q).
  10. If r = 0 and n < -0𝔽, return -0𝔽.
  11. Return 𝔽(r).
 Note 1

In C and C , the remainder operator accepts only integral operands; in ECMAScript, it also accepts floating-point operands.

 Note 2
The result of a floating-point remainder operation as computed by the % operator is not the same as the “remainder” operation defined by IEEE 754-2019. The IEEE 754-2019 “remainder” operation computes the remainder from a rounding division, not a truncating division, and so its behaviour is not analogous to that of the usual integer remainder operator. Instead the ECMAScript language defines % on floating-point operations to behave in a manner analogous to that of the Java integer remainder operator; this may be compared with the C library function fmod.

6.1.6.1.7 Number::add ( x, y )

The abstract operation Number::add takes arguments x (a Number) and y (a Number) and returns a Number. It performs addition according to the rules of IEEE 754-2019 binary double-precision arithmetic, producing the sum of its arguments. It performs the following steps when called:

  1. If x is NaN or y is NaN, return NaN.
  2. If x is ∞𝔽 and y is -∞𝔽, return NaN.
  3. If x is -∞𝔽 and y is ∞𝔽, return NaN.
  4. If x is either ∞𝔽 or -∞𝔽, return x.
  5. If y is either ∞𝔽 or -∞𝔽, return y.
  6. Assert: x and y are both finite.
  7. If x is -0𝔽 and y is -0𝔽, return -0𝔽.
  8. Return 𝔽(ℝ(x) ℝ(y)).
 Note

Finite-precision addition is commutative, but not always associative.

6.1.6.1.8 Number::subtract ( x, y )

The abstract operation Number::subtract takes arguments x (a Number) and y (a Number) and returns a Number. It performs subtraction, producing the difference of its operands; x is the minuend and y is the subtrahend. It performs the following steps when called:

  1. Return Number::add(x, Number::unaryMinus(y)).
 Note

It is always the case that x - y produces the same result as x (-y).

6.1.6.1.9 Number::leftShift ( x, y )

The abstract operation Number::leftShift takes arguments x (a Number) and y (a Number) and returns an integral Number. It performs the following steps when called:

  1. Let lnum be ! ToInt32(x).
  2. Let rnum be ! ToUint32(y).
  3. Let shiftCount be ℝ(rnum) modulo 32.
  4. Return the result of left shifting lnum by shiftCount bits. The mathematical value of the result is exactly representable as a 32-bit two's complement bit string.

6.1.6.1.10 Number::signedRightShift ( x, y )

The abstract operation Number::signedRightShift takes arguments x (a Number) and y (a Number) and returns an integral Number. It performs the following steps when called:

  1. Let lnum be ! ToInt32(x).
  2. Let rnum be ! ToUint32(y).
  3. Let shiftCount be ℝ(rnum) modulo 32.
  4. Return the result of performing a sign-extending right shift of lnum by shiftCount bits. The most significant bit is propagated. The mathematical value of the result is exactly representable as a 32-bit two's complement bit string.

6.1.6.1.11 Number::unsignedRightShift ( x, y )

The abstract operation Number::unsignedRightShift takes arguments x (a Number) and y (a Number) and returns an integral Number. It performs the following steps when called:

  1. Let lnum be ! ToUint32(x).
  2. Let rnum be ! ToUint32(y).
  3. Let shiftCount be ℝ(rnum) modulo 32.
  4. Return the result of performing a zero-filling right shift of lnum by shiftCount bits. Vacated bits are filled with zero. The mathematical value of the result is exactly representable as a 32-bit unsigned bit string.

6.1.6.1.12 Number::lessThan ( x, y )

The abstract operation Number::lessThan takes arguments x (a Number) and y (a Number) and returns a Boolean or undefined. It performs the following steps when called:

  1. If x is NaN, return undefined.
  2. If y is NaN, return undefined.
  3. If x is y, return false.
  4. If x is 0𝔽 and y is -0𝔽, return false.
  5. If x is -0𝔽 and y is 0𝔽, return false.
  6. If x is ∞𝔽, return false.
  7. If y is ∞𝔽, return true.
  8. If y is -∞𝔽, return false.
  9. If x is -∞𝔽, return true.
  10. Assert: x and y are finite and non-zero.
  11. If ℝ(x) < ℝ(y), return true. Otherwise, return false.

6.1.6.1.13 Number::equal ( x, y )

The abstract operation Number::equal takes arguments x (a Number) and y (a Number) and returns a Boolean. It performs the following steps when called:

  1. If x is NaN, return false.
  2. If y is NaN, return false.
  3. If x is y, return true.
  4. If x is 0𝔽 and y is -0𝔽, return true.
  5. If x is -0𝔽 and y is 0𝔽, return true.
  6. Return false.

6.1.6.1.14 Number::sameValue ( x, y )

The abstract operation Number::sameValue takes arguments x (a Number) and y (a Number) and returns a Boolean. It performs the following steps when called:

  1. If x is NaN and y is NaN, return true.
  2. If x is 0𝔽 and y is -0𝔽, return false.
  3. If x is -0𝔽 and y is 0𝔽, return false.
  4. If x is y, return true.
  5. Return false.

6.1.6.1.15 Number::sameValueZero ( x, y )

The abstract operation Number::sameValueZero takes arguments x (a Number) and y (a Number) and returns a Boolean. It performs the following steps when called:

  1. If x is NaN and y is NaN, return true.
  2. If x is 0𝔽 and y is -0𝔽, return true.
  3. If x is -0𝔽 and y is 0𝔽, return true.
  4. If x is y, return true.
  5. Return false.

6.1.6.1.16 NumberBitwiseOp ( op, x, y )

The abstract operation NumberBitwiseOp takes arguments op (&, ^, or |), x (a Number), and y (a Number) and returns an integral Number. It performs the following steps when called:

  1. Let lnum be ! ToInt32(x).
  2. Let rnum be ! ToInt32(y).
  3. Let lbits be the 32-bit two's complement bit string representing ℝ(lnum).
  4. Let rbits be the 32-bit two's complement bit string representing ℝ(rnum).
  5. If op is &, let result be the result of applying the bitwise AND operation to lbits and rbits.
  6. Else if op is ^, let result be the result of applying the bitwise exclusive OR (XOR) operation to lbits and rbits.
  7. Else, op is |. Let result be the result of applying the bitwise inclusive OR operation to lbits and rbits.
  8. Return the Number value for the integer represented by the 32-bit two's complement bit string result.

6.1.6.1.17 Number::bitwiseAND ( x, y )

The abstract operation Number::bitwiseAND takes arguments x (a Number) and y (a Number) and returns an integral Number. It performs the following steps when called:

  1. Return NumberBitwiseOp(&, x, y).

6.1.6.1.18 Number::bitwiseXOR ( x, y )

The abstract operation Number::bitwiseXOR takes arguments x (a Number) and y (a Number) and returns an integral Number. It performs the following steps when called:

  1. Return NumberBitwiseOp(^, x, y).

6.1.6.1.19 Number::bitwiseOR ( x, y )

The abstract operation Number::bitwiseOR takes arguments x (a Number) and y (a Number) and returns an integral Number. It performs the following steps when called:

  1. Return NumberBitwiseOp(|, x, y).

6.1.6.1.20 Number::toString ( x, radix )

The abstract operation Number::toString takes arguments x (a Number) and radix (an integer in the inclusive interval from 2 to 36) and returns a String. It represents x as a String using a positional numeral system with radix radix. The digits used in the representation of a number using radix r are taken from the first r code units of "0123456789abcdefghijklmnopqrstuvwxyz" in order. The representation of numbers with magnitude greater than or equal to 1𝔽 never includes leading zeroes. It performs the following steps when called:

  1. If x is NaN, return "NaN".
  2. If x is either 0𝔽 or -0𝔽, return "0".
  3. If x < -0𝔽, return the string-concatenation of "-" and Number::toString(-x, radix).
  4. If x is ∞𝔽, return "Infinity".
  5. Let n, k, and s be integers such that k ≥ 1, radixk - 1 ≤ s < radixk, 𝔽(s × radixn - k) is x, and k is as small as possible. Note that k is the number of digits in the representation of s using radix radix, that s is not divisible by radix, and that the least significant digit of s is not necessarily uniquely determined by these criteria.
  6. If radix ≠ 10 or n is in the inclusive interval from -5 to 21, then
    1. If n ≥ k, then
      1. Return the string-concatenation of:
        • the code units of the k digits of the representation of s using radix radix
        • n - k occurrences of the code unit 0x0030 (DIGIT ZERO)
    2. Else if n > 0, then
      1. Return the string-concatenation of:
        • the code units of the most significant n digits of the representation of s using radix radix
        • the code unit 0x002E (FULL STOP)
        • the code units of the remaining k - n digits of the representation of s using radix radix
    3. Else,
      1. Assert: n ≤ 0.
      2. Return the string-concatenation of:
        • the code unit 0x0030 (DIGIT ZERO)
        • the code unit 0x002E (FULL STOP)
        • -n occurrences of the code unit 0x0030 (DIGIT ZERO)
        • the code units of the k digits of the representation of s using radix radix
  7. NOTE: In this case, the input will be represented using scientific E notation, such as 1.2e 3.
  8. Assert: radix is 10.
  9. If n < 0, then
    1. Let exponentSign be the code unit 0x002D (HYPHEN-MINUS).
  10. Else,
    1. Let exponentSign be the code unit 0x002B (PLUS SIGN).
  11. If k = 1, then
    1. Return the string-concatenation of:
      • the code unit of the single digit of s
      • the code unit 0x0065 (LATIN SMALL LETTER E)
      • exponentSign
      • the code units of the decimal representation of abs(n - 1)
  12. Return the string-concatenation of:
    • the code unit of the most significant digit of the decimal representation of s
    • the code unit 0x002E (FULL STOP)
    • the code units of the remaining k - 1 digits of the decimal representation of s
    • the code unit 0x0065 (LATIN SMALL LETTER E)
    • exponentSign
    • the code units of the decimal representation of abs(n - 1)
 Note 1

The following observations may be useful as guidelines for implementations, but are not part of the normative requirements of this Standard:

  • If x is any Number value other than -0𝔽, then ToNumber(ToString(x)) is x.
  • The least significant digit of s is not always uniquely determined by the requirements listed in step 5.
Note 2

For implementations that provide more accurate conversions than required by the rules above, it is recommended that the following alternative version of step 5 be used as a guideline:

  1. Let n, k, and s be integers such that k ≥ 1, radixk - 1 ≤ s < radixk, 𝔽(s × radixn - k) is x, and k is as small as possible. If there are multiple possibilities for s, choose the value of s for which s × radixn - k is closest in value to ℝ(x). If there are two such possible values of s, choose the one that is even. Note that k is the number of digits in the representation of s using radix radix and that s is not divisible by radix.
Note 3

Implementers of ECMAScript may find useful the paper and code written by David M. Gay for binary-to-decimal conversion of floating-point numbers:

Gay, David M. Correctly Rounded Binary-Decimal and Decimal-Binary Conversions. Numerical Analysis, Manuscript 90-10. AT&T Bell Laboratories (Murray Hill, New Jersey). 30 November 1990. Available as
http://ampl.com/REFS/abstracts.html#rounding. Associated code available as
http://netlib.sandia.gov/fp/dtoa.c and as
http://netlib.sandia.gov/fp/g_fmt.c and may also be found at the various netlib mirror sites.

6.1.6.2 The BigInt Type

The BigInt type represents an integer value. The value may be any size and is not limited to a particular bit-width. Generally, where not otherwise noted, operations are designed to return exact mathematically-based answers. For binary operations, BigInts act as two's complement binary strings, with negative numbers treated as having bits set infinitely to the left.

6.1.6.2.1 BigInt::unaryMinus ( x )

The abstract operation BigInt::unaryMinus takes argument x (a BigInt) and returns a BigInt. It performs the following steps when called:

  1. If x is 0ℤ, return 0ℤ.
  2. Return the BigInt value that represents the negation of ℝ(x).

6.1.6.2.2 BigInt::bitwiseNOT ( x )

The abstract operation BigInt::bitwiseNOT takes argument x (a BigInt) and returns a BigInt. It returns the one's complement of x. It performs the following steps when called:

  1. Return -x - 1ℤ.

6.1.6.2.3 BigInt::exponentiate ( base, exponent )

The abstract operation BigInt::exponentiate takes arguments base (a BigInt) and exponent (a BigInt) and returns either a normal completion containing a BigInt or a throw completion. It performs the following steps when called:

  1. If exponent < 0ℤ, throw a RangeError exception.
  2. If base is 0ℤ and exponent is 0ℤ, return 1ℤ.
  3. Return the BigInt value that represents ℝ(base) raised to the power ℝ(exponent).

6.1.6.2.4 BigInt::multiply ( x, y )

The abstract operation BigInt::multiply takes arguments x (a BigInt) and y (a BigInt) and returns a BigInt. It performs the following steps when called:

  1. Return the BigInt value that represents the product of x and y.
 Note
Even if the result has a much larger bit width than the input, the exact mathematical answer is given.

6.1.6.2.5 BigInt::divide ( x, y )

The abstract operation BigInt::divide takes arguments x (a BigInt) and y (a BigInt) and returns either a normal completion containing a BigInt or a throw completion. It performs the following steps when called:

  1. If y is 0ℤ, throw a RangeError exception.
  2. Let quotient be ℝ(x) / ℝ(y).
  3. Return ℤ(truncate(quotient)).

6.1.6.2.6 BigInt::remainder ( n, d )

The abstract operation BigInt::remainder takes arguments n (a BigInt) and d (a BigInt) and returns either a normal completion containing a BigInt or a throw completion. It performs the following steps when called:

  1. If d is 0ℤ, throw a RangeError exception.
  2. If n is 0ℤ, return 0ℤ.
  3. Let quotient be ℝ(n) / ℝ(d).
  4. Let q be ℤ(truncate(quotient)).
  5. Return n - (d × q).
 Note
The sign of the result is the sign of the dividend.

6.1.6.2.7 BigInt::add ( x, y )

The abstract operation BigInt::add takes arguments x (a BigInt) and y (a BigInt) and returns a BigInt. It performs the following steps when called:

  1. Return the BigInt value that represents the sum of x and y.

6.1.6.2.8 BigInt::subtract ( x, y )

The abstract operation BigInt::subtract takes arguments x (a BigInt) and y (a BigInt) and returns a BigInt. It performs the following steps when called:

  1. Return the BigInt value that represents the difference x minus y.

6.1.6.2.9 BigInt::leftShift ( x, y )

The abstract operation BigInt::leftShift takes arguments x (a BigInt) and y (a BigInt) and returns a BigInt. It performs the following steps when called:

  1. If y < 0ℤ, then
    1. Return the BigInt value that represents ℝ(x) / 2-ℝ(y), rounding down to the nearest integer, including for negative numbers.
  2. Return the BigInt value that represents ℝ(x) × 2ℝ(y).
 Note
Semantics here should be equivalent to a bitwise shift, treating the BigInt as an infinite length string of binary two's complement digits.

6.1.6.2.10 BigInt::signedRightShift ( x, y )

The abstract operation BigInt::signedRightShift takes arguments x (a BigInt) and y (a BigInt) and returns a BigInt. It performs the following steps when called:

  1. Return BigInt::leftShift(x, -y).

6.1.6.2.11 BigInt::unsignedRightShift ( x, y )

The abstract operation BigInt::unsignedRightShift takes arguments x (a BigInt) and y (a BigInt) and returns a throw completion. It performs the following steps when called:

  1. Throw a TypeError exception.

6.1.6.2.12 BigInt::lessThan ( x, y )

The abstract operation BigInt::lessThan takes arguments x (a BigInt) and y (a BigInt) and returns a Boolean. It performs the following steps when called:

  1. If ℝ(x) < ℝ(y), return true; otherwise return false.

6.1.6.2.13 BigInt::equal ( x, y )

The abstract operation BigInt::equal takes arguments x (a BigInt) and y (a BigInt) and returns a Boolean. It performs the following steps when called:

  1. If ℝ(x) = ℝ(y), return true; otherwise return false.

6.1.6.2.14 BinaryAnd ( x, y )

The abstract operation BinaryAnd takes arguments x (0 or 1) and y (0 or 1) and returns 0 or 1. It performs the following steps when called:

  1. If x = 1 and y = 1, return 1.
  2. Else, return 0.

6.1.6.2.15 BinaryOr ( x, y )

The abstract operation BinaryOr takes arguments x (0 or 1) and y (0 or 1) and returns 0 or 1. It performs the following steps when called:

  1. If x = 1 or y = 1, return 1.
  2. Else, return 0.

6.1.6.2.16 BinaryXor ( x, y )

The abstract operation BinaryXor takes arguments x (0 or 1) and y (0 or 1) and returns 0 or 1. It performs the following steps when called:

  1. If x = 1 and y = 0, return 1.
  2. Else if x = 0 and y = 1, return 1.
  3. Else, return 0.

6.1.6.2.17 BigIntBitwiseOp ( op, x, y )

The abstract operation BigIntBitwiseOp takes arguments op (&, ^, or |), x (a BigInt), and y (a BigInt) and returns a BigInt. It performs the following steps when called:

  1. Set x to ℝ(x).
  2. Set y to ℝ(y).
  3. Let result be 0.
  4. Let shift be 0.
  5. Repeat, until (x = 0 or x = -1) and (y = 0 or y = -1),
    1. Let xDigit be x modulo 2.
    2. Let yDigit be y modulo 2.
    3. If op is &, set result to result 2shift × BinaryAnd(xDigit, yDigit).
    4. Else if op is |, set result to result 2shift × BinaryOr(xDigit, yDigit).
    5. Else,
      1. Assert: op is ^.
      2. Set result to result 2shift × BinaryXor(xDigit, yDigit).
    6. Set shift to shift 1.
    7. Set x to (x - xDigit) / 2.
    8. Set y to (y - yDigit) / 2.
  6. If op is &, let tmp be BinaryAnd(x modulo 2, y modulo 2).
  7. Else if op is |, let tmp be BinaryOr(x modulo 2, y modulo 2).
  8. Else,
    1. Assert: op is ^.
    2. Let tmp be BinaryXor(x modulo 2, y modulo 2).
  9. If tmp ≠ 0, then
    1. Set result to result - 2shift.
    2. NOTE: This extends the sign.
  10. Return the BigInt value for result.

6.1.6.2.18 BigInt::bitwiseAND ( x, y )

The abstract operation BigInt::bitwiseAND takes arguments x (a BigInt) and y (a BigInt) and returns a BigInt. It performs the following steps when called:

  1. Return BigIntBitwiseOp(&, x, y).

6.1.6.2.19 BigInt::bitwiseXOR ( x, y )

The abstract operation BigInt::bitwiseXOR takes arguments x (a BigInt) and y (a BigInt) and returns a BigInt. It performs the following steps when called:

  1. Return BigIntBitwiseOp(^, x, y).

6.1.6.2.20 BigInt::bitwiseOR ( x, y )

The abstract operation BigInt::bitwiseOR takes arguments x (a BigInt) and y (a BigInt) and returns a BigInt. It performs the following steps when called:

  1. Return BigIntBitwiseOp(|, x, y).

6.1.6.2.21 BigInt::toString ( x, radix )

The abstract operation BigInt::toString takes arguments x (a BigInt) and radix (an integer in the inclusive interval from 2 to 36) and returns a String. It represents x as a String using a positional numeral system with radix radix. The digits used in the representation of a BigInt using radix r are taken from the first r code units of "0123456789abcdefghijklmnopqrstuvwxyz" in order. The representation of BigInts other than 0ℤ never includes leading zeroes. It performs the following steps when called:

  1. If x < 0ℤ, return the string-concatenation of "-" and BigInt::toString(-x, radix).
  2. Return the String value consisting of the representation of x using radix radix.

6.1.7 The Object Type

An Object is logically a collection of properties. Each property is either a data property, or an accessor property:

  • A data property associates a key value with an ECMAScript language value and a set of Boolean attributes.
  • An accessor property associates a key value with one or two accessor functions, and a set of Boolean attributes. The accessor functions are used to store or retrieve an ECMAScript language value that is associated with the property.

Properties are identified using key values. A property key value is either an ECMAScript String value or a Symbol value. All String and Symbol values, including the empty String, are valid as property keys. A property name is a property key that is a String value.

An integer index is a String-valued property key that is a canonical numeric string and whose numeric value is either 0𝔽 or a positive integral Number ≤ 𝔽(253 - 1). An array index is an integer index whose numeric value i is in the range 0𝔽 ≤ i < 𝔽(232 - 1).

Property keys are used to access properties and their values. There are two kinds of access for properties: get and set, corresponding to value retrieval and assignment, respectively. The properties accessible via get and set access includes both own properties that are a direct part of an object and inherited properties which are provided by another associated object via a property inheritance relationship. Inherited properties may be either own or inherited properties of the associated object. Each own property of an object must each have a key value that is distinct from the key values of the other own properties of that object.

All objects are logically collections of properties, but there are multiple forms of objects that differ in their semantics for accessing and manipulating their properties. Please see 6.1.7.2 for definitions of the multiple forms of objects.

6.1.7.1 Property Attributes

Attributes are used in this specification to define and explain the state of Object properties as described in Table 3. Unless specified explicitly, the initial value of each attribute is its Default Value.

Table 3: Attributes of an Object property
Attribute Name Types of property for which it is present Value Domain Default Value Description
[[Value]] data property an ECMAScript language value undefined The value retrieved by a get access of the property.
[[Writable]] data property a Boolean false If false, attempts by ECMAScript code to change the property's [[Value]] attribute using [[Set]] will not succeed.
[[Get]] accessor property an Object or undefined undefined If the value is an Object it must be a function object. The function's [[Call]] internal method (Table 5) is called with an empty arguments list to retrieve the property value each time a get access of the property is performed.
[[Set]] accessor property an Object or undefined undefined If the value is an Object it must be a function object. The function's [[Call]] internal method (Table 5) is called with an arguments list containing the assigned value as its sole argument each time a set access of the property is performed. The effect of a property's [[Set]] internal method may, but is not required to, have an effect on the value returned by subsequent calls to the property's [[Get]] internal method.
[[Enumerable]] data property or accessor property a Boolean false If true, the property will be enumerated by a for-in enumeration (see 14.7.5). Otherwise, the property is said to be non-enumerable.
[[Configurable]] data property or accessor property a Boolean false If false, attempts to delete the property, change it from a data property to an accessor property or from an accessor property to a data property, or make any changes to its attributes (other than replacing an existing [[Value]] or setting [[Writable]] to false) will fail.

6.1.7.2 Object Internal Methods and Internal Slots

The actual semantics of objects, in ECMAScript, are specified via algorithms called internal methods. Each object in an ECMAScript engine is associated with a set of internal methods that defines its runtime behaviour. These internal methods are not part of the ECMAScript language. They are defined by this specification purely for expository purposes. However, each object within an implementation of ECMAScript must behave as specified by the internal methods associated with it. The exact manner in which this is accomplished is determined by the implementation.

Internal method names are polymorphic. This means that different object values may perform different algorithms when a common internal method name is invoked upon them. That actual object upon which an internal method is invoked is the “target” of the invocation. If, at runtime, the implementation of an algorithm attempts to use an internal method of an object that the object does not support, a TypeError exception is thrown.

Internal slots correspond to internal state that is associated with objects and used by various ECMAScript specification algorithms. Internal slots are not object properties and they are not inherited. Depending upon the specific internal slot specification, such state may consist of values of any ECMAScript language type or of specific ECMAScript specification type values. Unless explicitly specified otherwise, internal slots are allocated as part of the process of creating an object and may not be dynamically added to an object. Unless specified otherwise, the initial value of an internal slot is the value undefined. Various algorithms within this specification create objects that have internal slots. However, the ECMAScript language provides no direct way to associate internal slots with an object.

All objects have an internal slot named [[PrivateElements]], which is a List of PrivateElements. This List represents the values of the private fields, methods, and accessors for the object. Initially, it is an empty List.

Internal methods and internal slots are identified within this specification using names enclosed in double square brackets [[ ]].

Table 4 summarizes the essential internal methods used by this specification that are applicable to all objects created or manipulated by ECMAScript code. Every object must have algorithms for all of the essential internal methods. However, all objects do not necessarily use the same algorithms for those methods.

An ordinary object is an object that satisfies all of the following criteria:

  • For the internal methods listed in Table 4, the object uses those defined in 10.1.
  • If the object has a [[Call]] internal method, it uses either the one defined in 10.2.1 or the one defined in 10.3.1.
  • If the object has a [[Construct]] internal method, it uses either the one defined in 10.2.2 or the one defined in 10.3.2.

An exotic object is an object that is not an ordinary object.

This specification recognizes different kinds of exotic objects by those objects' internal methods. An object that is behaviourally equivalent to a particular kind of exotic object (such as an Array exotic object or a bound function exotic object), but does not have the same collection of internal methods specified for that kind, is not recognized as that kind of exotic object.

The “Signature” column of Table 4 and other similar tables describes the invocation pattern for each internal method. The invocation pattern always includes a parenthesized list of descriptive parameter names. If a parameter name is the same as an ECMAScript type name then the name describes the required type of the parameter value. If an internal method explicitly returns a value, its parameter list is followed by the symbol “→” and the type name of the returned value. The type names used in signatures refer to the types defined in clause 6 augmented by the following additional names. “any” means the value may be any ECMAScript language type.

In addition to its parameters, an internal method always has access to the object that is the target of the method invocation.

An internal method implicitly returns a Completion Record, either a normal completion that wraps a value of the return type shown in its invocation pattern, or a throw completion.

Table 4: Essential Internal Methods
Internal Method Signature Description
[[GetPrototypeOf]] ( ) → Object | Null Determine the object that provides inherited properties for this object. A null value indicates that there are no inherited properties.
[[SetPrototypeOf]] (Object | Null) → Boolean Associate this object with another object that provides inherited properties. Passing null indicates that there are no inherited properties. Returns true indicating that the operation was completed successfully or false indicating that the operation was not successful.
[[IsExtensible]] ( ) → Boolean Determine whether it is permitted to add additional properties to this object.
[[PreventExtensions]] ( ) → Boolean Control whether new properties may be added to this object. Returns true if the operation was successful or false if the operation was unsuccessful.
[[GetOwnProperty]] (propertyKey) → Undefined | Property Descriptor Return a Property Descriptor for the own property of this object whose key is propertyKey, or undefined if no such property exists.
[[DefineOwnProperty]] (propertyKey, PropertyDescriptor) → Boolean Create or alter the own property, whose key is propertyKey, to have the state described by PropertyDescriptor. Return true if that property was successfully created/updated or false if the property could not be created or updated.
[[HasProperty]] (propertyKey) → Boolean Return a Boolean value indicating whether this object already has either an own or inherited property whose key is propertyKey.
[[Get]] (propertyKey, Receiver) → any Return the value of the property whose key is propertyKey from this object. If any ECMAScript code must be executed to retrieve the property value, Receiver is used as the this value when evaluating the code.
[[Set]] (propertyKey, value, Receiver) → Boolean Set the value of the property whose key is propertyKey to value. If any ECMAScript code must be executed to set the property value, Receiver is used as the this value when evaluating the code. Returns true if the property value was set or false if it could not be set.
[[Delete]] (propertyKey) → Boolean Remove the own property whose key is propertyKey from this object. Return false if the property was not deleted and is still present. Return true if the property was deleted or is not present.
[[OwnPropertyKeys]] ( ) → List of property keys Return a List whose elements are all of the own property keys for the object.

Table 5 summarizes additional essential internal methods that are supported by objects that may be called as functions. A function object is an object that supports the [[Call]] internal method. A constructor is an object that supports the [[Construct]] internal method. Every object that supports [[Construct]] must support [[Call]]; that is, every constructor must be a function object. Therefore, a constructor may also be referred to as a constructor function or constructor function object.

Table 5: Additional Essential Internal Methods of Function Objects
Internal Method Signature Description
[[Call]] (any, a List of any) → any Executes code associated with this object. Invoked via a function call expression. The arguments to the internal method are a this value and a List whose elements are the arguments passed to the function by a call expression. Objects that implement this internal method are callable.
[[Construct]] (a List of any, Object) → Object Creates an object. Invoked via the new operator or a super call. The first argument to the internal method is a List whose elements are the arguments of the constructor invocation or the super call. The second argument is the object to which the new operator was initially applied. Objects that implement this internal method are called constructors. A function object is not necessarily a constructor and such non-constructor function objects do not have a [[Construct]] internal method.

The semantics of the essential internal methods for ordinary objects and standard exotic objects are specified in clause 10. If any specified use of an internal method of an exotic object is not supported by an implementation, that usage must throw a TypeError exception when attempted.

6.1.7.3 Invariants of the Essential Internal Methods

The Internal Methods of Objects of an ECMAScript engine must conform to the list of invariants specified below. Ordinary ECMAScript Objects as well as all standard exotic objects in this specification maintain these invariants. ECMAScript Proxy objects maintain these invariants by means of runtime checks on the result of traps invoked on the [[ProxyHandler]] object.

Any implementation provided exotic objects must also maintain these invariants for those objects. Violation of these invariants may cause ECMAScript code to have unpredictable behaviour and create security issues. However, violation of these invariants must never compromise the memory safety of an implementation.

An implementation must not allow these invariants to be circumvented in any manner such as by providing alternative interfaces that implement the functionality of the essential internal methods without enforcing their invariants.

Definitions:

  • The target of an internal method is the object upon which the internal method is called.
  • A target is non-extensible if it has been observed to return false from its [[IsExtensible]] internal method, or true from its [[PreventExtensions]] internal method.
  • A non-existent property is a property that does not exist as an own property on a non-extensible target.
  • All references to SameValue are according to the definition of the SameValue algorithm.

Return value:

The value returned by any internal method must be a Completion Record with either:

  • [[Type]] = normal, [[Target]] = empty, and [[Value]] = a value of the "normal return type" shown below for that internal method, or
  • [[Type]] = throw, [[Target]] = empty, and [[Value]] = any ECMAScript language value.
Note 1

An internal method must not return a continue completion, a break completion, or a return completion.

[[GetPrototypeOf]] ( )

  • The normal return type is either Object or Null.
  • If target is non-extensible, and [[GetPrototypeOf]] returns a value V, then any future calls to [[GetPrototypeOf]] should return the SameValue as V.
Note 2

An object's prototype chain should have finite length (that is, starting from any object, recursively applying the [[GetPrototypeOf]] internal method to its result should eventually lead to the value null). However, this requirement is not enforceable as an object level invariant if the prototype chain includes any exotic objects that do not use the ordinary object definition of [[GetPrototypeOf]]. Such a circular prototype chain may result in infinite loops when accessing object properties.

[[SetPrototypeOf]] ( V )

  • The normal return type is Boolean.
  • If target is non-extensible, [[SetPrototypeOf]] must return false, unless V is the SameValue as the target's observed [[GetPrototypeOf]] value.

[[IsExtensible]] ( )

  • The normal return type is Boolean.
  • If [[IsExtensible]] returns false, all future calls to [[IsExtensible]] on the target must return false.

[[PreventExtensions]] ( )

  • The normal return type is Boolean.
  • If [[PreventExtensions]] returns true, all future calls to [[IsExtensible]] on the target must return false and the target is now considered non-extensible.

[[GetOwnProperty]] ( P )

  • The normal return type is either Property Descriptor or Undefined.
  • If the Type of the return value is Property Descriptor, the return value must be a fully populated Property Descriptor.
  • If P is described as a non-configurable, non-writable own data property, all future calls to [[GetOwnProperty]] ( P ) must return Property Descriptor whose [[Value]] is SameValue as P's [[Value]] attribute.
  • If P's attributes other than [[Writable]] may change over time or if the property might be deleted, then P's [[Configurable]] attribute must be true.
  • If the [[Writable]] attribute may change from false to true, then the [[Configurable]] attribute must be true.
  • If the target is non-extensible and P is non-existent, then all future calls to [[GetOwnProperty]] (P) on the target must describe P as non-existent (i.e. [[GetOwnProperty]] (P) must return undefined).
Note 3

As a consequence of the third invariant, if a property is described as a data property and it may return different values over time, then either or both of the [[Writable]] and [[Configurable]] attributes must be true even if no mechanism to change the value is exposed via the other essential internal methods.

[[DefineOwnProperty]] ( P, Desc )

  • The normal return type is Boolean.
  • [[DefineOwnProperty]] must return false if P has previously been observed as a non-configurable own property of the target, unless either:
    1. P is a writable data property. A non-configurable writable data property can be changed into a non-configurable non-writable data property.
    2. All attributes of Desc are the SameValue as P's attributes.
  • [[DefineOwnProperty]] (P, Desc) must return false if target is non-extensible and P is a non-existent own property. That is, a non-extensible target object cannot be extended with new properties.

[[HasProperty]] ( P )

  • The normal return type is Boolean.
  • If P was previously observed as a non-configurable own data or accessor property of the target, [[HasProperty]] must return true.

[[Get]] ( P, Receiver )

  • The normal return type is any ECMAScript language type.
  • If P was previously observed as a non-configurable, non-writable own data property of the target with value V, then [[Get]] must return the SameValue as V.
  • If P was previously observed as a non-configurable own accessor property of the target whose [[Get]] attribute is undefined, the [[Get]] operation must return undefined.

[[Set]] ( P, V, Receiver )

  • The normal return type is Boolean.
  • If P was previously observed as a non-configurable, non-writable own data property of the target, then [[Set]] must return false unless V is the SameValue as P's [[Value]] attribute.
  • If P was previously observed as a non-configurable own accessor property of the target whose [[Set]] attribute is undefined, the [[Set]] operation must return false.

[[Delete]] ( P )

  • The normal return type is Boolean.
  • If P was previously observed as a non-configurable own data or accessor property of the target, [[Delete]] must return false.

[[OwnPropertyKeys]] ( )

  • The normal return type is List.
  • The returned List must not contain any duplicate entries.
  • The Type of each element of the returned List is either String or Symbol.
  • The returned List must contain at least the keys of all non-configurable own properties that have previously been observed.
  • If the target is non-extensible, the returned List must contain only the keys of all own properties of the target that are observable using [[GetOwnProperty]].

[[Call]] ( )

[[Construct]] ( )

  • The normal return type is Object.
  • The target must also have a [[Call]] internal method.

6.1.7.4 Well-Known Intrinsic Objects

Well-known intrinsics are built-in objects that are explicitly referenced by the algorithms of this specification and which usually have realm-specific identities. Unless otherwise specified each intrinsic object actually corresponds to a set of similar objects, one per realm.

Within this specification a reference such as %name% means the intrinsic object, associated with the current realm, corresponding to the name. A reference such as %name.a.b% means, as if the "b" property of the value of the "a" property of the intrinsic object %name% was accessed prior to any ECMAScript code being evaluated. Determination of the current realm and its intrinsics is described in 9.4. The well-known intrinsics are listed in Table 6.

Table 6: Well-Known Intrinsic Objects
Intrinsic Name Global Name ECMAScript Language Association
%AggregateError% AggregateError The AggregateError constructor (20.5.7.1)
%Array% Array The Array constructor (23.1.1)
%ArrayBuffer% ArrayBuffer The ArrayBuffer constructor (25.1.3)
%ArrayIteratorPrototype% The prototype of Array iterator objects (23.1.5)
%AsyncFromSyncIteratorPrototype% The prototype of async-from-sync iterator objects (27.1.4)
%AsyncFunction% The constructor of async function objects (27.7.1)
%AsyncGeneratorFunction% The constructor of async iterator objects (27.4.1)
%AsyncIteratorPrototype% An object that all standard built-in async iterator objects indirectly inherit from
%Atomics% Atomics The Atomics object (25.4)
%BigInt% BigInt The BigInt constructor (21.2.1)
%BigInt64Array% BigInt64Array The BigInt64Array constructor (23.2)
%BigUint64Array% BigUint64Array The BigUint64Array constructor (23.2)
%Boolean% Boolean The Boolean constructor (20.3.1)
�taView% DataView The DataView constructor (25.3.2)
�te% Date The Date constructor (21.4.2)
�codeURI% decodeURI The decodeURI function (19.2.6.1)
�codeURIComponent% decodeURIComponent The decodeURIComponent function (19.2.6.2)
%encodeURI% encodeURI The encodeURI function (19.2.6.3)
%encodeURIComponent% encodeURIComponent The encodeURIComponent function (19.2.6.4)
%Error% Error The Error constructor (20.5.1)
%eval% eval The eval function (19.2.1)
%EvalError% EvalError The EvalError constructor (20.5.5.1)
%FinalizationRegistry% FinalizationRegistry The FinalizationRegistry constructor (26.2.1)
%Float32Array% Float32Array The Float32Array constructor (23.2)
%Float64Array% Float64Array The Float64Array constructor (23.2)
%ForInIteratorPrototype% The prototype of For-In iterator objects (14.7.5.10)
%Function% Function The Function constructor (20.2.1)
%GeneratorFunction% The constructor of Generators (27.3.1)
%Int8Array% Int8Array The Int8Array constructor (23.2)
%Int16Array% Int16Array The Int16Array constructor (23.2)
%Int32Array% Int32Array The Int32Array constructor (23.2)
%isFinite% isFinite The isFinite function (19.2.2)
%isNaN% isNaN The isNaN function (19.2.3)
%IteratorPrototype% An object that all standard built-in iterator objects indirectly inherit from
%JSON% JSON The JSON object (25.5)
%Map% Map The Map constructor (24.1.1)
%MapIteratorPrototype% The prototype of Map iterator objects (24.1.5)
%Math% Math The Math object (21.3)
%Number% Number The Number constructor (21.1.1)
%Object% Object The Object constructor (20.1.1)
%parseFloat% parseFloat The parseFloat function (19.2.4)
%parseInt% parseInt The parseInt function (19.2.5)
%Promise% Promise The Promise constructor (27.2.3)
%Proxy% Proxy The Proxy constructor (28.2.1)
%RangeError% RangeError The RangeError constructor (20.5.5.2)
%ReferenceError% ReferenceError The ReferenceError constructor (20.5.5.3)
%Reflect% Reflect The Reflect object (28.1)
%RegExp% RegExp The RegExp constructor (22.2.4)
%RegExpStringIteratorPrototype% The prototype of RegExp String Iterator objects (22.2.9)
%Set% Set The Set constructor (24.2.1)
%SetIteratorPrototype% The prototype of Set iterator objects (24.2.5)
%SharedArrayBuffer% SharedArrayBuffer The SharedArrayBuffer constructor (25.2.2)
%String% String The String constructor (22.1.1)
%StringIteratorPrototype% The prototype of String iterator objects (22.1.5)
%Symbol% Symbol The Symbol constructor (20.4.1)
%SyntaxError% SyntaxError The SyntaxError constructor (20.5.5.4)
%ThrowTypeError% A function object that unconditionally throws a new instance of %TypeError%
%TypedArray% The super class of all typed Array constructors (23.2.1)
%TypeError% TypeError The TypeError constructor (20.5.5.5)
%Uint8Array% Uint8Array The Uint8Array constructor (23.2)
%Uint8ClampedArray% Uint8ClampedArray The Uint8ClampedArray constructor (23.2)
%Uint16Array% Uint16Array The Uint16Array constructor (23.2)
%Uint32Array% Uint32Array The Uint32Array constructor (23.2)
%URIError% URIError The URIError constructor (20.5.5.6)
%WeakMap% WeakMap The WeakMap constructor (24.3.1)
%WeakRef% WeakRef The WeakRef constructor (26.1.1)
%WeakSet% WeakSet The WeakSet constructor (24.4.1)
Note

Additional entries in Table 91.

6.2 ECMAScript Specification Types

A specification type corresponds to meta-values that are used within algorithms to describe the semantics of ECMAScript language constructs and ECMAScript language types. The specification types include Reference, List, Completion Record, Property Descriptor, Environment Record, Abstract Closure, and Data Block. Specification type values are specification artefacts that do not necessarily correspond to any specific entity within an ECMAScript implementation. Specification type values may be used to describe intermediate results of ECMAScript expression evaluation but such values cannot be stored as properties of objects or values of ECMAScript language variables.

6.2.1 The Enum Specification Type

Enums are values which are internal to the specification and not directly observable from ECMAScript code. Enums are denoted using a sans-serif typeface. For instance, a Completion Record's [[Type]] field takes on values like normal, return, or throw. Enums have no characteristics other than their name. The name of an enum serves no purpose other than to distinguish it from other enums, and implies nothing about its usage or meaning in context.

6.2.2 The List and Record Specification Types

The List type is used to explain the evaluation of argument lists (see 13.3.8) in new expressions, in function calls, and in other algorithms where a simple ordered list of values is needed. Values of the List type are simply ordered sequences of list elements containing the individual values. These sequences may be of any length. The elements of a list may be randomly accessed using 0-origin indices. For notational convenience an array-like syntax can be used to access List elements. For example, arguments[2] is shorthand for saying the 3rd element of the List arguments.

When an algorithm iterates over the elements of a List without specifying an order, the order used is the order of the elements in the List.

For notational convenience within this specification, a literal syntax can be used to express a new List value. For example, « 1, 2 » defines a List value that has two elements each of which is initialized to a specific value. A new empty List can be expressed as « ».

In this specification, the phrase "the list-concatenation of A, B, ..." (where each argument is a possibly empty List) denotes a new List value whose elements are the concatenation of the elements (in order) of each of the arguments (in order).

The Record type is used to describe data aggregations within the algorithms of this specification. A Record type value consists of one or more named fields. The value of each field is an ECMAScript language value or specification value. Field names are always enclosed in double brackets, for example [[Value]].

For notational convenience within this specification, an object literal-like syntax can be used to express a Record value. For example, { [[Field1]]: 42, [[Field2]]: false, [[Field3]]: empty } defines a Record value that has three fields, each of which is initialized to a specific value. Field name order is not significant. Any fields that are not explicitly listed are considered to be absent.

In specification text and algorithms, dot notation may be used to refer to a specific field of a Record value. For example, if R is the record shown in the previous paragraph then R.[[Field2]] is shorthand for “the field of R named [[Field2]]”.

Schema for commonly used Record field combinations may be named, and that name may be used as a prefix to a literal Record value to identify the specific kind of aggregations that is being described. For example: PropertyDescriptor { [[Value]]: 42, [[Writable]]: false, [[Configurable]]: true }.

6.2.3 The Set and Relation Specification Types

The Set type is used to explain a collection of unordered elements for use in the memory model. It is distinct from the ECMAScript collection type of the same name. To disambiguate, instances of the ECMAScript collection are consistently referred to as "Set objects" within this specification. Values of the Set type are simple collections of elements, where no element appears more than once. Elements may be added to and removed from Sets. Sets may be unioned, intersected, or subtracted from each other.

The Relation type is used to explain constraints on Sets. Values of the Relation type are Sets of ordered pairs of values from its value domain. For example, a Relation on events is a set of ordered pairs of events. For a Relation R and two values a and b in the value domain of R, a R b is shorthand for saying the ordered pair (a, b) is a member of R. A Relation is least with respect to some conditions when it is the smallest Relation that satisfies those conditions.

A strict partial order is a Relation value R that satisfies the following.

  • For all a, b, and c in R's domain:

    • It is not the case that a R a, and
    • If a R b and b R c, then a R c.
Note 1

The two properties above are called irreflexivity and transitivity, respectively.

A strict total order is a Relation value R that satisfies the following.

  • For all a, b, and c in R's domain:

    • a is b or a R b or b R a, and
    • It is not the case that a R a, and
    • If a R b and b R c, then a R c.
Note 2

The three properties above are called totality, irreflexivity, and transitivity, respectively.

6.2.4 The Completion Record Specification Type

The Completion Record specification type is used to explain the runtime propagation of values and control flow such as the behaviour of statements (break, continue, return and throw) that perform nonlocal transfers of control.

Completion Records have the fields defined in Table 7.

Table 7: Completion Record Fields
Field Name Value Meaning
[[Type]] normal, break, continue, return, or throw The type of completion that occurred.
[[Value]] any value except a Completion Record The value that was produced.
[[Target]] a String or empty The target label for directed control transfers.

The following shorthand terms are sometimes used to refer to Completion Records.

  • normal completion refers to any Completion Record with a [[Type]] value of normal.
  • break completion refers to any Completion Record with a [[Type]] value of break.
  • continue completion refers to any Completion Record with a [[Type]] value of continue.
  • return completion refers to any Completion Record with a [[Type]] value of return.
  • throw completion refers to any Completion Record with a [[Type]] value of throw.
  • abrupt completion refers to any Completion Record with a [[Type]] value other than normal.
  • a normal completion containing some type of value refers to a normal completion that has a value of that type in its [[Value]] field.

Callable objects that are defined in this specification only return a normal completion or a throw completion. Returning any other kind of Completion Record is considered an editorial error.

Implementation-defined callable objects must return either a normal completion or a throw completion.

6.2.4.1 NormalCompletion ( value )

The abstract operation NormalCompletion takes argument value (any value except a Completion Record) and returns a normal completion. It performs the following steps when called:

  1. Return Completion Record { [[Type]]: normal, [[Value]]: value, [[Target]]: empty }.

6.2.4.2 ThrowCompletion ( value )

The abstract operation ThrowCompletion takes argument value (an ECMAScript language value) and returns a throw completion. It performs the following steps when called:

  1. Return Completion Record { [[Type]]: throw, [[Value]]: value, [[Target]]: empty }.

6.2.4.3 UpdateEmpty ( completionRecord, value )

The abstract operation UpdateEmpty takes arguments completionRecord (a Completion Record) and value (any value except a Completion Record) and returns a Completion Record. It performs the following steps when called:

  1. Assert: If completionRecord.[[Type]] is either return or throw, then completionRecord.[[Value]] is not empty.
  2. If completionRecord.[[Value]] is not empty, return ? completionRecord.
  3. Return Completion Record { [[Type]]: completionRecord.[[Type]], [[Value]]: value, [[Target]]: completionRecord.[[Target]] }.

6.2.5 The Reference Record Specification Type

The Reference Record type is used to explain the behaviour of such operators as delete, typeof, the assignment operators, the super keyword and other language features. For example, the left-hand operand of an assignment is expected to produce a Reference Record.

A Reference Record is a resolved name or property binding; its fields are defined by Table 8.

Table 8: Reference Record Fields
Field Name Value Meaning
[[Base]] an ECMAScript language value, an Environment Record, or unresolvable The value or Environment Record which holds the binding. A [[Base]] of unresolvable indicates that the binding could not be resolved.
[[ReferencedName]] a String, a Symbol, or a Private Name The name of the binding. Always a String if [[Base]] value is an Environment Record.
[[Strict]] a Boolean true if the Reference Record originated in strict mode code, false otherwise.
[[ThisValue]] an ECMAScript language value or empty If not empty, the Reference Record represents a property binding that was expressed using the super keyword; it is called a Super Reference Record and its [[Base]] value will never be an Environment Record. In that case, the [[ThisValue]] field holds the this value at the time the Reference Record was created.

The following abstract operations are used in this specification to operate upon Reference Records:

6.2.5.1 IsPropertyReference ( V )

The abstract operation IsPropertyReference takes argument V (a Reference Record) and returns a Boolean. It performs the following steps when called:

  1. If V.[[Base]] is unresolvable, return false.
  2. If V.[[Base]] is an Environment Record, return false; otherwise return true.

6.2.5.2 IsUnresolvableReference ( V )

The abstract operation IsUnresolvableReference takes argument V (a Reference Record) and returns a Boolean. It performs the following steps when called:

  1. If V.[[Base]] is unresolvable, return true; otherwise return false.

6.2.5.3 IsSuperReference ( V )

The abstract operation IsSuperReference takes argument V (a Reference Record) and returns a Boolean. It performs the following steps when called:

  1. If V.[[ThisValue]] is not empty, return true; otherwise return false.

6.2.5.4 IsPrivateReference ( V )

The abstract operation IsPrivateReference takes argument V (a Reference Record) and returns a Boolean. It performs the following steps when called:

  1. If V.[[ReferencedName]] is a Private Name, return true; otherwise return false.

6.2.5.5 GetValue ( V )

The abstract operation GetValue takes argument V (a Reference Record or an ECMAScript language value) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It performs the following steps when called:

  1. If V is not a Reference Record, return V.
  2. If IsUnresolvableReference(V) is true, throw a ReferenceError exception.
  3. If IsPropertyReference(V) is true, then
    1. Let baseObj be ? ToObject(V.[[Base]]).
    2. If IsPrivateReference(V) is true, then
      1. Return ? PrivateGet(baseObj, V.[[ReferencedName]]).
    3. Return ? baseObj.[[Get]](V.[[ReferencedName]], GetThisValue(V)).
  4. Else,
    1. Let base be V.[[Base]].
    2. Assert: base is an Environment Record.
    3. Return ? base.GetBindingValue(V.[[ReferencedName]], V.[[Strict]]) (see 9.1).
 Note

The object that may be created in step 3.a is not accessible outside of the above abstract operation and the ordinary object [[Get]] internal method. An implementation might choose to avoid the actual creation of the object.

6.2.5.6 PutValue ( V, W )

The abstract operation PutValue takes arguments V (a Reference Record or an ECMAScript language value) and W (an ECMAScript language value) and returns either a normal completion containing unused or an abrupt completion. It performs the following steps when called:

  1. If V is not a Reference Record, throw a ReferenceError exception.
  2. If IsUnresolvableReference(V) is true, then
    1. If V.[[Strict]] is true, throw a ReferenceError exception.
    2. Let globalObj be GetGlobalObject().
    3. Perform ? Set(globalObj, V.[[ReferencedName]], W, false).
    4. Return unused.
  3. If IsPropertyReference(V) is true, then
    1. Let baseObj be ? ToObject(V.[[Base]]).
    2. If IsPrivateReference(V) is true, then
      1. Return ? PrivateSet(baseObj, V.[[ReferencedName]], W).
    3. Let succeeded be ? baseObj.[[Set]](V.[[ReferencedName]], W, GetThisValue(V)).
    4. If succeeded is false and V.[[Strict]] is true, throw a TypeError exception.
    5. Return unused.
  4. Else,
    1. Let base be V.[[Base]].
    2. Assert: base is an Environment Record.
    3. Return ? base.SetMutableBinding(V.[[ReferencedName]], W, V.[[Strict]]) (see 9.1).
 Note

The object that may be created in step 3.a is not accessible outside of the above abstract operation and the ordinary object [[Set]] internal method. An implementation might choose to avoid the actual creation of that object.

6.2.5.7 GetThisValue ( V )

The abstract operation GetThisValue takes argument V (a Reference Record) and returns an ECMAScript language value. It performs the following steps when called:

  1. Assert: IsPropertyReference(V) is true.
  2. If IsSuperReference(V) is true, return V.[[ThisValue]]; otherwise return V.[[Base]].

6.2.5.8 InitializeReferencedBinding ( V, W )

The abstract operation InitializeReferencedBinding takes arguments V (a Reference Record) and W (an ECMAScript language value) and returns either a normal completion containing unused or an abrupt completion. It performs the following steps when called:

  1. Assert: IsUnresolvableReference(V) is false.
  2. Let base be V.[[Base]].
  3. Assert: base is an Environment Record.
  4. Return ? base.InitializeBinding(V.[[ReferencedName]], W).

6.2.5.9 MakePrivateReference ( baseValue, privateIdentifier )

The abstract operation MakePrivateReference takes arguments baseValue (an ECMAScript language value) and privateIdentifier (a String) and returns a Reference Record. It performs the following steps when called:

  1. Let privEnv be the running execution context's PrivateEnvironment.
  2. Assert: privEnv is not null.
  3. Let privateName be ResolvePrivateIdentifier(privEnv, privateIdentifier).
  4. Return the Reference Record { [[Base]]: baseValue, [[ReferencedName]]: privateName, [[Strict]]: true, [[ThisValue]]: empty }.

6.2.6 The Property Descriptor Specification Type

The Property Descriptor type is used to explain the manipulation and reification of Object property attributes. A Property Descriptor is a Record with zero or more fields, where each field's name is an attribute name and its value is a corresponding attribute value as specified in 6.1.7.1. The schema name used within this specification to tag literal descriptions of Property Descriptor records is “PropertyDescriptor”.

Property Descriptor values may be further classified as data Property Descriptors and accessor Property Descriptors based upon the existence or use of certain fields. A data Property Descriptor is one that includes any fields named either [[Value]] or [[Writable]]. An accessor Property Descriptor is one that includes any fields named either [[Get]] or [[Set]]. Any Property Descriptor may have fields named [[Enumerable]] and [[Configurable]]. A Property Descriptor value may not be both a data Property Descriptor and an accessor Property Descriptor; however, it may be neither (in which case it is a generic Property Descriptor). A fully populated Property Descriptor is one that is either an accessor Property Descriptor or a data Property Descriptor and that has all of the corresponding fields defined in Table 3.

The following abstract operations are used in this specification to operate upon Property Descriptor values:

6.2.6.1 IsAccessorDescriptor ( Desc )

The abstract operation IsAccessorDescriptor takes argument Desc (a Property Descriptor or undefined) and returns a Boolean. It performs the following steps when called:

  1. If Desc is undefined, return false.
  2. If Desc has a [[Get]] field, return true.
  3. If Desc has a [[Set]] field, return true.
  4. Return false.

6.2.6.2 IsDataDescriptor ( Desc )

The abstract operation IsDataDescriptor takes argument Desc (a Property Descriptor or undefined) and returns a Boolean. It performs the following steps when called:

  1. If Desc is undefined, return false.
  2. If Desc has a [[Value]] field, return true.
  3. If Desc has a [[Writable]] field, return true.
  4. Return false.

6.2.6.3 IsGenericDescriptor ( Desc )

The abstract operation IsGenericDescriptor takes argument Desc (a Property Descriptor or undefined) and returns a Boolean. It performs the following steps when called:

  1. If Desc is undefined, return false.
  2. If IsAccessorDescriptor(Desc) is true, return false.
  3. If IsDataDescriptor(Desc) is true, return false.
  4. Return true.

6.2.6.4 FromPropertyDescriptor ( Desc )

The abstract operation FromPropertyDescriptor takes argument Desc (a Property Descriptor or undefined) and returns an Object or undefined. It performs the following steps when called:

  1. If Desc is undefined, return undefined.
  2. Let obj be OrdinaryObjectCreate(%Object.prototype%).
  3. Assert: obj is an extensible ordinary object with no own properties.
  4. If Desc has a [[Value]] field, then
    1. Perform ! CreateDataPropertyOrThrow(obj, "value", Desc.[[Value]]).
  5. If Desc has a [[Writable]] field, then
    1. Perform ! CreateDataPropertyOrThrow(obj, "writable", Desc.[[Writable]]).
  6. If Desc has a [[Get]] field, then
    1. Perform ! CreateDataPropertyOrThrow(obj, "get", Desc.[[Get]]).
  7. If Desc has a [[Set]] field, then
    1. Perform ! CreateDataPropertyOrThrow(obj, "set", Desc.[[Set]]).
  8. If Desc has an [[Enumerable]] field, then
    1. Perform ! CreateDataPropertyOrThrow(obj, "enumerable", Desc.[[Enumerable]]).
  9. If Desc has a [[Configurable]] field, then
    1. Perform ! CreateDataPropertyOrThrow(obj, "configurable", Desc.[[Configurable]]).
  10. Return obj.

6.2.6.5 ToPropertyDescriptor ( Obj )

The abstract operation ToPropertyDescriptor takes argument Obj (an ECMAScript language value) and returns either a normal completion containing a Property Descriptor or a throw completion. It performs the following steps when called:

  1. If Obj is not an Object, throw a TypeError exception.
  2. Let desc be a new Property Descriptor that initially has no fields.
  3. Let hasEnumerable be ? HasProperty(Obj, "enumerable").
  4. If hasEnumerable is true, then
    1. Let enumerable be ToBoolean(? Get(Obj, "enumerable")).
    2. Set desc.[[Enumerable]] to enumerable.
  5. Let hasConfigurable be ? HasProperty(Obj, "configurable").
  6. If hasConfigurable is true, then
    1. Let configurable be ToBoolean(? Get(Obj, "configurable")).
    2. Set desc.[[Configurable]] to configurable.
  7. Let hasValue be ? HasProperty(Obj, "value").
  8. If hasValue is true, then
    1. Let value be ? Get(Obj, "value").
    2. Set desc.[[Value]] to value.
  9. Let hasWritable be ? HasProperty(Obj, "writable").
  10. If hasWritable is true, then
    1. Let writable be ToBoolean(? Get(Obj, "writable")).
    2. Set desc.[[Writable]] to writable.
  11. Let hasGet be ? HasProperty(Obj, "get").
  12. If hasGet is true, then
    1. Let getter be ? Get(Obj, "get").
    2. If IsCallable(getter) is false and getter is not undefined, throw a TypeError exception.
    3. Set desc.[[Get]] to getter.
  13. Let hasSet be ? HasProperty(Obj, "set").
  14. If hasSet is true, then
    1. Let setter be ? Get(Obj, "set").
    2. If IsCallable(setter) is false and setter is not undefined, throw a TypeError exception.
    3. Set desc.[[Set]] to setter.
  15. If desc has a [[Get]] field or desc has a [[Set]] field, then
    1. If desc has a [[Value]] field or desc has a [[Writable]] field, throw a TypeError exception.
  16. Return desc.

6.2.6.6 CompletePropertyDescriptor ( Desc )

The abstract operation CompletePropertyDescriptor takes argument Desc (a Property Descriptor) and returns unused. It performs the following steps when called:

  1. Let like be the Record { [[Value]]: undefined, [[Writable]]: false, [[Get]]: undefined, [[Set]]: undefined, [[Enumerable]]: false, [[Configurable]]: false }.
  2. If IsGenericDescriptor(Desc) is true or IsDataDescriptor(Desc) is true, then
    1. If Desc does not have a [[Value]] field, set Desc.[[Value]] to like.[[Value]].
    2. If Desc does not have a [[Writable]] field, set Desc.[[Writable]] to like.[[Writable]].
  3. Else,
    1. If Desc does not have a [[Get]] field, set Desc.[[Get]] to like.[[Get]].
    2. If Desc does not have a [[Set]] field, set Desc.[[Set]] to like.[[Set]].
  4. If Desc does not have an [[Enumerable]] field, set Desc.[[Enumerable]] to like.[[Enumerable]].
  5. If Desc does not have a [[Configurable]] field, set Desc.[[Configurable]] to like.[[Configurable]].
  6. Return unused.

6.2.7 The Environment Record Specification Type

The Environment Record type is used to explain the behaviour of name resolution in nested functions and blocks. This type and the operations upon it are defined in 9.1.

6.2.8 The Abstract Closure Specification Type

The Abstract Closure specification type is used to refer to algorithm steps together with a collection of values. Abstract Closures are meta-values and are invoked using function application style such as closure(arg1, arg2). Like abstract operations, invocations perform the algorithm steps described by the Abstract Closure.

In algorithm steps that create an Abstract Closure, values are captured with the verb "capture" followed by a list of aliases. When an Abstract Closure is created, it captures the value that is associated with each alias at that time. In steps that specify the algorithm to be performed when an Abstract Closure is called, each captured value is referred to by the alias that was used to capture the value.

If an Abstract Closure returns a Completion Record, that Completion Record's [[Type]] must be either normal or throw.

Abstract Closures are created inline as part of other algorithms, shown in the following example.

  1. Let addend be 41.
  2. Let closure be a new Abstract Closure with parameters (x) that captures addend and performs the following steps when called:
    1. Return x addend.
  3. Let val be closure(1).
  4. Assert: val is 42.

6.2.9 Data Blocks

The Data Block specification type is used to describe a distinct and mutable sequence of byte-sized (8 bit) numeric values. A byte value is an integer in the inclusive interval from 0 to 255. A Data Block value is created with a fixed number of bytes that each have the initial value 0.

For notational convenience within this specification, an array-like syntax can be used to access the individual bytes of a Data Block value. This notation presents a Data Block value as a 0-origined integer-indexed sequence of bytes. For example, if db is a 5 byte Data Block value then db[2] can be used to access its 3rd byte.

A data block that resides in memory that can be referenced from multiple agents concurrently is designated a Shared Data Block. A Shared Data Block has an identity (for the purposes of equality testing Shared Data Block values) that is address-free: it is tied not to the virtual addresses the block is mapped to in any process, but to the set of locations in memory that the block represents. Two data blocks are equal only if the sets of the locations they contain are equal; otherwise, they are not equal and the intersection of the sets of locations they contain is empty. Finally, Shared Data Blocks can be distinguished from Data Blocks.

The semantics of Shared Data Blocks is defined using Shared Data Block events by the memory model. Abstract operations below introduce Shared Data Block events and act as the interface between evaluation semantics and the event semantics of the memory model. The events form a candidate execution, on which the memory model acts as a filter. Please consult the memory model for full semantics.

Shared Data Block events are modeled by Records, defined in the memory model.

The following abstract operations are used in this specification to operate upon Data Block values:

6.2.9.1 CreateByteDataBlock ( size )

The abstract operation CreateByteDataBlock takes argument size (a non-negative integer) and returns either a normal completion containing a Data Block or a throw completion. It performs the following steps when called:

  1. Let db be a new Data Block value consisting of size bytes. If it is impossible to create such a Data Block, throw a RangeError exception.
  2. Set all of the bytes of db to 0.
  3. Return db.

6.2.9.2 CreateSharedByteDataBlock ( size )

The abstract operation CreateSharedByteDataBlock takes argument size (a non-negative integer) and returns either a normal completion containing a Shared Data Block or a throw completion. It performs the following steps when called:

  1. Let db be a new Shared Data Block value consisting of size bytes. If it is impossible to create such a Shared Data Block, throw a RangeError exception.
  2. Let execution be the [[CandidateExecution]] field of the surrounding agent's Agent Record.
  3. Let eventsRecord be the Agent Events Record of execution.[[EventsRecords]] whose [[AgentSignifier]] is AgentSignifier().
  4. Let zero be « 0 ».
  5. For each index i of db, do
    1. Append WriteSharedMemory { [[Order]]: Init, [[NoTear]]: true, [[Block]]: db, [[ByteIndex]]: i, [[ElementSize]]: 1, [[Payload]]: zero } to eventsRecord.[[EventList]].
  6. Return db.

6.2.9.3 CopyDataBlockBytes ( toBlock, toIndex, fromBlock, fromIndex, count )

The abstract operation CopyDataBlockBytes takes arguments toBlock (a Data Block or a Shared Data Block), toIndex (a non-negative integer), fromBlock (a Data Block or a Shared Data Block), fromIndex (a non-negative integer), and count (a non-negative integer) and returns unused. It performs the following steps when called:

  1. Assert: fromBlock and toBlock are distinct values.
  2. Let fromSize be the number of bytes in fromBlock.
  3. Assert: fromIndex count ≤ fromSize.
  4. Let toSize be the number of bytes in toBlock.
  5. Assert: toIndex count ≤ toSize.
  6. Repeat, while count > 0,
    1. If fromBlock is a Shared Data Block, then
      1. Let execution be the [[CandidateExecution]] field of the surrounding agent's Agent Record.
      2. Let eventsRecord be the Agent Events Record of execution.[[EventsRecords]] whose [[AgentSignifier]] is AgentSignifier().
      3. Let bytes be a List whose sole element is a nondeterministically chosen byte value.
      4. NOTE: In implementations, bytes is the result of a non-atomic read instruction on the underlying hardware. The nondeterminism is a semantic prescription of the memory model to describe observable behaviour of hardware with weak consistency.
      5. Let readEvent be ReadSharedMemory { [[Order]]: Unordered, [[NoTear]]: true, [[Block]]: fromBlock, [[ByteIndex]]: fromIndex, [[ElementSize]]: 1 }.
      6. Append readEvent to eventsRecord.[[EventList]].
      7. Append Chosen Value Record { [[Event]]: readEvent, [[ChosenValue]]: bytes } to execution.[[ChosenValues]].
      8. If toBlock is a Shared Data Block, then
        1. Append WriteSharedMemory { [[Order]]: Unordered, [[NoTear]]: true, [[Block]]: toBlock, [[ByteIndex]]: toIndex, [[ElementSize]]: 1, [[Payload]]: bytes } to eventsRecord.[[EventList]].
      9. Else,
        1. Set toBlock[toIndex] to bytes[0].
    2. Else,
      1. Assert: toBlock is not a Shared Data Block.
      2. Set toBlock[toIndex] to fromBlock[fromIndex].
    3. Set toIndex to toIndex 1.
    4. Set fromIndex to fromIndex 1.
    5. Set count to count - 1.
  7. Return unused.

6.2.10 The PrivateElement Specification Type

The PrivateElement type is a Record used in the specification of private class fields, methods, and accessors. Although Property Descriptors are not used for private elements, private fields behave similarly to non-configurable, non-enumerable, writable data properties, private methods behave similarly to non-configurable, non-enumerable, non-writable data properties, and private accessors behave similarly to non-configurable, non-enumerable accessor properties.

Values of the PrivateElement type are Record values whose fields are defined by Table 9. Such values are referred to as PrivateElements.

Table 9: PrivateElement Fields
Field Name Values of the [[Kind]] field for which it is present Value Meaning
[[Key]] All a Private Name The name of the field, method, or accessor.
[[Kind]] All field, method, or accessor The kind of the element.
[[Value]] field and method an ECMAScript language value The value of the field.
[[Get]] accessor a function object or undefined The getter for a private accessor.
[[Set]] accessor a function object or undefined The setter for a private accessor.

6.2.11 The ClassFieldDefinition Record Specification Type

The ClassFieldDefinition type is a Record used in the specification of class fields.

Values of the ClassFieldDefinition type are Record values whose fields are defined by Table 10. Such values are referred to as ClassFieldDefinition Records.

Table 10: ClassFieldDefinition Record Fields
Field Name Value Meaning
[[Name]] a Private Name, a String, or a Symbol The name of the field.
[[Initializer]] a function object or empty The initializer of the field, if any.

6.2.12 Private Names

The Private Name specification type is used to describe a globally unique value (one which differs from any other Private Name, even if they are otherwise indistinguishable) which represents the key of a private class element (field, method, or accessor). Each Private Name has an associated immutable [[Description]] which is a String value. A Private Name may be installed on any ECMAScript object with PrivateFieldAdd or PrivateMethodOrAccessorAdd, and then read or written using PrivateGet and PrivateSet.

6.2.13 The ClassStaticBlockDefinition Record Specification Type

A ClassStaticBlockDefinition Record is a Record value used to encapsulate the executable code for a class static initialization block.

ClassStaticBlockDefinition Records have the fields listed in Table 11.

Table 11: ClassStaticBlockDefinition Record Fields
Field Name Value Meaning
[[BodyFunction]] a function object The function object to be called during static initialization of a class.

7 Abstract Operations

These operations are not a part of the ECMAScript language; they are defined here solely to aid the specification of the semantics of the ECMAScript language. Other, more specialized abstract operations are defined throughout this specification.

7.1 Type Conversion

The ECMAScript language implicitly performs automatic type conversion as needed. To clarify the semantics of certain constructs it is useful to define a set of conversion abstract operations. The conversion abstract operations are polymorphic; they can accept a value of any ECMAScript language type. But no other specification types are used with these operations.

The BigInt type has no implicit conversions in the ECMAScript language; programmers must call BigInt explicitly to convert values from other types.

7.1.1 ToPrimitive ( input [ , preferredType ] )

The abstract operation ToPrimitive takes argument input (an ECMAScript language value) and optional argument preferredType (string or number) and returns either a normal completion containing an ECMAScript language value or a throw completion. It converts its input argument to a non-Object type. If an object is capable of converting to more than one primitive type, it may use the optional hint preferredType to favour that type. It performs the following steps when called:

  1. If input is an Object, then
    1. Let exoticToPrim be ? GetMethod(input, @@toPrimitive).
    2. If exoticToPrim is not undefined, then
      1. If preferredType is not present, let hint be "default".
      2. Else if preferredType is string, let hint be "string".
      3. Else,
        1. Assert: preferredType is number.
        2. Let hint be "number".
      4. Let result be ? Call(exoticToPrim, input, « hint »).
      5. If result is not an Object, return result.
      6. Throw a TypeError exception.
    3. If preferredType is not present, let preferredType be number.
    4. Return ? OrdinaryToPrimitive(input, preferredType).
  2. Return input.
 Note

When ToPrimitive is called without a hint, then it generally behaves as if the hint were number. However, objects may over-ride this behaviour by defining a @@toPrimitive method. Of the objects defined in this specification only Dates (see 21.4.4.45) and Symbol objects (see 20.4.3.5) over-ride the default ToPrimitive behaviour. Dates treat the absence of a hint as if the hint were string.

7.1.1.1 OrdinaryToPrimitive ( O, hint )

The abstract operation OrdinaryToPrimitive takes arguments O (an Object) and hint (string or number) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

  1. If hint is string, then
    1. Let methodNames be « "toString", "valueOf" ».
  2. Else,
    1. Let methodNames be « "valueOf", "toString" ».
  3. For each element name of methodNames, do
    1. Let method be ? Get(O, name).
    2. If IsCallable(method) is true, then
      1. Let result be ? Call(method, O).
      2. If result is not an Object, return result.
  4. Throw a TypeError exception.

7.1.2 ToBoolean ( argument )

The abstract operation ToBoolean takes argument argument (an ECMAScript language value) and returns a Boolean. It converts argument to a value of type Boolean. It performs the following steps when called:

  1. If argument is a Boolean, return argument.
  2. If argument is one of undefined, null, 0𝔽, -0𝔽, NaN, 0ℤ, or the empty String, return false.
  3. NOTE: This step is replaced in section B.3.6.1.
  4. Return true.

7.1.3 ToNumeric ( value )

The abstract operation ToNumeric takes argument value (an ECMAScript language value) and returns either a normal completion containing either a Number or a BigInt, or a throw completion. It returns value converted to a Number or a BigInt. It performs the following steps when called:

  1. Let primValue be ? ToPrimitive(value, number).
  2. If primValue is a BigInt, return primValue.
  3. Return ? ToNumber(primValue).

7.1.4 ToNumber ( argument )

The abstract operation ToNumber takes argument argument (an ECMAScript language value) and returns either a normal completion containing a Number or a throw completion. It converts argument to a value of type Number. It performs the following steps when called:

  1. If argument is a Number, return argument.
  2. If argument is either a Symbol or a BigInt, throw a TypeError exception.
  3. If argument is undefined, return NaN.
  4. If argument is either null or false, return 0𝔽.
  5. If argument is true, return 1𝔽.
  6. If argument is a String, return StringToNumber(argument).
  7. Assert: argument is an Object.
  8. Let primValue be ? ToPrimitive(argument, number).
  9. Assert: primValue is not an Object.
  10. Return ? ToNumber(primValue).

7.1.4.1 ToNumber Applied to the String Type

The abstract operation StringToNumber specifies how to convert a String value to a Number value, using the following grammar.

Syntax

StringNumericLiteral ::: StrWhiteSpaceopt StrWhiteSpaceopt StrNumericLiteral StrWhiteSpaceopt StrWhiteSpace ::: StrWhiteSpaceChar StrWhiteSpaceopt StrWhiteSpaceChar ::: WhiteSpace LineTerminator StrNumericLiteral ::: StrDecimalLiteral NonDecimalIntegerLiteral[~Sep] StrDecimalLiteral ::: StrUnsignedDecimalLiteral StrUnsignedDecimalLiteral - StrUnsignedDecimalLiteral StrUnsignedDecimalLiteral ::: Infinity DecimalDigits[~Sep] . DecimalDigits[~Sep]opt ExponentPart[~Sep]opt . DecimalDigits[~Sep] ExponentPart[~Sep]opt DecimalDigits[~Sep] ExponentPart[~Sep]opt

All grammar symbols not explicitly defined above have the definitions used in the Lexical Grammar for numeric literals (12.9.3)

Note

Some differences should be noted between the syntax of a StringNumericLiteral and a NumericLiteral:

7.1.4.1.1 StringToNumber ( str )

The abstract operation StringToNumber takes argument str (a String) and returns a Number. It performs the following steps when called:

  1. Let text be StringToCodePoints(str).
  2. Let literal be ParseText(text, StringNumericLiteral).
  3. If literal is a List of errors, return NaN.
  4. Return StringNumericValue of literal.

7.1.4.1.2 Runtime Semantics: StringNumericValue

The syntax-directed operation StringNumericValue takes no arguments and returns a Number.

Note

The conversion of a StringNumericLiteral to a Number value is similar overall to the determination of the NumericValue of a NumericLiteral (see 12.9.3), but some of the details are different.

It is defined piecewise over the following productions:

StringNumericLiteral ::: StrWhiteSpaceopt
  1. Return 0𝔽.
 StringNumericLiteral ::: StrWhiteSpaceopt StrNumericLiteral StrWhiteSpaceopt
  1. Return StringNumericValue of StrNumericLiteral.
 StrNumericLiteral ::: NonDecimalIntegerLiteral
  1. Return 𝔽(MV of NonDecimalIntegerLiteral).
 StrDecimalLiteral ::: - StrUnsignedDecimalLiteral
  1. Let a be StringNumericValue of StrUnsignedDecimalLiteral.
  2. If a is 0𝔽, return -0𝔽.
  3. Return -a.
 StrUnsignedDecimalLiteral ::: Infinity
  1. Return ∞𝔽.
 StrUnsignedDecimalLiteral ::: DecimalDigits . DecimalDigitsopt ExponentPartopt
  1. Let a be MV of the first DecimalDigits.
  2. If the second DecimalDigits is present, then
    1. Let b be MV of the second DecimalDigits.
    2. Let n be the number of code points in the second DecimalDigits.
  3. Else,
    1. Let b be 0.
    2. Let n be 0.
  4. If ExponentPart is present, let e be MV of ExponentPart. Otherwise, let e be 0.
  5. Return RoundMVResult((a (b × 10-n)) × 10e).
 StrUnsignedDecimalLiteral ::: . DecimalDigits ExponentPartopt
  1. Let b be MV of DecimalDigits.
  2. If ExponentPart is present, let e be MV of ExponentPart. Otherwise, let e be 0.
  3. Let n be the number of code points in DecimalDigits.
  4. Return RoundMVResult(b × 10e - n).
 StrUnsignedDecimalLiteral ::: DecimalDigits ExponentPartopt
  1. Let a be MV of DecimalDigits.
  2. If ExponentPart is present, let e be MV of ExponentPart. Otherwise, let e be 0.
  3. Return RoundMVResult(a × 10e).

7.1.4.1.3 RoundMVResult ( n )

The abstract operation RoundMVResult takes argument n (a mathematical value) and returns a Number. It converts n to a Number in an implementation-defined manner. For the purposes of this abstract operation, a digit is significant if it is not zero or there is a non-zero digit to its left and there is a non-zero digit to its right. For the purposes of this abstract operation, "the mathematical value denoted by" a representation of a mathematical value is the inverse of "the decimal representation of" a mathematical value. It performs the following steps when called:

  1. If the decimal representation of n has 20 or fewer significant digits, return 𝔽(n).
  2. Let option1 be the mathematical value denoted by the result of replacing each significant digit in the decimal representation of n after the 20th with a 0 digit.
  3. Let option2 be the mathematical value denoted by the result of replacing each significant digit in the decimal representation of n after the 20th with a 0 digit and then incrementing it at the 20th position (with carrying as necessary).
  4. Let chosen be an implementation-defined choice of either option1 or option2.
  5. Return 𝔽(chosen).

7.1.5 ToIntegerOrInfinity ( argument )

The abstract operation ToIntegerOrInfinity takes argument argument (an ECMAScript language value) and returns either a normal completion containing either an integer, ∞, or -∞, or a throw completion. It converts argument to an integer representing its Number value with fractional part truncated, or to ∞ or -∞ when that Number value is infinite. It performs the following steps when called:

  1. Let number be ? ToNumber(argument).
  2. If number is one of NaN, 0𝔽, or -0𝔽, return 0.
  3. If number is ∞𝔽, return ∞.
  4. If number is -∞𝔽, return -∞.
  5. Return truncate(ℝ(number)).
 Note
𝔽(ToIntegerOrInfinity(x)) never returns -0𝔽 for any value of x. The truncation of the fractional part is performed after converting x to a mathematical value.

7.1.6 ToInt32 ( argument )

The abstract operation ToInt32 takes argument argument (an ECMAScript language value) and returns either a normal completion containing an integral Number or a throw completion. It converts argument to one of 232 integral Number values in the inclusive interval from 𝔽(-231) to 𝔽(231 - 1). It performs the following steps when called:

  1. Let number be ? ToNumber(argument).
  2. If number is not finite or number is either 0𝔽 or -0𝔽, return 0𝔽.
  3. Let int be truncate(ℝ(number)).
  4. Let int32bit be int modulo 232.
  5. If int32bit ≥ 231, return 𝔽(int32bit - 232); otherwise return 𝔽(int32bit).
 Note

Given the above definition of ToInt32:

  • The ToInt32 abstract operation is idempotent: if applied to a result that it produced, the second application leaves that value unchanged.
  • ToInt32(ToUint32(x)) is the same value as ToInt32(x) for all values of x. (It is to preserve this latter property that ∞𝔽 and -∞𝔽 are mapped to 0𝔽.)
  • ToInt32 maps -0𝔽 to 0𝔽.

7.1.7 ToUint32 ( argument )

The abstract operation ToUint32 takes argument argument (an ECMAScript language value) and returns either a normal completion containing an integral Number or a throw completion. It converts argument to one of 232 integral Number values in the inclusive interval from 0𝔽 to 𝔽(232 - 1). It performs the following steps when called:

  1. Let number be ? ToNumber(argument).
  2. If number is not finite or number is either 0𝔽 or -0𝔽, return 0𝔽.
  3. Let int be truncate(ℝ(number)).
  4. Let int32bit be int modulo 232.
  5. Return 𝔽(int32bit).
 Note

Given the above definition of ToUint32:

  • Step 5 is the only difference between ToUint32 and ToInt32.
  • The ToUint32 abstract operation is idempotent: if applied to a result that it produced, the second application leaves that value unchanged.
  • ToUint32(ToInt32(x)) is the same value as ToUint32(x) for all values of x. (It is to preserve this latter property that ∞𝔽 and -∞𝔽 are mapped to 0𝔽.)
  • ToUint32 maps -0𝔽 to 0𝔽.

7.1.8 ToInt16 ( argument )

The abstract operation ToInt16 takes argument argument (an ECMAScript language value) and returns either a normal completion containing an integral Number or a throw completion. It converts argument to one of 216 integral Number values in the inclusive interval from 𝔽(-215) to 𝔽(215 - 1). It performs the following steps when called:

  1. Let number be ? ToNumber(argument).
  2. If number is not finite or number is either 0𝔽 or -0𝔽, return 0𝔽.
  3. Let int be truncate(ℝ(number)).
  4. Let int16bit be int modulo 216.
  5. If int16bit ≥ 215, return 𝔽(int16bit - 216); otherwise return 𝔽(int16bit).

7.1.9 ToUint16 ( argument )

The abstract operation ToUint16 takes argument argument (an ECMAScript language value) and returns either a normal completion containing an integral Number or a throw completion. It converts argument to one of 216 integral Number values in the inclusive interval from 0𝔽 to 𝔽(216 - 1). It performs the following steps when called:

  1. Let number be ? ToNumber(argument).
  2. If number is not finite or number is either 0𝔽 or -0𝔽, return 0𝔽.
  3. Let int be truncate(ℝ(number)).
  4. Let int16bit be int modulo 216.
  5. Return 𝔽(int16bit).
 Note

Given the above definition of ToUint16:

  • The substitution of 216 for 232 in step 4 is the only difference between ToUint32 and ToUint16.
  • ToUint16 maps -0𝔽 to 0𝔽.

7.1.10 ToInt8 ( argument )

The abstract operation ToInt8 takes argument argument (an ECMAScript language value) and returns either a normal completion containing an integral Number or a throw completion. It converts argument to one of 28 integral Number values in the inclusive interval from -128𝔽 to 127𝔽. It performs the following steps when called:

  1. Let number be ? ToNumber(argument).
  2. If number is not finite or number is either 0𝔽 or -0𝔽, return 0𝔽.
  3. Let int be truncate(ℝ(number)).
  4. Let int8bit be int modulo 28.
  5. If int8bit ≥ 27, return 𝔽(int8bit - 28); otherwise return 𝔽(int8bit).

7.1.11 ToUint8 ( argument )

The abstract operation ToUint8 takes argument argument (an ECMAScript language value) and returns either a normal completion containing an integral Number or a throw completion. It converts argument to one of 28 integral Number values in the inclusive interval from 0𝔽 to 255𝔽. It performs the following steps when called:

  1. Let number be ? ToNumber(argument).
  2. If number is not finite or number is either 0𝔽 or -0𝔽, return 0𝔽.
  3. Let int be truncate(ℝ(number)).
  4. Let int8bit be int modulo 28.
  5. Return 𝔽(int8bit).

7.1.12 ToUint8Clamp ( argument )

The abstract operation ToUint8Clamp takes argument argument (an ECMAScript language value) and returns either a normal completion containing an integral Number or a throw completion. It converts argument to one of 28 integral Number values in the inclusive interval from 0𝔽 to 255𝔽. It performs the following steps when called:

  1. Let number be ? ToNumber(argument).
  2. If number is NaN, return 0𝔽.
  3. If ℝ(number) ≤ 0, return 0𝔽.
  4. If ℝ(number) ≥ 255, return 255𝔽.
  5. Let f be floor(ℝ(number)).
  6. If f 0.5 < ℝ(number), return 𝔽(f 1).
  7. If ℝ(number) < f 0.5, return 𝔽(f).
  8. If f is odd, return 𝔽(f 1).
  9. Return 𝔽(f).
 Note

Unlike the other ECMAScript integer conversion abstract operation, ToUint8Clamp rounds rather than truncates non-integral values and does not convert ∞𝔽 to 0𝔽. ToUint8Clamp does “round half to even” tie-breaking. This differs from Math.round which does “round half up” tie-breaking.

7.1.13 ToBigInt ( argument )

The abstract operation ToBigInt takes argument argument (an ECMAScript language value) and returns either a normal completion containing a BigInt or a throw completion. It converts argument to a BigInt value, or throws if an implicit conversion from Number would be required. It performs the following steps when called:

  1. Let prim be ? ToPrimitive(argument, number).
  2. Return the value that prim corresponds to in Table 12.
Table 12: BigInt Conversions
Argument Type Result
Undefined Throw a TypeError exception.
Null Throw a TypeError exception.
Boolean Return 1n if prim is true and 0n if prim is false.
BigInt Return prim.
Number Throw a TypeError exception.
String
  1. Let n be StringToBigInt(prim).
  2. If n is undefined, throw a SyntaxError exception.
  3. Return n.
Symbol Throw a TypeError exception.

7.1.14 StringToBigInt ( str )

The abstract operation StringToBigInt takes argument str (a String) and returns a BigInt or undefined. It performs the following steps when called:

  1. Let text be StringToCodePoints(str).
  2. Let literal be ParseText(text, StringIntegerLiteral).
  3. If literal is a List of errors, return undefined.
  4. Let mv be the MV of literal.
  5. Assert: mv is an integer.
  6. Return ℤ(mv).

7.1.14.1 StringIntegerLiteral Grammar

StringToBigInt uses the following grammar.

Syntax

StringIntegerLiteral ::: StrWhiteSpaceopt StrWhiteSpaceopt StrIntegerLiteral StrWhiteSpaceopt StrIntegerLiteral ::: SignedInteger[~Sep] NonDecimalIntegerLiteral[~Sep]

7.1.14.2 Runtime Semantics: MV

7.1.15 ToBigInt64 ( argument )

The abstract operation ToBigInt64 takes argument argument (an ECMAScript language value) and returns either a normal completion containing a BigInt or a throw completion. It converts argument to one of 264 BigInt values in the inclusive interval from ℤ(-263) to ℤ(263-1). It performs the following steps when called:

  1. Let n be ? ToBigInt(argument).
  2. Let int64bit be ℝ(n) modulo 264.
  3. If int64bit ≥ 263, return ℤ(int64bit - 264); otherwise return ℤ(int64bit).

7.1.16 ToBigUint64 ( argument )

The abstract operation ToBigUint64 takes argument argument (an ECMAScript language value) and returns either a normal completion containing a BigInt or a throw completion. It converts argument to one of 264 BigInt values in the inclusive interval from 0ℤ to ℤ(264-1). It performs the following steps when called:

  1. Let n be ? ToBigInt(argument).
  2. Let int64bit be ℝ(n) modulo 264.
  3. Return ℤ(int64bit).

7.1.17 ToString ( argument )

The abstract operation ToString takes argument argument (an ECMAScript language value) and returns either a normal completion containing a String or a throw completion. It converts argument to a value of type String. It performs the following steps when called:

  1. If argument is a String, return argument.
  2. If argument is a Symbol, throw a TypeError exception.
  3. If argument is undefined, return "undefined".
  4. If argument is null, return "null".
  5. If argument is true, return "true".
  6. If argument is false, return "false".
  7. If argument is a Number, return Number::toString(argument, 10).
  8. If argument is a BigInt, return BigInt::toString(argument, 10).
  9. Assert: argument is an Object.
  10. Let primValue be ? ToPrimitive(argument, string).
  11. Assert: primValue is not an Object.
  12. Return ? ToString(primValue).

7.1.18 ToObject ( argument )

The abstract operation ToObject takes argument argument (an ECMAScript language value) and returns either a normal completion containing an Object or a throw completion. It converts argument to a value of type Object according to Table 13:

Table 13: ToObject Conversions
Argument Type Result
Undefined Throw a TypeError exception.
Null Throw a TypeError exception.
Boolean Return a new Boolean object whose [[BooleanData]] internal slot is set to argument. See 20.3 for a description of Boolean objects.
Number Return a new Number object whose [[NumberData]] internal slot is set to argument. See 21.1 for a description of Number objects.
String Return a new String object whose [[StringData]] internal slot is set to argument. See 22.1 for a description of String objects.
Symbol Return a new Symbol object whose [[SymbolData]] internal slot is set to argument. See 20.4 for a description of Symbol objects.
BigInt Return a new BigInt object whose [[BigIntData]] internal slot is set to argument. See 21.2 for a description of BigInt objects.
Object Return argument.

7.1.19 ToPropertyKey ( argument )

The abstract operation ToPropertyKey takes argument argument (an ECMAScript language value) and returns either a normal completion containing a property key or a throw completion. It converts argument to a value that can be used as a property key. It performs the following steps when called:

  1. Let key be ? ToPrimitive(argument, string).
  2. If key is a Symbol, then
    1. Return key.
  3. Return ! ToString(key).

7.1.20 ToLength ( argument )

The abstract operation ToLength takes argument argument (an ECMAScript language value) and returns either a normal completion containing an integral Number or a throw completion. It clamps argument to an integral Number suitable for use as the length of an array-like object. It performs the following steps when called:

  1. Let len be ? ToIntegerOrInfinity(argument).
  2. If len ≤ 0, return 0𝔽.
  3. Return 𝔽(min(len, 253 - 1)).

7.1.21 CanonicalNumericIndexString ( argument )

The abstract operation CanonicalNumericIndexString takes argument argument (a String) and returns a Number or undefined. If argument is either "-0" or exactly matches the result of ToString(n) for some Number value n, it returns the respective Number value. Otherwise, it returns undefined. It performs the following steps when called:

  1. If argument is "-0", return -0𝔽.
  2. Let n be ! ToNumber(argument).
  3. If ! ToString(n) is argument, return n.
  4. Return undefined.

A canonical numeric string is any String value for which the CanonicalNumericIndexString abstract operation does not return undefined.

7.1.22 ToIndex ( value )

The abstract operation ToIndex takes argument value (an ECMAScript language value) and returns either a normal completion containing a non-negative integer or a throw completion. It converts value to a non-negative integer if the corresponding decimal representation, as a String, is an integer index. It performs the following steps when called:

  1. If value is undefined, then
    1. Return 0.
  2. Else,
    1. Let integer be ? ToIntegerOrInfinity(value).
    2. Let clamped be ! ToLength(𝔽(integer)).
    3. If SameValue(𝔽(integer), clamped) is false, throw a RangeError exception.
    4. Assert: 0 ≤ integer ≤ 253 - 1.
    5. Return integer.

7.2 Testing and Comparison Operations

7.2.1 RequireObjectCoercible ( argument )

The abstract operation RequireObjectCoercible takes argument argument (an ECMAScript language value) and returns either a normal completion containing an ECMAScript language value or a throw completion. It throws an error if argument is a value that cannot be converted to an Object using ToObject. It is defined by Table 14:

Table 14: RequireObjectCoercible Results
Argument Type Result
Undefined Throw a TypeError exception.
Null Throw a TypeError exception.
Boolean Return argument.
Number Return argument.
String Return argument.
Symbol Return argument.
BigInt Return argument.
Object Return argument.

7.2.2 IsArray ( argument )

The abstract operation IsArray takes argument argument (an ECMAScript language value) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

  1. If argument is not an Object, return false.
  2. If argument is an Array exotic object, return true.
  3. If argument is a Proxy exotic object, then
    1. Perform ? ValidateNonRevokedProxy(argument).
    2. Let proxyTarget be argument.[[ProxyTarget]].
    3. Return ? IsArray(proxyTarget).
  4. Return false.

7.2.3 IsCallable ( argument )

The abstract operation IsCallable takes argument argument (an ECMAScript language value) and returns a Boolean. It determines if argument is a callable function with a [[Call]] internal method. It performs the following steps when called:

  1. If argument is not an Object, return false.
  2. If argument has a [[Call]] internal method, return true.
  3. Return false.

7.2.4 IsConstructor ( argument )

The abstract operation IsConstructor takes argument argument (an ECMAScript language value) and returns a Boolean. It determines if argument is a function object with a [[Construct]] internal method. It performs the following steps when called:

  1. If argument is not an Object, return false.
  2. If argument has a [[Construct]] internal method, return true.
  3. Return false.

7.2.5 IsExtensible ( O )

The abstract operation IsExtensible takes argument O (an Object) and returns either a normal completion containing a Boolean or a throw completion. It is used to determine whether additional properties can be added to O. It performs the following steps when called:

  1. Return ? O.[[IsExtensible]]().

7.2.6 IsIntegralNumber ( argument )

The abstract operation IsIntegralNumber takes argument argument (an ECMAScript language value) and returns a Boolean. It determines if argument is a finite integral Number value. It performs the following steps when called:

  1. If argument is not a Number, return false.
  2. If argument is not finite, return false.
  3. If truncate(ℝ(argument)) ≠ ℝ(argument), return false.
  4. Return true.

7.2.7 IsPropertyKey ( argument )

The abstract operation IsPropertyKey takes argument argument (an ECMAScript language value) and returns a Boolean. It determines if argument is a value that may be used as a property key. It performs the following steps when called:

  1. If argument is a String, return true.
  2. If argument is a Symbol, return true.
  3. Return false.

7.2.8 IsRegExp ( argument )

The abstract operation IsRegExp takes argument argument (an ECMAScript language value) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

  1. If argument is not an Object, return false.
  2. Let matcher be ? Get(argument, @@match).
  3. If matcher is not undefined, return ToBoolean(matcher).
  4. If argument has a [[RegExpMatcher]] internal slot, return true.
  5. Return false.

7.2.9 Static Semantics: IsStringWellFormedUnicode ( string )

The abstract operation IsStringWellFormedUnicode takes argument string (a String) and returns a Boolean. It interprets string as a sequence of UTF-16 encoded code points, as described in 6.1.4, and determines whether it is a well formed UTF-16 sequence. It performs the following steps when called:

  1. Let len be the length of string.
  2. Let k be 0.
  3. Repeat, while k < len,
    1. Let cp be CodePointAt(string, k).
    2. If cp.[[IsUnpairedSurrogate]] is true, return false.
    3. Set k to k cp.[[CodeUnitCount]].
  4. Return true.

7.2.10 SameValue ( x, y )

The abstract operation SameValue takes arguments x (an ECMAScript language value) and y (an ECMAScript language value) and returns a Boolean. It determines whether or not the two arguments are the same value. It performs the following steps when called:

  1. If Type(x) is not Type(y), return false.
  2. If x is a Number, then
    1. Return Number::sameValue(x, y).
  3. Return SameValueNonNumber(x, y).
 Note

This algorithm differs from the IsStrictlyEqual Algorithm by treating all NaN values as equivalent and by differentiating 0𝔽 from -0𝔽.

7.2.11 SameValueZero ( x, y )

The abstract operation SameValueZero takes arguments x (an ECMAScript language value) and y (an ECMAScript language value) and returns a Boolean. It determines whether or not the two arguments are the same value (ignoring the difference between 0𝔽 and -0𝔽). It performs the following steps when called:

  1. If Type(x) is not Type(y), return false.
  2. If x is a Number, then
    1. Return Number::sameValueZero(x, y).
  3. Return SameValueNonNumber(x, y).
 Note

SameValueZero differs from SameValue only in that it treats 0𝔽 and -0𝔽 as equivalent.

7.2.12 SameValueNonNumber ( x, y )

The abstract operation SameValueNonNumber takes arguments x (an ECMAScript language value, but not a Number) and y (an ECMAScript language value, but not a Number) and returns a Boolean. It performs the following steps when called:

  1. Assert: Type(x) is Type(y).
  2. If x is either null or undefined, return true.
  3. If x is a BigInt, then
    1. Return BigInt::equal(x, y).
  4. If x is a String, then
    1. If x and y have the same length and the same code units in the same positions, return true; otherwise, return false.
  5. If x is a Boolean, then
    1. If x and y are both true or both false, return true; otherwise, return false.
  6. NOTE: All other ECMAScript language values are compared by identity.
  7. If x is y, return true; otherwise, return false.
 Note 1
For expository purposes, some cases are handled separately within this algorithm even if it is unnecessary to do so.
Note 2
The specifics of what "x is y" means are detailed in 5.2.7.

7.2.13 IsLessThan ( x, y, LeftFirst )

The abstract operation IsLessThan takes arguments x (an ECMAScript language value), y (an ECMAScript language value), and LeftFirst (a Boolean) and returns either a normal completion containing either a Boolean or undefined, or a throw completion. It provides the semantics for the comparison x < y, returning true, false, or undefined (which indicates that at least one operand is NaN). The LeftFirst flag is used to control the order in which operations with potentially visible side-effects are performed upon x and y. It is necessary because ECMAScript specifies left to right evaluation of expressions. If LeftFirst is true, the x parameter corresponds to an expression that occurs to the left of the y parameter's corresponding expression. If LeftFirst is false, the reverse is the case and operations must be performed upon y before x. It performs the following steps when called:

  1. If LeftFirst is true, then
    1. Let px be ? ToPrimitive(x, number).
    2. Let py be ? ToPrimitive(y, number).
  2. Else,
    1. NOTE: The order of evaluation needs to be reversed to preserve left to right evaluation.
    2. Let py be ? ToPrimitive(y, number).
    3. Let px be ? ToPrimitive(x, number).
  3. If px is a String and py is a String, then
    1. Let lx be the length of px.
    2. Let ly be the length of py.
    3. For each integer i such that 0 ≤ i < min(lx, ly), in ascending order, do
      1. Let cx be the numeric value of the code unit at index i within px.
      2. Let cy be the numeric value of the code unit at index i within py.
      3. If cx < cy, return true.
      4. If cx > cy, return false.
    4. If lx < ly, return true. Otherwise, return false.
  4. Else,
    1. If px is a BigInt and py is a String, then
      1. Let ny be StringToBigInt(py).
      2. If ny is undefined, return undefined.
      3. Return BigInt::lessThan(px, ny).
    2. If px is a String and py is a BigInt, then
      1. Let nx be StringToBigInt(px).
      2. If nx is undefined, return undefined.
      3. Return BigInt::lessThan(nx, py).
    3. NOTE: Because px and py are primitive values, evaluation order is not important.
    4. Let nx be ? ToNumeric(px).
    5. Let ny be ? ToNumeric(py).
    6. If Type(nx) is Type(ny), then
      1. If nx is a Number, then
        1. Return Number::lessThan(nx, ny).
      2. Else,
        1. Assert: nx is a BigInt.
        2. Return BigInt::lessThan(nx, ny).
    7. Assert: nx is a BigInt and ny is a Number, or nx is a Number and ny is a BigInt.
    8. If nx or ny is NaN, return undefined.
    9. If nx is -∞𝔽 or ny is ∞𝔽, return true.
    10. If nx is ∞𝔽 or ny is -∞𝔽, return false.
    11. If ℝ(nx) < ℝ(ny), return true; otherwise return false.
 Note 1

Step 3 differs from step 1.c in the algorithm that handles the addition operator (13.15.3) by using the logical-and operation instead of the logical-or operation.

 Note 2

The comparison of Strings uses a simple lexicographic ordering on sequences of UTF-16 code unit values. There is no attempt to use the more complex, semantically oriented definitions of character or string equality and collating order defined in the Unicode specification. Therefore String values that are canonically equal according to the Unicode Standard but not in the same normalization form could test as unequal. Also note that lexicographic ordering by code unit differs from ordering by code point for Strings containing surrogate pairs.

7.2.14 IsLooselyEqual ( x, y )

The abstract operation IsLooselyEqual takes arguments x (an ECMAScript language value) and y (an ECMAScript language value) and returns either a normal completion containing a Boolean or a throw completion. It provides the semantics for the == operator. It performs the following steps when called:

  1. If Type(x) is Type(y), then
    1. Return IsStrictlyEqual(x, y).
  2. If x is null and y is undefined, return true.
  3. If x is undefined and y is null, return true.
  4. NOTE: This step is replaced in section B.3.6.2.
  5. If x is a Number and y is a String, return ! IsLooselyEqual(x, ! ToNumber(y)).
  6. If x is a String and y is a Number, return ! IsLooselyEqual(! ToNumber(x), y).
  7. If x is a BigInt and y is a String, then
    1. Let n be StringToBigInt(y).
    2. If n is undefined, return false.
    3. Return ! IsLooselyEqual(x, n).
  8. If x is a String and y is a BigInt, return ! IsLooselyEqual(y, x).
  9. If x is a Boolean, return ! IsLooselyEqual(! ToNumber(x), y).
  10. If y is a Boolean, return ! IsLooselyEqual(x, ! ToNumber(y)).
  11. If x is either a String, a Number, a BigInt, or a Symbol and y is an Object, return ! IsLooselyEqual(x, ? ToPrimitive(y)).
  12. If x is an Object and y is either a String, a Number, a BigInt, or a Symbol, return ! IsLooselyEqual(? ToPrimitive(x), y).
  13. If x is a BigInt and y is a Number, or if x is a Number and y is a BigInt, then
    1. If x is not finite or y is not finite, return false.
    2. If ℝ(x) = ℝ(y), return true; otherwise return false.
  14. Return false.

7.2.15 IsStrictlyEqual ( x, y )

The abstract operation IsStrictlyEqual takes arguments x (an ECMAScript language value) and y (an ECMAScript language value) and returns a Boolean. It provides the semantics for the === operator. It performs the following steps when called:

  1. If Type(x) is not Type(y), return false.
  2. If x is a Number, then
    1. Return Number::equal(x, y).
  3. Return SameValueNonNumber(x, y).
 Note

This algorithm differs from the SameValue Algorithm in its treatment of signed zeroes and NaNs.

7.3 Operations on Objects

7.3.1 MakeBasicObject ( internalSlotsList )

The abstract operation MakeBasicObject takes argument internalSlotsList (a List of internal slot names) and returns an Object. It is the source of all ECMAScript objects that are created algorithmically, including both ordinary objects and exotic objects. It factors out common steps used in creating all objects, and centralizes object creation. It performs the following steps when called:

  1. Let obj be a newly created object with an internal slot for each name in internalSlotsList.
  2. Set obj's essential internal methods to the default ordinary object definitions specified in 10.1.
  3. Assert: If the caller will not be overriding both obj's [[GetPrototypeOf]] and [[SetPrototypeOf]] essential internal methods, then internalSlotsList contains [[Prototype]].
  4. Assert: If the caller will not be overriding all of obj's [[SetPrototypeOf]], [[IsExtensible]], and [[PreventExtensions]] essential internal methods, then internalSlotsList contains [[Extensible]].
  5. If internalSlotsList contains [[Extensible]], set obj.[[Extensible]] to true.
  6. Return obj.
 Note

Within this specification, exotic objects are created in abstract operations such as ArrayCreate and BoundFunctionCreate by first calling MakeBasicObject to obtain a basic, foundational object, and then overriding some or all of that object's internal methods. In order to encapsulate exotic object creation, the object's essential internal methods are never modified outside those operations.

7.3.2 Get ( O, P )

The abstract operation Get takes arguments O (an Object) and P (a property key) and returns either a normal completion containing an ECMAScript language value or a throw completion. It is used to retrieve the value of a specific property of an object. It performs the following steps when called:

  1. Return ? O.[[Get]](P, O).

7.3.3 GetV ( V, P )

The abstract operation GetV takes arguments V (an ECMAScript language value) and P (a property key) and returns either a normal completion containing an ECMAScript language value or a throw completion. It is used to retrieve the value of a specific property of an ECMAScript language value. If the value is not an object, the property lookup is performed using a wrapper object appropriate for the type of the value. It performs the following steps when called:

  1. Let O be ? ToObject(V).
  2. Return ? O.[[Get]](P, V).

7.3.4 Set ( O, P, V, Throw )

The abstract operation Set takes arguments O (an Object), P (a property key), V (an ECMAScript language value), and Throw (a Boolean) and returns either a normal completion containing unused or a throw completion. It is used to set the value of a specific property of an object. V is the new value for the property. It performs the following steps when called:

  1. Let success be ? O.[[Set]](P, V, O).
  2. If success is false and Throw is true, throw a TypeError exception.
  3. Return unused.

7.3.5 CreateDataProperty ( O, P, V )

The abstract operation CreateDataProperty takes arguments O (an Object), P (a property key), and V (an ECMAScript language value) and returns either a normal completion containing a Boolean or a throw completion. It is used to create a new own property of an object. It performs the following steps when called:

  1. Let newDesc be the PropertyDescriptor { [[Value]]: V, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true }.
  2. Return ? O.[[DefineOwnProperty]](P, newDesc).
 Note

This abstract operation creates a property whose attributes are set to the same defaults used for properties created by the ECMAScript language assignment operator. Normally, the property will not already exist. If it does exist and is not configurable or if O is not extensible, [[DefineOwnProperty]] will return false.

7.3.6 CreateMethodProperty ( O, P, V )

The abstract operation CreateMethodProperty takes arguments O (an Object), P (a property key), and V (an ECMAScript language value) and returns unused. It is used to create a new own property of an ordinary object. It performs the following steps when called:

  1. Assert: O is an ordinary, extensible object with no non-configurable properties.
  2. Let newDesc be the PropertyDescriptor { [[Value]]: V, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }.
  3. Perform ! DefinePropertyOrThrow(O, P, newDesc).
  4. Return unused.
 Note

This abstract operation creates a property whose attributes are set to the same defaults used for built-in methods and methods defined using class declaration syntax. Normally, the property will not already exist. If it does exist, DefinePropertyOrThrow is guaranteed to complete normally.

7.3.7 CreateDataPropertyOrThrow ( O, P, V )

The abstract operation CreateDataPropertyOrThrow takes arguments O (an Object), P (a property key), and V (an ECMAScript language value) and returns either a normal completion containing unused or a throw completion. It is used to create a new own property of an object. It throws a TypeError exception if the requested property update cannot be performed. It performs the following steps when called:

  1. Let success be ? CreateDataProperty(O, P, V).
  2. If success is false, throw a TypeError exception.
  3. Return unused.
 Note

This abstract operation creates a property whose attributes are set to the same defaults used for properties created by the ECMAScript language assignment operator. Normally, the property will not already exist. If it does exist and is not configurable or if O is not extensible, [[DefineOwnProperty]] will return false causing this operation to throw a TypeError exception.

7.3.8 CreateNonEnumerableDataPropertyOrThrow ( O, P, V )

The abstract operation CreateNonEnumerableDataPropertyOrThrow takes arguments O (an Object), P (a property key), and V (an ECMAScript language value) and returns unused. It is used to create a new non-enumerable own property of an ordinary object. It performs the following steps when called:

  1. Assert: O is an ordinary, extensible object with no non-configurable properties.
  2. Let newDesc be the PropertyDescriptor { [[Value]]: V, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }.
  3. Perform ! DefinePropertyOrThrow(O, P, newDesc).
  4. Return unused.
 Note

This abstract operation creates a property whose attributes are set to the same defaults used for properties created by the ECMAScript language assignment operator except it is not enumerable. Normally, the property will not already exist. If it does exist, DefinePropertyOrThrow is guaranteed to complete normally.

7.3.9 DefinePropertyOrThrow ( O, P, desc )

The abstract operation DefinePropertyOrThrow takes arguments O (an Object), P (a property key), and desc (a Property Descriptor) and returns either a normal completion containing unused or a throw completion. It is used to call the [[DefineOwnProperty]] internal method of an object in a manner that will throw a TypeError exception if the requested property update cannot be performed. It performs the following steps when called:

  1. Let success be ? O.[[DefineOwnProperty]](P, desc).
  2. If success is false, throw a TypeError exception.
  3. Return unused.

7.3.10 DeletePropertyOrThrow ( O, P )

The abstract operation DeletePropertyOrThrow takes arguments O (an Object) and P (a property key) and returns either a normal completion containing unused or a throw completion. It is used to remove a specific own property of an object. It throws an exception if the property is not configurable. It performs the following steps when called:

  1. Let success be ? O.[[Delete]](P).
  2. If success is false, throw a TypeError exception.
  3. Return unused.

7.3.11 GetMethod ( V, P )

The abstract operation GetMethod takes arguments V (an ECMAScript language value) and P (a property key) and returns either a normal completion containing either a function object or undefined, or a throw completion. It is used to get the value of a specific property of an ECMAScript language value when the value of the property is expected to be a function. It performs the following steps when called:

  1. Let func be ? GetV(V, P).
  2. If func is either undefined or null, return undefined.
  3. If IsCallable(func) is false, throw a TypeError exception.
  4. Return func.

7.3.12 HasProperty ( O, P )

The abstract operation HasProperty takes arguments O (an Object) and P (a property key) and returns either a normal completion containing a Boolean or a throw completion. It is used to determine whether an object has a property with the specified property key. The property may be either own or inherited. It performs the following steps when called:

  1. Return ? O.[[HasProperty]](P).

7.3.13 HasOwnProperty ( O, P )

The abstract operation HasOwnProperty takes arguments O (an Object) and P (a property key) and returns either a normal completion containing a Boolean or a throw completion. It is used to determine whether an object has an own property with the specified property key. It performs the following steps when called:

  1. Let desc be ? O.[[GetOwnProperty]](P).
  2. If desc is undefined, return false.
  3. Return true.

7.3.14 Call ( F, V [ , argumentsList ] )

The abstract operation Call takes arguments F (an ECMAScript language value) and V (an ECMAScript language value) and optional argument argumentsList (a List of ECMAScript language values) and returns either a normal completion containing an ECMAScript language value or a throw completion. It is used to call the [[Call]] internal method of a function object. F is the function object, V is an ECMAScript language value that is the this value of the [[Call]], and argumentsList is the value passed to the corresponding argument of the internal method. If argumentsList is not present, a new empty List is used as its value. It performs the following steps when called:

  1. If argumentsList is not present, set argumentsList to a new empty List.
  2. If IsCallable(F) is false, throw a TypeError exception.
  3. Return ? F.[[Call]](V, argumentsList).

7.3.15 Construct ( F [ , argumentsList [ , newTarget ] ] )

The abstract operation Construct takes argument F (a constructor) and optional arguments argumentsList (a List of ECMAScript language values) and newTarget (a constructor) and returns either a normal completion containing an Object or a throw completion. It is used to call the [[Construct]] internal method of a function object. argumentsList and newTarget are the values to be passed as the corresponding arguments of the internal method. If argumentsList is not present, a new empty List is used as its value. If newTarget is not present, F is used as its value. It performs the following steps when called:

  1. If newTarget is not present, set newTarget to F.
  2. If argumentsList is not present, set argumentsList to a new empty List.
  3. Return ? F.[[Construct]](argumentsList, newTarget).
 Note

If newTarget is not present, this operation is equivalent to: new F(...argumentsList)

7.3.16 SetIntegrityLevel ( O, level )

The abstract operation SetIntegrityLevel takes arguments O (an Object) and level (sealed or frozen) and returns either a normal completion containing a Boolean or a throw completion. It is used to fix the set of own properties of an object. It performs the following steps when called:

  1. Let status be ? O.[[PreventExtensions]]().
  2. If status is false, return false.
  3. Let keys be ? O.[[OwnPropertyKeys]]().
  4. If level is sealed, then
    1. For each element k of keys, do
      1. Perform ? DefinePropertyOrThrow(O, k, PropertyDescriptor { [[Configurable]]: false }).
  5. Else,
    1. Assert: level is frozen.
    2. For each element k of keys, do
      1. Let currentDesc be ? O.[[GetOwnProperty]](k).
      2. If currentDesc is not undefined, then
        1. If IsAccessorDescriptor(currentDesc) is true, then
          1. Let desc be the PropertyDescriptor { [[Configurable]]: false }.
        2. Else,
          1. Let desc be the PropertyDescriptor { [[Configurable]]: false, [[Writable]]: false }.
        3. Perform ? DefinePropertyOrThrow(O, k, desc).
  6. Return true.

7.3.17 TestIntegrityLevel ( O, level )

The abstract operation TestIntegrityLevel takes arguments O (an Object) and level (sealed or frozen) and returns either a normal completion containing a Boolean or a throw completion. It is used to determine if the set of own properties of an object are fixed. It performs the following steps when called:

  1. Let extensible be ? IsExtensible(O).
  2. If extensible is true, return false.
  3. NOTE: If the object is extensible, none of its properties are examined.
  4. Let keys be ? O.[[OwnPropertyKeys]]().
  5. For each element k of keys, do
    1. Let currentDesc be ? O.[[GetOwnProperty]](k).
    2. If currentDesc is not undefined, then
      1. If currentDesc.[[Configurable]] is true, return false.
      2. If level is frozen and IsDataDescriptor(currentDesc) is true, then
        1. If currentDesc.[[Writable]] is true, return false.
  6. Return true.

7.3.18 CreateArrayFromList ( elements )

The abstract operation CreateArrayFromList takes argument elements (a List of ECMAScript language values) and returns an Array. It is used to create an Array whose elements are provided by elements. It performs the following steps when called:

  1. Let array be ! ArrayCreate(0).
  2. Let n be 0.
  3. For each element e of elements, do
    1. Perform ! CreateDataPropertyOrThrow(array, ! ToString(𝔽(n)), e).
    2. Set n to n 1.
  4. Return array.

7.3.19 LengthOfArrayLike ( obj )

The abstract operation LengthOfArrayLike takes argument obj (an Object) and returns either a normal completion containing a non-negative integer or a throw completion. It returns the value of the "length" property of an array-like object. It performs the following steps when called:

  1. Return ℝ(? ToLength(? Get(obj, "length"))).

An array-like object is any object for which this operation returns a normal completion.

Note 1
Typically, an array-like object would also have some properties with integer index names. However, that is not a requirement of this definition.
Note 2
Arrays and String objects are examples of array-like objects.

7.3.20 CreateListFromArrayLike ( obj [ , elementTypes ] )

The abstract operation CreateListFromArrayLike takes argument obj (an ECMAScript language value) and optional argument elementTypes (a List of names of ECMAScript Language Types) and returns either a normal completion containing a List of ECMAScript language values or a throw completion. It is used to create a List value whose elements are provided by the indexed properties of obj. elementTypes contains the names of ECMAScript Language Types that are allowed for element values of the List that is created. It performs the following steps when called:

  1. If elementTypes is not present, set elementTypes to « Undefined, Null, Boolean, String, Symbol, Number, BigInt, Object ».
  2. If obj is not an Object, throw a TypeError exception.
  3. Let len be ? LengthOfArrayLike(obj).
  4. Let list be a new empty List.
  5. Let index be 0.
  6. Repeat, while index < len,
    1. Let indexName be ! ToString(𝔽(index)).
    2. Let next be ? Get(obj, indexName).
    3. If elementTypes does not contain Type(next), throw a TypeError exception.
    4. Append next to list.
    5. Set index to index 1.
  7. Return list.

7.3.21 Invoke ( V, P [ , argumentsList ] )

The abstract operation Invoke takes arguments V (an ECMAScript language value) and P (a property key) and optional argument argumentsList (a List of ECMAScript language values) and returns either a normal completion containing an ECMAScript language value or a throw completion. It is used to call a method property of an ECMAScript language value. V serves as both the lookup point for the property and the this value of the call. argumentsList is the list of arguments values passed to the method. If argumentsList is not present, a new empty List is used as its value. It performs the following steps when called:

  1. If argumentsList is not present, set argumentsList to a new empty List.
  2. Let func be ? GetV(V, P).
  3. Return ? Call(func, V, argumentsList).

7.3.22 OrdinaryHasInstance ( C, O )

The abstract operation OrdinaryHasInstance takes arguments C (an ECMAScript language value) and O (an ECMAScript language value) and returns either a normal completion containing a Boolean or a throw completion. It implements the default algorithm for determining if O inherits from the instance object inheritance path provided by C. It performs the following steps when called:

  1. If IsCallable(C) is false, return false.
  2. If C has a [[BoundTargetFunction]] internal slot, then
    1. Let BC be C.[[BoundTargetFunction]].
    2. Return ? InstanceofOperator(O, BC).
  3. If O is not an Object, return false.
  4. Let P be ? Get(C, "prototype").
  5. If P is not an Object, throw a TypeError exception.
  6. Repeat,
    1. Set O to ? O.[[GetPrototypeOf]]().
    2. If O is null, return false.
    3. If SameValue(P, O) is true, return true.

7.3.23 SpeciesConstructor ( O, defaultConstructor )

The abstract operation SpeciesConstructor takes arguments O (an Object) and defaultConstructor (a constructor) and returns either a normal completion containing a constructor or a throw completion. It is used to retrieve the constructor that should be used to create new objects that are derived from O. defaultConstructor is the constructor to use if a constructor @@species property cannot be found starting from O. It performs the following steps when called:

  1. Let C be ? Get(O, "constructor").
  2. If C is undefined, return defaultConstructor.
  3. If C is not an Object, throw a TypeError exception.
  4. Let S be ? Get(C, @@species).
  5. If S is either undefined or null, return defaultConstructor.
  6. If IsConstructor(S) is true, return S.
  7. Throw a TypeError exception.

7.3.24 EnumerableOwnProperties ( O, kind )

The abstract operation EnumerableOwnProperties takes arguments O (an Object) and kind (key, value, or key value) and returns either a normal completion containing a List of ECMAScript language values or a throw completion. It performs the following steps when called:

  1. Let ownKeys be ? O.[[OwnPropertyKeys]]().
  2. Let results be a new empty List.
  3. For each element key of ownKeys, do
    1. If key is a String, then
      1. Let desc be ? O.[[GetOwnProperty]](key).
      2. If desc is not undefined and desc.[[Enumerable]] is true, then
        1. If kind is key, append key to results.
        2. Else,
          1. Let value be ? Get(O, key).
          2. If kind is value, append value to results.
          3. Else,
            1. Assert: kind is key value.
            2. Let entry be CreateArrayFromList(« key, value »).
            3. Append entry to results.
  4. Return results.

7.3.25 GetFunctionRealm ( obj )

The abstract operation GetFunctionRealm takes argument obj (a function object) and returns either a normal completion containing a Realm Record or a throw completion. It performs the following steps when called:

  1. If obj has a [[Realm]] internal slot, then
    1. Return obj.[[Realm]].
  2. If obj is a bound function exotic object, then
    1. Let boundTargetFunction be obj.[[BoundTargetFunction]].
    2. Return ? GetFunctionRealm(boundTargetFunction).
  3. If obj is a Proxy exotic object, then
    1. Perform ? ValidateNonRevokedProxy(obj).
    2. Let proxyTarget be obj.[[ProxyTarget]].
    3. Return ? GetFunctionRealm(proxyTarget).
  4. Return the current Realm Record.
 Note

Step 4 will only be reached if obj is a non-standard function exotic object that does not have a [[Realm]] internal slot.

7.3.26 CopyDataProperties ( target, source, excludedItems )

The abstract operation CopyDataProperties takes arguments target (an Object), source (an ECMAScript language value), and excludedItems (a List of property keys) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

  1. If source is either undefined or null, return unused.
  2. Let from be ! ToObject(source).
  3. Let keys be ? from.[[OwnPropertyKeys]]().
  4. For each element nextKey of keys, do
    1. Let excluded be false.
    2. For each element e of excludedItems, do
      1. If SameValue(e, nextKey) is true, then
        1. Set excluded to true.
    3. If excluded is false, then
      1. Let desc be ? from.[[GetOwnProperty]](nextKey).
      2. If desc is not undefined and desc.[[Enumerable]] is true, then
        1. Let propValue be ? Get(from, nextKey).
        2. Perform ! CreateDataPropertyOrThrow(target, nextKey, propValue).
  5. Return unused.
 Note

The target passed in here is always a newly created object which is not directly accessible in case of an error being thrown.

7.3.27 PrivateElementFind ( O, P )

The abstract operation PrivateElementFind takes arguments O (an Object) and P (a Private Name) and returns a PrivateElement or empty. It performs the following steps when called:

  1. If O.[[PrivateElements]] contains a PrivateElement pe such that pe.[[Key]] is P, then
    1. Return pe.
  2. Return empty.

7.3.28 PrivateFieldAdd ( O, P, value )

The abstract operation PrivateFieldAdd takes arguments O (an Object), P (a Private Name), and value (an ECMAScript language value) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

  1. If the host is a web browser, then
    1. Perform ? HostEnsureCanAddPrivateElement(O).
  2. Let entry be PrivateElementFind(O, P).
  3. If entry is not empty, throw a TypeError exception.
  4. Append PrivateElement { [[Key]]: P, [[Kind]]: field, [[Value]]: value } to O.[[PrivateElements]].
  5. Return unused.

7.3.29 PrivateMethodOrAccessorAdd ( O, method )

The abstract operation PrivateMethodOrAccessorAdd takes arguments O (an Object) and method (a PrivateElement) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

  1. Assert: method.[[Kind]] is either method or accessor.
  2. If the host is a web browser, then
    1. Perform ? HostEnsureCanAddPrivateElement(O).
  3. Let entry be PrivateElementFind(O, method.[[Key]]).
  4. If entry is not empty, throw a TypeError exception.
  5. Append method to O.[[PrivateElements]].
  6. Return unused.
 Note

The values for private methods and accessors are shared across instances. This operation does not create a new copy of the method or accessor.

7.3.30 HostEnsureCanAddPrivateElement ( O )

The host-defined abstract operation HostEnsureCanAddPrivateElement takes argument O (an Object) and returns either a normal completion containing unused or a throw completion. It allows host environments to prevent the addition of private elements to particular host-defined exotic objects.

An implementation of HostEnsureCanAddPrivateElement must conform to the following requirements:

The default implementation of HostEnsureCanAddPrivateElement is to return NormalCompletion(unused).

This abstract operation is only invoked by ECMAScript hosts that are web browsers.

7.3.31 PrivateGet ( O, P )

The abstract operation PrivateGet takes arguments O (an Object) and P (a Private Name) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

  1. Let entry be PrivateElementFind(O, P).
  2. If entry is empty, throw a TypeError exception.
  3. If entry.[[Kind]] is either field or method, then
    1. Return entry.[[Value]].
  4. Assert: entry.[[Kind]] is accessor.
  5. If entry.[[Get]] is undefined, throw a TypeError exception.
  6. Let getter be entry.[[Get]].
  7. Return ? Call(getter, O).

7.3.32 PrivateSet ( O, P, value )

The abstract operation PrivateSet takes arguments O (an Object), P (a Private Name), and value (an ECMAScript language value) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

  1. Let entry be PrivateElementFind(O, P).
  2. If entry is empty, throw a TypeError exception.
  3. If entry.[[Kind]] is field, then
    1. Set entry.[[Value]] to value.
  4. Else if entry.[[Kind]] is method, then
    1. Throw a TypeError exception.
  5. Else,
    1. Assert: entry.[[Kind]] is accessor.
    2. If entry.[[Set]] is undefined, throw a TypeError exception.
    3. Let setter be entry.[[Set]].
    4. Perform ? Call(setter, O, « value »).
  6. Return unused.

7.3.33 DefineField ( receiver, fieldRecord )

The abstract operation DefineField takes arguments receiver (an Object) and fieldRecord (a ClassFieldDefinition Record) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

  1. Let fieldName be fieldRecord.[[Name]].
  2. Let initializer be fieldRecord.[[Initializer]].
  3. If initializer is not empty, then
    1. Let initValue be ? Call(initializer, receiver).
  4. Else, let initValue be undefined.
  5. If fieldName is a Private Name, then
    1. Perform ? PrivateFieldAdd(receiver, fieldName, initValue).
  6. Else,
    1. Assert: IsPropertyKey(fieldName) is true.
    2. Perform ? CreateDataPropertyOrThrow(receiver, fieldName, initValue).
  7. Return unused.

7.3.34 InitializeInstanceElements ( O, constructor )

The abstract operation InitializeInstanceElements takes arguments O (an Object) and constructor (an ECMAScript function object) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

  1. Let methods be the value of constructor.[[PrivateMethods]].
  2. For each PrivateElement method of methods, do
    1. Perform ? PrivateMethodOrAccessorAdd(O, method).
  3. Let fields be the value of constructor.[[Fields]].
  4. For each element fieldRecord of fields, do
    1. Perform ? DefineField(O, fieldRecord).
  5. Return unused.

7.4 Operations on Iterator Objects

See Common Iteration Interfaces (27.1).

7.4.1 Iterator Records

An Iterator Record is a Record value used to encapsulate an Iterator or AsyncIterator along with the next method.

Iterator Records have the fields listed in Table 15.

Table 15: Iterator Record Fields
Field Name Value Meaning
[[Iterator]] an Object An object that conforms to the Iterator or AsyncIterator interface.
[[NextMethod]] a function object The next method of the [[Iterator]] object.
[[Done]] a Boolean Whether the iterator has been closed.

7.4.2 GetIteratorFromMethod ( obj, method )

The abstract operation GetIteratorFromMethod takes arguments obj (an ECMAScript language value) and method (a function object) and returns either a normal completion containing an Iterator Record or a throw completion. It performs the following steps when called:

  1. Let iterator be ? Call(method, obj).
  2. If iterator is not an Object, throw a TypeError exception.
  3. Let nextMethod be ? GetV(iterator, "next").
  4. Let iteratorRecord be the Iterator Record { [[Iterator]]: iterator, [[NextMethod]]: nextMethod, [[Done]]: false }.
  5. Return iteratorRecord.

7.4.3 GetIterator ( obj, kind )

The abstract operation GetIterator takes arguments obj (an ECMAScript language value) and kind (sync or async) and returns either a normal completion containing an Iterator Record or a throw completion. It performs the following steps when called:

  1. If kind is async, then
    1. Let method be ? GetMethod(obj, @@asyncIterator).
    2. If method is undefined, then
      1. Let syncMethod be ? GetMethod(obj, @@iterator).
      2. If syncMethod is undefined, throw a TypeError exception.
      3. Let syncIteratorRecord be ? GetIteratorFromMethod(obj, syncMethod).
      4. Return CreateAsyncFromSyncIterator(syncIteratorRecord).
  2. Otherwise, let method be ? GetMethod(obj, @@iterator).
  3. If method is undefined, throw a TypeError exception.
  4. Return ? GetIteratorFromMethod(obj, method).

7.4.4 IteratorNext ( iteratorRecord [ , value ] )

The abstract operation IteratorNext takes argument iteratorRecord (an Iterator Record) and optional argument value (an ECMAScript language value) and returns either a normal completion containing an Object or a throw completion. It performs the following steps when called:

  1. If value is not present, then
    1. Let result be ? Call(iteratorRecord.[[NextMethod]], iteratorRecord.[[Iterator]]).
  2. Else,
    1. Let result be ? Call(iteratorRecord.[[NextMethod]], iteratorRecord.[[Iterator]], « value »).
  3. If result is not an Object, throw a TypeError exception.
  4. Return result.

7.4.5 IteratorComplete ( iterResult )

The abstract operation IteratorComplete takes argument iterResult (an Object) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

  1. Return ToBoolean(? Get(iterResult, "done")).

7.4.6 IteratorValue ( iterResult )

The abstract operation IteratorValue takes argument iterResult (an Object) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

  1. Return ? Get(iterResult, "value").

7.4.7 IteratorStep ( iteratorRecord )

The abstract operation IteratorStep takes argument iteratorRecord (an Iterator Record) and returns either a normal completion containing either an Object or false, or a throw completion. It requests the next value from iteratorRecord.[[Iterator]] by calling iteratorRecord.[[NextMethod]] and returns either false indicating that the iterator has reached its end or the IteratorResult object if a next value is available. It performs the following steps when called:

  1. Let result be ? IteratorNext(iteratorRecord).
  2. Let done be ? IteratorComplete(result).
  3. If done is true, return false.
  4. Return result.

7.4.8 IteratorClose ( iteratorRecord, completion )

The abstract operation IteratorClose takes arguments iteratorRecord (an Iterator Record) and completion (a Completion Record) and returns a Completion Record. It is used to notify an iterator that it should perform any actions it would normally perform when it has reached its completed state. It performs the following steps when called:

  1. Assert: iteratorRecord.[[Iterator]] is an Object.
  2. Let iterator be iteratorRecord.[[Iterator]].
  3. Let innerResult be Completion(GetMethod(iterator, "return")).
  4. If innerResult.[[Type]] is normal, then
    1. Let return be innerResult.[[Value]].
    2. If return is undefined, return ? completion.
    3. Set innerResult to Completion(Call(return, iterator)).
  5. If completion.[[Type]] is throw, return ? completion.
  6. If innerResult.[[Type]] is throw, return ? innerResult.
  7. If innerResult.[[Value]] is not an Object, throw a TypeError exception.
  8. Return ? completion.

7.4.9 IfAbruptCloseIterator ( value, iteratorRecord )

IfAbruptCloseIterator is a shorthand for a sequence of algorithm steps that use an Iterator Record. An algorithm step of the form:

  1. IfAbruptCloseIterator(value, iteratorRecord).

means the same thing as:

  1. Assert: value is a Completion Record.
  2. If value is an abrupt completion, return ? IteratorClose(iteratorRecord, value).
  3. Else, set value to value.[[Value]].

7.4.10 AsyncIteratorClose ( iteratorRecord, completion )

The abstract operation AsyncIteratorClose takes arguments iteratorRecord (an Iterator Record) and completion (a Completion Record) and returns a Completion Record. It is used to notify an async iterator that it should perform any actions it would normally perform when it has reached its completed state. It performs the following steps when called:

  1. Assert: iteratorRecord.[[Iterator]] is an Object.
  2. Let iterator be iteratorRecord.[[Iterator]].
  3. Let innerResult be Completion(GetMethod(iterator, "return")).
  4. If innerResult.[[Type]] is normal, then
    1. Let return be innerResult.[[Value]].
    2. If return is undefined, return ? completion.
    3. Set innerResult to Completion(Call(return, iterator)).
    4. If innerResult.[[Type]] is normal, set innerResult to Completion(Await(innerResult.[[Value]])).
  5. If completion.[[Type]] is throw, return ? completion.
  6. If innerResult.[[Type]] is throw, return ? innerResult.
  7. If innerResult.[[Value]] is not an Object, throw a TypeError exception.
  8. Return ? completion.

7.4.11 CreateIterResultObject ( value, done )

The abstract operation CreateIterResultObject takes arguments value (an ECMAScript language value) and done (a Boolean) and returns an Object that conforms to the IteratorResult interface. It creates an object that conforms to the IteratorResult interface. It performs the following steps when called:

  1. Let obj be OrdinaryObjectCreate(%Object.prototype%).
  2. Perform ! CreateDataPropertyOrThrow(obj, "value", value).
  3. Perform ! CreateDataPropertyOrThrow(obj, "done", done).
  4. Return obj.

7.4.12 CreateListIteratorRecord ( list )

The abstract operation CreateListIteratorRecord takes argument list (a List of ECMAScript language values) and returns an Iterator Record. It creates an Iterator (27.1.1.2) object record whose next method returns the successive elements of list. It performs the following steps when called:

  1. Let closure be a new Abstract Closure with no parameters that captures list and performs the following steps when called:
    1. For each element E of list, do
      1. Perform ? GeneratorYield(CreateIterResultObject(E, false)).
    2. Return NormalCompletion(undefined).
  2. Let iterator be CreateIteratorFromClosure(closure, empty, %IteratorPrototype%).
  3. Return the Iterator Record { [[Iterator]]: iterator, [[NextMethod]]: %GeneratorFunction.prototype.prototype.next%, [[Done]]: false }.
 Note

The list iterator object is never directly accessible to ECMAScript code.

7.4.13 IteratorToList ( iteratorRecord )

The abstract operation IteratorToList takes argument iteratorRecord (an Iterator Record) and returns either a normal completion containing a List of ECMAScript language values or a throw completion. It performs the following steps when called:

  1. Let values be a new empty List.
  2. Let next be true.
  3. Repeat, while next is not false,
    1. Set next to ? IteratorStep(iteratorRecord).
    2. If next is not false, then
      1. Let nextValue be ? IteratorValue(next).
      2. Append nextValue to values.
  4. Return values.

8 Syntax-Directed Operations

In addition to those defined in this section, specialized syntax-directed operations are defined throughout this specification.

8.1 Runtime Semantics: Evaluation

The syntax-directed operation Evaluation takes no arguments and returns a Completion Record.

Note
The definitions for this operation are distributed over the "ECMAScript Language" sections of this specification. Each definition appears after the defining occurrence of the relevant productions.

8.2 Scope Analysis

8.2.1 Static Semantics: BoundNames

The syntax-directed operation BoundNames takes no arguments and returns a List of Strings.

Note

"*default*" is used within this specification as a synthetic name for a module's default export when it does not have another name. An entry in the module's [[Environment]] is created with that name and holds the corresponding value, and resolving the export named "default" by calling ResolveExport ( exportName [ , resolveSet ] ) for the module will return a ResolvedBinding Record whose [[BindingName]] is "*default*", which will then resolve in the module's [[Environment]] to the above-mentioned value. This is done only for ease of specification, so that anonymous default exports can be resolved like any other export. This "*default*" string is never accessible to ECMAScript code or to the module linking algorithm.

It is defined piecewise over the following productions:

BindingIdentifier : Identifier
  1. Return a List whose sole element is the StringValue of Identifier.
 BindingIdentifier : yield
  1. Return « "yield" ».
 BindingIdentifier : await
  1. Return « "await" ».
 LexicalDeclaration : LetOrConst BindingList ;
  1. Return the BoundNames of BindingList.
 BindingList : BindingList , LexicalBinding
  1. Let names1 be the BoundNames of BindingList.
  2. Let names2 be the BoundNames of LexicalBinding.
  3. Return the list-concatenation of names1 and names2.
 LexicalBinding : BindingIdentifier Initializeropt
  1. Return the BoundNames of BindingIdentifier.
 LexicalBinding : BindingPattern Initializer
  1. Return the BoundNames of BindingPattern.
 VariableDeclarationList : VariableDeclarationList , VariableDeclaration
  1. Let names1 be BoundNames of VariableDeclarationList.
  2. Let names2 be BoundNames of VariableDeclaration.
  3. Return the list-concatenation of names1 and names2.
 VariableDeclaration : BindingIdentifier Initializeropt
  1. Return the BoundNames of BindingIdentifier.
 VariableDeclaration : BindingPattern Initializer
  1. Return the BoundNames of BindingPattern.
 ObjectBindingPattern : { }
  1. Return a new empty List.
 ObjectBindingPattern : { BindingPropertyList , BindingRestProperty }
  1. Let names1 be BoundNames of BindingPropertyList.
  2. Let names2 be BoundNames of BindingRestProperty.
  3. Return the list-concatenation of names1 and names2.
 ArrayBindingPattern : [ Elisionopt ]
  1. Return a new empty List.
 ArrayBindingPattern : [ Elisionopt BindingRestElement ]
  1. Return the BoundNames of BindingRestElement.
 ArrayBindingPattern : [ BindingElementList , Elisionopt ]
  1. Return the BoundNames of BindingElementList.
 ArrayBindingPattern : [ BindingElementList , Elisionopt BindingRestElement ]
  1. Let names1 be BoundNames of BindingElementList.
  2. Let names2 be BoundNames of BindingRestElement.
  3. Return the list-concatenation of names1 and names2.
 BindingPropertyList : BindingPropertyList , BindingProperty
  1. Let names1 be BoundNames of BindingPropertyList.
  2. Let names2 be BoundNames of BindingProperty.
  3. Return the list-concatenation of names1 and names2.
 BindingElementList : BindingElementList , BindingElisionElement
  1. Let names1 be BoundNames of BindingElementList.
  2. Let names2 be BoundNames of BindingElisionElement.
  3. Return the list-concatenation of names1 and names2.
 BindingElisionElement : Elisionopt BindingElement
  1. Return BoundNames of BindingElement.
 BindingProperty : PropertyName : BindingElement
  1. Return the BoundNames of BindingElement.
 SingleNameBinding : BindingIdentifier Initializeropt
  1. Return the BoundNames of BindingIdentifier.
 BindingElement : BindingPattern Initializeropt
  1. Return the BoundNames of BindingPattern.
 ForDeclaration : LetOrConst ForBinding
  1. Return the BoundNames of ForBinding.
 FunctionDeclaration : function BindingIdentifier ( FormalParameters ) { FunctionBody }
  1. Return the BoundNames of BindingIdentifier.
 FunctionDeclaration : function ( FormalParameters ) { FunctionBody }
  1. Return « "*default*" ».
 FormalParameters : [empty]
  1. Return a new empty List.
 FormalParameters : FormalParameterList , FunctionRestParameter
  1. Let names1 be BoundNames of FormalParameterList.
  2. Let names2 be BoundNames of FunctionRestParameter.
  3. Return the list-concatenation of names1 and names2.
 FormalParameterList : FormalParameterList , FormalParameter
  1. Let names1 be BoundNames of FormalParameterList.
  2. Let names2 be BoundNames of FormalParameter.
  3. Return the list-concatenation of names1 and names2.
 ArrowParameters : CoverParenthesizedExpressionAndArrowParameterList
  1. Let formals be the ArrowFormalParameters that is covered by CoverParenthesizedExpressionAndArrowParameterList.
  2. Return the BoundNames of formals.
 GeneratorDeclaration : function * BindingIdentifier ( FormalParameters ) { GeneratorBody }
  1. Return the BoundNames of BindingIdentifier.
 GeneratorDeclaration : function * ( FormalParameters ) { GeneratorBody }
  1. Return « "*default*" ».
 AsyncGeneratorDeclaration : async function * BindingIdentifier ( FormalParameters ) { AsyncGeneratorBody }
  1. Return the BoundNames of BindingIdentifier.
 AsyncGeneratorDeclaration : async function * ( FormalParameters ) { AsyncGeneratorBody }
  1. Return « "*default*" ».
 ClassDeclaration : class BindingIdentifier ClassTail
  1. Return the BoundNames of BindingIdentifier.
 ClassDeclaration : class ClassTail
  1. Return « "*default*" ».
 AsyncFunctionDeclaration : async function BindingIdentifier ( FormalParameters ) { AsyncFunctionBody }
  1. Return the BoundNames of BindingIdentifier.
 AsyncFunctionDeclaration : async function ( FormalParameters ) { AsyncFunctionBody }
  1. Return « "*default*" ».
 CoverCallExpressionAndAsyncArrowHead : MemberExpression Arguments
  1. Let head be the AsyncArrowHead that is covered by CoverCallExpressionAndAsyncArrowHead.
  2. Return the BoundNames of head.
 ImportDeclaration : import ImportClause FromClause ;
  1. Return the BoundNames of ImportClause.
 ImportDeclaration : import ModuleSpecifier ;
  1. Return a new empty List.
 ImportClause : ImportedDefaultBinding , NameSpaceImport
  1. Let names1 be the BoundNames of ImportedDefaultBinding.
  2. Let names2 be the BoundNames of NameSpaceImport.
  3. Return the list-concatenation of names1 and names2.
 ImportClause : ImportedDefaultBinding , NamedImports
  1. Let names1 be the BoundNames of ImportedDefaultBinding.
  2. Let names2 be the BoundNames of NamedImports.
  3. Return the list-concatenation of names1 and names2.
 NamedImports : { }
  1. Return a new empty List.
 ImportsList : ImportsList , ImportSpecifier
  1. Let names1 be the BoundNames of ImportsList.
  2. Let names2 be the BoundNames of ImportSpecifier.
  3. Return the list-concatenation of names1 and names2.
 ImportSpecifier : ModuleExportName as ImportedBinding
  1. Return the BoundNames of ImportedBinding.
 ExportDeclaration : export ExportFromClause FromClause ; export NamedExports ;
  1. Return a new empty List.
 ExportDeclaration : export VariableStatement
  1. Return the BoundNames of VariableStatement.
 ExportDeclaration : export Declaration
  1. Return the BoundNames of Declaration.
 ExportDeclaration : export default HoistableDeclaration
  1. Let declarationNames be the BoundNames of HoistableDeclaration.
  2. If declarationNames does not include the element "*default*", append "*default*" to declarationNames.
  3. Return declarationNames.
 ExportDeclaration : export default ClassDeclaration
  1. Let declarationNames be the BoundNames of ClassDeclaration.
  2. If declarationNames does not include the element "*default*", append "*default*" to declarationNames.
  3. Return declarationNames.
 ExportDeclaration : export default AssignmentExpression ;
  1. Return « "*default*" ».

8.2.2 Static Semantics: DeclarationPart

The syntax-directed operation DeclarationPart takes no arguments and returns a Parse Node. It is defined piecewise over the following productions:

HoistableDeclaration : FunctionDeclaration
  1. Return FunctionDeclaration.
 HoistableDeclaration : GeneratorDeclaration
  1. Return GeneratorDeclaration.
 HoistableDeclaration : AsyncFunctionDeclaration
  1. Return AsyncFunctionDeclaration.
 HoistableDeclaration : AsyncGeneratorDeclaration
  1. Return AsyncGeneratorDeclaration.
 Declaration : ClassDeclaration
  1. Return ClassDeclaration.
 Declaration : LexicalDeclaration
  1. Return LexicalDeclaration.

8.2.3 Static Semantics: IsConstantDeclaration

The syntax-directed operation IsConstantDeclaration takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

LexicalDeclaration : LetOrConst BindingList ;
  1. Return IsConstantDeclaration of LetOrConst.
 LetOrConst : let
  1. Return false.
 LetOrConst : const
  1. Return true.
 FunctionDeclaration : function BindingIdentifier ( FormalParameters ) { FunctionBody } function ( FormalParameters ) { FunctionBody } GeneratorDeclaration : function * BindingIdentifier ( FormalParameters ) { GeneratorBody } function * ( FormalParameters ) { GeneratorBody } AsyncGeneratorDeclaration : async function * BindingIdentifier ( FormalParameters ) { AsyncGeneratorBody } async function * ( FormalParameters ) { AsyncGeneratorBody } AsyncFunctionDeclaration : async function BindingIdentifier ( FormalParameters ) { AsyncFunctionBody } async function ( FormalParameters ) { AsyncFunctionBody }
  1. Return false.
 ClassDeclaration : class BindingIdentifier ClassTail class ClassTail
  1. Return false.
 ExportDeclaration : export ExportFromClause FromClause ; export NamedExports ; export default AssignmentExpression ;
  1. Return false.
 Note

It is not necessary to treat export default AssignmentExpression as a constant declaration because there is no syntax that permits assignment to the internal bound name used to reference a module's default object.

8.2.4 Static Semantics: LexicallyDeclaredNames

The syntax-directed operation LexicallyDeclaredNames takes no arguments and returns a List of Strings. It is defined piecewise over the following productions:

Block : { }
  1. Return a new empty List.
 StatementList : StatementList StatementListItem
  1. Let names1 be LexicallyDeclaredNames of StatementList.
  2. Let names2 be LexicallyDeclaredNames of StatementListItem.
  3. Return the list-concatenation of names1 and names2.
 StatementListItem : Statement
  1. If Statement is Statement : LabelledStatement , return LexicallyDeclaredNames of LabelledStatement.
  2. Return a new empty List.
 StatementListItem : Declaration
  1. Return the BoundNames of Declaration.
 CaseBlock : { }
  1. Return a new empty List.
 CaseBlock : { CaseClausesopt DefaultClause CaseClausesopt }
  1. If the first CaseClauses is present, let names1 be the LexicallyDeclaredNames of the first CaseClauses.
  2. Else, let names1 be a new empty List.
  3. Let names2 be LexicallyDeclaredNames of DefaultClause.
  4. If the second CaseClauses is present, let names3 be the LexicallyDeclaredNames of the second CaseClauses.
  5. Else, let names3 be a new empty List.
  6. Return the list-concatenation of names1, names2, and names3.
 CaseClauses : CaseClauses CaseClause
  1. Let names1 be LexicallyDeclaredNames of CaseClauses.
  2. Let names2 be LexicallyDeclaredNames of CaseClause.
  3. Return the list-concatenation of names1 and names2.
 CaseClause : case Expression : StatementListopt
  1. If the StatementList is present, return the LexicallyDeclaredNames of StatementList.
  2. Return a new empty List.
 DefaultClause : default : StatementListopt
  1. If the StatementList is present, return the LexicallyDeclaredNames of StatementList.
  2. Return a new empty List.
 LabelledStatement : LabelIdentifier : LabelledItem
  1. Return the LexicallyDeclaredNames of LabelledItem.
 LabelledItem : Statement
  1. Return a new empty List.
 LabelledItem : FunctionDeclaration
  1. Return BoundNames of FunctionDeclaration.
 FunctionStatementList : [empty]
  1. Return a new empty List.
 FunctionStatementList : StatementList
  1. Return TopLevelLexicallyDeclaredNames of StatementList.
 ClassStaticBlockStatementList : [empty]
  1. Return a new empty List.
 ClassStaticBlockStatementList : StatementList
  1. Return the TopLevelLexicallyDeclaredNames of StatementList.
 ConciseBody : ExpressionBody
  1. Return a new empty List.
 AsyncConciseBody : ExpressionBody
  1. Return a new empty List.
 Script : [empty]
  1. Return a new empty List.
 ScriptBody : StatementList
  1. Return TopLevelLexicallyDeclaredNames of StatementList.
 Note 1

At the top level of a Script, function declarations are treated like var declarations rather than like lexical declarations.

 Note 2

The LexicallyDeclaredNames of a Module includes the names of all of its imported bindings.

 ModuleItemList : ModuleItemList ModuleItem
  1. Let names1 be LexicallyDeclaredNames of ModuleItemList.
  2. Let names2 be LexicallyDeclaredNames of ModuleItem.
  3. Return the list-concatenation of names1 and names2.
 ModuleItem : ImportDeclaration
  1. Return the BoundNames of ImportDeclaration.
 ModuleItem : ExportDeclaration
  1. If ExportDeclaration is export VariableStatement, return a new empty List.
  2. Return the BoundNames of ExportDeclaration.
 ModuleItem : StatementListItem
  1. Return LexicallyDeclaredNames of StatementListItem.
 Note 3

At the top level of a Module, function declarations are treated like lexical declarations rather than like var declarations.

8.2.5 Static Semantics: LexicallyScopedDeclarations

The syntax-directed operation LexicallyScopedDeclarations takes no arguments and returns a List of Parse Nodes. It is defined piecewise over the following productions:

StatementList : StatementList StatementListItem
  1. Let declarations1 be LexicallyScopedDeclarations of StatementList.
  2. Let declarations2 be LexicallyScopedDeclarations of StatementListItem.
  3. Return the list-concatenation of declarations1 and declarations2.
 StatementListItem : Statement
  1. If Statement is Statement : LabelledStatement , return LexicallyScopedDeclarations of LabelledStatement.
  2. Return a new empty List.
 StatementListItem : Declaration
  1. Return a List whose sole element is DeclarationPart of Declaration.
 CaseBlock : { }
  1. Return a new empty List.
 CaseBlock : { CaseClausesopt DefaultClause CaseClausesopt }
  1. If the first CaseClauses is present, let declarations1 be the LexicallyScopedDeclarations of the first CaseClauses.
  2. Else, let declarations1 be a new empty List.
  3. Let declarations2 be LexicallyScopedDeclarations of DefaultClause.
  4. If the second CaseClauses is present, let declarations3 be the LexicallyScopedDeclarations of the second CaseClauses.
  5. Else, let declarations3 be a new empty List.
  6. Return the list-concatenation of declarations1, declarations2, and declarations3.
 CaseClauses : CaseClauses CaseClause
  1. Let declarations1 be LexicallyScopedDeclarations of CaseClauses.
  2. Let declarations2 be LexicallyScopedDeclarations of CaseClause.
  3. Return the list-concatenation of declarations1 and declarations2.
 CaseClause : case Expression : StatementListopt
  1. If the StatementList is present, return the LexicallyScopedDeclarations of StatementList.
  2. Return a new empty List.
 DefaultClause : default : StatementListopt
  1. If the StatementList is present, return the LexicallyScopedDeclarations of StatementList.
  2. Return a new empty List.
 LabelledStatement : LabelIdentifier : LabelledItem
  1. Return the LexicallyScopedDeclarations of LabelledItem.
 LabelledItem : Statement
  1. Return a new empty List.
 LabelledItem : FunctionDeclaration
  1. Return « FunctionDeclaration ».
 FunctionStatementList : [empty]
  1. Return a new empty List.
 FunctionStatementList : StatementList
  1. Return the TopLevelLexicallyScopedDeclarations of StatementList.
 ClassStaticBlockStatementList : [empty]
  1. Return a new empty List.
 ClassStaticBlockStatementList : StatementList
  1. Return the TopLevelLexicallyScopedDeclarations of StatementList.
 ConciseBody : ExpressionBody
  1. Return a new empty List.
 AsyncConciseBody : ExpressionBody
  1. Return a new empty List.
 Script : [empty]
  1. Return a new empty List.
 ScriptBody : StatementList
  1. Return TopLevelLexicallyScopedDeclarations of StatementList.
 Module : [empty]
  1. Return a new empty List.
 ModuleItemList : ModuleItemList ModuleItem
  1. Let declarations1 be LexicallyScopedDeclarations of ModuleItemList.
  2. Let declarations2 be LexicallyScopedDeclarations of ModuleItem.
  3. Return the list-concatenation of declarations1 and declarations2.
 ModuleItem : ImportDeclaration
  1. Return a new empty List.
 ExportDeclaration : export ExportFromClause FromClause ; export NamedExports ; export VariableStatement
  1. Return a new empty List.
 ExportDeclaration : export Declaration
  1. Return a List whose sole element is DeclarationPart of Declaration.
 ExportDeclaration : export default HoistableDeclaration
  1. Return a List whose sole element is DeclarationPart of HoistableDeclaration.
 ExportDeclaration : export default ClassDeclaration
  1. Return a List whose sole element is ClassDeclaration.
 ExportDeclaration : export default AssignmentExpression ;
  1. Return a List whose sole element is this ExportDeclaration.

8.2.6 Static Semantics: VarDeclaredNames

The syntax-directed operation VarDeclaredNames takes no arguments and returns a List of Strings. It is defined piecewise over the following productions:

Statement : EmptyStatement ExpressionStatement ContinueStatement BreakStatement ReturnStatement ThrowStatement DebuggerStatement
  1. Return a new empty List.
 Block : { }
  1. Return a new empty List.
 StatementList : StatementList StatementListItem
  1. Let names1 be VarDeclaredNames of StatementList.
  2. Let names2 be VarDeclaredNames of StatementListItem.
  3. Return the list-concatenation of names1 and names2.
 StatementListItem : Declaration
  1. Return a new empty List.
 VariableStatement : var VariableDeclarationList ;
  1. Return BoundNames of VariableDeclarationList.
 IfStatement : if ( Expression ) Statement else Statement
  1. Let names1 be VarDeclaredNames of the first Statement.
  2. Let names2 be VarDeclaredNames of the second Statement.
  3. Return the list-concatenation of names1 and names2.
 IfStatement : if ( Expression ) Statement
  1. Return the VarDeclaredNames of Statement.
 DoWhileStatement : do Statement while ( Expression ) ;
  1. Return the VarDeclaredNames of Statement.
 WhileStatement : while ( Expression ) Statement
  1. Return the VarDeclaredNames of Statement.
 ForStatement : for ( Expressionopt ; Expressionopt ; Expressionopt ) Statement
  1. Return the VarDeclaredNames of Statement.
 ForStatement : for ( var VariableDeclarationList ; Expressionopt ; Expressionopt ) Statement
  1. Let names1 be BoundNames of VariableDeclarationList.
  2. Let names2 be VarDeclaredNames of Statement.
  3. Return the list-concatenation of names1 and names2.
 ForStatement : for ( LexicalDeclaration Expressionopt ; Expressionopt ) Statement
  1. Return the VarDeclaredNames of Statement.
 ForInOfStatement : for ( LeftHandSideExpression in Expression ) Statement for ( ForDeclaration in Expression ) Statement for ( LeftHandSideExpression of AssignmentExpression ) Statement for ( ForDeclaration of AssignmentExpression ) Statement for await ( LeftHandSideExpression of AssignmentExpression ) Statement for await ( ForDeclaration of AssignmentExpression ) Statement
  1. Return the VarDeclaredNames of Statement.
 ForInOfStatement : for ( var ForBinding in Expression ) Statement for ( var ForBinding of AssignmentExpression ) Statement for await ( var ForBinding of AssignmentExpression ) Statement
  1. Let names1 be the BoundNames of ForBinding.
  2. Let names2 be the VarDeclaredNames of Statement.
  3. Return the list-concatenation of names1 and names2.
 Note

This section is extended by Annex B.3.5.

 WithStatement : with ( Expression ) Statement
  1. Return the VarDeclaredNames of Statement.
 SwitchStatement : switch ( Expression ) CaseBlock
  1. Return the VarDeclaredNames of CaseBlock.
 CaseBlock : { }
  1. Return a new empty List.
 CaseBlock : { CaseClausesopt DefaultClause CaseClausesopt }
  1. If the first CaseClauses is present, let names1 be the VarDeclaredNames of the first CaseClauses.
  2. Else, let names1 be a new empty List.
  3. Let names2 be VarDeclaredNames of DefaultClause.
  4. If the second CaseClauses is present, let names3 be the VarDeclaredNames of the second CaseClauses.
  5. Else, let names3 be a new empty List.
  6. Return the list-concatenation of names1, names2, and names3.
 CaseClauses : CaseClauses CaseClause
  1. Let names1 be VarDeclaredNames of CaseClauses.
  2. Let names2 be VarDeclaredNames of CaseClause.
  3. Return the list-concatenation of names1 and names2.
 CaseClause : case Expression : StatementListopt
  1. If the StatementList is present, return the VarDeclaredNames of StatementList.
  2. Return a new empty List.
 DefaultClause : default : StatementListopt
  1. If the StatementList is present, return the VarDeclaredNames of StatementList.
  2. Return a new empty List.
 LabelledStatement : LabelIdentifier : LabelledItem
  1. Return the VarDeclaredNames of LabelledItem.
 LabelledItem : FunctionDeclaration
  1. Return a new empty List.
 TryStatement : try Block Catch
  1. Let names1 be VarDeclaredNames of Block.
  2. Let names2 be VarDeclaredNames of Catch.
  3. Return the list-concatenation of names1 and names2.
 TryStatement : try Block Finally
  1. Let names1 be VarDeclaredNames of Block.
  2. Let names2 be VarDeclaredNames of Finally.
  3. Return the list-concatenation of names1 and names2.
 TryStatement : try Block Catch Finally
  1. Let names1 be VarDeclaredNames of Block.
  2. Let names2 be VarDeclaredNames of Catch.
  3. Let names3 be VarDeclaredNames of Finally.
  4. Return the list-concatenation of names1, names2, and names3.
 Catch : catch ( CatchParameter ) Block
  1. Return the VarDeclaredNames of Block.
 FunctionStatementList : [empty]
  1. Return a new empty List.
 FunctionStatementList : StatementList
  1. Return TopLevelVarDeclaredNames of StatementList.
 ClassStaticBlockStatementList : [empty]
  1. Return a new empty List.
 ClassStaticBlockStatementList : StatementList
  1. Return the TopLevelVarDeclaredNames of StatementList.
 ConciseBody : ExpressionBody
  1. Return a new empty List.
 AsyncConciseBody : ExpressionBody
  1. Return a new empty List.
 Script : [empty]
  1. Return a new empty List.
 ScriptBody : StatementList
  1. Return TopLevelVarDeclaredNames of StatementList.
 ModuleItemList : ModuleItemList ModuleItem
  1. Let names1 be VarDeclaredNames of ModuleItemList.
  2. Let names2 be VarDeclaredNames of ModuleItem.
  3. Return the list-concatenation of names1 and names2.
 ModuleItem : ImportDeclaration
  1. Return a new empty List.
 ModuleItem : ExportDeclaration
  1. If ExportDeclaration is export VariableStatement, return BoundNames of ExportDeclaration.
  2. Return a new empty List.

8.2.7 Static Semantics: VarScopedDeclarations

The syntax-directed operation VarScopedDeclarations takes no arguments and returns a List of Parse Nodes. It is defined piecewise over the following productions:

Statement : EmptyStatement ExpressionStatement ContinueStatement BreakStatement ReturnStatement ThrowStatement DebuggerStatement
  1. Return a new empty List.
 Block : { }
  1. Return a new empty List.
 StatementList : StatementList StatementListItem
  1. Let declarations1 be VarScopedDeclarations of StatementList.
  2. Let declarations2 be VarScopedDeclarations of StatementListItem.
  3. Return the list-concatenation of declarations1 and declarations2.
 StatementListItem : Declaration
  1. Return a new empty List.
 VariableDeclarationList : VariableDeclaration
  1. Return « VariableDeclaration ».
 VariableDeclarationList : VariableDeclarationList , VariableDeclaration
  1. Let declarations1 be VarScopedDeclarations of VariableDeclarationList.
  2. Return the list-concatenation of declarations1 and « VariableDeclaration ».
 IfStatement : if ( Expression ) Statement else Statement
  1. Let declarations1 be VarScopedDeclarations of the first Statement.
  2. Let declarations2 be VarScopedDeclarations of the second Statement.
  3. Return the list-concatenation of declarations1 and declarations2.
 IfStatement : if ( Expression ) Statement
  1. Return the VarScopedDeclarations of Statement.
 DoWhileStatement : do Statement while ( Expression ) ;
  1. Return the VarScopedDeclarations of Statement.
 WhileStatement : while ( Expression ) Statement
  1. Return the VarScopedDeclarations of Statement.
 ForStatement : for ( Expressionopt ; Expressionopt ; Expressionopt ) Statement
  1. Return the VarScopedDeclarations of Statement.
 ForStatement : for ( var VariableDeclarationList ; Expressionopt ; Expressionopt ) Statement
  1. Let declarations1 be VarScopedDeclarations of VariableDeclarationList.
  2. Let declarations2 be VarScopedDeclarations of Statement.
  3. Return the list-concatenation of declarations1 and declarations2.
 ForStatement : for ( LexicalDeclaration Expressionopt ; Expressionopt ) Statement
  1. Return the VarScopedDeclarations of Statement.
 ForInOfStatement : for ( LeftHandSideExpression in Expression ) Statement for ( ForDeclaration in Expression ) Statement for ( LeftHandSideExpression of AssignmentExpression ) Statement for ( ForDeclaration of AssignmentExpression ) Statement for await ( LeftHandSideExpression of AssignmentExpression ) Statement for await ( ForDeclaration of AssignmentExpression ) Statement
  1. Return the VarScopedDeclarations of Statement.
 ForInOfStatement : for ( var ForBinding in Expression ) Statement for ( var ForBinding of AssignmentExpression ) Statement for await ( var ForBinding of AssignmentExpression ) Statement
  1. Let declarations1 be « ForBinding ».
  2. Let declarations2 be VarScopedDeclarations of Statement.
  3. Return the list-concatenation of declarations1 and declarations2.
 Note

This section is extended by Annex B.3.5.

 WithStatement : with ( Expression ) Statement
  1. Return the VarScopedDeclarations of Statement.
 SwitchStatement : switch ( Expression ) CaseBlock
  1. Return the VarScopedDeclarations of CaseBlock.
 CaseBlock : { }
  1. Return a new empty List.
 CaseBlock : { CaseClausesopt DefaultClause CaseClausesopt }
  1. If the first CaseClauses is present, let declarations1 be the VarScopedDeclarations of the first CaseClauses.
  2. Else, let declarations1 be a new empty List.
  3. Let declarations2 be VarScopedDeclarations of DefaultClause.
  4. If the second CaseClauses is present, let declarations3 be the VarScopedDeclarations of the second CaseClauses.
  5. Else, let declarations3 be a new empty List.
  6. Return the list-concatenation of declarations1, declarations2, and declarations3.
 CaseClauses : CaseClauses CaseClause
  1. Let declarations1 be VarScopedDeclarations of CaseClauses.
  2. Let declarations2 be VarScopedDeclarations of CaseClause.
  3. Return the list-concatenation of declarations1 and declarations2.
 CaseClause : case Expression : StatementListopt
  1. If the StatementList is present, return the VarScopedDeclarations of StatementList.
  2. Return a new empty List.
 DefaultClause : default : StatementListopt
  1. If the StatementList is present, return the VarScopedDeclarations of StatementList.
  2. Return a new empty List.
 LabelledStatement : LabelIdentifier : LabelledItem
  1. Return the VarScopedDeclarations of LabelledItem.
 LabelledItem : FunctionDeclaration
  1. Return a new empty List.
 TryStatement : try Block Catch
  1. Let declarations1 be VarScopedDeclarations of Block.
  2. Let declarations2 be VarScopedDeclarations of Catch.
  3. Return the list-concatenation of declarations1 and declarations2.
 TryStatement : try Block Finally
  1. Let declarations1 be VarScopedDeclarations of Block.
  2. Let declarations2 be VarScopedDeclarations of Finally.
  3. Return the list-concatenation of declarations1 and declarations2.
 TryStatement : try Block Catch Finally
  1. Let declarations1 be VarScopedDeclarations of Block.
  2. Let declarations2 be VarScopedDeclarations of Catch.
  3. Let declarations3 be VarScopedDeclarations of Finally.
  4. Return the list-concatenation of declarations1, declarations2, and declarations3.
 Catch : catch ( CatchParameter ) Block
  1. Return the VarScopedDeclarations of Block.
 FunctionStatementList : [empty]
  1. Return a new empty List.
 FunctionStatementList : StatementList
  1. Return the TopLevelVarScopedDeclarations of StatementList.
 ClassStaticBlockStatementList : [empty]
  1. Return a new empty List.
 ClassStaticBlockStatementList : StatementList
  1. Return the TopLevelVarScopedDeclarations of StatementList.
 ConciseBody : ExpressionBody
  1. Return a new empty List.
 AsyncConciseBody : ExpressionBody
  1. Return a new empty List.
 Script : [empty]
  1. Return a new empty List.
 ScriptBody : StatementList
  1. Return TopLevelVarScopedDeclarations of StatementList.
 Module : [empty]
  1. Return a new empty List.
 ModuleItemList : ModuleItemList ModuleItem
  1. Let declarations1 be VarScopedDeclarations of ModuleItemList.
  2. Let declarations2 be VarScopedDeclarations of ModuleItem.
  3. Return the list-concatenation of declarations1 and declarations2.
 ModuleItem : ImportDeclaration
  1. Return a new empty List.
 ModuleItem : ExportDeclaration
  1. If ExportDeclaration is export VariableStatement, return VarScopedDeclarations of VariableStatement.
  2. Return a new empty List.

8.2.8 Static Semantics: TopLevelLexicallyDeclaredNames

The syntax-directed operation TopLevelLexicallyDeclaredNames takes no arguments and returns a List of Strings. It is defined piecewise over the following productions:

StatementList : StatementList StatementListItem
  1. Let names1 be TopLevelLexicallyDeclaredNames of StatementList.
  2. Let names2 be TopLevelLexicallyDeclaredNames of StatementListItem.
  3. Return the list-concatenation of names1 and names2.
 StatementListItem : Statement
  1. Return a new empty List.
 StatementListItem : Declaration
  1. If Declaration is Declaration : HoistableDeclaration , then
    1. Return a new empty List.
  2. Return the BoundNames of Declaration.
 Note

At the top level of a function, or script, function declarations are treated like var declarations rather than like lexical declarations.

8.2.9 Static Semantics: TopLevelLexicallyScopedDeclarations

The syntax-directed operation TopLevelLexicallyScopedDeclarations takes no arguments and returns a List of Parse Nodes. It is defined piecewise over the following productions:

StatementList : StatementList StatementListItem
  1. Let declarations1 be TopLevelLexicallyScopedDeclarations of StatementList.
  2. Let declarations2 be TopLevelLexicallyScopedDeclarations of StatementListItem.
  3. Return the list-concatenation of declarations1 and declarations2.
 StatementListItem : Statement
  1. Return a new empty List.
 StatementListItem : Declaration
  1. If Declaration is Declaration : HoistableDeclaration , then
    1. Return a new empty List.
  2. Return « Declaration ».

8.2.10 Static Semantics: TopLevelVarDeclaredNames

The syntax-directed operation TopLevelVarDeclaredNames takes no arguments and returns a List of Strings. It is defined piecewise over the following productions:

StatementList : StatementList StatementListItem
  1. Let names1 be TopLevelVarDeclaredNames of StatementList.
  2. Let names2 be TopLevelVarDeclaredNames of StatementListItem.
  3. Return the list-concatenation of names1 and names2.
 StatementListItem : Declaration
  1. If Declaration is Declaration : HoistableDeclaration , then
    1. Return the BoundNames of HoistableDeclaration.
  2. Return a new empty List.
 StatementListItem : Statement
  1. If Statement is Statement : LabelledStatement , return TopLevelVarDeclaredNames of Statement.
  2. Return VarDeclaredNames of Statement.
 Note

At the top level of a function or script, inner function declarations are treated like var declarations.

 LabelledStatement : LabelIdentifier : LabelledItem
  1. Return the TopLevelVarDeclaredNames of LabelledItem.
 LabelledItem : Statement
  1. If Statement is Statement : LabelledStatement , return TopLevelVarDeclaredNames of Statement.
  2. Return VarDeclaredNames of Statement.
 LabelledItem : FunctionDeclaration
  1. Return BoundNames of FunctionDeclaration.

8.2.11 Static Semantics: TopLevelVarScopedDeclarations

The syntax-directed operation TopLevelVarScopedDeclarations takes no arguments and returns a List of Parse Nodes. It is defined piecewise over the following productions:

StatementList : StatementList StatementListItem
  1. Let declarations1 be TopLevelVarScopedDeclarations of StatementList.
  2. Let declarations2 be TopLevelVarScopedDeclarations of StatementListItem.
  3. Return the list-concatenation of declarations1 and declarations2.
 StatementListItem : Statement
  1. If Statement is Statement : LabelledStatement , return TopLevelVarScopedDeclarations of Statement.
  2. Return VarScopedDeclarations of Statement.
 StatementListItem : Declaration
  1. If Declaration is Declaration : HoistableDeclaration , then
    1. Let declaration be DeclarationPart of HoistableDeclaration.
    2. Return « declaration ».
  2. Return a new empty List.
 LabelledStatement : LabelIdentifier : LabelledItem
  1. Return the TopLevelVarScopedDeclarations of LabelledItem.
 LabelledItem : Statement
  1. If Statement is Statement : LabelledStatement , return TopLevelVarScopedDeclarations of Statement.
  2. Return VarScopedDeclarations of Statement.
 LabelledItem : FunctionDeclaration
  1. Return « FunctionDeclaration ».

8.3 Labels

8.3.1 Static Semantics: ContainsDuplicateLabels

The syntax-directed operation ContainsDuplicateLabels takes argument labelSet (a List of Strings) and returns a Boolean. It is defined piecewise over the following productions:

Statement : VariableStatement EmptyStatement ExpressionStatement ContinueStatement BreakStatement ReturnStatement ThrowStatement DebuggerStatement Block : { } StatementListItem : Declaration
  1. Return false.
 StatementList : StatementList StatementListItem
  1. Let hasDuplicates be ContainsDuplicateLabels of StatementList with argument labelSet.
  2. If hasDuplicates is true, return true.
  3. Return ContainsDuplicateLabels of StatementListItem with argument labelSet.
 IfStatement : if ( Expression ) Statement else Statement
  1. Let hasDuplicate be ContainsDuplicateLabels of the first Statement with argument labelSet.
  2. If hasDuplicate is true, return true.
  3. Return ContainsDuplicateLabels of the second Statement with argument labelSet.
 IfStatement : if ( Expression ) Statement
  1. Return ContainsDuplicateLabels of Statement with argument labelSet.
 DoWhileStatement : do Statement while ( Expression ) ;
  1. Return ContainsDuplicateLabels of Statement with argument labelSet.
 WhileStatement : while ( Expression ) Statement
  1. Return ContainsDuplicateLabels of Statement with argument labelSet.
 ForStatement : for ( Expressionopt ; Expressionopt ; Expressionopt ) Statement for ( var VariableDeclarationList ; Expressionopt ; Expressionopt ) Statement for ( LexicalDeclaration Expressionopt ; Expressionopt ) Statement
  1. Return ContainsDuplicateLabels of Statement with argument labelSet.
 ForInOfStatement : for ( LeftHandSideExpression in Expression ) Statement for ( var ForBinding in Expression ) Statement for ( ForDeclaration in Expression ) Statement for ( LeftHandSideExpression of AssignmentExpression ) Statement for ( var ForBinding of AssignmentExpression ) Statement for ( ForDeclaration of AssignmentExpression ) Statement for await ( LeftHandSideExpression of AssignmentExpression ) Statement for await ( var ForBinding of AssignmentExpression ) Statement for await ( ForDeclaration of AssignmentExpression ) Statement
  1. Return ContainsDuplicateLabels of Statement with argument labelSet.
 Note

This section is extended by Annex B.3.5.

 WithStatement : with ( Expression ) Statement
  1. Return ContainsDuplicateLabels of Statement with argument labelSet.
 SwitchStatement : switch ( Expression ) CaseBlock
  1. Return ContainsDuplicateLabels of CaseBlock with argument labelSet.
 CaseBlock : { }
  1. Return false.
 CaseBlock : { CaseClausesopt DefaultClause CaseClausesopt }
  1. If the first CaseClauses is present, then
    1. If ContainsDuplicateLabels of the first CaseClauses with argument labelSet is true, return true.
  2. If ContainsDuplicateLabels of DefaultClause with argument labelSet is true, return true.
  3. If the second CaseClauses is not present, return false.
  4. Return ContainsDuplicateLabels of the second CaseClauses with argument labelSet.
 CaseClauses : CaseClauses CaseClause
  1. Let hasDuplicates be ContainsDuplicateLabels of CaseClauses with argument labelSet.
  2. If hasDuplicates is true, return true.
  3. Return ContainsDuplicateLabels of CaseClause with argument labelSet.
 CaseClause : case Expression : StatementListopt
  1. If the StatementList is present, return ContainsDuplicateLabels of StatementList with argument labelSet.
  2. Return false.
 DefaultClause : default : StatementListopt
  1. If the StatementList is present, return ContainsDuplicateLabels of StatementList with argument labelSet.
  2. Return false.
 LabelledStatement : LabelIdentifier : LabelledItem
  1. Let label be the StringValue of LabelIdentifier.
  2. If labelSet contains label, return true.
  3. Let newLabelSet be the list-concatenation of labelSet and « label ».
  4. Return ContainsDuplicateLabels of LabelledItem with argument newLabelSet.
 LabelledItem : FunctionDeclaration
  1. Return false.
 TryStatement : try Block Catch
  1. Let hasDuplicates be ContainsDuplicateLabels of Block with argument labelSet.
  2. If hasDuplicates is true, return true.
  3. Return ContainsDuplicateLabels of Catch with argument labelSet.
 TryStatement : try Block Finally
  1. Let hasDuplicates be ContainsDuplicateLabels of Block with argument labelSet.
  2. If hasDuplicates is true, return true.
  3. Return ContainsDuplicateLabels of Finally with argument labelSet.
 TryStatement : try Block Catch Finally
  1. If ContainsDuplicateLabels of Block with argument labelSet is true, return true.
  2. If ContainsDuplicateLabels of Catch with argument labelSet is true, return true.
  3. Return ContainsDuplicateLabels of Finally with argument labelSet.
 Catch : catch ( CatchParameter ) Block
  1. Return ContainsDuplicateLabels of Block with argument labelSet.
 FunctionStatementList : [empty]
  1. Return false.
 ClassStaticBlockStatementList : [empty]
  1. Return false.
 ModuleItemList : ModuleItemList ModuleItem
  1. Let hasDuplicates be ContainsDuplicateLabels of ModuleItemList with argument labelSet.
  2. If hasDuplicates is true, return true.
  3. Return ContainsDuplicateLabels of ModuleItem with argument labelSet.
 ModuleItem : ImportDeclaration ExportDeclaration
  1. Return false.

8.3.2 Static Semantics: ContainsUndefinedBreakTarget

The syntax-directed operation ContainsUndefinedBreakTarget takes argument labelSet (a List of Strings) and returns a Boolean. It is defined piecewise over the following productions:

Statement : VariableStatement EmptyStatement ExpressionStatement ContinueStatement ReturnStatement ThrowStatement DebuggerStatement Block : { } StatementListItem : Declaration
  1. Return false.
 StatementList : StatementList StatementListItem
  1. Let hasUndefinedLabels be ContainsUndefinedBreakTarget of StatementList with argument labelSet.
  2. If hasUndefinedLabels is true, return true.
  3. Return ContainsUndefinedBreakTarget of StatementListItem with argument labelSet.
 IfStatement : if ( Expression ) Statement else Statement
  1. Let hasUndefinedLabels be ContainsUndefinedBreakTarget of the first Statement with argument labelSet.
  2. If hasUndefinedLabels is true, return true.
  3. Return ContainsUndefinedBreakTarget of the second Statement with argument labelSet.
 IfStatement : if ( Expression ) Statement
  1. Return ContainsUndefinedBreakTarget of Statement with argument labelSet.
 DoWhileStatement : do Statement while ( Expression ) ;
  1. Return ContainsUndefinedBreakTarget of Statement with argument labelSet.
 WhileStatement : while ( Expression ) Statement
  1. Return ContainsUndefinedBreakTarget of Statement with argument labelSet.
 ForStatement : for ( Expressionopt ; Expressionopt ; Expressionopt ) Statement for ( var VariableDeclarationList ; Expressionopt ; Expressionopt ) Statement for ( LexicalDeclaration Expressionopt ; Expressionopt ) Statement
  1. Return ContainsUndefinedBreakTarget of Statement with argument labelSet.
 ForInOfStatement : for ( LeftHandSideExpression in Expression ) Statement for ( var ForBinding in Expression ) Statement for ( ForDeclaration in Expression ) Statement for ( LeftHandSideExpression of AssignmentExpression ) Statement for ( var ForBinding of AssignmentExpression ) Statement for ( ForDeclaration of AssignmentExpression ) Statement for await ( LeftHandSideExpression of AssignmentExpression ) Statement for await ( var ForBinding of AssignmentExpression ) Statement for await ( ForDeclaration of AssignmentExpression ) Statement
  1. Return ContainsUndefinedBreakTarget of Statement with argument labelSet.
 Note

This section is extended by Annex B.3.5.

 BreakStatement : break ;
  1. Return false.
 BreakStatement : break LabelIdentifier ;
  1. If labelSet does not contain the StringValue of LabelIdentifier, return true.
  2. Return false.
 WithStatement : with ( Expression ) Statement
  1. Return ContainsUndefinedBreakTarget of Statement with argument labelSet.
 SwitchStatement : switch ( Expression ) CaseBlock
  1. Return ContainsUndefinedBreakTarget of CaseBlock with argument labelSet.
 CaseBlock : { }
  1. Return false.
 CaseBlock : { CaseClausesopt DefaultClause CaseClausesopt }
  1. If the first CaseClauses is present, then
    1. If ContainsUndefinedBreakTarget of the first CaseClauses with argument labelSet is true, return true.
  2. If ContainsUndefinedBreakTarget of DefaultClause with argument labelSet is true, return true.
  3. If the second CaseClauses is not present, return false.
  4. Return ContainsUndefinedBreakTarget of the second CaseClauses with argument labelSet.
 CaseClauses : CaseClauses CaseClause
  1. Let hasUndefinedLabels be ContainsUndefinedBreakTarget of CaseClauses with argument labelSet.
  2. If hasUndefinedLabels is true, return true.
  3. Return ContainsUndefinedBreakTarget of CaseClause with argument labelSet.
 CaseClause : case Expression : StatementListopt
  1. If the StatementList is present, return ContainsUndefinedBreakTarget of StatementList with argument labelSet.
  2. Return false.
 DefaultClause : default : StatementListopt
  1. If the StatementList is present, return ContainsUndefinedBreakTarget of StatementList with argument labelSet.
  2. Return false.
 LabelledStatement : LabelIdentifier : LabelledItem
  1. Let label be the StringValue of LabelIdentifier.
  2. Let newLabelSet be the list-concatenation of labelSet and « label ».
  3. Return ContainsUndefinedBreakTarget of LabelledItem with argument newLabelSet.
 LabelledItem : FunctionDeclaration
  1. Return false.
 TryStatement : try Block Catch
  1. Let hasUndefinedLabels be ContainsUndefinedBreakTarget of Block with argument labelSet.
  2. If hasUndefinedLabels is true, return true.
  3. Return ContainsUndefinedBreakTarget of Catch with argument labelSet.
 TryStatement : try Block Finally
  1. Let hasUndefinedLabels be ContainsUndefinedBreakTarget of Block with argument labelSet.
  2. If hasUndefinedLabels is true, return true.
  3. Return ContainsUndefinedBreakTarget of Finally with argument labelSet.
 TryStatement : try Block Catch Finally
  1. If ContainsUndefinedBreakTarget of Block with argument labelSet is true, return true.
  2. If ContainsUndefinedBreakTarget of Catch with argument labelSet is true, return true.
  3. Return ContainsUndefinedBreakTarget of Finally with argument labelSet.
 Catch : catch ( CatchParameter ) Block
  1. Return ContainsUndefinedBreakTarget of Block with argument labelSet.
 FunctionStatementList : [empty]
  1. Return false.
 ClassStaticBlockStatementList : [empty]
  1. Return false.
 ModuleItemList : ModuleItemList ModuleItem
  1. Let hasUndefinedLabels be ContainsUndefinedBreakTarget of ModuleItemList with argument labelSet.
  2. If hasUndefinedLabels is true, return true.
  3. Return ContainsUndefinedBreakTarget of ModuleItem with argument labelSet.
 ModuleItem : ImportDeclaration ExportDeclaration
  1. Return false.

8.3.3 Static Semantics: ContainsUndefinedContinueTarget

The syntax-directed operation ContainsUndefinedContinueTarget takes arguments iterationSet (a List of Strings) and labelSet (a List of Strings) and returns a Boolean. It is defined piecewise over the following productions:

Statement : VariableStatement EmptyStatement ExpressionStatement BreakStatement ReturnStatement ThrowStatement DebuggerStatement Block : { } StatementListItem : Declaration
  1. Return false.
 Statement : BlockStatement
  1. Return ContainsUndefinedContinueTarget of BlockStatement with arguments iterationSet and « ».
 BreakableStatement : IterationStatement
  1. Let newIterationSet be the list-concatenation of iterationSet and labelSet.
  2. Return ContainsUndefinedContinueTarget of IterationStatement with arguments newIterationSet and « ».
 StatementList : StatementList StatementListItem
  1. Let hasUndefinedLabels be ContainsUndefinedContinueTarget of StatementList with arguments iterationSet and « ».
  2. If hasUndefinedLabels is true, return true.
  3. Return ContainsUndefinedContinueTarget of StatementListItem with arguments iterationSet and « ».
 IfStatement : if ( Expression ) Statement else Statement
  1. Let hasUndefinedLabels be ContainsUndefinedContinueTarget of the first Statement with arguments iterationSet and « ».
  2. If hasUndefinedLabels is true, return true.
  3. Return ContainsUndefinedContinueTarget of the second Statement with arguments iterationSet and « ».
 IfStatement : if ( Expression ) Statement
  1. Return ContainsUndefinedContinueTarget of Statement with arguments iterationSet and « ».
 DoWhileStatement : do Statement while ( Expression ) ;
  1. Return ContainsUndefinedContinueTarget of Statement with arguments iterationSet and « ».
 WhileStatement : while ( Expression ) Statement
  1. Return ContainsUndefinedContinueTarget of Statement with arguments iterationSet and « ».
 ForStatement : for ( Expressionopt ; Expressionopt ; Expressionopt ) Statement for ( var VariableDeclarationList ; Expressionopt ; Expressionopt ) Statement for ( LexicalDeclaration Expressionopt ; Expressionopt ) Statement
  1. Return ContainsUndefinedContinueTarget of Statement with arguments iterationSet and « ».
 ForInOfStatement : for ( LeftHandSideExpression in Expression ) Statement for ( var ForBinding in Expression ) Statement for ( ForDeclaration in Expression ) Statement for ( LeftHandSideExpression of AssignmentExpression ) Statement for ( var ForBinding of AssignmentExpression ) Statement for ( ForDeclaration of AssignmentExpression ) Statement for await ( LeftHandSideExpression of AssignmentExpression ) Statement for await ( var ForBinding of AssignmentExpression ) Statement for await ( ForDeclaration of AssignmentExpression ) Statement
  1. Return ContainsUndefinedContinueTarget of Statement with arguments iterationSet and « ».
 Note

This section is extended by Annex B.3.5.

 ContinueStatement : continue ;
  1. Return false.
 ContinueStatement : continue LabelIdentifier ;
  1. If iterationSet does not contain the StringValue of LabelIdentifier, return true.
  2. Return false.
 WithStatement : with ( Expression ) Statement
  1. Return ContainsUndefinedContinueTarget of Statement with arguments iterationSet and « ».
 SwitchStatement : switch ( Expression ) CaseBlock
  1. Return ContainsUndefinedContinueTarget of CaseBlock with arguments iterationSet and « ».
 CaseBlock : { }
  1. Return false.
 CaseBlock : { CaseClausesopt DefaultClause CaseClausesopt }
  1. If the first CaseClauses is present, then
    1. If ContainsUndefinedContinueTarget of the first CaseClauses with arguments iterationSet and « » is true, return true.
  2. If ContainsUndefinedContinueTarget of DefaultClause with arguments iterationSet and « » is true, return true.
  3. If the second CaseClauses is not present, return false.
  4. Return ContainsUndefinedContinueTarget of the second CaseClauses with arguments iterationSet and « ».
 CaseClauses : CaseClauses CaseClause
  1. Let hasUndefinedLabels be ContainsUndefinedContinueTarget of CaseClauses with arguments iterationSet and « ».
  2. If hasUndefinedLabels is true, return true.
  3. Return ContainsUndefinedContinueTarget of CaseClause with arguments iterationSet and « ».
 CaseClause : case Expression : StatementListopt
  1. If the StatementList is present, return ContainsUndefinedContinueTarget of StatementList with arguments iterationSet and « ».
  2. Return false.
 DefaultClause : default : StatementListopt
  1. If the StatementList is present, return ContainsUndefinedContinueTarget of StatementList with arguments iterationSet and « ».
  2. Return false.
 LabelledStatement : LabelIdentifier : LabelledItem
  1. Let label be the StringValue of LabelIdentifier.
  2. Let newLabelSet be the list-concatenation of labelSet and « label ».
  3. Return ContainsUndefinedContinueTarget of LabelledItem with arguments iterationSet and newLabelSet.
 LabelledItem : FunctionDeclaration
  1. Return false.
 TryStatement : try Block Catch
  1. Let hasUndefinedLabels be ContainsUndefinedContinueTarget of Block with arguments iterationSet and « ».
  2. If hasUndefinedLabels is true, return true.
  3. Return ContainsUndefinedContinueTarget of Catch with arguments iterationSet and « ».
 TryStatement : try Block Finally
  1. Let hasUndefinedLabels be ContainsUndefinedContinueTarget of Block with arguments iterationSet and « ».
  2. If hasUndefinedLabels is true, return true.
  3. Return ContainsUndefinedContinueTarget of Finally with arguments iterationSet and « ».
 TryStatement : try Block Catch Finally
  1. If ContainsUndefinedContinueTarget of Block with arguments iterationSet and « » is true, return true.
  2. If ContainsUndefinedContinueTarget of Catch with arguments iterationSet and « » is true, return true.
  3. Return ContainsUndefinedContinueTarget of Finally with arguments iterationSet and « ».
 Catch : catch ( CatchParameter ) Block
  1. Return ContainsUndefinedContinueTarget of Block with arguments iterationSet and « ».
 FunctionStatementList : [empty]
  1. Return false.
 ClassStaticBlockStatementList : [empty]
  1. Return false.
 ModuleItemList : ModuleItemList ModuleItem
  1. Let hasUndefinedLabels be ContainsUndefinedContinueTarget of ModuleItemList with arguments iterationSet and « ».
  2. If hasUndefinedLabels is true, return true.
  3. Return ContainsUndefinedContinueTarget of ModuleItem with arguments iterationSet and « ».
 ModuleItem : ImportDeclaration ExportDeclaration
  1. Return false.

8.4 Function Name Inference

8.4.1 Static Semantics: HasName

The syntax-directed operation HasName takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList
  1. Let expr be the ParenthesizedExpression that is covered by CoverParenthesizedExpressionAndArrowParameterList.
  2. If IsFunctionDefinition of expr is false, return false.
  3. Return HasName of expr.
 FunctionExpression : function ( FormalParameters ) { FunctionBody } GeneratorExpression : function * ( FormalParameters ) { GeneratorBody } AsyncGeneratorExpression : async function * ( FormalParameters ) { AsyncGeneratorBody } AsyncFunctionExpression : async function ( FormalParameters ) { AsyncFunctionBody } ArrowFunction : ArrowParameters => ConciseBody AsyncArrowFunction : async AsyncArrowBindingIdentifier => AsyncConciseBody CoverCallExpressionAndAsyncArrowHead => AsyncConciseBody ClassExpression : class ClassTail
  1. Return false.
 FunctionExpression : function BindingIdentifier ( FormalParameters ) { FunctionBody } GeneratorExpression : function * BindingIdentifier ( FormalParameters ) { GeneratorBody } AsyncGeneratorExpression : async function * BindingIdentifier ( FormalParameters ) { AsyncGeneratorBody } AsyncFunctionExpression : async function BindingIdentifier ( FormalParameters ) { AsyncFunctionBody } ClassExpression : class BindingIdentifier ClassTail
  1. Return true.

8.4.2 Static Semantics: IsFunctionDefinition

The syntax-directed operation IsFunctionDefinition takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList
  1. Let expr be the ParenthesizedExpression that is covered by CoverParenthesizedExpressionAndArrowParameterList.
  2. Return IsFunctionDefinition of expr.
 PrimaryExpression : this IdentifierReference Literal ArrayLiteral ObjectLiteral RegularExpressionLiteral TemplateLiteral MemberExpression : MemberExpression [ Expression ] MemberExpression . IdentifierName MemberExpression TemplateLiteral SuperProperty MetaProperty new MemberExpression Arguments MemberExpression . PrivateIdentifier NewExpression : new NewExpression LeftHandSideExpression : CallExpression OptionalExpression UpdateExpression : LeftHandSideExpression LeftHandSideExpression -- UnaryExpression -- UnaryExpression UnaryExpression : delete UnaryExpression void UnaryExpression typeof UnaryExpression UnaryExpression - UnaryExpression ~ UnaryExpression ! UnaryExpression AwaitExpression ExponentiationExpression : UpdateExpression ** ExponentiationExpression MultiplicativeExpression : MultiplicativeExpression MultiplicativeOperator ExponentiationExpression AdditiveExpression : AdditiveExpression MultiplicativeExpression AdditiveExpression - MultiplicativeExpression ShiftExpression : ShiftExpression << AdditiveExpression ShiftExpression >> AdditiveExpression ShiftExpression >>> AdditiveExpression RelationalExpression : RelationalExpression < ShiftExpression RelationalExpression > ShiftExpression RelationalExpression <= ShiftExpression RelationalExpression >= ShiftExpression RelationalExpression instanceof ShiftExpression RelationalExpression in ShiftExpression PrivateIdentifier in ShiftExpression EqualityExpression : EqualityExpression == RelationalExpression EqualityExpression != RelationalExpression EqualityExpression === RelationalExpression EqualityExpression !== RelationalExpression BitwiseANDExpression : BitwiseANDExpression & EqualityExpression BitwiseXORExpression : BitwiseXORExpression ^ BitwiseANDExpression BitwiseORExpression : BitwiseORExpression | BitwiseXORExpression LogicalANDExpression : LogicalANDExpression && BitwiseORExpression LogicalORExpression : LogicalORExpression || LogicalANDExpression CoalesceExpression : CoalesceExpressionHead ?? BitwiseORExpression ConditionalExpression : ShortCircuitExpression ? AssignmentExpression : AssignmentExpression AssignmentExpression : YieldExpression LeftHandSideExpression = AssignmentExpression LeftHandSideExpression AssignmentOperator AssignmentExpression LeftHandSideExpression &&= AssignmentExpression LeftHandSideExpression ||= AssignmentExpression LeftHandSideExpression ??= AssignmentExpression Expression : Expression , AssignmentExpression
  1. Return false.
 AssignmentExpression : ArrowFunction AsyncArrowFunction FunctionExpression : function BindingIdentifieropt ( FormalParameters ) { FunctionBody } GeneratorExpression : function * BindingIdentifieropt ( FormalParameters ) { GeneratorBody } AsyncGeneratorExpression : async function * BindingIdentifieropt ( FormalParameters ) { AsyncGeneratorBody } AsyncFunctionExpression : async function BindingIdentifieropt ( FormalParameters ) { AsyncFunctionBody } ClassExpression : class BindingIdentifieropt ClassTail
  1. Return true.

8.4.3 Static Semantics: IsAnonymousFunctionDefinition ( expr )

The abstract operation IsAnonymousFunctionDefinition takes argument expr (an AssignmentExpression Parse Node or an Initializer Parse Node) and returns a Boolean. It determines if its argument is a function definition that does not bind a name. It performs the following steps when called:

  1. If IsFunctionDefinition of expr is false, return false.
  2. Let hasName be HasName of expr.
  3. If hasName is true, return false.
  4. Return true.

8.4.4 Static Semantics: IsIdentifierRef

The syntax-directed operation IsIdentifierRef takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

PrimaryExpression : IdentifierReference
  1. Return true.
 PrimaryExpression : this Literal ArrayLiteral ObjectLiteral FunctionExpression ClassExpression GeneratorExpression AsyncFunctionExpression AsyncGeneratorExpression RegularExpressionLiteral TemplateLiteral CoverParenthesizedExpressionAndArrowParameterList MemberExpression : MemberExpression [ Expression ] MemberExpression . IdentifierName MemberExpression TemplateLiteral SuperProperty MetaProperty new MemberExpression Arguments MemberExpression . PrivateIdentifier NewExpression : new NewExpression LeftHandSideExpression : CallExpression OptionalExpression
  1. Return false.

8.4.5 Runtime Semantics: NamedEvaluation

The syntax-directed operation NamedEvaluation takes argument name (a property key or a Private Name) and returns either a normal completion containing a function object or an abrupt completion. It is defined piecewise over the following productions:

PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList
  1. Let expr be the ParenthesizedExpression that is covered by CoverParenthesizedExpressionAndArrowParameterList.
  2. Return ? NamedEvaluation of expr with argument name.
 ParenthesizedExpression : ( Expression )
  1. Assert: IsAnonymousFunctionDefinition(Expression) is true.
  2. Return ? NamedEvaluation of Expression with argument name.
 FunctionExpression : function ( FormalParameters ) { FunctionBody }
  1. Return InstantiateOrdinaryFunctionExpression of FunctionExpression with argument name.
 GeneratorExpression : function * ( FormalParameters ) { GeneratorBody }
  1. Return InstantiateGeneratorFunctionExpression of GeneratorExpression with argument name.
 AsyncGeneratorExpression : async function * ( FormalParameters ) { AsyncGeneratorBody }
  1. Return InstantiateAsyncGeneratorFunctionExpression of AsyncGeneratorExpression with argument name.
 AsyncFunctionExpression : async function ( FormalParameters ) { AsyncFunctionBody }
  1. Return InstantiateAsyncFunctionExpression of AsyncFunctionExpression with argument name.
 ArrowFunction : ArrowParameters => ConciseBody
  1. Return InstantiateArrowFunctionExpression of ArrowFunction with argument name.
 AsyncArrowFunction : async AsyncArrowBindingIdentifier => AsyncConciseBody CoverCallExpressionAndAsyncArrowHead => AsyncConciseBody
  1. Return InstantiateAsyncArrowFunctionExpression of AsyncArrowFunction with argument name.
 ClassExpression : class ClassTail
  1. Let value be ? ClassDefinitionEvaluation of ClassTail with arguments undefined and name.
  2. Set value.[[SourceText]] to the source text matched by ClassExpression.
  3. Return value.

8.5 Contains

8.5.1 Static Semantics: Contains

The syntax-directed operation Contains takes argument symbol (a grammar symbol) and returns a Boolean.

Every grammar production alternative in this specification which is not listed below implicitly has the following default definition of Contains:

  1. For each child node child of this Parse Node, do
    1. If child is an instance of symbol, return true.
    2. If child is an instance of a nonterminal, then
      1. Let contained be the result of child Contains symbol.
      2. If contained is true, return true.
  2. Return false.
 FunctionDeclaration : function BindingIdentifier ( FormalParameters ) { FunctionBody } function ( FormalParameters ) { FunctionBody } FunctionExpression : function BindingIdentifieropt ( FormalParameters ) { FunctionBody } GeneratorDeclaration : function * BindingIdentifier ( FormalParameters ) { GeneratorBody } function * ( FormalParameters ) { GeneratorBody } GeneratorExpression : function * BindingIdentifieropt ( FormalParameters ) { GeneratorBody } AsyncGeneratorDeclaration : async function * BindingIdentifier ( FormalParameters ) { AsyncGeneratorBody } async function * ( FormalParameters ) { AsyncGeneratorBody } AsyncGeneratorExpression : async function * BindingIdentifieropt ( FormalParameters ) { AsyncGeneratorBody } AsyncFunctionDeclaration : async function BindingIdentifier ( FormalParameters ) { AsyncFunctionBody } async function ( FormalParameters ) { AsyncFunctionBody } AsyncFunctionExpression : async function BindingIdentifieropt ( FormalParameters ) { AsyncFunctionBody }
  1. Return false.
 Note 1

Static semantic rules that depend upon substructure generally do not look into function definitions.

 ClassTail : ClassHeritageopt { ClassBody }
  1. If symbol is ClassBody, return true.
  2. If symbol is ClassHeritage, then
    1. If ClassHeritage is present, return true; otherwise return false.
  3. If ClassHeritage is present, then
    1. If ClassHeritage Contains symbol is true, return true.
  4. Return the result of ComputedPropertyContains of ClassBody with argument symbol.
 Note 2

Static semantic rules that depend upon substructure generally do not look into class bodies except for PropertyNames.

 ClassStaticBlock : static { ClassStaticBlockBody }
  1. Return false.
 Note 3

Static semantic rules that depend upon substructure generally do not look into static initialization blocks.

 ArrowFunction : ArrowParameters => ConciseBody
  1. If symbol is not one of NewTarget, SuperProperty, SuperCall, super, or this, return false.
  2. If ArrowParameters Contains symbol is true, return true.
  3. Return ConciseBody Contains symbol.
 ArrowParameters : CoverParenthesizedExpressionAndArrowParameterList
  1. Let formals be the ArrowFormalParameters that is covered by CoverParenthesizedExpressionAndArrowParameterList.
  2. Return formals Contains symbol.
 AsyncArrowFunction : async AsyncArrowBindingIdentifier => AsyncConciseBody
  1. If symbol is not one of NewTarget, SuperProperty, SuperCall, super, or this, return false.
  2. Return AsyncConciseBody Contains symbol.
 AsyncArrowFunction : CoverCallExpressionAndAsyncArrowHead => AsyncConciseBody
  1. If symbol is not one of NewTarget, SuperProperty, SuperCall, super, or this, return false.
  2. Let head be the AsyncArrowHead that is covered by CoverCallExpressionAndAsyncArrowHead.
  3. If head Contains symbol is true, return true.
  4. Return AsyncConciseBody Contains symbol.
 Note 4

Contains is used to detect new.target, this, and super usage within an ArrowFunction or AsyncArrowFunction.

 PropertyDefinition : MethodDefinition
  1. If symbol is MethodDefinition, return true.
  2. Return the result of ComputedPropertyContains of MethodDefinition with argument symbol.
 LiteralPropertyName : IdentifierName
  1. Return false.
 MemberExpression : MemberExpression . IdentifierName
  1. If MemberExpression Contains symbol is true, return true.
  2. Return false.
 SuperProperty : super . IdentifierName
  1. If symbol is the ReservedWord super, return true.
  2. Return false.
 CallExpression : CallExpression . IdentifierName
  1. If CallExpression Contains symbol is true, return true.
  2. Return false.
 OptionalChain : ?. IdentifierName
  1. Return false.
 OptionalChain : OptionalChain . IdentifierName
  1. If OptionalChain Contains symbol is true, return true.
  2. Return false.

8.5.2 Static Semantics: ComputedPropertyContains

The syntax-directed operation ComputedPropertyContains takes argument symbol (a grammar symbol) and returns a Boolean. It is defined piecewise over the following productions:

ClassElementName : PrivateIdentifier PropertyName : LiteralPropertyName
  1. Return false.
 PropertyName : ComputedPropertyName
  1. Return the result of ComputedPropertyName Contains symbol.
 MethodDefinition : ClassElementName ( UniqueFormalParameters ) { FunctionBody } get ClassElementName ( ) { FunctionBody } set ClassElementName ( PropertySetParameterList ) { FunctionBody }
  1. Return the result of ComputedPropertyContains of ClassElementName with argument symbol.
 GeneratorMethod : * ClassElementName ( UniqueFormalParameters ) { GeneratorBody }
  1. Return the result of ComputedPropertyContains of ClassElementName with argument symbol.
 AsyncGeneratorMethod : async * ClassElementName ( UniqueFormalParameters ) { AsyncGeneratorBody }
  1. Return the result of ComputedPropertyContains of ClassElementName with argument symbol.
 ClassElementList : ClassElementList ClassElement
  1. Let inList be ComputedPropertyContains of ClassElementList with argument symbol.
  2. If inList is true, return true.
  3. Return the result of ComputedPropertyContains of ClassElement with argument symbol.
 ClassElement : ClassStaticBlock
  1. Return false.
 ClassElement : ;
  1. Return false.
 AsyncMethod : async ClassElementName ( UniqueFormalParameters ) { AsyncFunctionBody }
  1. Return the result of ComputedPropertyContains of ClassElementName with argument symbol.
 FieldDefinition : ClassElementName Initializeropt
  1. Return the result of ComputedPropertyContains of ClassElementName with argument symbol.

8.6 Miscellaneous

These operations are used in multiple places throughout the specification.

8.6.1 Runtime Semantics: InstantiateFunctionObject

The syntax-directed operation InstantiateFunctionObject takes arguments env (an Environment Record) and privateEnv (a PrivateEnvironment Record or null) and returns a function object. It is defined piecewise over the following productions:

FunctionDeclaration : function BindingIdentifier ( FormalParameters ) { FunctionBody } function ( FormalParameters ) { FunctionBody }
  1. Return InstantiateOrdinaryFunctionObject of FunctionDeclaration with arguments env and privateEnv.
 GeneratorDeclaration : function * BindingIdentifier ( FormalParameters ) { GeneratorBody } function * ( FormalParameters ) { GeneratorBody }
  1. Return InstantiateGeneratorFunctionObject of GeneratorDeclaration with arguments env and privateEnv.
 AsyncGeneratorDeclaration : async function * BindingIdentifier ( FormalParameters ) { AsyncGeneratorBody } async function * ( FormalParameters ) { AsyncGeneratorBody }
  1. Return InstantiateAsyncGeneratorFunctionObject of AsyncGeneratorDeclaration with arguments env and privateEnv.
 AsyncFunctionDeclaration : async function BindingIdentifier ( FormalParameters ) { AsyncFunctionBody } async function ( FormalParameters ) { AsyncFunctionBody }
  1. Return InstantiateAsyncFunctionObject of AsyncFunctionDeclaration with arguments env and privateEnv.

8.6.2 Runtime Semantics: BindingInitialization

The syntax-directed operation BindingInitialization takes arguments value (an ECMAScript language value) and environment (an Environment Record or undefined) and returns either a normal completion containing unused or an abrupt completion.

Note

undefined is passed for environment to indicate that a PutValue operation should be used to assign the initialization value. This is the case for var statements and formal parameter lists of some non-strict functions (See 10.2.11). In those cases a lexical binding is hoisted and preinitialized prior to evaluation of its initializer.

It is defined piecewise over the following productions:

BindingIdentifier : Identifier
  1. Let name be StringValue of Identifier.
  2. Return ? InitializeBoundName(name, value, environment).
 BindingIdentifier : yield
  1. Return ? InitializeBoundName("yield", value, environment).
 BindingIdentifier : await
  1. Return ? InitializeBoundName("await", value, environment).
 BindingPattern : ObjectBindingPattern
  1. Perform ? RequireObjectCoercible(value).
  2. Return ? BindingInitialization of ObjectBindingPattern with arguments value and environment.
 BindingPattern : ArrayBindingPattern
  1. Let iteratorRecord be ? GetIterator(value, sync).
  2. Let result be Completion(IteratorBindingInitialization of ArrayBindingPattern with arguments iteratorRecord and environment).
  3. If iteratorRecord.[[Done]] is false, return ? IteratorClose(iteratorRecord, result).
  4. Return ? result.
 ObjectBindingPattern : { }
  1. Return unused.
 ObjectBindingPattern : { BindingPropertyList } { BindingPropertyList , }
  1. Perform ? PropertyBindingInitialization of BindingPropertyList with arguments value and environment.
  2. Return unused.
 ObjectBindingPattern : { BindingRestProperty }
  1. Let excludedNames be a new empty List.
  2. Return ? RestBindingInitialization of BindingRestProperty with arguments value, environment, and excludedNames.
 ObjectBindingPattern : { BindingPropertyList , BindingRestProperty }
  1. Let excludedNames be ? PropertyBindingInitialization of BindingPropertyList with arguments value and environment.
  2. Return ? RestBindingInitialization of BindingRestProperty with arguments value, environment, and excludedNames.

8.6.2.1 InitializeBoundName ( name, value, environment )

The abstract operation InitializeBoundName takes arguments name (a String), value (an ECMAScript language value), and environment (an Environment Record or undefined) and returns either a normal completion containing unused or an abrupt completion. It performs the following steps when called:

  1. If environment is not undefined, then
    1. Perform ! environment.InitializeBinding(name, value).
    2. Return unused.
  2. Else,
    1. Let lhs be ? ResolveBinding(name).
    2. Return ? PutValue(lhs, value).

8.6.3 Runtime Semantics: IteratorBindingInitialization

The syntax-directed operation IteratorBindingInitialization takes arguments iteratorRecord (an Iterator Record) and environment (an Environment Record or undefined) and returns either a normal completion containing unused or an abrupt completion.

Note

When undefined is passed for environment it indicates that a PutValue operation should be used to assign the initialization value. This is the case for formal parameter lists of non-strict functions. In that case the formal parameter bindings are preinitialized in order to deal with the possibility of multiple parameters with the same name.

It is defined piecewise over the following productions:

ArrayBindingPattern : [ ]
  1. Return unused.
 ArrayBindingPattern : [ Elision ]
  1. Return ? IteratorDestructuringAssignmentEvaluation of Elision with argument iteratorRecord.
 ArrayBindingPattern : [ Elisionopt BindingRestElement ]
  1. If Elision is present, then
    1. Perform ? IteratorDestructuringAssignmentEvaluation of Elision with argument iteratorRecord.
  2. Return ? IteratorBindingInitialization of BindingRestElement with arguments iteratorRecord and environment.
 ArrayBindingPattern : [ BindingElementList , Elision ]
  1. Perform ? IteratorBindingInitialization of BindingElementList with arguments iteratorRecord and environment.
  2. Return ? IteratorDestructuringAssignmentEvaluation of Elision with argument iteratorRecord.
 ArrayBindingPattern : [ BindingElementList , Elisionopt BindingRestElement ]
  1. Perform ? IteratorBindingInitialization of BindingElementList with arguments iteratorRecord and environment.
  2. If Elision is present, then
    1. Perform ? IteratorDestructuringAssignmentEvaluation of Elision with argument iteratorRecord.
  3. Return ? IteratorBindingInitialization of BindingRestElement with arguments iteratorRecord and environment.
 BindingElementList : BindingElementList , BindingElisionElement
  1. Perform ? IteratorBindingInitialization of BindingElementList with arguments iteratorRecord and environment.
  2. Return ? IteratorBindingInitialization of BindingElisionElement with arguments iteratorRecord and environment.
 BindingElisionElement : Elision BindingElement
  1. Perform ? IteratorDestructuringAssignmentEvaluation of Elision with argument iteratorRecord.
  2. Return ? IteratorBindingInitialization of BindingElement with arguments iteratorRecord and environment.
 SingleNameBinding : BindingIdentifier Initializeropt
  1. Let bindingId be StringValue of BindingIdentifier.
  2. Let lhs be ? ResolveBinding(bindingId, environment).
  3. Let v be undefined.
  4. If iteratorRecord.[[Done]] is false, then
    1. Let next be Completion(IteratorStep(iteratorRecord)).
    2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
    3. ReturnIfAbrupt(next).
    4. If next is false, set iteratorRecord.[[Done]] to true.
    5. Else,
      1. Set v to Completion(IteratorValue(next)).
      2. If v is an abrupt completion, set iteratorRecord.[[Done]] to true.
      3. ReturnIfAbrupt(v).
  5. If Initializer is present and v is undefined, then
    1. If IsAnonymousFunctionDefinition(Initializer) is true, then
      1. Set v to ? NamedEvaluation of Initializer with argument bindingId.
    2. Else,
      1. Let defaultValue be ? Evaluation of Initializer.
      2. Set v to ? GetValue(defaultValue).
  6. If environment is undefined, return ? PutValue(lhs, v).
  7. Return ? InitializeReferencedBinding(lhs, v).
 BindingElement : BindingPattern Initializeropt
  1. Let v be undefined.
  2. If iteratorRecord.[[Done]] is false, then
    1. Let next be Completion(IteratorStep(iteratorRecord)).
    2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
    3. ReturnIfAbrupt(next).
    4. If next is false, set iteratorRecord.[[Done]] to true.
    5. Else,
      1. Set v to Completion(IteratorValue(next)).
      2. If v is an abrupt completion, set iteratorRecord.[[Done]] to true.
      3. ReturnIfAbrupt(v).
  3. If Initializer is present and v is undefined, then
    1. Let defaultValue be ? Evaluation of Initializer.
    2. Set v to ? GetValue(defaultValue).
  4. Return ? BindingInitialization of BindingPattern with arguments v and environment.
 BindingRestElement : ... BindingIdentifier
  1. Let lhs be ? ResolveBinding(StringValue of BindingIdentifier, environment).
  2. Let A be ! ArrayCreate(0).
  3. Let n be 0.
  4. Repeat,
    1. If iteratorRecord.[[Done]] is false, then
      1. Let next be Completion(IteratorStep(iteratorRecord)).
      2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
      3. ReturnIfAbrupt(next).
      4. If next is false, set iteratorRecord.[[Done]] to true.
    2. If iteratorRecord.[[Done]] is true, then
      1. If environment is undefined, return ? PutValue(lhs, A).
      2. Return ? InitializeReferencedBinding(lhs, A).
    3. Let nextValue be Completion(IteratorValue(next)).
    4. If nextValue is an abrupt completion, set iteratorRecord.[[Done]] to true.
    5. ReturnIfAbrupt(nextValue).
    6. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(n)), nextValue).
    7. Set n to n 1.
 BindingRestElement : ... BindingPattern
  1. Let A be ! ArrayCreate(0).
  2. Let n be 0.
  3. Repeat,
    1. If iteratorRecord.[[Done]] is false, then
      1. Let next be Completion(IteratorStep(iteratorRecord)).
      2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
      3. ReturnIfAbrupt(next).
      4. If next is false, set iteratorRecord.[[Done]] to true.
    2. If iteratorRecord.[[Done]] is true, then
      1. Return ? BindingInitialization of BindingPattern with arguments A and environment.
    3. Let nextValue be Completion(IteratorValue(next)).
    4. If nextValue is an abrupt completion, set iteratorRecord.[[Done]] to true.
    5. ReturnIfAbrupt(nextValue).
    6. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(n)), nextValue).
    7. Set n to n 1.
 FormalParameters : [empty]
  1. Return unused.
 FormalParameters : FormalParameterList , FunctionRestParameter
  1. Perform ? IteratorBindingInitialization of FormalParameterList with arguments iteratorRecord and environment.
  2. Return ? IteratorBindingInitialization of FunctionRestParameter with arguments iteratorRecord and environment.
 FormalParameterList : FormalParameterList , FormalParameter
  1. Perform ? IteratorBindingInitialization of FormalParameterList with arguments iteratorRecord and environment.
  2. Return ? IteratorBindingInitialization of FormalParameter with arguments iteratorRecord and environment.
 ArrowParameters : BindingIdentifier
  1. Let v be undefined.
  2. Assert: iteratorRecord.[[Done]] is false.
  3. Let next be Completion(IteratorStep(iteratorRecord)).
  4. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
  5. ReturnIfAbrupt(next).
  6. If next is false, set iteratorRecord.[[Done]] to true.
  7. Else,
    1. Set v to Completion(IteratorValue(next)).
    2. If v is an abrupt completion, set iteratorRecord.[[Done]] to true.
    3. ReturnIfAbrupt(v).
  8. Return ? BindingInitialization of BindingIdentifier with arguments v and environment.
 ArrowParameters : CoverParenthesizedExpressionAndArrowParameterList
  1. Let formals be the ArrowFormalParameters that is covered by CoverParenthesizedExpressionAndArrowParameterList.
  2. Return ? IteratorBindingInitialization of formals with arguments iteratorRecord and environment.
 AsyncArrowBindingIdentifier : BindingIdentifier
  1. Let v be undefined.
  2. Assert: iteratorRecord.[[Done]] is false.
  3. Let next be Completion(IteratorStep(iteratorRecord)).
  4. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
  5. ReturnIfAbrupt(next).
  6. If next is false, set iteratorRecord.[[Done]] to true.
  7. Else,
    1. Set v to Completion(IteratorValue(next)).
    2. If v is an abrupt completion, set iteratorRecord.[[Done]] to true.
    3. ReturnIfAbrupt(v).
  8. Return ? BindingInitialization of BindingIdentifier with arguments v and environment.

8.6.4 Static Semantics: AssignmentTargetType

The syntax-directed operation AssignmentTargetType takes no arguments and returns simple or invalid. It is defined piecewise over the following productions:

IdentifierReference : Identifier
  1. If this IdentifierReference is contained in strict mode code and StringValue of Identifier is either "eval" or "arguments", return invalid.
  2. Return simple.
 IdentifierReference : yield await CallExpression : CallExpression [ Expression ] CallExpression . IdentifierName CallExpression . PrivateIdentifier MemberExpression : MemberExpression [ Expression ] MemberExpression . IdentifierName SuperProperty MemberExpression . PrivateIdentifier
  1. Return simple.
 PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList
  1. Let expr be the ParenthesizedExpression that is covered by CoverParenthesizedExpressionAndArrowParameterList.
  2. Return AssignmentTargetType of expr.
 PrimaryExpression : this Literal ArrayLiteral ObjectLiteral FunctionExpression ClassExpression GeneratorExpression AsyncFunctionExpression AsyncGeneratorExpression RegularExpressionLiteral TemplateLiteral CallExpression : CoverCallExpressionAndAsyncArrowHead SuperCall ImportCall CallExpression Arguments CallExpression TemplateLiteral NewExpression : new NewExpression MemberExpression : MemberExpression TemplateLiteral new MemberExpression Arguments NewTarget : new . target ImportMeta : import . meta LeftHandSideExpression : OptionalExpression UpdateExpression : LeftHandSideExpression LeftHandSideExpression -- UnaryExpression -- UnaryExpression UnaryExpression : delete UnaryExpression void UnaryExpression typeof UnaryExpression UnaryExpression - UnaryExpression ~ UnaryExpression ! UnaryExpression AwaitExpression ExponentiationExpression : UpdateExpression ** ExponentiationExpression MultiplicativeExpression : MultiplicativeExpression MultiplicativeOperator ExponentiationExpression AdditiveExpression : AdditiveExpression MultiplicativeExpression AdditiveExpression - MultiplicativeExpression ShiftExpression : ShiftExpression << AdditiveExpression ShiftExpression >> AdditiveExpression ShiftExpression >>> AdditiveExpression RelationalExpression : RelationalExpression < ShiftExpression RelationalExpression > ShiftExpression RelationalExpression <= ShiftExpression RelationalExpression >= ShiftExpression RelationalExpression instanceof ShiftExpression RelationalExpression in ShiftExpression PrivateIdentifier in ShiftExpression EqualityExpression : EqualityExpression == RelationalExpression EqualityExpression != RelationalExpression EqualityExpression === RelationalExpression EqualityExpression !== RelationalExpression BitwiseANDExpression : BitwiseANDExpression & EqualityExpression BitwiseXORExpression : BitwiseXORExpression ^ BitwiseANDExpression BitwiseORExpression : BitwiseORExpression | BitwiseXORExpression LogicalANDExpression : LogicalANDExpression && BitwiseORExpression LogicalORExpression : LogicalORExpression || LogicalANDExpression CoalesceExpression : CoalesceExpressionHead ?? BitwiseORExpression ConditionalExpression : ShortCircuitExpression ? AssignmentExpression : AssignmentExpression AssignmentExpression : YieldExpression ArrowFunction AsyncArrowFunction LeftHandSideExpression = AssignmentExpression LeftHandSideExpression AssignmentOperator AssignmentExpression LeftHandSideExpression &&= AssignmentExpression LeftHandSideExpression ||= AssignmentExpression LeftHandSideExpression ??= AssignmentExpression Expression : Expression , AssignmentExpression
  1. Return invalid.

8.6.5 Static Semantics: PropName

The syntax-directed operation PropName takes no arguments and returns a String or empty. It is defined piecewise over the following productions:

PropertyDefinition : IdentifierReference
  1. Return StringValue of IdentifierReference.
 PropertyDefinition : ... AssignmentExpression
  1. Return empty.
 PropertyDefinition : PropertyName : AssignmentExpression
  1. Return PropName of PropertyName.
 LiteralPropertyName : IdentifierName
  1. Return StringValue of IdentifierName.
 LiteralPropertyName : StringLiteral
  1. Return the SV of StringLiteral.
 LiteralPropertyName : NumericLiteral
  1. Let nbr be the NumericValue of NumericLiteral.
  2. Return ! ToString(nbr).
 ComputedPropertyName : [ AssignmentExpression ]
  1. Return empty.
 MethodDefinition : ClassElementName ( UniqueFormalParameters ) { FunctionBody } get ClassElementName ( ) { FunctionBody } set ClassElementName ( PropertySetParameterList ) { FunctionBody }
  1. Return PropName of ClassElementName.
 GeneratorMethod : * ClassElementName ( UniqueFormalParameters ) { GeneratorBody }
  1. Return PropName of ClassElementName.
 AsyncGeneratorMethod : async * ClassElementName ( UniqueFormalParameters ) { AsyncGeneratorBody }
  1. Return PropName of ClassElementName.
 ClassElement : ClassStaticBlock
  1. Return empty.
 ClassElement : ;
  1. Return empty.
 AsyncMethod : async ClassElementName ( UniqueFormalParameters ) { AsyncFunctionBody }
  1. Return PropName of ClassElementName.
 FieldDefinition : ClassElementName Initializeropt
  1. Return PropName of ClassElementName.
 ClassElementName : PrivateIdentifier
  1. Return empty.

9 Executable Code and Execution Contexts

9.1 Environment Records

Environment Record is a specification type used to define the association of Identifiers to specific variables and functions, based upon the lexical nesting structure of ECMAScript code. Usually an Environment Record is associated with some specific syntactic structure of ECMAScript code such as a FunctionDeclaration, a BlockStatement, or a Catch clause of a TryStatement. Each time such code is evaluated, a new Environment Record is created to record the identifier bindings that are created by that code.

Every Environment Record has an [[OuterEnv]] field, which is either null or a reference to an outer Environment Record. This is used to model the logical nesting of Environment Record values. The outer reference of an (inner) Environment Record is a reference to the Environment Record that logically surrounds the inner Environment Record. An outer Environment Record may, of course, have its own outer Environment Record. An Environment Record may serve as the outer environment for multiple inner Environment Records. For example, if a FunctionDeclaration contains two nested FunctionDeclarations then the Environment Records of each of the nested functions will have as their outer Environment Record the Environment Record of the current evaluation of the surrounding function.

Environment Records are purely specification mechanisms and need not correspond to any specific artefact of an ECMAScript implementation. It is impossible for an ECMAScript program to directly access or manipulate such values.

9.1.1 The Environment Record Type Hierarchy

Environment Records can be thought of as existing in a simple object-oriented hierarchy where Environment Record is an abstract class with three concrete subclasses: Declarative Environment Record, Object Environment Record, and Global Environment Record. Function Environment Records and Module Environment Records are subclasses of Declarative Environment Record.

The Environment Record abstract class includes the abstract specification methods defined in Table 16. These abstract methods have distinct concrete algorithms for each of the concrete subclasses.

Table 16: Abstract Methods of Environment Records
Method Purpose
HasBinding(N) Determine if an Environment Record has a binding for the String value N. Return true if it does and false if it does not.
CreateMutableBinding(N, D) Create a new but uninitialized mutable binding in an Environment Record. The String value N is the text of the bound name. If the Boolean argument D is true the binding may be subsequently deleted.
CreateImmutableBinding(N, S) Create a new but uninitialized immutable binding in an Environment Record. The String value N is the text of the bound name. If S is true then attempts to set it after it has been initialized will always throw an exception, regardless of the strict mode setting of operations that reference that binding.
InitializeBinding(N, V) Set the value of an already existing but uninitialized binding in an Environment Record. The String value N is the text of the bound name. V is the value for the binding and is a value of any ECMAScript language type.
SetMutableBinding(N, V, S) Set the value of an already existing mutable binding in an Environment Record. The String value N is the text of the bound name. V is the value for the binding and may be a value of any ECMAScript language type. S is a Boolean flag. If S is true and the binding cannot be set throw a TypeError exception.
GetBindingValue(N, S) Returns the value of an already existing binding from an Environment Record. The String value N is the text of the bound name. S is used to identify references originating in strict mode code or that otherwise require strict mode reference semantics. If S is true and the binding does not exist throw a ReferenceError exception. If the binding exists but is uninitialized a ReferenceError is thrown, regardless of the value of S.
DeleteBinding(N) Delete a binding from an Environment Record. The String value N is the text of the bound name. If a binding for N exists, remove the binding and return true. If the binding exists but cannot be removed return false. If the binding does not exist return true.
HasThisBinding() Determine if an Environment Record establishes a this binding. Return true if it does and false if it does not.
HasSuperBinding() Determine if an Environment Record establishes a super method binding. Return true if it does and false if it does not.
WithBaseObject() If this Environment Record is associated with a with statement, return the with object. Otherwise, return undefined.

9.1.1.1 Declarative Environment Records

Each Declarative Environment Record is associated with an ECMAScript program scope containing variable, constant, let, class, module, import, and/or function declarations. A Declarative Environment Record binds the set of identifiers defined by the declarations contained within its scope.

The behaviour of the concrete specification methods for Declarative Environment Records is defined by the following algorithms.

9.1.1.1.1 HasBinding ( N )

The HasBinding concrete method of a Declarative Environment Record envRec takes argument N (a String) and returns a normal completion containing a Boolean. It determines if the argument identifier is one of the identifiers bound by the record. It performs the following steps when called:

  1. If envRec has a binding for N, return true.
  2. Return false.

9.1.1.1.2 CreateMutableBinding ( N, D )

The CreateMutableBinding concrete method of a Declarative Environment Record envRec takes arguments N (a String) and D (a Boolean) and returns a normal completion containing unused. It creates a new mutable binding for the name N that is uninitialized. A binding must not already exist in this Environment Record for N. If D is true, the new binding is marked as being subject to deletion. It performs the following steps when called:

  1. Assert: envRec does not already have a binding for N.
  2. Create a mutable binding in envRec for N and record that it is uninitialized. If D is true, record that the newly created binding may be deleted by a subsequent DeleteBinding call.
  3. Return unused.

9.1.1.1.3 CreateImmutableBinding ( N, S )

The CreateImmutableBinding concrete method of a Declarative Environment Record envRec takes arguments N (a String) and S (a Boolean) and returns a normal completion containing unused. It creates a new immutable binding for the name N that is uninitialized. A binding must not already exist in this Environment Record for N. If S is true, the new binding is marked as a strict binding. It performs the following steps when called:

  1. Assert: envRec does not already have a binding for N.
  2. Create an immutable binding in envRec for N and record that it is uninitialized. If S is true, record that the newly created binding is a strict binding.
  3. Return unused.

9.1.1.1.4 InitializeBinding ( N, V )

The InitializeBinding concrete method of a Declarative Environment Record envRec takes arguments N (a String) and V (an ECMAScript language value) and returns a normal completion containing unused. It is used to set the bound value of the current binding of the identifier whose name is N to the value V. An uninitialized binding for N must already exist. It performs the following steps when called:

  1. Assert: envRec must have an uninitialized binding for N.
  2. Set the bound value for N in envRec to V.
  3. Record that the binding for N in envRec has been initialized.
  4. Return unused.

9.1.1.1.5 SetMutableBinding ( N, V, S )

The SetMutableBinding concrete method of a Declarative Environment Record envRec takes arguments N (a String), V (an ECMAScript language value), and S (a Boolean) and returns either a normal completion containing unused or a throw completion. It attempts to change the bound value of the current binding of the identifier whose name is N to the value V. A binding for N normally already exists, but in rare cases it may not. If the binding is an immutable binding, a TypeError is thrown if S is true. It performs the following steps when called:

  1. If envRec does not have a binding for N, then
    1. If S is true, throw a ReferenceError exception.
    2. Perform ! envRec.CreateMutableBinding(N, true).
    3. Perform ! envRec.InitializeBinding(N, V).
    4. Return unused.
  2. If the binding for N in envRec is a strict binding, set S to true.
  3. If the binding for N in envRec has not yet been initialized, throw a ReferenceError exception.
  4. Else if the binding for N in envRec is a mutable binding, change its bound value to V.
  5. Else,
    1. Assert: This is an attempt to change the value of an immutable binding.
    2. If S is true, throw a TypeError exception.
  6. Return unused.
 Note

An example of ECMAScript code that results in a missing binding at step 1 is:

function f() { eval("var x; x = (delete x, 0);"); }

9.1.1.1.6 GetBindingValue ( N, S )

The GetBindingValue concrete method of a Declarative Environment Record envRec takes arguments N (a String) and S (a Boolean) and returns either a normal completion containing an ECMAScript language value or a throw completion. It returns the value of its bound identifier whose name is N. If the binding exists but is uninitialized a ReferenceError is thrown, regardless of the value of S. It performs the following steps when called:

  1. Assert: envRec has a binding for N.
  2. If the binding for N in envRec is an uninitialized binding, throw a ReferenceError exception.
  3. Return the value currently bound to N in envRec.

9.1.1.1.7 DeleteBinding ( N )

The DeleteBinding concrete method of a Declarative Environment Record envRec takes argument N (a String) and returns a normal completion containing a Boolean. It can only delete bindings that have been explicitly designated as being subject to deletion. It performs the following steps when called:

  1. Assert: envRec has a binding for N.
  2. If the binding for N in envRec cannot be deleted, return false.
  3. Remove the binding for N from envRec.
  4. Return true.

9.1.1.1.8 HasThisBinding ( )

The HasThisBinding concrete method of a Declarative Environment Record envRec takes no arguments and returns false. It performs the following steps when called:

  1. Return false.
 Note

A regular Declarative Environment Record (i.e., one that is neither a Function Environment Record nor a Module Environment Record) does not provide a this binding.

9.1.1.1.9 HasSuperBinding ( )

The HasSuperBinding concrete method of a Declarative Environment Record envRec takes no arguments and returns false. It performs the following steps when called:

  1. Return false.
 Note

A regular Declarative Environment Record (i.e., one that is neither a Function Environment Record nor a Module Environment Record) does not provide a super binding.

9.1.1.1.10 WithBaseObject ( )

The WithBaseObject concrete method of a Declarative Environment Record envRec takes no arguments and returns undefined. It performs the following steps when called:

  1. Return undefined.

9.1.1.2 Object Environment Records

Each Object Environment Record is associated with an object called its binding object. An Object Environment Record binds the set of string identifier names that directly correspond to the property names of its binding object. Property keys that are not strings in the form of an IdentifierName are not included in the set of bound identifiers. Both own and inherited properties are included in the set regardless of the setting of their [[Enumerable]] attribute. Because properties can be dynamically added and deleted from objects, the set of identifiers bound by an Object Environment Record may potentially change as a side-effect of any operation that adds or deletes properties. Any bindings that are created as a result of such a side-effect are considered to be a mutable binding even if the Writable attribute of the corresponding property is false. Immutable bindings do not exist for Object Environment Records.

Object Environment Records created for with statements (14.11) can provide their binding object as an implicit this value for use in function calls. The capability is controlled by a Boolean [[IsWithEnvironment]] field.

Object Environment Records have the additional state fields listed in Table 17.

Table 17: Additional Fields of Object Environment Records
Field Name Value Meaning
[[BindingObject]] an Object The binding object of this Environment Record.
[[IsWithEnvironment]] a Boolean Indicates whether this Environment Record is created for a with statement.

The behaviour of the concrete specification methods for Object Environment Records is defined by the following algorithms.

9.1.1.2.1 HasBinding ( N )

The HasBinding concrete method of an Object Environment Record envRec takes argument N (a String) and returns either a normal completion containing a Boolean or a throw completion. It determines if its associated binding object has a property whose name is N. It performs the following steps when called:

  1. Let bindingObject be envRec.[[BindingObject]].
  2. Let foundBinding be ? HasProperty(bindingObject, N).
  3. If foundBinding is false, return false.
  4. If envRec.[[IsWithEnvironment]] is false, return true.
  5. Let unscopables be ? Get(bindingObject, @@unscopables).
  6. If unscopables is an Object, then
    1. Let blocked be ToBoolean(? Get(unscopables, N)).
    2. If blocked is true, return false.
  7. Return true.

9.1.1.2.2 CreateMutableBinding ( N, D )

The CreateMutableBinding concrete method of an Object Environment Record envRec takes arguments N (a String) and D (a Boolean) and returns either a normal completion containing unused or a throw completion. It creates in an Environment Record's associated binding object a property whose name is N and initializes it to the value undefined. If D is true, the new property's [[Configurable]] attribute is set to true; otherwise it is set to false. It performs the following steps when called:

  1. Let bindingObject be envRec.[[BindingObject]].
  2. Perform ? DefinePropertyOrThrow(bindingObject, N, PropertyDescriptor { [[Value]]: undefined, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: D }).
  3. Return unused.
 Note

Normally envRec will not have a binding for N but if it does, the semantics of DefinePropertyOrThrow may result in an existing binding being replaced or shadowed or cause an abrupt completion to be returned.

9.1.1.2.3 CreateImmutableBinding ( N, S )

The CreateImmutableBinding concrete method of an Object Environment Record is never used within this specification.

9.1.1.2.4 InitializeBinding ( N, V )

The InitializeBinding concrete method of an Object Environment Record envRec takes arguments N (a String) and V (an ECMAScript language value) and returns either a normal completion containing unused or a throw completion. It is used to set the bound value of the current binding of the identifier whose name is N to the value V. It performs the following steps when called:

  1. Perform ? envRec.SetMutableBinding(N, V, false).
  2. Return unused.
 Note

In this specification, all uses of CreateMutableBinding for Object Environment Records are immediately followed by a call to InitializeBinding for the same name. Hence, this specification does not explicitly track the initialization state of bindings in Object Environment Records.

9.1.1.2.5 SetMutableBinding ( N, V, S )

The SetMutableBinding concrete method of an Object Environment Record envRec takes arguments N (a String), V (an ECMAScript language value), and S (a Boolean) and returns either a normal completion containing unused or a throw completion. It attempts to set the value of the Environment Record's associated binding object's property whose name is N to the value V. A property named N normally already exists but if it does not or is not currently writable, error handling is determined by S. It performs the following steps when called:

  1. Let bindingObject be envRec.[[BindingObject]].
  2. Let stillExists be ? HasProperty(bindingObject, N).
  3. If stillExists is false and S is true, throw a ReferenceError exception.
  4. Perform ? Set(bindingObject, N, V, S).
  5. Return unused.

9.1.1.2.6 GetBindingValue ( N, S )

The GetBindingValue concrete method of an Object Environment Record envRec takes arguments N (a String) and S (a Boolean) and returns either a normal completion containing an ECMAScript language value or a throw completion. It returns the value of its associated binding object's property whose name is N. The property should already exist but if it does not the result depends upon S. It performs the following steps when called:

  1. Let bindingObject be envRec.[[BindingObject]].
  2. Let value be ? HasProperty(bindingObject, N).
  3. If value is false, then
    1. If S is false, return undefined; otherwise throw a ReferenceError exception.
  4. Return ? Get(bindingObject, N).

9.1.1.2.7 DeleteBinding ( N )

The DeleteBinding concrete method of an Object Environment Record envRec takes argument N (a String) and returns either a normal completion containing a Boolean or a throw completion. It can only delete bindings that correspond to properties of the environment object whose [[Configurable]] attribute have the value true. It performs the following steps when called:

  1. Let bindingObject be envRec.[[BindingObject]].
  2. Return ? bindingObject.[[Delete]](N).

9.1.1.2.8 HasThisBinding ( )

The HasThisBinding concrete method of an Object Environment Record envRec takes no arguments and returns false. It performs the following steps when called:

  1. Return false.
 Note

Object Environment Records do not provide a this binding.

9.1.1.2.9 HasSuperBinding ( )

The HasSuperBinding concrete method of an Object Environment Record envRec takes no arguments and returns false. It performs the following steps when called:

  1. Return false.
 Note

Object Environment Records do not provide a super binding.

9.1.1.2.10 WithBaseObject ( )

The WithBaseObject concrete method of an Object Environment Record envRec takes no arguments and returns an Object or undefined. It performs the following steps when called:

  1. If envRec.[[IsWithEnvironment]] is true, return envRec.[[BindingObject]].
  2. Otherwise, return undefined.

9.1.1.3 Function Environment Records

A Function Environment Record is a Declarative Environment Record that is used to represent the top-level scope of a function and, if the function is not an ArrowFunction, provides a this binding. If a function is not an ArrowFunction function and references super, its Function Environment Record also contains the state that is used to perform super method invocations from within the function.

Function Environment Records have the additional state fields listed in Table 18.

Table 18: Additional Fields of Function Environment Records
Field Name Value Meaning
[[ThisValue]] an ECMAScript language value This is the this value used for this invocation of the function.
[[ThisBindingStatus]] lexical, initialized, or uninitialized If the value is lexical, this is an ArrowFunction and does not have a local this value.
[[FunctionObject]] an ECMAScript function object The function object whose invocation caused this Environment Record to be created.
[[NewTarget]] an Object or undefined If this Environment Record was created by the [[Construct]] internal method, [[NewTarget]] is the value of the [[Construct]] newTarget parameter. Otherwise, its value is undefined.

Function Environment Records support all of the Declarative Environment Record methods listed in Table 16 and share the same specifications for all of those methods except for HasThisBinding and HasSuperBinding. In addition, Function Environment Records support the methods listed in Table 19:

Table 19: Additional Methods of Function Environment Records
Method Purpose
BindThisValue(V) Set the [[ThisValue]] and record that it has been initialized.
GetThisBinding() Return the value of this Environment Record's this binding. Throws a ReferenceError if the this binding has not been initialized.
GetSuperBase() Return the object that is the base for super property accesses bound in this Environment Record. The value undefined indicates that such accesses will produce runtime errors.

The behaviour of the additional concrete specification methods for Function Environment Records is defined by the following algorithms:

9.1.1.3.1 BindThisValue ( V )

The BindThisValue concrete method of a Function Environment Record envRec takes argument V (an ECMAScript language value) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

  1. Assert: envRec.[[ThisBindingStatus]] is not lexical.
  2. If envRec.[[ThisBindingStatus]] is initialized, throw a ReferenceError exception.
  3. Set envRec.[[ThisValue]] to V.
  4. Set envRec.[[ThisBindingStatus]] to initialized.
  5. Return V.

9.1.1.3.2 HasThisBinding ( )

The HasThisBinding concrete method of a Function Environment Record envRec takes no arguments and returns a Boolean. It performs the following steps when called:

  1. If envRec.[[ThisBindingStatus]] is lexical, return false; otherwise, return true.

9.1.1.3.3 HasSuperBinding ( )

The HasSuperBinding concrete method of a Function Environment Record envRec takes no arguments and returns a Boolean. It performs the following steps when called:

  1. If envRec.[[ThisBindingStatus]] is lexical, return false.
  2. If envRec.[[FunctionObject]].[[HomeObject]] is undefined, return false; otherwise, return true.

9.1.1.3.4 GetThisBinding ( )

The GetThisBinding concrete method of a Function Environment Record envRec takes no arguments and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

  1. Assert: envRec.[[ThisBindingStatus]] is not lexical.
  2. If envRec.[[ThisBindingStatus]] is uninitialized, throw a ReferenceError exception.
  3. Return envRec.[[ThisValue]].

9.1.1.3.5 GetSuperBase ( )

The GetSuperBase concrete method of a Function Environment Record envRec takes no arguments and returns either a normal completion containing either an Object, null, or undefined, or a throw completion. It performs the following steps when called:

  1. Let home be envRec.[[FunctionObject]].[[HomeObject]].
  2. If home is undefined, return undefined.
  3. Assert: home is an Object.
  4. Return ? home.[[GetPrototypeOf]]().

9.1.1.4 Global Environment Records

A Global Environment Record is used to represent the outer most scope that is shared by all of the ECMAScript Script elements that are processed in a common realm. A Global Environment Record provides the bindings for built-in globals (clause 19), properties of the global object, and for all top-level declarations (8.2.9, 8.2.11) that occur within a Script.

A Global Environment Record is logically a single record but it is specified as a composite encapsulating an Object Environment Record and a Declarative Environment Record. The Object Environment Record has as its base object the global object of the associated Realm Record. This global object is the value returned by the Global Environment Record's GetThisBinding concrete method. The Object Environment Record component of a Global Environment Record contains the bindings for all built-in globals (clause 19) and all bindings introduced by a FunctionDeclaration, GeneratorDeclaration, AsyncFunctionDeclaration, AsyncGeneratorDeclaration, or VariableStatement contained in global code. The bindings for all other ECMAScript declarations in global code are contained in the Declarative Environment Record component of the Global Environment Record.

Properties may be created directly on a global object. Hence, the Object Environment Record component of a Global Environment Record may contain both bindings created explicitly by FunctionDeclaration, GeneratorDeclaration, AsyncFunctionDeclaration, AsyncGeneratorDeclaration, or VariableDeclaration declarations and bindings created implicitly as properties of the global object. In order to identify which bindings were explicitly created using declarations, a Global Environment Record maintains a list of the names bound using its CreateGlobalVarBinding and CreateGlobalFunctionBinding concrete methods.

Global Environment Records have the additional fields listed in Table 20 and the additional methods listed in Table 21.

Table 20: Additional Fields of Global Environment Records
Field Name Value Meaning
[[ObjectRecord]] an Object Environment Record Binding object is the global object. It contains global built-in bindings as well as FunctionDeclaration, GeneratorDeclaration, AsyncFunctionDeclaration, AsyncGeneratorDeclaration, and VariableDeclaration bindings in global code for the associated realm.
[[GlobalThisValue]] an Object The value returned by this in global scope. Hosts may provide any ECMAScript Object value.
[[DeclarativeRecord]] a Declarative Environment Record Contains bindings for all declarations in global code for the associated realm code except for FunctionDeclaration, GeneratorDeclaration, AsyncFunctionDeclaration, AsyncGeneratorDeclaration, and VariableDeclaration bindings.
[[VarNames]] a List of Strings The string names bound by FunctionDeclaration, GeneratorDeclaration, AsyncFunctionDeclaration, AsyncGeneratorDeclaration, and VariableDeclaration declarations in global code for the associated realm.
Table 21: Additional Methods of Global Environment Records
Method Purpose
GetThisBinding() Return the value of this Environment Record's this binding.
HasVarDeclaration (N) Determines if the argument identifier has a binding in this Environment Record that was created using a VariableDeclaration, FunctionDeclaration, GeneratorDeclaration, AsyncFunctionDeclaration, or AsyncGeneratorDeclaration.
HasLexicalDeclaration (N) Determines if the argument identifier has a binding in this Environment Record that was created using a lexical declaration such as a LexicalDeclaration or a ClassDeclaration.
HasRestrictedGlobalProperty (N) Determines if the argument is the name of a global object property that may not be shadowed by a global lexical binding.
CanDeclareGlobalVar (N) Determines if a corresponding CreateGlobalVarBinding call would succeed if called for the same argument N.
CanDeclareGlobalFunction (N) Determines if a corresponding CreateGlobalFunctionBinding call would succeed if called for the same argument N.
CreateGlobalVarBinding(N, D) Used to create and initialize to undefined a global var binding in the [[ObjectRecord]] component of a Global Environment Record. The binding will be a mutable binding. The corresponding global object property will have attribute values appropriate for a var. The String value N is the bound name. If D is true, the binding may be deleted. Logically equivalent to CreateMutableBinding followed by a SetMutableBinding but it allows var declarations to receive special treatment.
CreateGlobalFunctionBinding(N, V, D) Create and initialize a global function binding in the [[ObjectRecord]] component of a Global Environment Record. The binding will be a mutable binding. The corresponding global object property will have attribute values appropriate for a function. The String value N is the bound name. V is the initialization value. If the Boolean argument D is true, the binding may be deleted. Logically equivalent to CreateMutableBinding followed by a SetMutableBinding but it allows function declarations to receive special treatment.

The behaviour of the concrete specification methods for Global Environment Records is defined by the following algorithms.

9.1.1.4.1 HasBinding ( N )

The HasBinding concrete method of a Global Environment Record envRec takes argument N (a String) and returns either a normal completion containing a Boolean or a throw completion. It determines if the argument identifier is one of the identifiers bound by the record. It performs the following steps when called:

  1. Let DclRec be envRec.[[DeclarativeRecord]].
  2. If ! DclRec.HasBinding(N) is true, return true.
  3. Let ObjRec be envRec.[[ObjectRecord]].
  4. Return ? ObjRec.HasBinding(N).

9.1.1.4.2 CreateMutableBinding ( N, D )

The CreateMutableBinding concrete method of a Global Environment Record envRec takes arguments N (a String) and D (a Boolean) and returns either a normal completion containing unused or a throw completion. It creates a new mutable binding for the name N that is uninitialized. The binding is created in the associated DeclarativeRecord. A binding for N must not already exist in the DeclarativeRecord. If D is true, the new binding is marked as being subject to deletion. It performs the following steps when called:

  1. Let DclRec be envRec.[[DeclarativeRecord]].
  2. If ! DclRec.HasBinding(N) is true, throw a TypeError exception.
  3. Return ! DclRec.CreateMutableBinding(N, D).

9.1.1.4.3 CreateImmutableBinding ( N, S )

The CreateImmutableBinding concrete method of a Global Environment Record envRec takes arguments N (a String) and S (a Boolean) and returns either a normal completion containing unused or a throw completion. It creates a new immutable binding for the name N that is uninitialized. A binding must not already exist in this Environment Record for N. If S is true, the new binding is marked as a strict binding. It performs the following steps when called:

  1. Let DclRec be envRec.[[DeclarativeRecord]].
  2. If ! DclRec.HasBinding(N) is true, throw a TypeError exception.
  3. Return ! DclRec.CreateImmutableBinding(N, S).

9.1.1.4.4 InitializeBinding ( N, V )

The InitializeBinding concrete method of a Global Environment Record envRec takes arguments N (a String) and V (an ECMAScript language value) and returns either a normal completion containing unused or a throw completion. It is used to set the bound value of the current binding of the identifier whose name is N to the value V. An uninitialized binding for N must already exist. It performs the following steps when called:

  1. Let DclRec be envRec.[[DeclarativeRecord]].
  2. If ! DclRec.HasBinding(N) is true, then
    1. Return ! DclRec.InitializeBinding(N, V).
  3. Assert: If the binding exists, it must be in the Object Environment Record.
  4. Let ObjRec be envRec.[[ObjectRecord]].
  5. Return ? ObjRec.InitializeBinding(N, V).

9.1.1.4.5 SetMutableBinding ( N, V, S )

The SetMutableBinding concrete method of a Global Environment Record envRec takes arguments N (a String), V (an ECMAScript language value), and S (a Boolean) and returns either a normal completion containing unused or a throw completion. It attempts to change the bound value of the current binding of the identifier whose name is N to the value V. If the binding is an immutable binding and S is true, a TypeError is thrown. A property named N normally already exists but if it does not or is not currently writable, error handling is determined by S. It performs the following steps when called:

  1. Let DclRec be envRec.[[DeclarativeRecord]].
  2. If ! DclRec.HasBinding(N) is true, then
    1. Return ? DclRec.SetMutableBinding(N, V, S).
  3. Let ObjRec be envRec.[[ObjectRecord]].
  4. Return ? ObjRec.SetMutableBinding(N, V, S).

9.1.1.4.6 GetBindingValue ( N, S )

The GetBindingValue concrete method of a Global Environment Record envRec takes arguments N (a String) and S (a Boolean) and returns either a normal completion containing an ECMAScript language value or a throw completion. It returns the value of its bound identifier whose name is N. If the binding is an uninitialized binding throw a ReferenceError exception. A property named N normally already exists but if it does not or is not currently writable, error handling is determined by S. It performs the following steps when called:

  1. Let DclRec be envRec.[[DeclarativeRecord]].
  2. If ! DclRec.HasBinding(N) is true, then
    1. Return ? DclRec.GetBindingValue(N, S).
  3. Let ObjRec be envRec.[[ObjectRecord]].
  4. Return ? ObjRec.GetBindingValue(N, S).

9.1.1.4.7 DeleteBinding ( N )

The DeleteBinding concrete method of a Global Environment Record envRec takes argument N (a String) and returns either a normal completion containing a Boolean or a throw completion. It can only delete bindings that have been explicitly designated as being subject to deletion. It performs the following steps when called:

  1. Let DclRec be envRec.[[DeclarativeRecord]].
  2. If ! DclRec.HasBinding(N) is true, then
    1. Return ! DclRec.DeleteBinding(N).
  3. Let ObjRec be envRec.[[ObjectRecord]].
  4. Let globalObject be ObjRec.[[BindingObject]].
  5. Let existingProp be ? HasOwnProperty(globalObject, N).
  6. If existingProp is true, then
    1. Let status be ? ObjRec.DeleteBinding(N).
    2. If status is true and envRec.[[VarNames]] contains N, then
      1. Remove N from envRec.[[VarNames]].
    3. Return status.
  7. Return true.

9.1.1.4.8 HasThisBinding ( )

The HasThisBinding concrete method of a Global Environment Record envRec takes no arguments and returns true. It performs the following steps when called:

  1. Return true.
 Note

Global Environment Records always provide a this binding.

9.1.1.4.9 HasSuperBinding ( )

The HasSuperBinding concrete method of a Global Environment Record envRec takes no arguments and returns false. It performs the following steps when called:

  1. Return false.
 Note

Global Environment Records do not provide a super binding.

9.1.1.4.10 WithBaseObject ( )

The WithBaseObject concrete method of a Global Environment Record envRec takes no arguments and returns undefined. It performs the following steps when called:

  1. Return undefined.

9.1.1.4.11 GetThisBinding ( )

The GetThisBinding concrete method of a Global Environment Record envRec takes no arguments and returns a normal completion containing an Object. It performs the following steps when called:

  1. Return envRec.[[GlobalThisValue]].

9.1.1.4.12 HasVarDeclaration ( N )

The HasVarDeclaration concrete method of a Global Environment Record envRec takes argument N (a String) and returns a Boolean. It determines if the argument identifier has a binding in this record that was created using a VariableStatement or a FunctionDeclaration. It performs the following steps when called:

  1. Let varDeclaredNames be envRec.[[VarNames]].
  2. If varDeclaredNames contains N, return true.
  3. Return false.

9.1.1.4.13 HasLexicalDeclaration ( N )

The HasLexicalDeclaration concrete method of a Global Environment Record envRec takes argument N (a String) and returns a Boolean. It determines if the argument identifier has a binding in this record that was created using a lexical declaration such as a LexicalDeclaration or a ClassDeclaration. It performs the following steps when called:

  1. Let DclRec be envRec.[[DeclarativeRecord]].
  2. Return ! DclRec.HasBinding(N).

9.1.1.4.14 HasRestrictedGlobalProperty ( N )

The HasRestrictedGlobalProperty concrete method of a Global Environment Record envRec takes argument N (a String) and returns either a normal completion containing a Boolean or a throw completion. It determines if the argument identifier is the name of a property of the global object that must not be shadowed by a global lexical binding. It performs the following steps when called:

  1. Let ObjRec be envRec.[[ObjectRecord]].
  2. Let globalObject be ObjRec.[[BindingObject]].
  3. Let existingProp be ? globalObject.[[GetOwnProperty]](N).
  4. If existingProp is undefined, return false.
  5. If existingProp.[[Configurable]] is true, return false.
  6. Return true.
 Note

Properties may exist upon a global object that were directly created rather than being declared using a var or function declaration. A global lexical binding may not be created that has the same name as a non-configurable property of the global object. The global property "undefined" is an example of such a property.

9.1.1.4.15 CanDeclareGlobalVar ( N )

The CanDeclareGlobalVar concrete method of a Global Environment Record envRec takes argument N (a String) and returns either a normal completion containing a Boolean or a throw completion. It determines if a corresponding CreateGlobalVarBinding call would succeed if called for the same argument N. Redundant var declarations and var declarations for pre-existing global object properties are allowed. It performs the following steps when called:

  1. Let ObjRec be envRec.[[ObjectRecord]].
  2. Let globalObject be ObjRec.[[BindingObject]].
  3. Let hasProperty be ? HasOwnProperty(globalObject, N).
  4. If hasProperty is true, return true.
  5. Return ? IsExtensible(globalObject).

9.1.1.4.16 CanDeclareGlobalFunction ( N )

The CanDeclareGlobalFunction concrete method of a Global Environment Record envRec takes argument N (a String) and returns either a normal completion containing a Boolean or a throw completion. It determines if a corresponding CreateGlobalFunctionBinding call would succeed if called for the same argument N. It performs the following steps when called:

  1. Let ObjRec be envRec.[[ObjectRecord]].
  2. Let globalObject be ObjRec.[[BindingObject]].
  3. Let existingProp be ? globalObject.[[GetOwnProperty]](N).
  4. If existingProp is undefined, return ? IsExtensible(globalObject).
  5. If existingProp.[[Configurable]] is true, return true.
  6. If IsDataDescriptor(existingProp) is true and existingProp has attribute values { [[Writable]]: true, [[Enumerable]]: true }, return true.
  7. Return false.

9.1.1.4.17 CreateGlobalVarBinding ( N, D )

The CreateGlobalVarBinding concrete method of a Global Environment Record envRec takes arguments N (a String) and D (a Boolean) and returns either a normal completion containing unused or a throw completion. It creates and initializes a mutable binding in the associated Object Environment Record and records the bound name in the associated [[VarNames]] List. If a binding already exists, it is reused and assumed to be initialized. It performs the following steps when called:

  1. Let ObjRec be envRec.[[ObjectRecord]].
  2. Let globalObject be ObjRec.[[BindingObject]].
  3. Let hasProperty be ? HasOwnProperty(globalObject, N).
  4. Let extensible be ? IsExtensible(globalObject).
  5. If hasProperty is false and extensible is true, then
    1. Perform ? ObjRec.CreateMutableBinding(N, D).
    2. Perform ? ObjRec.InitializeBinding(N, undefined).
  6. If envRec.[[VarNames]] does not contain N, then
    1. Append N to envRec.[[VarNames]].
  7. Return unused.

9.1.1.4.18 CreateGlobalFunctionBinding ( N, V, D )

The CreateGlobalFunctionBinding concrete method of a Global Environment Record envRec takes arguments N (a String), V (an ECMAScript language value), and D (a Boolean) and returns either a normal completion containing unused or a throw completion. It creates and initializes a mutable binding in the associated Object Environment Record and records the bound name in the associated [[VarNames]] List. If a binding already exists, it is replaced. It performs the following steps when called:

  1. Let ObjRec be envRec.[[ObjectRecord]].
  2. Let globalObject be ObjRec.[[BindingObject]].
  3. Let existingProp be ? globalObject.[[GetOwnProperty]](N).
  4. If existingProp is undefined or existingProp.[[Configurable]] is true, then
    1. Let desc be the PropertyDescriptor { [[Value]]: V, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: D }.
  5. Else,
    1. Let desc be the PropertyDescriptor { [[Value]]: V }.
  6. Perform ? DefinePropertyOrThrow(globalObject, N, desc).
  7. Perform ? Set(globalObject, N, V, false).
  8. If envRec.[[VarNames]] does not contain N, then
    1. Append N to envRec.[[VarNames]].
  9. Return unused.
 Note

Global function declarations are always represented as own properties of the global object. If possible, an existing own property is reconfigured to have a standard set of attribute values. Step 7 is equivalent to what calling the InitializeBinding concrete method would do and if globalObject is a Proxy will produce the same sequence of Proxy trap calls.

9.1.1.5 Module Environment Records

A Module Environment Record is a Declarative Environment Record that is used to represent the outer scope of an ECMAScript Module. In additional to normal mutable and immutable bindings, Module Environment Records also provide immutable import bindings which are bindings that provide indirect access to a target binding that exists in another Environment Record.

Module Environment Records support all of the Declarative Environment Record methods listed in Table 16 and share the same specifications for all of those methods except for GetBindingValue, DeleteBinding, HasThisBinding and GetThisBinding. In addition, Module Environment Records support the methods listed in Table 22:

Table 22: Additional Methods of Module Environment Records
Method Purpose
CreateImportBinding(N, M, N2) Create an immutable indirect binding in a Module Environment Record. The String value N is the text of the bound name. M is a Module Record, and N2 is a binding that exists in M's Module Environment Record.
GetThisBinding() Return the value of this Environment Record's this binding.

The behaviour of the additional concrete specification methods for Module Environment Records are defined by the following algorithms:

9.1.1.5.1 GetBindingValue ( N, S )

The GetBindingValue concrete method of a Module Environment Record envRec takes arguments N (a String) and S (a Boolean) and returns either a normal completion containing an ECMAScript language value or a throw completion. It returns the value of its bound identifier whose name is N. However, if the binding is an indirect binding the value of the target binding is returned. If the binding exists but is uninitialized a ReferenceError is thrown. It performs the following steps when called:

  1. Assert: S is true.
  2. Assert: envRec has a binding for N.
  3. If the binding for N is an indirect binding, then
    1. Let M and N2 be the indirection values provided when this binding for N was created.
    2. Let targetEnv be M.[[Environment]].
    3. If targetEnv is empty, throw a ReferenceError exception.
    4. Return ? targetEnv.GetBindingValue(N2, true).
  4. If the binding for N in envRec is an uninitialized binding, throw a ReferenceError exception.
  5. Return the value currently bound to N in envRec.
 Note

S will always be true because a Module is always strict mode code.

9.1.1.5.2 DeleteBinding ( N )

The DeleteBinding concrete method of a Module Environment Record is never used within this specification.

Note

Module Environment Records are only used within strict code and an early error rule prevents the delete operator, in strict code, from being applied to a Reference Record that would resolve to a Module Environment Record binding. See 13.5.1.1.

9.1.1.5.3 HasThisBinding ( )

The HasThisBinding concrete method of a Module Environment Record envRec takes no arguments and returns true. It performs the following steps when called:

  1. Return true.
 Note

Module Environment Records always provide a this binding.

9.1.1.5.4 GetThisBinding ( )

The GetThisBinding concrete method of a Module Environment Record envRec takes no arguments and returns a normal completion containing undefined. It performs the following steps when called:

  1. Return undefined.

9.1.1.5.5 CreateImportBinding ( N, M, N2 )

The CreateImportBinding concrete method of a Module Environment Record envRec takes arguments N (a String), M (a Module Record), and N2 (a String) and returns unused. It creates a new initialized immutable indirect binding for the name N. A binding must not already exist in this Environment Record for N. N2 is the name of a binding that exists in M's Module Environment Record. Accesses to the value of the new binding will indirectly access the bound value of the target binding. It performs the following steps when called:

  1. Assert: envRec does not already have a binding for N.
  2. Assert: When M.[[Environment]] is instantiated, it will have a direct binding for N2.
  3. Create an immutable indirect binding in envRec for N that references M and N2 as its target binding and record that the binding is initialized.
  4. Return unused.

9.1.2 Environment Record Operations

The following abstract operations are used in this specification to operate upon Environment Records:

9.1.2.1 GetIdentifierReference ( env, name, strict )

The abstract operation GetIdentifierReference takes arguments env (an Environment Record or null), name (a String), and strict (a Boolean) and returns either a normal completion containing a Reference Record or a throw completion. It performs the following steps when called:

  1. If env is null, then
    1. Return the Reference Record { [[Base]]: unresolvable, [[ReferencedName]]: name, [[Strict]]: strict, [[ThisValue]]: empty }.
  2. Let exists be ? env.HasBinding(name).
  3. If exists is true, then
    1. Return the Reference Record { [[Base]]: env, [[ReferencedName]]: name, [[Strict]]: strict, [[ThisValue]]: empty }.
  4. Else,
    1. Let outer be env.[[OuterEnv]].
    2. Return ? GetIdentifierReference(outer, name, strict).

9.1.2.2 NewDeclarativeEnvironment ( E )

The abstract operation NewDeclarativeEnvironment takes argument E (an Environment Record or null) and returns a Declarative Environment Record. It performs the following steps when called:

  1. Let env be a new Declarative Environment Record containing no bindings.
  2. Set env.[[OuterEnv]] to E.
  3. Return env.

9.1.2.3 NewObjectEnvironment ( O, W, E )

The abstract operation NewObjectEnvironment takes arguments O (an Object), W (a Boolean), and E (an Environment Record or null) and returns an Object Environment Record. It performs the following steps when called:

  1. Let env be a new Object Environment Record.
  2. Set env.[[BindingObject]] to O.
  3. Set env.[[IsWithEnvironment]] to W.
  4. Set env.[[OuterEnv]] to E.
  5. Return env.

9.1.2.4 NewFunctionEnvironment ( F, newTarget )

The abstract operation NewFunctionEnvironment takes arguments F (an ECMAScript function) and newTarget (an Object or undefined) and returns a Function Environment Record. It performs the following steps when called:

  1. Let env be a new Function Environment Record containing no bindings.
  2. Set env.[[FunctionObject]] to F.
  3. If F.[[ThisMode]] is lexical, set env.[[ThisBindingStatus]] to lexical.
  4. Else, set env.[[ThisBindingStatus]] to uninitialized.
  5. Set env.[[NewTarget]] to newTarget.
  6. Set env.[[OuterEnv]] to F.[[Environment]].
  7. Return env.

9.1.2.5 NewGlobalEnvironment ( G, thisValue )

The abstract operation NewGlobalEnvironment takes arguments G (an Object) and thisValue (an Object) and returns a Global Environment Record. It performs the following steps when called:

  1. Let objRec be NewObjectEnvironment(G, false, null).
  2. Let dclRec be NewDeclarativeEnvironment(null).
  3. Let env be a new Global Environment Record.
  4. Set env.[[ObjectRecord]] to objRec.
  5. Set env.[[GlobalThisValue]] to thisValue.
  6. Set env.[[DeclarativeRecord]] to dclRec.
  7. Set env.[[VarNames]] to a new empty List.
  8. Set env.[[OuterEnv]] to null.
  9. Return env.

9.1.2.6 NewModuleEnvironment ( E )

The abstract operation NewModuleEnvironment takes argument E (an Environment Record) and returns a Module Environment Record. It performs the following steps when called:

  1. Let env be a new Module Environment Record containing no bindings.
  2. Set env.[[OuterEnv]] to E.
  3. Return env.

9.2 PrivateEnvironment Records

A PrivateEnvironment Record is a specification mechanism used to track Private Names based upon the lexical nesting structure of ClassDeclarations and ClassExpressions in ECMAScript code. They are similar to, but distinct from, Environment Records. Each PrivateEnvironment Record is associated with a ClassDeclaration or ClassExpression. Each time such a class is evaluated, a new PrivateEnvironment Record is created to record the Private Names declared by that class.

Each PrivateEnvironment Record has the fields defined in Table 23.

Table 23: PrivateEnvironment Record Fields
Field Name Value Type Meaning
[[OuterPrivateEnvironment]] a PrivateEnvironment Record or null The PrivateEnvironment Record of the nearest containing class. null if the class with which this PrivateEnvironment Record is associated is not contained in any other class.
[[Names]] a List of Private Names The Private Names declared by this class.

9.2.1 PrivateEnvironment Record Operations

The following abstract operations are used in this specification to operate upon PrivateEnvironment Records:

9.2.1.1 NewPrivateEnvironment ( outerPrivEnv )

The abstract operation NewPrivateEnvironment takes argument outerPrivEnv (a PrivateEnvironment Record or null) and returns a PrivateEnvironment Record. It performs the following steps when called:

  1. Let names be a new empty List.
  2. Return the PrivateEnvironment Record { [[OuterPrivateEnvironment]]: outerPrivEnv, [[Names]]: names }.

9.2.1.2 ResolvePrivateIdentifier ( privEnv, identifier )

The abstract operation ResolvePrivateIdentifier takes arguments privEnv (a PrivateEnvironment Record) and identifier (a String) and returns a Private Name. It performs the following steps when called:

  1. Let names be privEnv.[[Names]].
  2. For each Private Name pn of names, do
    1. If pn.[[Description]] is identifier, then
      1. Return pn.
  3. Let outerPrivEnv be privEnv.[[OuterPrivateEnvironment]].
  4. Assert: outerPrivEnv is not null.
  5. Return ResolvePrivateIdentifier(outerPrivEnv, identifier).

9.3 Realms

Before it is evaluated, all ECMAScript code must be associated with a realm. Conceptually, a realm consists of a set of intrinsic objects, an ECMAScript global environment, all of the ECMAScript code that is loaded within the scope of that global environment, and other associated state and resources.

A realm is represented in this specification as a Realm Record with the fields specified in Table 24:

Table 24: Realm Record Fields
Field Name Value Meaning
[[Intrinsics]] a Record whose field names are intrinsic keys and whose values are objects The intrinsic values used by code associated with this realm
[[GlobalObject]] an Object or undefined The global object for this realm
[[GlobalEnv]] a Global Environment Record The global environment for this realm
[[TemplateMap]] a List of Records with fields [[Site]] (a TemplateLiteral Parse Node) and [[Array]] (an Array)

Template objects are canonicalized separately for each realm using its Realm Record's [[TemplateMap]]. Each [[Site]] value is a Parse Node that is a TemplateLiteral. The associated [[Array]] value is the corresponding template object that is passed to a tag function.

Note 1
Once a Parse Node becomes unreachable, the corresponding [[Array]] is also unreachable, and it would be unobservable if an implementation removed the pair from the [[TemplateMap]] list.
[[LoadedModules]] a List of Records with fields [[Specifier]] (a String) and [[Module]] (a Module Record)

A map from the specifier strings imported by this realm to the resolved Module Record. The list does not contain two different Records with the same [[Specifier]].

Note 2
As mentioned in HostLoadImportedModule (16.2.1.8 Note 1), [[LoadedModules]] in Realm Records is only used when running an import() expression in a context where there is no active script or module.
[[HostDefined]] anything (default value is undefined) Field reserved for use by hosts that need to associate additional information with a Realm Record.

9.3.1 CreateRealm ( )

The abstract operation CreateRealm takes no arguments and returns a Realm Record. It performs the following steps when called:

  1. Let realmRec be a new Realm Record.
  2. Perform CreateIntrinsics(realmRec).
  3. Set realmRec.[[GlobalObject]] to undefined.
  4. Set realmRec.[[GlobalEnv]] to undefined.
  5. Set realmRec.[[TemplateMap]] to a new empty List.
  6. Return realmRec.

9.3.2 CreateIntrinsics ( realmRec )

The abstract operation CreateIntrinsics takes argument realmRec (a Realm Record) and returns unused. It performs the following steps when called:

  1. Set realmRec.[[Intrinsics]] to a new Record.
  2. Set fields of realmRec.[[Intrinsics]] with the values listed in Table 6. The field names are the names listed in column one of the table. The value of each field is a new object value fully and recursively populated with property values as defined by the specification of each object in clauses 19 through 28. All object property values are newly created object values. All values that are built-in function objects are created by performing CreateBuiltinFunction(steps, length, name, slots, realmRec, prototype) where steps is the definition of that function provided by this specification, name is the initial value of the function's "name" property, length is the initial value of the function's "length" property, slots is a list of the names, if any, of the function's specified internal slots, and prototype is the specified value of the function's [[Prototype]] internal slot. The creation of the intrinsics and their properties must be ordered to avoid any dependencies upon objects that have not yet been created.
  3. Perform AddRestrictedFunctionProperties(realmRec.[[Intrinsics]].[[%Function.prototype%]], realmRec).
  4. Return unused.

9.3.3 SetRealmGlobalObject ( realmRec, globalObj, thisValue )

The abstract operation SetRealmGlobalObject takes arguments realmRec (a Realm Record), globalObj (an Object or undefined), and thisValue (an Object or undefined) and returns unused. It performs the following steps when called:

  1. If globalObj is undefined, then
    1. Let intrinsics be realmRec.[[Intrinsics]].
    2. Set globalObj to OrdinaryObjectCreate(intrinsics.[[%Object.prototype%]]).
  2. Assert: globalObj is an Object.
  3. If thisValue is undefined, set thisValue to globalObj.
  4. Set realmRec.[[GlobalObject]] to globalObj.
  5. Let newGlobalEnv be NewGlobalEnvironment(globalObj, thisValue).
  6. Set realmRec.[[GlobalEnv]] to newGlobalEnv.
  7. Return unused.

9.3.4 SetDefaultGlobalBindings ( realmRec )

The abstract operation SetDefaultGlobalBindings takes argument realmRec (a Realm Record) and returns either a normal completion containing an Object or a throw completion. It performs the following steps when called:

  1. Let global be realmRec.[[GlobalObject]].
  2. For each property of the Global Object specified in clause 19, do
    1. Let name be the String value of the property name.
    2. Let desc be the fully populated data Property Descriptor for the property, containing the specified attributes for the property. For properties listed in 19.2, 19.3, or 19.4 the value of the [[Value]] attribute is the corresponding intrinsic object from realmRec.
    3. Perform ? DefinePropertyOrThrow(global, name, desc).
  3. Return global.

9.4 Execution Contexts

An execution context is a specification device that is used to track the runtime evaluation of code by an ECMAScript implementation. At any point in time, there is at most one execution context per agent that is actually executing code. This is known as the agent's running execution context. All references to the running execution context in this specification denote the running execution context of the surrounding agent.

The execution context stack is used to track execution contexts. The running execution context is always the top element of this stack. A new execution context is created whenever control is transferred from the executable code associated with the currently running execution context to executable code that is not associated with that execution context. The newly created execution context is pushed onto the stack and becomes the running execution context.

An execution context contains whatever implementation specific state is necessary to track the execution progress of its associated code. Each execution context has at least the state components listed in Table 25.

Table 25: State Components for All Execution Contexts
Component Purpose
code evaluation state Any state needed to perform, suspend, and resume evaluation of the code associated with this execution context.
Function If this execution context is evaluating the code of a function object, then the value of this component is that function object. If the context is evaluating the code of a Script or Module, the value is null.
Realm The Realm Record from which associated code accesses ECMAScript resources.
ScriptOrModule The Module Record or Script Record from which associated code originates. If there is no originating script or module, as is the case for the original execution context created in InitializeHostDefinedRealm, the value is null.

Evaluation of code by the running execution context may be suspended at various points defined within this specification. Once the running execution context has been suspended a different execution context may become the running execution context and commence evaluating its code. At some later time a suspended execution context may again become the running execution context and continue evaluating its code at the point where it had previously been suspended. Transition of the running execution context status among execution contexts usually occurs in stack-like last-in/first-out manner. However, some ECMAScript features require non-LIFO transitions of the running execution context.

The value of the Realm component of the running execution context is also called the current Realm Record. The value of the Function component of the running execution context is also called the active function object.

ECMAScript code execution contexts have the additional state components listed in Table 26.

Table 26: Additional State Components for ECMAScript Code Execution Contexts
Component Purpose
LexicalEnvironment Identifies the Environment Record used to resolve identifier references made by code within this execution context.
VariableEnvironment Identifies the Environment Record that holds bindings created by VariableStatements within this execution context.
PrivateEnvironment Identifies the PrivateEnvironment Record that holds Private Names created by ClassElements in the nearest containing class. null if there is no containing class.

The LexicalEnvironment and VariableEnvironment components of an execution context are always Environment Records.

Execution contexts representing the evaluation of Generators have the additional state components listed in Table 27.

Table 27: Additional State Components for Generator Execution Contexts
Component Purpose
Generator The Generator that this execution context is evaluating.

In most situations only the running execution context (the top of the execution context stack) is directly manipulated by algorithms within this specification. Hence when the terms “LexicalEnvironment”, and “VariableEnvironment” are used without qualification they are in reference to those components of the running execution context.

An execution context is purely a specification mechanism and need not correspond to any particular artefact of an ECMAScript implementation. It is impossible for ECMAScript code to directly access or observe an execution context.

9.4.1 GetActiveScriptOrModule ( )

The abstract operation GetActiveScriptOrModule takes no arguments and returns a Script Record, a Module Record, or null. It is used to determine the running script or module, based on the running execution context. It performs the following steps when called:

  1. If the execution context stack is empty, return null.
  2. Let ec be the topmost execution context on the execution context stack whose ScriptOrModule component is not null.
  3. If no such execution context exists, return null. Otherwise, return ec's ScriptOrModule.

9.4.2 ResolveBinding ( name [ , env ] )

The abstract operation ResolveBinding takes argument name (a String) and optional argument env (an Environment Record or undefined) and returns either a normal completion containing a Reference Record or a throw completion. It is used to determine the binding of name. env can be used to explicitly provide the Environment Record that is to be searched for the binding. It performs the following steps when called:

  1. If env is not present or env is undefined, then
    1. Set env to the running execution context's LexicalEnvironment.
  2. Assert: env is an Environment Record.
  3. If the source text matched by the syntactic production that is being evaluated is contained in strict mode code, let strict be true; else let strict be false.
  4. Return ? GetIdentifierReference(env, name, strict).
 Note

The result of ResolveBinding is always a Reference Record whose [[ReferencedName]] field is name.

9.4.3 GetThisEnvironment ( )

The abstract operation GetThisEnvironment takes no arguments and returns an Environment Record. It finds the Environment Record that currently supplies the binding of the keyword this. It performs the following steps when called:

  1. Let env be the running execution context's LexicalEnvironment.
  2. Repeat,
    1. Let exists be env.HasThisBinding().
    2. If exists is true, return env.
    3. Let outer be env.[[OuterEnv]].
    4. Assert: outer is not null.
    5. Set env to outer.
 Note

The loop in step 2 will always terminate because the list of environments always ends with the global environment which has a this binding.

9.4.4 ResolveThisBinding ( )

The abstract operation ResolveThisBinding takes no arguments and returns either a normal completion containing an ECMAScript language value or a throw completion. It determines the binding of the keyword this using the LexicalEnvironment of the running execution context. It performs the following steps when called:

  1. Let envRec be GetThisEnvironment().
  2. Return ? envRec.GetThisBinding().

9.4.5 GetNewTarget ( )

The abstract operation GetNewTarget takes no arguments and returns an Object or undefined. It determines the NewTarget value using the LexicalEnvironment of the running execution context. It performs the following steps when called:

  1. Let envRec be GetThisEnvironment().
  2. Assert: envRec has a [[NewTarget]] field.
  3. Return envRec.[[NewTarget]].

9.4.6 GetGlobalObject ( )

The abstract operation GetGlobalObject takes no arguments and returns an Object. It returns the global object used by the currently running execution context. It performs the following steps when called:

  1. Let currentRealm be the current Realm Record.
  2. Return currentRealm.[[GlobalObject]].

9.5 Jobs and Host Operations to Enqueue Jobs

A Job is an Abstract Closure with no parameters that initiates an ECMAScript computation when no other ECMAScript computation is currently in progress.

Jobs are scheduled for execution by ECMAScript host environments. This specification describes the host hook HostEnqueuePromiseJob to schedule one kind of job; hosts may define additional abstract operations which schedule jobs. Such operations accept a Job Abstract Closure as the parameter and schedule it to be performed at some future time. Their implementations must conform to the following requirements:

Note 1
Host environments are not required to treat Jobs uniformly with respect to scheduling. For example, web browsers and Node.js treat Promise-handling Jobs as a higher priority than other work; future features may add Jobs that are not treated at such a high priority.

At any particular time, scriptOrModule (a Script Record, a Module Record, or null) is the active script or module if all of the following conditions are true:

At any particular time, an execution is prepared to evaluate ECMAScript code if all of the following conditions are true:

Note 2

Host environments may prepare an execution to evaluate code by pushing execution contexts onto the execution context stack. The specific steps are implementation-defined.

The specific choice of Realm is up to the host environment. This initial execution context and Realm is only in use before any callback function is invoked. When a callback function related to a Job, like a Promise handler, is invoked, the invocation pushes its own execution context and Realm.

Particular kinds of Jobs have additional conformance requirements.

9.5.1 JobCallback Records

A JobCallback Record is a Record value used to store a function object and a host-defined value. Function objects that are invoked via a Job enqueued by the host may have additional host-defined context. To propagate the state, Job Abstract Closures should not capture and call function objects directly. Instead, use HostMakeJobCallback and HostCallJobCallback.

Note

The WHATWG HTML specification (https://html.spec.whatwg.org/), for example, uses the host-defined value to propagate the incumbent settings object for Promise callbacks.

JobCallback Records have the fields listed in Table 28.

Table 28: JobCallback Record Fields
Field Name Value Meaning
[[Callback]] a function object The function to invoke when the Job is invoked.
[[HostDefined]] anything (default value is empty) Field reserved for use by hosts.

9.5.2 HostMakeJobCallback ( callback )

The host-defined abstract operation HostMakeJobCallback takes argument callback (a function object) and returns a JobCallback Record.

An implementation of HostMakeJobCallback must conform to the following requirements:

The default implementation of HostMakeJobCallback performs the following steps when called:

  1. Return the JobCallback Record { [[Callback]]: callback, [[HostDefined]]: empty }.

ECMAScript hosts that are not web browsers must use the default implementation of HostMakeJobCallback.

Note

This is called at the time that the callback is passed to the function that is responsible for its being eventually scheduled and run. For example, promise.then(thenAction) calls MakeJobCallback on thenAction at the time of invoking Promise.prototype.then, not at the time of scheduling the reaction Job.

9.5.3 HostCallJobCallback ( jobCallback, V, argumentsList )

The host-defined abstract operation HostCallJobCallback takes arguments jobCallback (a JobCallback Record), V (an ECMAScript language value), and argumentsList (a List of ECMAScript language values) and returns either a normal completion containing an ECMAScript language value or a throw completion.

An implementation of HostCallJobCallback must conform to the following requirements:

  • It must perform and return the result of Call(jobCallback.[[Callback]], V, argumentsList).
Note

This requirement means that hosts cannot change the [[Call]] behaviour of function objects defined in this specification.

The default implementation of HostCallJobCallback performs the following steps when called:

  1. Assert: IsCallable(jobCallback.[[Callback]]) is true.
  2. Return ? Call(jobCallback.[[Callback]], V, argumentsList).

ECMAScript hosts that are not web browsers must use the default implementation of HostCallJobCallback.

9.5.4 HostEnqueuePromiseJob ( job, realm )

The host-defined abstract operation HostEnqueuePromiseJob takes arguments job (a Job Abstract Closure) and realm (a Realm Record or null) and returns unused. It schedules job to be performed at some future time. The Abstract Closures used with this algorithm are intended to be related to the handling of Promises, or otherwise, to be scheduled with equal priority to Promise handling operations.

An implementation of HostEnqueuePromiseJob must conform to the requirements in 9.5 as well as the following:

Note

The realm for Jobs returned by NewPromiseResolveThenableJob is usually the result of calling GetFunctionRealm on the then function object. The realm for Jobs returned by NewPromiseReactionJob is usually the result of calling GetFunctionRealm on the handler if the handler is not undefined. If the handler is undefined, realm is null. For both kinds of Jobs, when GetFunctionRealm completes abnormally (i.e. called on a revoked Proxy), realm is the current Realm at the time of the GetFunctionRealm call. When the realm is null, no user ECMAScript code will be evaluated and no new ECMAScript objects (e.g. Error objects) will be created. The WHATWG HTML specification (https://html.spec.whatwg.org/), for example, uses realm to check for the ability to run script and for the entry concept.

9.6 InitializeHostDefinedRealm ( )

The abstract operation InitializeHostDefinedRealm takes no arguments and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

  1. Let realm be CreateRealm().
  2. Let newContext be a new execution context.
  3. Set the Function of newContext to null.
  4. Set the Realm of newContext to realm.
  5. Set the ScriptOrModule of newContext to null.
  6. Push newContext onto the execution context stack; newContext is now the running execution context.
  7. If the host requires use of an exotic object to serve as realm's global object, let global be such an object created in a host-defined manner. Otherwise, let global be undefined, indicating that an ordinary object should be created as the global object.
  8. If the host requires that the this binding in realm's global scope return an object other than the global object, let thisValue be such an object created in a host-defined manner. Otherwise, let thisValue be undefined, indicating that realm's global this binding should be the global object.
  9. Perform SetRealmGlobalObject(realm, global, thisValue).
  10. Let globalObj be ? SetDefaultGlobalBindings(realm).
  11. Create any host-defined global object properties on globalObj.
  12. Return unused.

9.7 Agents

An agent comprises a set of ECMAScript execution contexts, an execution context stack, a running execution context, an Agent Record, and an executing thread. Except for the executing thread, the constituents of an agent belong exclusively to that agent.

An agent's executing thread executes a job on the agent's execution contexts independently of other agents, except that an executing thread may be used as the executing thread by multiple agents, provided none of the agents sharing the thread have an Agent Record whose [[CanBlock]] field is true.

Note 1

Some web browsers share a single executing thread across multiple unrelated tabs of a browser window, for example.

While an agent's executing thread executes jobs, the agent is the surrounding agent for the code in those jobs. The code uses the surrounding agent to access the specification-level execution objects held within the agent: the running execution context, the execution context stack, and the Agent Record's fields.

An agent signifier is a globally-unique opaque value used to identify an Agent.

Table 29: Agent Record Fields
Field Name Value Meaning
[[LittleEndian]] a Boolean The default value computed for the isLittleEndian parameter when it is needed by the algorithms GetValueFromBuffer and SetValueInBuffer. The choice is implementation-defined and should be the alternative that is most efficient for the implementation. Once the value has been observed it cannot change.
[[CanBlock]] a Boolean Determines whether the agent can block or not.
[[Signifier]] an agent signifier Uniquely identifies the agent within its agent cluster.
[[IsLockFree1]] a Boolean true if atomic operations on one-byte values are lock-free, false otherwise.
[[IsLockFree2]] a Boolean true if atomic operations on two-byte values are lock-free, false otherwise.
[[IsLockFree8]] a Boolean true if atomic operations on eight-byte values are lock-free, false otherwise.
[[CandidateExecution]] a candidate execution Record See the memory model.
[[KeptAlive]] a List of either Objects or Symbols Initially a new empty List, representing the list of objects and/or symbols to be kept alive until the end of the current Job

Once the values of [[Signifier]], [[IsLockFree1]], and [[IsLockFree2]] have been observed by any agent in the agent cluster they cannot change.

Note 2

The values of [[IsLockFree1]] and [[IsLockFree2]] are not necessarily determined by the hardware, but may also reflect implementation choices that can vary over time and between ECMAScript implementations.

There is no [[IsLockFree4]] field: 4-byte atomic operations are always lock-free.

In practice, if an atomic operation is implemented with any type of lock the operation is not lock-free. Lock-free does not imply wait-free: there is no upper bound on how many machine steps may be required to complete a lock-free atomic operation.

That an atomic access of size n is lock-free does not imply anything about the (perceived) atomicity of non-atomic accesses of size n, specifically, non-atomic accesses may still be performed as a sequence of several separate memory accesses. See ReadSharedMemory and WriteSharedMemory for details.

 Note 3

An agent is a specification mechanism and need not correspond to any particular artefact of an ECMAScript implementation.

9.7.1 AgentSignifier ( )

The abstract operation AgentSignifier takes no arguments and returns an agent signifier. It performs the following steps when called:

  1. Let AR be the Agent Record of the surrounding agent.
  2. Return AR.[[Signifier]].

9.7.2 AgentCanSuspend ( )

The abstract operation AgentCanSuspend takes no arguments and returns a Boolean. It performs the following steps when called:

  1. Let AR be the Agent Record of the surrounding agent.
  2. Return AR.[[CanBlock]].
 Note

In some environments it may not be reasonable for a given agent to suspend. For example, in a web browser environment, it may be reasonable to disallow suspending a document's main event handling thread, while still allowing workers' event handling threads to suspend.

9.8 Agent Clusters

An agent cluster is a maximal set of agents that can communicate by operating on shared memory.

Note 1

Programs within different agents may share memory by unspecified means. At a minimum, the backing memory for SharedArrayBuffers can be shared among the agents in the cluster.

There may be agents that can communicate by message passing that cannot share memory; they are never in the same agent cluster.

Every agent belongs to exactly one agent cluster.

Note 2

The agents in a cluster need not all be alive at some particular point in time. If agent A creates another agent B, after which A terminates and B creates agent C, the three agents are in the same cluster if A could share some memory with B and B could share some memory with C.

All agents within a cluster must have the same value for the [[LittleEndian]] field in their respective Agent Records.

Note 3

If different agents within an agent cluster have different values of [[LittleEndian]] it becomes hard to use shared memory for multi-byte data.

All agents within a cluster must have the same values for the [[IsLockFree1]] field in their respective Agent Records; similarly for the [[IsLockFree2]] field.

All agents within a cluster must have different values for the [[Signifier]] field in their respective Agent Records.

An embedding may deactivate (stop forward progress) or activate (resume forward progress) an agent without the agent's knowledge or cooperation. If the embedding does so, it must not leave some agents in the cluster active while other agents in the cluster are deactivated indefinitely.

Note 4

The purpose of the preceding restriction is to avoid a situation where an agent deadlocks or starves because another agent has been deactivated. For example, if an HTML shared worker that has a lifetime independent of documents in any windows were allowed to share memory with the dedicated worker of such an independent document, and the document and its dedicated worker were to be deactivated while the dedicated worker holds a lock (say, the document is pushed into its window's history), and the shared worker then tries to acquire the lock, then the shared worker will be blocked until the dedicated worker is activated again, if ever. Meanwhile other workers trying to access the shared worker from other windows will starve.

The implication of the restriction is that it will not be possible to share memory between agents that don't belong to the same suspend/wake collective within the embedding.

An embedding may terminate an agent without any of the agent's cluster's other agents' prior knowledge or cooperation. If an agent is terminated not by programmatic action of its own or of another agent in the cluster but by forces external to the cluster, then the embedding must choose one of two strategies: Either terminate all the agents in the cluster, or provide reliable APIs that allow the agents in the cluster to coordinate so that at least one remaining member of the cluster will be able to detect the termination, with the termination data containing enough information to identify the agent that was terminated.

Note 5

Examples of that type of termination are: operating systems or users terminating agents that are running in separate processes; the embedding itself terminating an agent that is running in-process with the other agents when per-agent resource accounting indicates that the agent is runaway.

Prior to any evaluation of any ECMAScript code by any agent in a cluster, the [[CandidateExecution]] field of the Agent Record for all agents in the cluster is set to the initial candidate execution. The initial candidate execution is an empty candidate execution whose [[EventsRecords]] field is a List containing, for each agent, an Agent Events Record whose [[AgentSignifier]] field is that agent's agent signifier, and whose [[EventList]] and [[AgentSynchronizesWith]] fields are empty Lists.

Note 6

All agents in an agent cluster share the same candidate execution in its Agent Record's [[CandidateExecution]] field. The candidate execution is a specification mechanism used by the memory model.

 Note 7

An agent cluster is a specification mechanism and need not correspond to any particular artefact of an ECMAScript implementation.

9.9 Forward Progress

For an agent to make forward progress is for it to perform an evaluation step according to this specification.

An agent becomes blocked when its running execution context waits synchronously and indefinitely for an external event. Only agents whose Agent Record's [[CanBlock]] field is true can become blocked in this sense. An unblocked agent is one that is not blocked.

Implementations must ensure that:

  • every unblocked agent with a dedicated executing thread eventually makes forward progress
  • in a set of agents that share an executing thread, one agent eventually makes forward progress
  • an agent does not cause another agent to become blocked except via explicit APIs that provide blocking.
Note

This, along with the liveness guarantee in the memory model, ensures that all SeqCst writes eventually become observable to all agents.

9.10 Processing Model of WeakRef and FinalizationRegistry Targets

9.10.1 Objectives

This specification does not make any guarantees that any object or symbol will be garbage collected. Objects or symbols which are not live may be released after long periods of time, or never at all. For this reason, this specification uses the term "may" when describing behaviour triggered by garbage collection.

The semantics of WeakRefs and FinalizationRegistrys is based on two operations which happen at particular points in time:

  • When WeakRef.prototype.deref is called, the referent (if undefined is not returned) is kept alive so that subsequent, synchronous accesses also return the same value. This list is reset when synchronous work is done using the ClearKeptObjects abstract operation.
  • When an object or symbol which is registered with a FinalizationRegistry becomes unreachable, a call of the FinalizationRegistry's cleanup callback may eventually be made, after synchronous ECMAScript execution completes. The FinalizationRegistry cleanup is performed with the CleanupFinalizationRegistry abstract operation.

Neither of these actions (ClearKeptObjects or CleanupFinalizationRegistry) may interrupt synchronous ECMAScript execution. Because hosts may assemble longer, synchronous ECMAScript execution runs, this specification defers the scheduling of ClearKeptObjects and CleanupFinalizationRegistry to the host environment.

Some ECMAScript implementations include garbage collector implementations which run in the background, including when ECMAScript is idle. Letting the host environment schedule CleanupFinalizationRegistry allows it to resume ECMAScript execution in order to run finalizer work, which may free up held values, reducing overall memory usage.

9.10.2 Liveness

For some set of objects and/or symbols S a hypothetical WeakRef-oblivious execution with respect to S is an execution whereby the abstract operation WeakRefDeref of a WeakRef whose referent is an element of S always returns undefined.

Note 1
WeakRef-obliviousness, together with liveness, capture two notions. One, that a WeakRef itself does not keep its referent alive. Two, that cycles in liveness does not imply that a value is live. To be concrete, if determining v's liveness depends on determining the liveness of a WeakRef referent, r, r's liveness cannot assume v's liveness, which would be circular reasoning.
Note 2
WeakRef-obliviousness is defined on sets of objects or symbols instead of individual values to account for cycles. If it were defined on individual values, then a WeakRef referent in a cycle will be considered live even though its identity is only observed via other WeakRef referents in the cycle.
Note 3
Colloquially, we say that an individual object or symbol is live if every set containing it is live.

At any point during evaluation, a set of objects and/or symbols S is considered live if either of the following conditions is met:

  • Any element in S is included in any agent's [[KeptAlive]] List.
  • There exists a valid future hypothetical WeakRef-oblivious execution with respect to S that observes the identity of any value in S.
Note 4
The second condition above intends to capture the intuition that a value is live if its identity is observable via non-WeakRef means. A value's identity may be observed by observing a strict equality comparison or observing the value being used as key in a Map.
Note 5

Presence of an object or a symbol in a field, an internal slot, or a property does not imply that the value is live. For example if the value in question is never passed back to the program, then it cannot be observed.

This is the case for keys in a WeakMap, members of a WeakSet, as well as the [[WeakRefTarget]] and [[UnregisterToken]] fields of a FinalizationRegistry Cell record.

The above definition implies that, if a key in a WeakMap is not live, then its corresponding value is not necessarily live either.

 Note 6
Liveness is the lower bound for guaranteeing which WeakRefs engines must not empty. Liveness as defined here is undecidable. In practice, engines use conservative approximations such as reachability. There is expected to be significant implementation leeway.

9.10.3 Execution

At any time, if a set of objects and/or symbols S is not live, an ECMAScript implementation may perform the following steps atomically:

  1. For each element value of S, do
    1. For each WeakRef ref such that ref.[[WeakRefTarget]] is value, do
      1. Set ref.[[WeakRefTarget]] to empty.
    2. For each FinalizationRegistry fg such that fg.[[Cells]] contains a Record cell such that cell.[[WeakRefTarget]] is value, do
      1. Set cell.[[WeakRefTarget]] to empty.
      2. Optionally, perform HostEnqueueFinalizationRegistryCleanupJob(fg).
    3. For each WeakMap map such that map.[[WeakMapData]] contains a Record r such that r.[[Key]] is value, do
      1. Set r.[[Key]] to empty.
      2. Set r.[[Value]] to empty.
    4. For each WeakSet set such that set.[[WeakSetData]] contains value, do
      1. Replace the element of set.[[WeakSetData]] whose value is value with an element whose value is empty.
 Note 1

Together with the definition of liveness, this clause prescribes optimizations that an implementation may apply regarding WeakRefs.

It is possible to access an object without observing its identity. Optimizations such as dead variable elimination and scalar replacement on properties of non-escaping objects whose identity is not observed are allowed. These optimizations are thus allowed to observably empty WeakRefs that point to such objects.

On the other hand, if an object's identity is observable, and that object is in the [[WeakRefTarget]] internal slot of a WeakRef, optimizations such as rematerialization that observably empty the WeakRef are prohibited.

Because calling HostEnqueueFinalizationRegistryCleanupJob is optional, registered objects in a FinalizationRegistry do not necessarily hold that FinalizationRegistry live. Implementations may omit FinalizationRegistry callbacks for any reason, e.g., if the FinalizationRegistry itself becomes dead, or if the application is shutting down.

 Note 2

Implementations are not obligated to empty WeakRefs for maximal sets of non-live objects or symbols.

If an implementation chooses a non-live set S in which to empty WeakRefs, this definition requires that it empties WeakRefs for all values in S simultaneously. In other words, it is not conformant for an implementation to empty a WeakRef pointing to a value v without emptying out other WeakRefs that, if not emptied, could result in an execution that observes the value of v.

9.10.4 Host Hooks

9.10.4.1 HostEnqueueFinalizationRegistryCleanupJob ( finalizationRegistry )

The host-defined abstract operation HostEnqueueFinalizationRegistryCleanupJob takes argument finalizationRegistry (a FinalizationRegistry) and returns unused.

Let cleanupJob be a new Job Abstract Closure with no parameters that captures finalizationRegistry and performs the following steps when called:

  1. Let cleanupResult be Completion(CleanupFinalizationRegistry(finalizationRegistry)).
  2. If cleanupResult is an abrupt completion, perform any host-defined steps for reporting the error.
  3. Return unused.

An implementation of HostEnqueueFinalizationRegistryCleanupJob schedules cleanupJob to be performed at some future time, if possible. It must also conform to the requirements in 9.5.

9.11 ClearKeptObjects ( )

The abstract operation ClearKeptObjects takes no arguments and returns unused. ECMAScript implementations are expected to call ClearKeptObjects when a synchronous sequence of ECMAScript executions completes. It performs the following steps when called:

  1. Let agentRecord be the surrounding agent's Agent Record.
  2. Set agentRecord.[[KeptAlive]] to a new empty List.
  3. Return unused.

9.12 AddToKeptObjects ( value )

The abstract operation AddToKeptObjects takes argument value (an Object or a Symbol) and returns unused. It performs the following steps when called:

  1. Let agentRecord be the surrounding agent's Agent Record.
  2. Append value to agentRecord.[[KeptAlive]].
  3. Return unused.
 Note
When the abstract operation AddToKeptObjects is called with a target object or symbol, it adds the target to a list that will point strongly at the target until ClearKeptObjects is called.

9.13 CleanupFinalizationRegistry ( finalizationRegistry )

The abstract operation CleanupFinalizationRegistry takes argument finalizationRegistry (a FinalizationRegistry) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

  1. Assert: finalizationRegistry has [[Cells]] and [[CleanupCallback]] internal slots.
  2. Let callback be finalizationRegistry.[[CleanupCallback]].
  3. While finalizationRegistry.[[Cells]] contains a Record cell such that cell.[[WeakRefTarget]] is empty, an implementation may perform the following steps:
    1. Choose any such cell.
    2. Remove cell from finalizationRegistry.[[Cells]].
    3. Perform ? HostCallJobCallback(callback, undefined, « cell.[[HeldValue]] »).
  4. Return unused.

9.14 CanBeHeldWeakly ( v )

The abstract operation CanBeHeldWeakly takes argument v (an ECMAScript language value) and returns a Boolean. It returns true if and only if v is suitable for use as a weak reference. Only values that are suitable for use as a weak reference may be a key of a WeakMap, an element of a WeakSet, the target of a WeakRef, or one of the targets of a FinalizationRegistry. It performs the following steps when called:

  1. If v is an Object, return true.
  2. If v is a Symbol and KeyForSymbol(v) is undefined, return true.
  3. Return false.
 Note

A language value without language identity can be manifested without prior reference and is unsuitable for use as a weak reference. A Symbol value produced by Symbol.for, unlike other Symbol values, does not have language identity and is unsuitable for use as a weak reference. Well-known symbols are likely to never be collected, but are nonetheless treated as suitable for use as a weak reference because they are limited in number and therefore manageable by a variety of implementation approaches. However, any value associated to a well-known symbol in a live WeakMap is unlikely to be collected and could “leak” memory resources in implementations.

10 Ordinary and Exotic Objects Behaviours

10.1 Ordinary Object Internal Methods and Internal Slots

All ordinary objects have an internal slot called [[Prototype]]. The value of this internal slot is either null or an object and is used for implementing inheritance. Assume a property named P is missing from an ordinary object O but exists on its [[Prototype]] object. If P refers to a data property on the [[Prototype]] object, O inherits it for get access, making it behave as if P was a property of O. If P refers to a writable data property on the [[Prototype]] object, set access of P on O creates a new data property named P on O. If P refers to a non-writable data property on the [[Prototype]] object, set access of P on O fails. If P refers to an accessor property on the [[Prototype]] object, the accessor is inherited by O for both get access and set access.

Every ordinary object has a Boolean-valued [[Extensible]] internal slot which is used to fulfill the extensibility-related internal method invariants specified in 6.1.7.3. Namely, once the value of an object's [[Extensible]] internal slot has been set to false, it is no longer possible to add properties to the object, to modify the value of the object's [[Prototype]] internal slot, or to subsequently change the value of [[Extensible]] to true.

In the following algorithm descriptions, assume O is an ordinary object, P is a property key value, V is any ECMAScript language value, and Desc is a Property Descriptor record.

Each ordinary object internal method delegates to a similarly-named abstract operation. If such an abstract operation depends on another internal method, then the internal method is invoked on O rather than calling the similarly-named abstract operation directly. These semantics ensure that exotic objects have their overridden internal methods invoked when ordinary object internal methods are applied to them.

10.1.1 [[GetPrototypeOf]] ( )

The [[GetPrototypeOf]] internal method of an ordinary object O takes no arguments and returns a normal completion containing either an Object or null. It performs the following steps when called:

  1. Return OrdinaryGetPrototypeOf(O).

10.1.1.1 OrdinaryGetPrototypeOf ( O )

The abstract operation OrdinaryGetPrototypeOf takes argument O (an Object) and returns an Object or null. It performs the following steps when called:

  1. Return O.[[Prototype]].

10.1.2 [[SetPrototypeOf]] ( V )

The [[SetPrototypeOf]] internal method of an ordinary object O takes argument V (an Object or null) and returns a normal completion containing a Boolean. It performs the following steps when called:

  1. Return OrdinarySetPrototypeOf(O, V).

10.1.2.1 OrdinarySetPrototypeOf ( O, V )

The abstract operation OrdinarySetPrototypeOf takes arguments O (an Object) and V (an Object or null) and returns a Boolean. It performs the following steps when called:

  1. Let current be O.[[Prototype]].
  2. If SameValue(V, current) is true, return true.
  3. Let extensible be O.[[Extensible]].
  4. If extensible is false, return false.
  5. Let p be V.
  6. Let done be false.
  7. Repeat, while done is false,
    1. If p is null, set done to true.
    2. Else if SameValue(p, O) is true, return false.
    3. Else,
      1. If p.[[GetPrototypeOf]] is not the ordinary object internal method defined in 10.1.1, set done to true.
      2. Else, set p to p.[[Prototype]].
  8. Set O.[[Prototype]] to V.
  9. Return true.
 Note

The loop in step 7 guarantees that there will be no circularities in any prototype chain that only includes objects that use the ordinary object definitions for [[GetPrototypeOf]] and [[SetPrototypeOf]].

10.1.3 [[IsExtensible]] ( )

The [[IsExtensible]] internal method of an ordinary object O takes no arguments and returns a normal completion containing a Boolean. It performs the following steps when called:

  1. Return OrdinaryIsExtensible(O).

10.1.3.1 OrdinaryIsExtensible ( O )

The abstract operation OrdinaryIsExtensible takes argument O (an Object) and returns a Boolean. It performs the following steps when called:

  1. Return O.[[Extensible]].

10.1.4 [[PreventExtensions]] ( )

The [[PreventExtensions]] internal method of an ordinary object O takes no arguments and returns a normal completion containing true. It performs the following steps when called:

  1. Return OrdinaryPreventExtensions(O).

10.1.4.1 OrdinaryPreventExtensions ( O )

The abstract operation OrdinaryPreventExtensions takes argument O (an Object) and returns true. It performs the following steps when called:

  1. Set O.[[Extensible]] to false.
  2. Return true.

10.1.5 [[GetOwnProperty]] ( P )

The [[GetOwnProperty]] internal method of an ordinary object O takes argument P (a property key) and returns a normal completion containing either a Property Descriptor or undefined. It performs the following steps when called:

  1. Return OrdinaryGetOwnProperty(O, P).

10.1.5.1 OrdinaryGetOwnProperty ( O, P )

The abstract operation OrdinaryGetOwnProperty takes arguments O (an Object) and P (a property key) and returns a Property Descriptor or undefined. It performs the following steps when called:

  1. If O does not have an own property with key P, return undefined.
  2. Let D be a newly created Property Descriptor with no fields.
  3. Let X be O's own property whose key is P.
  4. If X is a data property, then
    1. Set D.[[Value]] to the value of X's [[Value]] attribute.
    2. Set D.[[Writable]] to the value of X's [[Writable]] attribute.
  5. Else,
    1. Assert: X is an accessor property.
    2. Set D.[[Get]] to the value of X's [[Get]] attribute.
    3. Set D.[[Set]] to the value of X's [[Set]] attribute.
  6. Set D.[[Enumerable]] to the value of X's [[Enumerable]] attribute.
  7. Set D.[[Configurable]] to the value of X's [[Configurable]] attribute.
  8. Return D.

10.1.6 [[DefineOwnProperty]] ( P, Desc )

The [[DefineOwnProperty]] internal method of an ordinary object O takes arguments P (a property key) and Desc (a Property Descriptor) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

  1. Return ? OrdinaryDefineOwnProperty(O, P, Desc).

10.1.6.1 OrdinaryDefineOwnProperty ( O, P, Desc )

The abstract operation OrdinaryDefineOwnProperty takes arguments O (an Object), P (a property key), and Desc (a Property Descriptor) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

  1. Let current be ? O.[[GetOwnProperty]](P).
  2. Let extensible be ? IsExtensible(O).
  3. Return ValidateAndApplyPropertyDescriptor(O, P, extensible, Desc, current).

10.1.6.2 IsCompatiblePropertyDescriptor ( Extensible, Desc, Current )

The abstract operation IsCompatiblePropertyDescriptor takes arguments Extensible (a Boolean), Desc (a Property Descriptor), and Current (a Property Descriptor) and returns a Boolean. It performs the following steps when called:

  1. Return ValidateAndApplyPropertyDescriptor(undefined, "", Extensible, Desc, Current).

10.1.6.3 ValidateAndApplyPropertyDescriptor ( O, P, extensible, Desc, current )

The abstract operation ValidateAndApplyPropertyDescriptor takes arguments O (an Object or undefined), P (a property key), extensible (a Boolean), Desc (a Property Descriptor), and current (a Property Descriptor or undefined) and returns a Boolean. It returns true if and only if Desc can be applied as the property of an object with specified extensibility and current property current while upholding invariants. When such application is possible and O is not undefined, it is performed for the property named P (which is created if necessary). It performs the following steps when called:

  1. Assert: IsPropertyKey(P) is true.
  2. If current is undefined, then
    1. If extensible is false, return false.
    2. If O is undefined, return true.
    3. If IsAccessorDescriptor(Desc) is true, then
      1. Create an own accessor property named P of object O whose [[Get]], [[Set]], [[Enumerable]], and [[Configurable]] attributes are set to the value of the corresponding field in Desc if Desc has that field, or to the attribute's default value otherwise.
    4. Else,
      1. Create an own data property named P of object O whose [[Value]], [[Writable]], [[Enumerable]], and [[Configurable]] attributes are set to the value of the corresponding field in Desc if Desc has that field, or to the attribute's default value otherwise.
    5. Return true.
  3. Assert: current is a fully populated Property Descriptor.
  4. If Desc does not have any fields, return true.
  5. If current.[[Configurable]] is false, then
    1. If Desc has a [[Configurable]] field and Desc.[[Configurable]] is true, return false.
    2. If Desc has an [[Enumerable]] field and SameValue(Desc.[[Enumerable]], current.[[Enumerable]]) is false, return false.
    3. If IsGenericDescriptor(Desc) is false and SameValue(IsAccessorDescriptor(Desc), IsAccessorDescriptor(current)) is false, return false.
    4. If IsAccessorDescriptor(current) is true, then
      1. If Desc has a [[Get]] field and SameValue(Desc.[[Get]], current.[[Get]]) is false, return false.
      2. If Desc has a [[Set]] field and SameValue(Desc.[[Set]], current.[[Set]]) is false, return false.
    5. Else if current.[[Writable]] is false, then
      1. If Desc has a [[Writable]] field and Desc.[[Writable]] is true, return false.
      2. If Desc has a [[Value]] field and SameValue(Desc.[[Value]], current.[[Value]]) is false, return false.
  6. If O is not undefined, then
    1. If IsDataDescriptor(current) is true and IsAccessorDescriptor(Desc) is true, then
      1. If Desc has a [[Configurable]] field, let configurable be Desc.[[Configurable]]; else let configurable be current.[[Configurable]].
      2. If Desc has a [[Enumerable]] field, let enumerable be Desc.[[Enumerable]]; else let enumerable be current.[[Enumerable]].
      3. Replace the property named P of object O with an accessor property whose [[Configurable]] and [[Enumerable]] attributes are set to configurable and enumerable, respectively, and whose [[Get]] and [[Set]] attributes are set to the value of the corresponding field in Desc if Desc has that field, or to the attribute's default value otherwise.
    2. Else if IsAccessorDescriptor(current) is true and IsDataDescriptor(Desc) is true, then
      1. If Desc has a [[Configurable]] field, let configurable be Desc.[[Configurable]]; else let configurable be current.[[Configurable]].
      2. If Desc has a [[Enumerable]] field, let enumerable be Desc.[[Enumerable]]; else let enumerable be current.[[Enumerable]].
      3. Replace the property named P of object O with a data property whose [[Configurable]] and [[Enumerable]] attributes are set to configurable and enumerable, respectively, and whose [[Value]] and [[Writable]] attributes are set to the value of the corresponding field in Desc if Desc has that field, or to the attribute's default value otherwise.
    3. Else,
      1. For each field of Desc, set the corresponding attribute of the property named P of object O to the value of the field.
  7. Return true.

10.1.7 [[HasProperty]] ( P )

The [[HasProperty]] internal method of an ordinary object O takes argument P (a property key) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

  1. Return ? OrdinaryHasProperty(O, P).

10.1.7.1 OrdinaryHasProperty ( O, P )

The abstract operation OrdinaryHasProperty takes arguments O (an Object) and P (a property key) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

  1. Let hasOwn be ? O.[[GetOwnProperty]](P).
  2. If hasOwn is not undefined, return true.
  3. Let parent be ? O.[[GetPrototypeOf]]().
  4. If parent is not null, then
    1. Return ? parent.[[HasProperty]](P).
  5. Return false.

10.1.8 [[Get]] ( P, Receiver )

The [[Get]] internal method of an ordinary object O takes arguments P (a property key) and Receiver (an ECMAScript language value) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

  1. Return ? OrdinaryGet(O, P, Receiver).

10.1.8.1 OrdinaryGet ( O, P, Receiver )

The abstract operation OrdinaryGet takes arguments O (an Object), P (a property key), and Receiver (an ECMAScript language value) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

  1. Let desc be ? O.[[GetOwnProperty]](P).
  2. If desc is undefined, then
    1. Let parent be ? O.[[GetPrototypeOf]]().
    2. If parent is null, return undefined.
    3. Return ? parent.[[Get]](P, Receiver).
  3. If IsDataDescriptor(desc) is true, return desc.[[Value]].
  4. Assert: IsAccessorDescriptor(desc) is true.
  5. Let getter be desc.[[Get]].
  6. If getter is undefined, return undefined.
  7. Return ? Call(getter, Receiver).

10.1.9 [[Set]] ( P, V, Receiver )

The [[Set]] internal method of an ordinary object O takes arguments P (a property key), V (an ECMAScript language value), and Receiver (an ECMAScript language value) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

  1. Return ? OrdinarySet(O, P, V, Receiver).

10.1.9.1 OrdinarySet ( O, P, V, Receiver )

The abstract operation OrdinarySet takes arguments O (an Object), P (a property key), V (an ECMAScript language value), and Receiver (an ECMAScript language value) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

  1. Let ownDesc be ? O.[[GetOwnProperty]](P).
  2. Return ? OrdinarySetWithOwnDescriptor(O, P, V, Receiver, ownDesc).

10.1.9.2 OrdinarySetWithOwnDescriptor ( O, P, V, Receiver, ownDesc )

The abstract operation OrdinarySetWithOwnDescriptor takes arguments O (an Object), P (a property key), V (an ECMAScript language value), Receiver (an ECMAScript language value), and ownDesc (a Property Descriptor or undefined) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

  1. If ownDesc is undefined, then
    1. Let parent be ? O.[[GetPrototypeOf]]().
    2. If parent is not null, then
      1. Return ? parent.[[Set]](P, V, Receiver).
    3. Else,
      1. Set ownDesc to the PropertyDescriptor { [[Value]]: undefined, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true }.
  2. If IsDataDescriptor(ownDesc) is true, then
    1. If ownDesc.[[Writable]] is false, return false.
    2. If Receiver is not an Object, return false.
    3. Let existingDescriptor be ? Receiver.[[GetOwnProperty]](P).
    4. If existingDescriptor is not undefined, then
      1. If IsAccessorDescriptor(existingDescriptor) is true, return false.
      2. If existingDescriptor.[[Writable]] is false, return false.
      3. Let valueDesc be the PropertyDescriptor { [[Value]]: V }.
      4. Return ? Receiver.[[DefineOwnProperty]](P, valueDesc).
    5. Else,
      1. Assert: Receiver does not currently have a property P.
      2. Return ? CreateDataProperty(Receiver, P, V).
  3. Assert: IsAccessorDescriptor(ownDesc) is true.
  4. Let setter be ownDesc.[[Set]].
  5. If setter is undefined, return false.
  6. Perform ? Call(setter, Receiver, « V »).
  7. Return true.

10.1.10 [[Delete]] ( P )

The [[Delete]] internal method of an ordinary object O takes argument P (a property key) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

  1. Return ? OrdinaryDelete(O, P).

10.1.10.1 OrdinaryDelete ( O, P )

The abstract operation OrdinaryDelete takes arguments O (an Object) and P (a property key) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

  1. Let desc be ? O.[[GetOwnProperty]](P).
  2. If desc is undefined, return true.
  3. If desc.[[Configurable]] is true, then
    1. Remove the own property with name P from O.
    2. Return true.
  4. Return false.

10.1.11 [[OwnPropertyKeys]] ( )

The [[OwnPropertyKeys]] internal method of an ordinary object O takes no arguments and returns a normal completion containing a List of property keys. It performs the following steps when called:

  1. Return OrdinaryOwnPropertyKeys(O).

10.1.11.1 OrdinaryOwnPropertyKeys ( O )

The abstract operation OrdinaryOwnPropertyKeys takes argument O (an Object) and returns a List of property keys. It performs the following steps when called:

  1. Let keys be a new empty List.
  2. For each own property key P of O such that P is an array index, in ascending numeric index order, do
    1. Append P to keys.
  3. For each own property key P of O such that P is a String and P is not an array index, in ascending chronological order of property creation, do
    1. Append P to keys.
  4. For each own property key P of O such that P is a Symbol, in ascending chronological order of property creation, do
    1. Append P to keys.
  5. Return keys.

10.1.12 OrdinaryObjectCreate ( proto [ , additionalInternalSlotsList ] )

The abstract operation OrdinaryObjectCreate takes argument proto (an Object or null) and optional argument additionalInternalSlotsList (a List of names of internal slots) and returns an Object. It is used to specify the runtime creation of new ordinary objects. additionalInternalSlotsList contains the names of additional internal slots that must be defined as part of the object, beyond [[Prototype]] and [[Extensible]]. If additionalInternalSlotsList is not provided, a new empty List is used. It performs the following steps when called:

  1. Let internalSlotsList be « [[Prototype]], [[Extensible]] ».
  2. If additionalInternalSlotsList is present, set internalSlotsList to the list-concatenation of internalSlotsList and additionalInternalSlotsList.
  3. Let O be MakeBasicObject(internalSlotsList).
  4. Set O.[[Prototype]] to proto.
  5. Return O.
 Note

Although OrdinaryObjectCreate does little more than call MakeBasicObject, its use communicates the intention to create an ordinary object, and not an exotic one. Thus, within this specification, it is not called by any algorithm that subsequently modifies the internal methods of the object in ways that would make the result non-ordinary. Operations that create exotic objects invoke MakeBasicObject directly.

10.1.13 OrdinaryCreateFromConstructor ( constructor, intrinsicDefaultProto [ , internalSlotsList ] )

The abstract operation OrdinaryCreateFromConstructor takes arguments constructor (a constructor) and intrinsicDefaultProto (a String) and optional argument internalSlotsList (a List of names of internal slots) and returns either a normal completion containing an Object or a throw completion. It creates an ordinary object whose [[Prototype]] value is retrieved from a constructor's "prototype" property, if it exists. Otherwise the intrinsic named by intrinsicDefaultProto is used for [[Prototype]]. internalSlotsList contains the names of additional internal slots that must be defined as part of the object. If internalSlotsList is not provided, a new empty List is used. It performs the following steps when called:

  1. Assert: intrinsicDefaultProto is this specification's name of an intrinsic object. The corresponding object must be an intrinsic that is intended to be used as the [[Prototype]] value of an object.
  2. Let proto be ? GetPrototypeFromConstructor(constructor, intrinsicDefaultProto).
  3. If internalSlotsList is present, let slotsList be internalSlotsList.
  4. Else, let slotsList be a new empty List.
  5. Return OrdinaryObjectCreate(proto, slotsList).

10.1.14 GetPrototypeFromConstructor ( constructor, intrinsicDefaultProto )

The abstract operation GetPrototypeFromConstructor takes arguments constructor (a function object) and intrinsicDefaultProto (a String) and returns either a normal completion containing an Object or a throw completion. It determines the [[Prototype]] value that should be used to create an object corresponding to a specific constructor. The value is retrieved from the constructor's "prototype" property, if it exists. Otherwise the intrinsic named by intrinsicDefaultProto is used for [[Prototype]]. It performs the following steps when called:

  1. Assert: intrinsicDefaultProto is this specification's name of an intrinsic object. The corresponding object must be an intrinsic that is intended to be used as the [[Prototype]] value of an object.
  2. Let proto be ? Get(constructor, "prototype").
  3. If proto is not an Object, then
    1. Let realm be ? GetFunctionRealm(constructor).
    2. Set proto to realm's intrinsic object named intrinsicDefaultProto.
  4. Return proto.
 Note

If constructor does not supply a [[Prototype]] value, the default value that is used is obtained from the realm of the constructor function rather than from the running execution context.

10.1.15 RequireInternalSlot ( O, internalSlot )

The abstract operation RequireInternalSlot takes arguments O (an ECMAScript language value) and internalSlot (an internal slot name) and returns either a normal completion containing unused or a throw completion. It throws an exception unless O is an Object and has the given internal slot. It performs the following steps when called:

  1. If O is not an Object, throw a TypeError exception.
  2. If O does not have an internalSlot internal slot, throw a TypeError exception.
  3. Return unused.

10.2 ECMAScript Function Objects

ECMAScript function objects encapsulate parameterized ECMAScript code closed over a lexical environment and support the dynamic evaluation of that code. An ECMAScript function object is an ordinary object and has the same internal slots and the same internal methods as other ordinary objects. The code of an ECMAScript function object may be either strict mode code (11.2.2) or non-strict code. An ECMAScript function object whose code is strict mode code is called a strict function. One whose code is not strict mode code is called a non-strict function.

In addition to [[Extensible]] and [[Prototype]], ECMAScript function objects also have the internal slots listed in Table 30.

Table 30: Internal Slots of ECMAScript Function Objects
Internal Slot Type Description
[[Environment]] an Environment Record The Environment Record that the function was closed over. Used as the outer environment when evaluating the code of the function.
[[PrivateEnvironment]] a PrivateEnvironment Record or null The PrivateEnvironment Record for Private Names that the function was closed over. null if this function is not syntactically contained within a class. Used as the outer PrivateEnvironment for inner classes when evaluating the code of the function.
[[FormalParameters]] a Parse Node The root parse node of the source text that defines the function's formal parameter list.
[[ECMAScriptCode]] a Parse Node The root parse node of the source text that defines the function's body.
[[ConstructorKind]] base or derived Whether or not the function is a derived class constructor.
[[Realm]] a Realm Record The realm in which the function was created and which provides any intrinsic objects that are accessed when evaluating the function.
[[ScriptOrModule]] a Script Record or a Module Record The script or module in which the function was created.
[[ThisMode]] lexical, strict, or global Defines how this references are interpreted within the formal parameters and code body of the function. lexical means that this refers to the this value of a lexically enclosing function. strict means that the this value is used exactly as provided by an invocation of the function. global means that a this value of undefined or null is interpreted as a reference to the global object, and any other this value is first passed to ToObject.
[[Strict]] a Boolean true if this is a strict function, false if this is a non-strict function.
[[HomeObject]] an Object If the function uses super, this is the object whose [[GetPrototypeOf]] provides the object where super property lookups begin.
[[SourceText]] a sequence of Unicode code points The source text that defines the function.
[[Fields]] a List of ClassFieldDefinition Records If the function is a class, this is a list of Records representing the non-static fields and corresponding initializers of the class.
[[PrivateMethods]] a List of PrivateElements If the function is a class, this is a list representing the non-static private methods and accessors of the class.
[[ClassFieldInitializerName]] a String, a Symbol, a Private Name, or empty If the function is created as the initializer of a class field, the name to use for NamedEvaluation of the field; empty otherwise.
[[IsClassConstructor]] a Boolean Indicates whether the function is a class constructor. (If true, invoking the function's [[Call]] will immediately throw a TypeError exception.)

All ECMAScript function objects have the [[Call]] internal method defined here. ECMAScript functions that are also constructors in addition have the [[Construct]] internal method.

10.2.1 [[Call]] ( thisArgument, argumentsList )

The [[Call]] internal method of an ECMAScript function object F takes arguments thisArgument (an ECMAScript language value) and argumentsList (a List of ECMAScript language values) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

  1. Let callerContext be the running execution context.
  2. Let calleeContext be PrepareForOrdinaryCall(F, undefined).
  3. Assert: calleeContext is now the running execution context.
  4. If F.[[IsClassConstructor]] is true, then
    1. Let error be a newly created TypeError object.
    2. NOTE: error is created in calleeContext with F's associated Realm Record.
    3. Remove calleeContext from the execution context stack and restore callerContext as the running execution context.
    4. Return ThrowCompletion(error).
  5. Perform OrdinaryCallBindThis(F, calleeContext, thisArgument).
  6. Let result be Completion(OrdinaryCallEvaluateBody(F, argumentsList)).
  7. Remove calleeContext from the execution context stack and restore callerContext as the running execution context.
  8. If result.[[Type]] is return, return result.[[Value]].
  9. ReturnIfAbrupt(result).
  10. Return undefined.
 Note

When calleeContext is removed from the execution context stack in step 7 it must not be destroyed if it is suspended and retained for later resumption by an accessible Generator.

10.2.1.1 PrepareForOrdinaryCall ( F, newTarget )

The abstract operation PrepareForOrdinaryCall takes arguments F (a function object) and newTarget (an Object or undefined) and returns an execution context. It performs the following steps when called:

  1. Let callerContext be the running execution context.
  2. Let calleeContext be a new ECMAScript code execution context.
  3. Set the Function of calleeContext to F.
  4. Let calleeRealm be F.[[Realm]].
  5. Set the Realm of calleeContext to calleeRealm.
  6. Set the ScriptOrModule of calleeContext to F.[[ScriptOrModule]].
  7. Let localEnv be NewFunctionEnvironment(F, newTarget).
  8. Set the LexicalEnvironment of calleeContext to localEnv.
  9. Set the VariableEnvironment of calleeContext to localEnv.
  10. Set the PrivateEnvironment of calleeContext to F.[[PrivateEnvironment]].
  11. If callerContext is not already suspended, suspend callerContext.
  12. Push calleeContext onto the execution context stack; calleeContext is now the running execution context.
  13. NOTE: Any exception objects produced after this point are associated with calleeRealm.
  14. Return calleeContext.

10.2.1.2 OrdinaryCallBindThis ( F, calleeContext, thisArgument )

The abstract operation OrdinaryCallBindThis takes arguments F (a function object), calleeContext (an execution context), and thisArgument (an ECMAScript language value) and returns unused. It performs the following steps when called:

  1. Let thisMode be F.[[ThisMode]].
  2. If thisMode is lexical, return unused.
  3. Let calleeRealm be F.[[Realm]].
  4. Let localEnv be the LexicalEnvironment of calleeContext.
  5. If thisMode is strict, let thisValue be thisArgument.
  6. Else,
    1. If thisArgument is either undefined or null, then
      1. Let globalEnv be calleeRealm.[[GlobalEnv]].
      2. Assert: globalEnv is a Global Environment Record.
      3. Let thisValue be globalEnv.[[GlobalThisValue]].
    2. Else,
      1. Let thisValue be ! ToObject(thisArgument).
      2. NOTE: ToObject produces wrapper objects using calleeRealm.
  7. Assert: localEnv is a Function Environment Record.
  8. Assert: The next step never returns an abrupt completion because localEnv.[[ThisBindingStatus]] is not initialized.
  9. Perform ! localEnv.BindThisValue(thisValue).
  10. Return unused.

10.2.1.3 Runtime Semantics: EvaluateBody

The syntax-directed operation EvaluateBody takes arguments functionObject (a function object) and argumentsList (a List of ECMAScript language values) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It is defined piecewise over the following productions:

FunctionBody : FunctionStatementList
  1. Return ? EvaluateFunctionBody of FunctionBody with arguments functionObject and argumentsList.
 ConciseBody : ExpressionBody
  1. Return ? EvaluateConciseBody of ConciseBody with arguments functionObject and argumentsList.
 GeneratorBody : FunctionBody
  1. Return ? EvaluateGeneratorBody of GeneratorBody with arguments functionObject and argumentsList.
 AsyncGeneratorBody : FunctionBody
  1. Return ? EvaluateAsyncGeneratorBody of AsyncGeneratorBody with arguments functionObject and argumentsList.
 AsyncFunctionBody : FunctionBody
  1. Return ? EvaluateAsyncFunctionBody of AsyncFunctionBody with arguments functionObject and argumentsList.
 AsyncConciseBody : ExpressionBody
  1. Return ? EvaluateAsyncConciseBody of AsyncConciseBody with arguments functionObject and argumentsList.
 Initializer : = AssignmentExpression
  1. Assert: argumentsList is empty.
  2. Assert: functionObject.[[ClassFieldInitializerName]] is not empty.
  3. If IsAnonymousFunctionDefinition(AssignmentExpression) is true, then
    1. Let value be ? NamedEvaluation of Initializer with argument functionObject.[[ClassFieldInitializerName]].
  4. Else,
    1. Let rhs be ? Evaluation of AssignmentExpression.
    2. Let value be ? GetValue(rhs).
  5. Return Completion Record { [[Type]]: return, [[Value]]: value, [[Target]]: empty }.
 Note

Even though field initializers constitute a function boundary, calling FunctionDeclarationInstantiation does not have any observable effect and so is omitted.

 ClassStaticBlockBody : ClassStaticBlockStatementList
  1. Assert: argumentsList is empty.
  2. Return ? EvaluateClassStaticBlockBody of ClassStaticBlockBody with argument functionObject.

10.2.1.4 OrdinaryCallEvaluateBody ( F, argumentsList )

The abstract operation OrdinaryCallEvaluateBody takes arguments F (a function object) and argumentsList (a List of ECMAScript language values) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It performs the following steps when called:

  1. Return ? EvaluateBody of F.[[ECMAScriptCode]] with arguments F and argumentsList.

10.2.2 [[Construct]] ( argumentsList, newTarget )

The [[Construct]] internal method of an ECMAScript function object F takes arguments argumentsList (a List of ECMAScript language values) and newTarget (a constructor) and returns either a normal completion containing an Object or a throw completion. It performs the following steps when called:

  1. Let callerContext be the running execution context.
  2. Let kind be F.[[ConstructorKind]].
  3. If kind is base, then
    1. Let thisArgument be ? OrdinaryCreateFromConstructor(newTarget, "%Object.prototype%").
  4. Let calleeContext be PrepareForOrdinaryCall(F, newTarget).
  5. Assert: calleeContext is now the running execution context.
  6. If kind is base, then
    1. Perform OrdinaryCallBindThis(F, calleeContext, thisArgument).
    2. Let initializeResult be Completion(InitializeInstanceElements(thisArgument, F)).
    3. If initializeResult is an abrupt completion, then
      1. Remove calleeContext from the execution context stack and restore callerContext as the running execution context.
      2. Return ? initializeResult.
  7. Let constructorEnv be the LexicalEnvironment of calleeContext.
  8. Let result be Completion(OrdinaryCallEvaluateBody(F, argumentsList)).
  9. Remove calleeContext from the execution context stack and restore callerContext as the running execution context.
  10. If result.[[Type]] is return, then
    1. If result.[[Value]] is an Object, return result.[[Value]].
    2. If kind is base, return thisArgument.
    3. If result.[[Value]] is not undefined, throw a TypeError exception.
  11. Else, ReturnIfAbrupt(result).
  12. Let thisBinding be ? constructorEnv.GetThisBinding().
  13. Assert: thisBinding is an Object.
  14. Return thisBinding.

10.2.3 OrdinaryFunctionCreate ( functionPrototype, sourceText, ParameterList, Body, thisMode, env, privateEnv )

The abstract operation OrdinaryFunctionCreate takes arguments functionPrototype (an Object), sourceText (a sequence of Unicode code points), ParameterList (a Parse Node), Body (a Parse Node), thisMode (lexical-this or non-lexical-this), env (an Environment Record), and privateEnv (a PrivateEnvironment Record or null) and returns a function object. It is used to specify the runtime creation of a new function with a default [[Call]] internal method and no [[Construct]] internal method (although one may be subsequently added by an operation such as MakeConstructor). sourceText is the source text of the syntactic definition of the function to be created. It performs the following steps when called:

  1. Let internalSlotsList be the internal slots listed in Table 30.
  2. Let F be OrdinaryObjectCreate(functionPrototype, internalSlotsList).
  3. Set F.[[Call]] to the definition specified in 10.2.1.
  4. Set F.[[SourceText]] to sourceText.
  5. Set F.[[FormalParameters]] to ParameterList.
  6. Set F.[[ECMAScriptCode]] to Body.
  7. If the source text matched by Body is strict mode code, let Strict be true; else let Strict be false.
  8. Set F.[[Strict]] to Strict.
  9. If thisMode is lexical-this, set F.[[ThisMode]] to lexical.
  10. Else if Strict is true, set F.[[ThisMode]] to strict.
  11. Else, set F.[[ThisMode]] to global.
  12. Set F.[[IsClassConstructor]] to false.
  13. Set F.[[Environment]] to env.
  14. Set F.[[PrivateEnvironment]] to privateEnv.
  15. Set F.[[ScriptOrModule]] to GetActiveScriptOrModule().
  16. Set F.[[Realm]] to the current Realm Record.
  17. Set F.[[HomeObject]] to undefined.
  18. Set F.[[Fields]] to a new empty List.
  19. Set F.[[PrivateMethods]] to a new empty List.
  20. Set F.[[ClassFieldInitializerName]] to empty.
  21. Let len be the ExpectedArgumentCount of ParameterList.
  22. Perform SetFunctionLength(F, len).
  23. Return F.

10.2.4 AddRestrictedFunctionProperties ( F, realm )

The abstract operation AddRestrictedFunctionProperties takes arguments F (a function object) and realm (a Realm Record) and returns unused. It performs the following steps when called:

  1. Assert: realm.[[Intrinsics]].[[%ThrowTypeError%]] exists and has been initialized.
  2. Let thrower be realm.[[Intrinsics]].[[%ThrowTypeError%]].
  3. Perform ! DefinePropertyOrThrow(F, "caller", PropertyDescriptor { [[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false, [[Configurable]]: true }).
  4. Perform ! DefinePropertyOrThrow(F, "arguments", PropertyDescriptor { [[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false, [[Configurable]]: true }).
  5. Return unused.

10.2.4.1 %ThrowTypeError% ( )

This function is the %ThrowTypeError% intrinsic object.

It is an anonymous built-in function object that is defined once for each realm.

It performs the following steps when called:

  1. Throw a TypeError exception.

The value of the [[Extensible]] internal slot of this function is false.

The "length" property of this function has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

The "name" property of this function has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

10.2.5 MakeConstructor ( F [ , writablePrototype [ , prototype ] ] )

The abstract operation MakeConstructor takes argument F (an ECMAScript function object or a built-in function object) and optional arguments writablePrototype (a Boolean) and prototype (an Object) and returns unused. It converts F into a constructor. It performs the following steps when called:

  1. If F is an ECMAScript function object, then
    1. Assert: IsConstructor(F) is false.
    2. Assert: F is an extensible object that does not have a "prototype" own property.
    3. Set F.[[Construct]] to the definition specified in 10.2.2.
  2. Else,
    1. Set F.[[Construct]] to the definition specified in 10.3.2.
  3. Set F.[[ConstructorKind]] to base.
  4. If writablePrototype is not present, set writablePrototype to true.
  5. If prototype is not present, then
    1. Set prototype to OrdinaryObjectCreate(%Object.prototype%).
    2. Perform ! DefinePropertyOrThrow(prototype, "constructor", PropertyDescriptor { [[Value]]: F, [[Writable]]: writablePrototype, [[Enumerable]]: false, [[Configurable]]: true }).
  6. Perform ! DefinePropertyOrThrow(F, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: writablePrototype, [[Enumerable]]: false, [[Configurable]]: false }).
  7. Return unused.

10.2.6 MakeClassConstructor ( F )

The abstract operation MakeClassConstructor takes argument F (an ECMAScript function object) and returns unused. It performs the following steps when called:

  1. Assert: F.[[IsClassConstructor]] is false.
  2. Set F.[[IsClassConstructor]] to true.
  3. Return unused.

10.2.7 MakeMethod ( F, homeObject )

The abstract operation MakeMethod takes arguments F (an ECMAScript function object) and homeObject (an Object) and returns unused. It configures F as a method. It performs the following steps when called:

  1. Set F.[[HomeObject]] to homeObject.
  2. Return unused.

10.2.8 DefineMethodProperty ( homeObject, key, closure, enumerable )

The abstract operation DefineMethodProperty takes arguments homeObject (an Object), key (a property key or Private Name), closure (a function object), and enumerable (a Boolean) and returns a PrivateElement or unused. It performs the following steps when called:

  1. Assert: homeObject is an ordinary, extensible object with no non-configurable properties.
  2. If key is a Private Name, then
    1. Return PrivateElement { [[Key]]: key, [[Kind]]: method, [[Value]]: closure }.
  3. Else,
    1. Let desc be the PropertyDescriptor { [[Value]]: closure, [[Writable]]: true, [[Enumerable]]: enumerable, [[Configurable]]: true }.
    2. Perform ! DefinePropertyOrThrow(homeObject, key, desc).
    3. Return unused.

10.2.9 SetFunctionName ( F, name [ , prefix ] )

The abstract operation SetFunctionName takes arguments F (a function object) and name (a property key or Private Name) and optional argument prefix (a String) and returns unused. It adds a "name" property to F. It performs the following steps when called:

  1. Assert: F is an extensible object that does not have a "name" own property.
  2. If name is a Symbol, then
    1. Let description be name's [[Description]] value.
    2. If description is undefined, set name to the empty String.
    3. Else, set name to the string-concatenation of "[", description, and "]".
  3. Else if name is a Private Name, then
    1. Set name to name.[[Description]].
  4. If F has an [[InitialName]] internal slot, then
    1. Set F.[[InitialName]] to name.
  5. If prefix is present, then
    1. Set name to the string-concatenation of prefix, the code unit 0x0020 (SPACE), and name.
    2. If F has an [[InitialName]] internal slot, then
      1. Optionally, set F.[[InitialName]] to name.
  6. Perform ! DefinePropertyOrThrow(F, "name", PropertyDescriptor { [[Value]]: name, [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }).
  7. Return unused.

10.2.10 SetFunctionLength ( F, length )

The abstract operation SetFunctionLength takes arguments F (a function object) and length (a non-negative integer or ∞) and returns unused. It adds a "length" property to F. It performs the following steps when called:

  1. Assert: F is an extensible object that does not have a "length" own property.
  2. Perform ! DefinePropertyOrThrow(F, "length", PropertyDescriptor { [[Value]]: 𝔽(length), [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }).
  3. Return unused.

10.2.11 FunctionDeclarationInstantiation ( func, argumentsList )

The abstract operation FunctionDeclarationInstantiation takes arguments func (a function object) and argumentsList (a List of ECMAScript language values) and returns either a normal completion containing unused or an abrupt completion. func is the function object for which the execution context is being established.

Note 1

When an execution context is established for evaluating an ECMAScript function a new Function Environment Record is created and bindings for each formal parameter are instantiated in that Environment Record. Each declaration in the function body is also instantiated. If the function's formal parameters do not include any default value initializers then the body declarations are instantiated in the same Environment Record as the parameters. If default value parameter initializers exist, a second Environment Record is created for the body declarations. Formal parameters and functions are initialized as part of FunctionDeclarationInstantiation. All other bindings are initialized during evaluation of the function body.

It performs the following steps when called:

  1. Let calleeContext be the running execution context.
  2. Let code be func.[[ECMAScriptCode]].
  3. Let strict be func.[[Strict]].
  4. Let formals be func.[[FormalParameters]].
  5. Let parameterNames be the BoundNames of formals.
  6. If parameterNames has any duplicate entries, let hasDuplicates be true. Otherwise, let hasDuplicates be false.
  7. Let simpleParameterList be IsSimpleParameterList of formals.
  8. Let hasParameterExpressions be ContainsExpression of formals.
  9. Let varNames be the VarDeclaredNames of code.
  10. Let varDeclarations be the VarScopedDeclarations of code.
  11. Let lexicalNames be the LexicallyDeclaredNames of code.
  12. Let functionNames be a new empty List.
  13. Let functionsToInitialize be a new empty List.
  14. For each element d of varDeclarations, in reverse List order, do
    1. If d is neither a VariableDeclaration nor a ForBinding nor a BindingIdentifier, then
      1. Assert: d is either a FunctionDeclaration, a GeneratorDeclaration, an AsyncFunctionDeclaration, or an AsyncGeneratorDeclaration.
      2. Let fn be the sole element of the BoundNames of d.
      3. If functionNames does not contain fn, then
        1. Insert fn as the first element of functionNames.
        2. NOTE: If there are multiple function declarations for the same name, the last declaration is used.
        3. Insert d as the first element of functionsToInitialize.
  15. Let argumentsObjectNeeded be true.
  16. If func.[[ThisMode]] is lexical, then
    1. NOTE: Arrow functions never have an arguments object.
    2. Set argumentsObjectNeeded to false.
  17. Else if parameterNames contains "arguments", then
    1. Set argumentsObjectNeeded to false.
  18. Else if hasParameterExpressions is false, then
    1. If functionNames contains "arguments" or lexicalNames contains "arguments", then
      1. Set argumentsObjectNeeded to false.
  19. If strict is true or hasParameterExpressions is false, then
    1. NOTE: Only a single Environment Record is needed for the parameters, since calls to eval in strict mode code cannot create new bindings which are visible outside of the eval.
    2. Let env be the LexicalEnvironment of calleeContext.
  20. Else,
    1. NOTE: A separate Environment Record is needed to ensure that bindings created by direct eval calls in the formal parameter list are outside the environment where parameters are declared.
    2. Let calleeEnv be the LexicalEnvironment of calleeContext.
    3. Let env be NewDeclarativeEnvironment(calleeEnv).
    4. Assert: The VariableEnvironment of calleeContext is calleeEnv.
    5. Set the LexicalEnvironment of calleeContext to env.
  21. For each String paramName of parameterNames, do
    1. Let alreadyDeclared be ! env.HasBinding(paramName).
    2. NOTE: Early errors ensure that duplicate parameter names can only occur in non-strict functions that do not have parameter default values or rest parameters.
    3. If alreadyDeclared is false, then
      1. Perform ! env.CreateMutableBinding(paramName, false).
      2. If hasDuplicates is true, then
        1. Perform ! env.InitializeBinding(paramName, undefined).
  22. If argumentsObjectNeeded is true, then
    1. If strict is true or simpleParameterList is false, then
      1. Let ao be CreateUnmappedArgumentsObject(argumentsList).
    2. Else,
      1. NOTE: A mapped argument object is only provided for non-strict functions that don't have a rest parameter, any parameter default value initializers, or any destructured parameters.
      2. Let ao be CreateMappedArgumentsObject(func, formals, argumentsList, env).
    3. If strict is true, then
      1. Perform ! env.CreateImmutableBinding("arguments", false).
      2. NOTE: In strict mode code early errors prevent attempting to assign to this binding, so its mutability is not observable.
    4. Else,
      1. Perform ! env.CreateMutableBinding("arguments", false).
    5. Perform ! env.InitializeBinding("arguments", ao).
    6. Let parameterBindings be the list-concatenation of parameterNames and « "arguments" ».
  23. Else,
    1. Let parameterBindings be parameterNames.
  24. Let iteratorRecord be CreateListIteratorRecord(argumentsList).
  25. If hasDuplicates is true, then
    1. Perform ? IteratorBindingInitialization of formals with arguments iteratorRecord and undefined.
  26. Else,
    1. Perform ? IteratorBindingInitialization of formals with arguments iteratorRecord and env.
  27. If hasParameterExpressions is false, then
    1. NOTE: Only a single Environment Record is needed for the parameters and top-level vars.
    2. Let instantiatedVarNames be a copy of the List parameterBindings.
    3. For each element n of varNames, do
      1. If instantiatedVarNames does not contain n, then
        1. Append n to instantiatedVarNames.
        2. Perform ! env.CreateMutableBinding(n, false).
        3. Perform ! env.InitializeBinding(n, undefined).
    4. Let varEnv be env.
  28. Else,
    1. NOTE: A separate Environment Record is needed to ensure that closures created by expressions in the formal parameter list do not have visibility of declarations in the function body.
    2. Let varEnv be NewDeclarativeEnvironment(env).
    3. Set the VariableEnvironment of calleeContext to varEnv.
    4. Let instantiatedVarNames be a new empty List.
    5. For each element n of varNames, do
      1. If instantiatedVarNames does not contain n, then
        1. Append n to instantiatedVarNames.
        2. Perform ! varEnv.CreateMutableBinding(n, false).
        3. If parameterBindings does not contain n, or if functionNames contains n, let initialValue be undefined.
        4. Else,
          1. Let initialValue be ! env.GetBindingValue(n, false).
        5. Perform ! varEnv.InitializeBinding(n, initialValue).
        6. NOTE: A var with the same name as a formal parameter initially has the same value as the corresponding initialized parameter.
  29. NOTE: Annex B.3.2.1 adds additional steps at this point.
  30. If strict is false, then
    1. Let lexEnv be NewDeclarativeEnvironment(varEnv).
    2. NOTE: Non-strict functions use a separate Environment Record for top-level lexical declarations so that a direct eval can determine whether any var scoped declarations introduced by the eval code conflict with pre-existing top-level lexically scoped declarations. This is not needed for strict functions because a strict direct eval always places all declarations into a new Environment Record.
  31. Else, let lexEnv be varEnv.
  32. Set the LexicalEnvironment of calleeContext to lexEnv.
  33. Let lexDeclarations be the LexicallyScopedDeclarations of code.
  34. For each element d of lexDeclarations, do
    1. NOTE: A lexically declared name cannot be the same as a function/generator declaration, formal parameter, or a var name. Lexically declared names are only instantiated here but not initialized.
    2. For each element dn of the BoundNames of d, do
      1. If IsConstantDeclaration of d is true, then
        1. Perform ! lexEnv.CreateImmutableBinding(dn, true).
      2. Else,
        1. Perform ! lexEnv.CreateMutableBinding(dn, false).
  35. Let privateEnv be the PrivateEnvironment of calleeContext.
  36. For each Parse Node f of functionsToInitialize, do
    1. Let fn be the sole element of the BoundNames of f.
    2. Let fo be InstantiateFunctionObject of f with arguments lexEnv and privateEnv.
    3. Perform ! varEnv.SetMutableBinding(fn, fo, false).
  37. Return unused.
 Note 2

B.3.2 provides an extension to the above algorithm that is necessary for backwards compatibility with web browser implementations of ECMAScript that predate ECMAScript 2015.

10.3 Built-in Function Objects

The built-in function objects defined in this specification may be implemented as either ECMAScript function objects (10.2) whose behaviour is provided using ECMAScript code or as function objects whose behaviour is provided in some other manner. In either case, the effect of calling such functions must conform to their specifications. An implementation may also provide additional built-in function objects that are not defined in this specification.

If a built-in function object is implemented as an ECMAScript function object, it must have all the internal slots described in 10.2 ([[Prototype]], [[Extensible]], and the slots listed in Table 30), and [[InitialName]]. The value of the [[InitialName]] internal slot is a String value that is the initial name of the function. It is used by 20.2.3.5.

Built-in function objects must have the ordinary object behaviour specified in 10.1. All such function objects have [[Prototype]], [[Extensible]], [[Realm]], and [[InitialName]] internal slots, with the same meanings as above.

Unless otherwise specified every built-in function object has the %Function.prototype% object as the initial value of its [[Prototype]] internal slot.

The behaviour specified for each built-in function via algorithm steps or other means is the specification of the function body behaviour for both [[Call]] and [[Construct]] invocations of the function. However, [[Construct]] invocation is not supported by all built-in functions. For each built-in function, when invoked with [[Call]], the [[Call]] thisArgument provides the this value, the [[Call]] argumentsList provides the named parameters, and the NewTarget value is undefined. When invoked with [[Construct]], the this value is uninitialized, the [[Construct]] argumentsList provides the named parameters, and the [[Construct]] newTarget parameter provides the NewTarget value. If the built-in function is implemented as an ECMAScript function object then this specified behaviour must be implemented by the ECMAScript code that is the body of the function. Built-in functions that are ECMAScript function objects must be strict functions. If a built-in constructor has any [[Call]] behaviour other than throwing a TypeError exception, an ECMAScript implementation of the function must be done in a manner that does not cause the function's [[IsClassConstructor]] internal slot to have the value true.

Built-in function objects that are not identified as constructors do not implement the [[Construct]] internal method unless otherwise specified in the description of a particular function. When a built-in constructor is called as part of a new expression the argumentsList parameter of the invoked [[Construct]] internal method provides the values for the built-in constructor's named parameters.

Built-in functions that are not constructors do not have a "prototype" property unless otherwise specified in the description of a particular function.

If a built-in function object is not implemented as an ECMAScript function it must provide [[Call]] and [[Construct]] internal methods that conform to the following definitions:

10.3.1 [[Call]] ( thisArgument, argumentsList )

The [[Call]] internal method of a built-in function object F takes arguments thisArgument (an ECMAScript language value) and argumentsList (a List of ECMAScript language values) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

  1. Let callerContext be the running execution context.
  2. If callerContext is not already suspended, suspend callerContext.
  3. Let calleeContext be a new execution context.
  4. Set the Function of calleeContext to F.
  5. Let calleeRealm be F.[[Realm]].
  6. Set the Realm of calleeContext to calleeRealm.
  7. Set the ScriptOrModule of calleeContext to null.
  8. Perform any necessary implementation-defined initialization of calleeContext.
  9. Push calleeContext onto the execution context stack; calleeContext is now the running execution context.
  10. Let result be the Completion Record that is the result of evaluating F in a manner that conforms to the specification of F. thisArgument is the this value, argumentsList provides the named parameters, and the NewTarget value is undefined.
  11. Remove calleeContext from the execution context stack and restore callerContext as the running execution context.
  12. Return ? result.
 Note

When calleeContext is removed from the execution context stack it must not be destroyed if it has been suspended and retained by an accessible Generator for later resumption.

10.3.2 [[Construct]] ( argumentsList, newTarget )

The [[Construct]] internal method of a built-in function object F (when the method is present) takes arguments argumentsList (a List of ECMAScript language values) and newTarget (a constructor) and returns either a normal completion containing an Object or a throw completion. The steps performed are the same as [[Call]] (see 10.3.1) except that step 10 is replaced by:

  1. Let result be the Completion Record that is the result of evaluating F in a manner that conforms to the specification of F. The this value is uninitialized, argumentsList provides the named parameters, and newTarget provides the NewTarget value.

10.3.3 CreateBuiltinFunction ( behaviour, length, name, additionalInternalSlotsList [ , realm [ , prototype [ , prefix ] ] ] )

The abstract operation CreateBuiltinFunction takes arguments behaviour (an Abstract Closure, a set of algorithm steps, or some other definition of a function's behaviour provided in this specification), length (a non-negative integer or ∞), name (a property key or a Private Name), and additionalInternalSlotsList (a List of names of internal slots) and optional arguments realm (a Realm Record), prototype (an Object or null), and prefix (a String) and returns a function object. additionalInternalSlotsList contains the names of additional internal slots that must be defined as part of the object. This operation creates a built-in function object. It performs the following steps when called:

  1. If realm is not present, set realm to the current Realm Record.
  2. If prototype is not present, set prototype to realm.[[Intrinsics]].[[%Function.prototype%]].
  3. Let internalSlotsList be a List containing the names of all the internal slots that 10.3 requires for the built-in function object that is about to be created.
  4. Append to internalSlotsList the elements of additionalInternalSlotsList.
  5. Let func be a new built-in function object that, when called, performs the action described by behaviour using the provided arguments as the values of the corresponding parameters specified by behaviour. The new function object has internal slots whose names are the elements of internalSlotsList, and an [[InitialName]] internal slot.
  6. Set func.[[Prototype]] to prototype.
  7. Set func.[[Extensible]] to true.
  8. Set func.[[Realm]] to realm.
  9. Set func.[[InitialName]] to null.
  10. Perform SetFunctionLength(func, length).
  11. If prefix is not present, then
    1. Perform SetFunctionName(func, name).
  12. Else,
    1. Perform SetFunctionName(func, name, prefix).
  13. Return func.

Each built-in function defined in this specification is created by calling the CreateBuiltinFunction abstract operation.

10.4 Built-in Exotic Object Internal Methods and Slots

This specification defines several kinds of built-in exotic objects. These objects generally behave similar to ordinary objects except for a few specific situations. The following exotic objects use the ordinary object internal methods except where it is explicitly specified otherwise below:

10.4.1 Bound Function Exotic Objects

A bound function exotic object is an exotic object that wraps another function object. A bound function exotic object is callable (it has a [[Call]] internal method and may have a [[Construct]] internal method). Calling a bound function exotic object generally results in a call of its wrapped function.

An object is a bound function exotic object if its [[Call]] and (if applicable) [[Construct]] internal methods use the following implementations, and its other essential internal methods use the definitions found in 10.1. These methods are installed in BoundFunctionCreate.

Bound function exotic objects do not have the internal slots of ECMAScript function objects listed in Table 30. Instead they have the internal slots listed in Table 31, in addition to [[Prototype]] and [[Extensible]].

Table 31: Internal Slots of Bound Function Exotic Objects
Internal Slot Type Description
[[BoundTargetFunction]] a callable Object The wrapped function object.
[[BoundThis]] an ECMAScript language value The value that is always passed as the this value when calling the wrapped function.
[[BoundArguments]] a List of ECMAScript language values A list of values whose elements are used as the first arguments to any call to the wrapped function.

10.4.1.1 [[Call]] ( thisArgument, argumentsList )

The [[Call]] internal method of a bound function exotic object F takes arguments thisArgument (an ECMAScript language value) and argumentsList (a List of ECMAScript language values) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

  1. Let target be F.[[BoundTargetFunction]].
  2. Let boundThis be F.[[BoundThis]].
  3. Let boundArgs be F.[[BoundArguments]].
  4. Let args be the list-concatenation of boundArgs and argumentsList.
  5. Return ? Call(target, boundThis, args).

10.4.1.2 [[Construct]] ( argumentsList, newTarget )

The [[Construct]] internal method of a bound function exotic object F takes arguments argumentsList (a List of ECMAScript language values) and newTarget (a constructor) and returns either a normal completion containing an Object or a throw completion. It performs the following steps when called:

  1. Let target be F.[[BoundTargetFunction]].
  2. Assert: IsConstructor(target) is true.
  3. Let boundArgs be F.[[BoundArguments]].
  4. Let args be the list-concatenation of boundArgs and argumentsList.
  5. If SameValue(F, newTarget) is true, set newTarget to target.
  6. Return ? Construct(target, args, newTarget).

10.4.1.3 BoundFunctionCreate ( targetFunction, boundThis, boundArgs )

The abstract operation BoundFunctionCreate takes arguments targetFunction (a function object), boundThis (an ECMAScript language value), and boundArgs (a List of ECMAScript language values) and returns either a normal completion containing a function object or a throw completion. It is used to specify the creation of new bound function exotic objects. It performs the following steps when called:

  1. Let proto be ? targetFunction.[[GetPrototypeOf]]().
  2. Let internalSlotsList be the list-concatenation of « [[Prototype]], [[Extensible]] » and the internal slots listed in Table 31.
  3. Let obj be MakeBasicObject(internalSlotsList).
  4. Set obj.[[Prototype]] to proto.
  5. Set obj.[[Call]] as described in 10.4.1.1.
  6. If IsConstructor(targetFunction) is true, then
    1. Set obj.[[Construct]] as described in 10.4.1.2.
  7. Set obj.[[BoundTargetFunction]] to targetFunction.
  8. Set obj.[[BoundThis]] to boundThis.
  9. Set obj.[[BoundArguments]] to boundArgs.
  10. Return obj.

10.4.2 Array Exotic Objects

An Array is an exotic object that gives special treatment to array index property keys (see 6.1.7). A property whose property name is an array index is also called an element. Every Array has a non-configurable "length" property whose value is always a non-negative integral Number whose mathematical value is strictly less than 232. The value of the "length" property is numerically greater than the name of every own property whose name is an array index; whenever an own property of an Array is created or changed, other properties are adjusted as necessary to maintain this invariant. Specifically, whenever an own property is added whose name is an array index, the value of the "length" property is changed, if necessary, to be one more than the numeric value of that array index; and whenever the value of the "length" property is changed, every own property whose name is an array index whose value is not smaller than the new length is deleted. This constraint applies only to own properties of an Array and is unaffected by "length" or array index properties that may be inherited from its prototypes.

Note

A String property name P is an array index if and only if ToString(ToUint32(P)) is P and ToUint32(P) is not 𝔽(232 - 1).

An object is an Array exotic object (or simply, an Array) if its [[DefineOwnProperty]] internal method uses the following implementation, and its other essential internal methods use the definitions found in 10.1. These methods are installed in ArrayCreate.

10.4.2.1 [[DefineOwnProperty]] ( P, Desc )

The [[DefineOwnProperty]] internal method of an Array exotic object A takes arguments P (a property key) and Desc (a Property Descriptor) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

  1. If P is "length", then
    1. Return ? ArraySetLength(A, Desc).
  2. Else if P is an array index, then
    1. Let lengthDesc be OrdinaryGetOwnProperty(A, "length").
    2. Assert: IsDataDescriptor(lengthDesc) is true.
    3. Assert: lengthDesc.[[Configurable]] is false.
    4. Let length be lengthDesc.[[Value]].
    5. Assert: length is a non-negative integral Number.
    6. Let index be ! ToUint32(P).
    7. If index ≥ length and lengthDesc.[[Writable]] is false, return false.
    8. Let succeeded be ! OrdinaryDefineOwnProperty(A, P, Desc).
    9. If succeeded is false, return false.
    10. If index ≥ length, then
      1. Set lengthDesc.[[Value]] to index 1𝔽.
      2. Set succeeded to ! OrdinaryDefineOwnProperty(A, "length", lengthDesc).
      3. Assert: succeeded is true.
    11. Return true.
  3. Return ? OrdinaryDefineOwnProperty(A, P, Desc).

10.4.2.2 ArrayCreate ( length [ , proto ] )

The abstract operation ArrayCreate takes argument length (a non-negative integer) and optional argument proto (an Object) and returns either a normal completion containing an Array exotic object or a throw completion. It is used to specify the creation of new Arrays. It performs the following steps when called:

  1. If length > 232 - 1, throw a RangeError exception.
  2. If proto is not present, set proto to %Array.prototype%.
  3. Let A be MakeBasicObject(« [[Prototype]], [[Extensible]] »).
  4. Set A.[[Prototype]] to proto.
  5. Set A.[[DefineOwnProperty]] as specified in 10.4.2.1.
  6. Perform ! OrdinaryDefineOwnProperty(A, "length", PropertyDescriptor { [[Value]]: 𝔽(length), [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
  7. Return A.

10.4.2.3 ArraySpeciesCreate ( originalArray, length )

The abstract operation ArraySpeciesCreate takes arguments originalArray (an Object) and length (a non-negative integer) and returns either a normal completion containing an Object or a throw completion. It is used to specify the creation of a new Array or similar object using a constructor function that is derived from originalArray. It does not enforce that the constructor function returns an Array. It performs the following steps when called:

  1. Let isArray be ? IsArray(originalArray).
  2. If isArray is false, return ? ArrayCreate(length).
  3. Let C be ? Get(originalArray, "constructor").
  4. If IsConstructor(C) is true, then
    1. Let thisRealm be the current Realm Record.
    2. Let realmC be ? GetFunctionRealm(C).
    3. If thisRealm and realmC are not the same Realm Record, then
      1. If SameValue(C, realmC.[[Intrinsics]].[[%Array%]]) is true, set C to undefined.
  5. If C is an Object, then
    1. Set C to ? Get(C, @@species).
    2. If C is null, set C to undefined.
  6. If C is undefined, return ? ArrayCreate(length).
  7. If IsConstructor(C) is false, throw a TypeError exception.
  8. Return ? Construct(C, « 𝔽(length) »).
 Note

If originalArray was created using the standard built-in Array constructor for a realm that is not the realm of the running execution context, then a new Array is created using the realm of the running execution context. This maintains compatibility with Web browsers that have historically had that behaviour for the Array.prototype methods that now are defined using ArraySpeciesCreate.

10.4.2.4 ArraySetLength ( A, Desc )

The abstract operation ArraySetLength takes arguments A (an Array) and Desc (a Property Descriptor) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

  1. If Desc does not have a [[Value]] field, then
    1. Return ! OrdinaryDefineOwnProperty(A, "length", Desc).
  2. Let newLenDesc be a copy of Desc.
  3. Let newLen be ? ToUint32(Desc.[[Value]]).
  4. Let numberLen be ? ToNumber(Desc.[[Value]]).
  5. If SameValueZero(newLen, numberLen) is false, throw a RangeError exception.
  6. Set newLenDesc.[[Value]] to newLen.
  7. Let oldLenDesc be OrdinaryGetOwnProperty(A, "length").
  8. Assert: IsDataDescriptor(oldLenDesc) is true.
  9. Assert: oldLenDesc.[[Configurable]] is false.
  10. Let oldLen be oldLenDesc.[[Value]].
  11. If newLen ≥ oldLen, then
    1. Return ! OrdinaryDefineOwnProperty(A, "length", newLenDesc).
  12. If oldLenDesc.[[Writable]] is false, return false.
  13. If newLenDesc does not have a [[Writable]] field or newLenDesc.[[Writable]] is true, let newWritable be true.
  14. Else,
    1. NOTE: Setting the [[Writable]] attribute to false is deferred in case any elements cannot be deleted.
    2. Let newWritable be false.
    3. Set newLenDesc.[[Writable]] to true.
  15. Let succeeded be ! OrdinaryDefineOwnProperty(A, "length", newLenDesc).
  16. If succeeded is false, return false.
  17. For each own property key P of A such that P is an array index and ! ToUint32(P) ≥ newLen, in descending numeric index order, do
    1. Let deleteSucceeded be ! A.[[Delete]](P).
    2. If deleteSucceeded is false, then
      1. Set newLenDesc.[[Value]] to ! ToUint32(P) 1𝔽.
      2. If newWritable is false, set newLenDesc.[[Writable]] to false.
      3. Perform ! OrdinaryDefineOwnProperty(A, "length", newLenDesc).
      4. Return false.
  18. If newWritable is false, then
    1. Set succeeded to ! OrdinaryDefineOwnProperty(A, "length", PropertyDescriptor { [[Writable]]: false }).
    2. Assert: succeeded is true.
  19. Return true.
 Note

In steps 3 and 4, if Desc.[[Value]] is an object then its valueOf method is called twice. This is legacy behaviour that was specified with this effect starting with the 2nd Edition of this specification.

10.4.3 String Exotic Objects

A String object is an exotic object that encapsulates a String value and exposes virtual integer-indexed data properties corresponding to the individual code unit elements of the String value. String exotic objects always have a data property named "length" whose value is the length of the encapsulated String value. Both the code unit data properties and the "length" property are non-writable and non-configurable.

An object is a String exotic object (or simply, a String object) if its [[GetOwnProperty]], [[DefineOwnProperty]], and [[OwnPropertyKeys]] internal methods use the following implementations, and its other essential internal methods use the definitions found in 10.1. These methods are installed in StringCreate.

String exotic objects have the same internal slots as ordinary objects. They also have a [[StringData]] internal slot.

10.4.3.1 [[GetOwnProperty]] ( P )

The [[GetOwnProperty]] internal method of a String exotic object S takes argument P (a property key) and returns a normal completion containing either a Property Descriptor or undefined. It performs the following steps when called:

  1. Let desc be OrdinaryGetOwnProperty(S, P).
  2. If desc is not undefined, return desc.
  3. Return StringGetOwnProperty(S, P).

10.4.3.2 [[DefineOwnProperty]] ( P, Desc )

The [[DefineOwnProperty]] internal method of a String exotic object S takes arguments P (a property key) and Desc (a Property Descriptor) and returns a normal completion containing a Boolean. It performs the following steps when called:

  1. Let stringDesc be StringGetOwnProperty(S, P).
  2. If stringDesc is not undefined, then
    1. Let extensible be S.[[Extensible]].
    2. Return IsCompatiblePropertyDescriptor(extensible, Desc, stringDesc).
  3. Return ! OrdinaryDefineOwnProperty(S, P, Desc).

10.4.3.3 [[OwnPropertyKeys]] ( )

The [[OwnPropertyKeys]] internal method of a String exotic object O takes no arguments and returns a normal completion containing a List of property keys. It performs the following steps when called:

  1. Let keys be a new empty List.
  2. Let str be O.[[StringData]].
  3. Assert: str is a String.
  4. Let len be the length of str.
  5. For each integer i such that 0 ≤ i < len, in ascending order, do
    1. Append ! ToString(𝔽(i)) to keys.
  6. For each own property key P of O such that P is an array index and ! ToIntegerOrInfinity(P) ≥ len, in ascending numeric index order, do
    1. Append P to keys.
  7. For each own property key P of O such that P is a String and P is not an array index, in ascending chronological order of property creation, do
    1. Append P to keys.
  8. For each own property key P of O such that P is a Symbol, in ascending chronological order of property creation, do
    1. Append P to keys.
  9. Return keys.

10.4.3.4 StringCreate ( value, prototype )

The abstract operation StringCreate takes arguments value (a String) and prototype (an Object) and returns a String exotic object. It is used to specify the creation of new String exotic objects. It performs the following steps when called:

  1. Let S be MakeBasicObject(« [[Prototype]], [[Extensible]], [[StringData]] »).
  2. Set S.[[Prototype]] to prototype.
  3. Set S.[[StringData]] to value.
  4. Set S.[[GetOwnProperty]] as specified in 10.4.3.1.
  5. Set S.[[DefineOwnProperty]] as specified in 10.4.3.2.
  6. Set S.[[OwnPropertyKeys]] as specified in 10.4.3.3.
  7. Let length be the length of value.
  8. Perform ! DefinePropertyOrThrow(S, "length", PropertyDescriptor { [[Value]]: 𝔽(length), [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }).
  9. Return S.

10.4.3.5 StringGetOwnProperty ( S, P )

The abstract operation StringGetOwnProperty takes arguments S (an Object that has a [[StringData]] internal slot) and P (a property key) and returns a Property Descriptor or undefined. It performs the following steps when called:

  1. If P is not a String, return undefined.
  2. Let index be CanonicalNumericIndexString(P).
  3. If index is undefined, return undefined.
  4. If IsIntegralNumber(index) is false, return undefined.
  5. If index is -0𝔽, return undefined.
  6. Let str be S.[[StringData]].
  7. Assert: str is a String.
  8. Let len be the length of str.
  9. If ℝ(index) < 0 or len ≤ ℝ(index), return undefined.
  10. Let resultStr be the substring of str from ℝ(index) to ℝ(index) 1.
  11. Return the PropertyDescriptor { [[Value]]: resultStr, [[Writable]]: false, [[Enumerable]]: true, [[Configurable]]: false }.

10.4.4 Arguments Exotic Objects

Most ECMAScript functions make an arguments object available to their code. Depending upon the characteristics of the function definition, its arguments object is either an ordinary object or an arguments exotic object. An arguments exotic object is an exotic object whose array index properties map to the formal parameters bindings of an invocation of its associated ECMAScript function.

An object is an arguments exotic object if its internal methods use the following implementations, with the ones not specified here using those found in 10.1. These methods are installed in CreateMappedArgumentsObject.

Note 1

While CreateUnmappedArgumentsObject is grouped into this clause, it creates an ordinary object, not an arguments exotic object.

Arguments exotic objects have the same internal slots as ordinary objects. They also have a [[ParameterMap]] internal slot. Ordinary arguments objects also have a [[ParameterMap]] internal slot whose value is always undefined. For ordinary argument objects the [[ParameterMap]] internal slot is only used by Object.prototype.toString (20.1.3.6) to identify them as such.

Note 2

The integer-indexed data properties of an arguments exotic object whose numeric name values are less than the number of formal parameters of the corresponding function object initially share their values with the corresponding argument bindings in the function's execution context. This means that changing the property changes the corresponding value of the argument binding and vice-versa. This correspondence is broken if such a property is deleted and then redefined or if the property is changed into an accessor property. If the arguments object is an ordinary object, the values of its properties are simply a copy of the arguments passed to the function and there is no dynamic linkage between the property values and the formal parameter values.

 Note 3

The ParameterMap object and its property values are used as a device for specifying the arguments object correspondence to argument bindings. The ParameterMap object and the objects that are the values of its properties are not directly observable from ECMAScript code. An ECMAScript implementation does not need to actually create or use such objects to implement the specified semantics.

 Note 4

Ordinary arguments objects define a non-configurable accessor property named "callee" which throws a TypeError exception on access. The "callee" property has a more specific meaning for arguments exotic objects, which are created only for some class of non-strict functions. The definition of this property in the ordinary variant exists to ensure that it is not defined in any other manner by conforming ECMAScript implementations.

 Note 5

ECMAScript implementations of arguments exotic objects have historically contained an accessor property named "caller". Prior to ECMAScript 2017, this specification included the definition of a throwing "caller" property on ordinary arguments objects. Since implementations do not contain this extension any longer, ECMAScript 2017 dropped the requirement for a throwing "caller" accessor.

10.4.4.1 [[GetOwnProperty]] ( P )

The [[GetOwnProperty]] internal method of an arguments exotic object args takes argument P (a property key) and returns a normal completion containing either a Property Descriptor or undefined. It performs the following steps when called:

  1. Let desc be OrdinaryGetOwnProperty(args, P).
  2. If desc is undefined, return desc.
  3. Let map be args.[[ParameterMap]].
  4. Let isMapped be ! HasOwnProperty(map, P).
  5. If isMapped is true, then
    1. Set desc.[[Value]] to ! Get(map, P).
  6. Return desc.

10.4.4.2 [[DefineOwnProperty]] ( P, Desc )

The [[DefineOwnProperty]] internal method of an arguments exotic object args takes arguments P (a property key) and Desc (a Property Descriptor) and returns a normal completion containing a Boolean. It performs the following steps when called:

  1. Let map be args.[[ParameterMap]].
  2. Let isMapped be ! HasOwnProperty(map, P).
  3. Let newArgDesc be Desc.
  4. If isMapped is true and IsDataDescriptor(Desc) is true, then
    1. If Desc does not have a [[Value]] field, Desc has a [[Writable]] field, and Desc.[[Writable]] is false, then
      1. Set newArgDesc to a copy of Desc.
      2. Set newArgDesc.[[Value]] to ! Get(map, P).
  5. Let allowed be ! OrdinaryDefineOwnProperty(args, P, newArgDesc).
  6. If allowed is false, return false.
  7. If isMapped is true, then
    1. If IsAccessorDescriptor(Desc) is true, then
      1. Perform ! map.[[Delete]](P).
    2. Else,
      1. If Desc has a [[Value]] field, then
        1. Assert: The following Set will succeed, since formal parameters mapped by arguments objects are always writable.
        2. Perform ! Set(map, P, Desc.[[Value]], false).
      2. If Desc has a [[Writable]] field and Desc.[[Writable]] is false, then
        1. Perform ! map.[[Delete]](P).
  8. Return true.

10.4.4.3 [[Get]] ( P, Receiver )

The [[Get]] internal method of an arguments exotic object args takes arguments P (a property key) and Receiver (an ECMAScript language value) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

  1. Let map be args.[[ParameterMap]].
  2. Let isMapped be ! HasOwnProperty(map, P).
  3. If isMapped is false, then
    1. Return ? OrdinaryGet(args, P, Receiver).
  4. Else,
    1. Assert: map contains a formal parameter mapping for P.
    2. Return ! Get(map, P).

10.4.4.4 [[Set]] ( P, V, Receiver )

The [[Set]] internal method of an arguments exotic object args takes arguments P (a property key), V (an ECMAScript language value), and Receiver (an ECMAScript language value) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

  1. If SameValue(args, Receiver) is false, then
    1. Let isMapped be false.
  2. Else,
    1. Let map be args.[[ParameterMap]].
    2. Let isMapped be ! HasOwnProperty(map, P).
  3. If isMapped is true, then
    1. Assert: The following Set will succeed, since formal parameters mapped by arguments objects are always writable.
    2. Perform ! Set(map, P, V, false).
  4. Return ? OrdinarySet(args, P, V, Receiver).

10.4.4.5 [[Delete]] ( P )

The [[Delete]] internal method of an arguments exotic object args takes argument P (a property key) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

  1. Let map be args.[[ParameterMap]].
  2. Let isMapped be ! HasOwnProperty(map, P).
  3. Let result be ? OrdinaryDelete(args, P).
  4. If result is true and isMapped is true, then
    1. Perform ! map.[[Delete]](P).
  5. Return result.

10.4.4.6 CreateUnmappedArgumentsObject ( argumentsList )

The abstract operation CreateUnmappedArgumentsObject takes argument argumentsList (a List of ECMAScript language values) and returns an ordinary object. It performs the following steps when called:

  1. Let len be the number of elements in argumentsList.
  2. Let obj be OrdinaryObjectCreate(%Object.prototype%, « [[ParameterMap]] »).
  3. Set obj.[[ParameterMap]] to undefined.
  4. Perform ! DefinePropertyOrThrow(obj, "length", PropertyDescriptor { [[Value]]: 𝔽(len), [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }).
  5. Let index be 0.
  6. Repeat, while index < len,
    1. Let val be argumentsList[index].
    2. Perform ! CreateDataPropertyOrThrow(obj, ! ToString(𝔽(index)), val).
    3. Set index to index 1.
  7. Perform ! DefinePropertyOrThrow(obj, @@iterator, PropertyDescriptor { [[Value]]: %Array.prototype.values%, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }).
  8. Perform ! DefinePropertyOrThrow(obj, "callee", PropertyDescriptor { [[Get]]: %ThrowTypeError%, [[Set]]: %ThrowTypeError%, [[Enumerable]]: false, [[Configurable]]: false }).
  9. Return obj.

10.4.4.7 CreateMappedArgumentsObject ( func, formals, argumentsList, env )

The abstract operation CreateMappedArgumentsObject takes arguments func (an Object), formals (a Parse Node), argumentsList (a List of ECMAScript language values), and env (an Environment Record) and returns an arguments exotic object. It performs the following steps when called:

  1. Assert: formals does not contain a rest parameter, any binding patterns, or any initializers. It may contain duplicate identifiers.
  2. Let len be the number of elements in argumentsList.
  3. Let obj be MakeBasicObject(« [[Prototype]], [[Extensible]], [[ParameterMap]] »).
  4. Set obj.[[GetOwnProperty]] as specified in 10.4.4.1.
  5. Set obj.[[DefineOwnProperty]] as specified in 10.4.4.2.
  6. Set obj.[[Get]] as specified in 10.4.4.3.
  7. Set obj.[[Set]] as specified in 10.4.4.4.
  8. Set obj.[[Delete]] as specified in 10.4.4.5.
  9. Set obj.[[Prototype]] to %Object.prototype%.
  10. Let map be OrdinaryObjectCreate(null).
  11. Set obj.[[ParameterMap]] to map.
  12. Let parameterNames be the BoundNames of formals.
  13. Let numberOfParameters be the number of elements in parameterNames.
  14. Let index be 0.
  15. Repeat, while index < len,
    1. Let val be argumentsList[index].
    2. Perform ! CreateDataPropertyOrThrow(obj, ! ToString(𝔽(index)), val).
    3. Set index to index 1.
  16. Perform ! DefinePropertyOrThrow(obj, "length", PropertyDescriptor { [[Value]]: 𝔽(len), [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }).
  17. Let mappedNames be a new empty List.
  18. Set index to numberOfParameters - 1.
  19. Repeat, while index ≥ 0,
    1. Let name be parameterNames[index].
    2. If mappedNames does not contain name, then
      1. Append name to mappedNames.
      2. If index < len, then
        1. Let g be MakeArgGetter(name, env).
        2. Let p be MakeArgSetter(name, env).
        3. Perform ! map.[[DefineOwnProperty]](! ToString(𝔽(index)), PropertyDescriptor { [[Set]]: p, [[Get]]: g, [[Enumerable]]: false, [[Configurable]]: true }).
    3. Set index to index - 1.
  20. Perform ! DefinePropertyOrThrow(obj, @@iterator, PropertyDescriptor { [[Value]]: %Array.prototype.values%, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }).
  21. Perform ! DefinePropertyOrThrow(obj, "callee", PropertyDescriptor { [[Value]]: func, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }).
  22. Return obj.

10.4.4.7.1 MakeArgGetter ( name, env )

The abstract operation MakeArgGetter takes arguments name (a String) and env (an Environment Record) and returns a function object. It creates a built-in function object that when executed returns the value bound for name in env. It performs the following steps when called:

  1. Let getterClosure be a new Abstract Closure with no parameters that captures name and env and performs the following steps when called:
    1. Return env.GetBindingValue(name, false).
  2. Let getter be CreateBuiltinFunction(getterClosure, 0, "", « »).
  3. NOTE: getter is never directly accessible to ECMAScript code.
  4. Return getter.

10.4.4.7.2 MakeArgSetter ( name, env )

The abstract operation MakeArgSetter takes arguments name (a String) and env (an Environment Record) and returns a function object. It creates a built-in function object that when executed sets the value bound for name in env. It performs the following steps when called:

  1. Let setterClosure be a new Abstract Closure with parameters (value) that captures name and env and performs the following steps when called:
    1. Return ! env.SetMutableBinding(name, value, false).
  2. Let setter be CreateBuiltinFunction(setterClosure, 1, "", « »).
  3. NOTE: setter is never directly accessible to ECMAScript code.
  4. Return setter.

10.4.5 Integer-Indexed Exotic Objects

An Integer-Indexed exotic object is an exotic object that performs special handling of integer index property keys.

Integer-Indexed exotic objects have the same internal slots as ordinary objects and additionally [[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]], [[ContentType]], and [[TypedArrayName]] internal slots.

An object is an Integer-Indexed exotic object if its [[GetOwnProperty]], [[HasProperty]], [[DefineOwnProperty]], [[Get]], [[Set]], [[Delete]], and [[OwnPropertyKeys]] internal methods use the definitions in this section, and its other essential internal methods use the definitions found in 10.1. These methods are installed by IntegerIndexedObjectCreate.

10.4.5.1 [[GetOwnProperty]] ( P )

The [[GetOwnProperty]] internal method of an Integer-Indexed exotic object O takes argument P (a property key) and returns a normal completion containing either a Property Descriptor or undefined. It performs the following steps when called:

  1. If P is a String, then
    1. Let numericIndex be CanonicalNumericIndexString(P).
    2. If numericIndex is not undefined, then
      1. Let value be IntegerIndexedElementGet(O, numericIndex).
      2. If value is undefined, return undefined.
      3. Return the PropertyDescriptor { [[Value]]: value, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true }.
  2. Return OrdinaryGetOwnProperty(O, P).

10.4.5.2 [[HasProperty]] ( P )

The [[HasProperty]] internal method of an Integer-Indexed exotic object O takes argument P (a property key) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

  1. If P is a String, then
    1. Let numericIndex be CanonicalNumericIndexString(P).
    2. If numericIndex is not undefined, return IsValidIntegerIndex(O, numericIndex).
  2. Return ? OrdinaryHasProperty(O, P).

10.4.5.3 [[DefineOwnProperty]] ( P, Desc )

The [[DefineOwnProperty]] internal method of an Integer-Indexed exotic object O takes arguments P (a property key) and Desc (a Property Descriptor) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

  1. If P is a String, then
    1. Let numericIndex be CanonicalNumericIndexString(P).
    2. If numericIndex is not undefined, then
      1. If IsValidIntegerIndex(O, numericIndex) is false, return false.
      2. If Desc has a [[Configurable]] field and Desc.[[Configurable]] is false, return false.
      3. If Desc has an [[Enumerable]] field and Desc.[[Enumerable]] is false, return false.
      4. If IsAccessorDescriptor(Desc) is true, return false.
      5. If Desc has a [[Writable]] field and Desc.[[Writable]] is false, return false.
      6. If Desc has a [[Value]] field, perform ? IntegerIndexedElementSet(O, numericIndex, Desc.[[Value]]).
      7. Return true.
  2. Return ! OrdinaryDefineOwnProperty(O, P, Desc).

10.4.5.4 [[Get]] ( P, Receiver )

The [[Get]] internal method of an Integer-Indexed exotic object O takes arguments P (a property key) and Receiver (an ECMAScript language value) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

  1. If P is a String, then
    1. Let numericIndex be CanonicalNumericIndexString(P).
    2. If numericIndex is not undefined, then
      1. Return IntegerIndexedElementGet(O, numericIndex).
  2. Return ? OrdinaryGet(O, P, Receiver).

10.4.5.5 [[Set]] ( P, V, Receiver )

The [[Set]] internal method of an Integer-Indexed exotic object O takes arguments P (a property key), V (an ECMAScript language value), and Receiver (an ECMAScript language value) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

  1. If P is a String, then
    1. Let numericIndex be CanonicalNumericIndexString(P).
    2. If numericIndex is not undefined, then
      1. If SameValue(O, Receiver) is true, then
        1. Perform ? IntegerIndexedElementSet(O, numericIndex, V).
        2. Return true.
      2. If IsValidIntegerIndex(O, numericIndex) is false, return true.
  2. Return ? OrdinarySet(O, P, V, Receiver).

10.4.5.6 [[Delete]] ( P )

The [[Delete]] internal method of an Integer-Indexed exotic object O takes argument P (a property key) and returns a normal completion containing a Boolean. It performs the following steps when called:

  1. If P is a String, then
    1. Let numericIndex be CanonicalNumericIndexString(P).
    2. If numericIndex is not undefined, then
      1. If IsValidIntegerIndex(O, numericIndex) is false, return true; else return false.
  2. Return ! OrdinaryDelete(O, P).

10.4.5.7 [[OwnPropertyKeys]] ( )

The [[OwnPropertyKeys]] internal method of an Integer-Indexed exotic object O takes no arguments and returns a normal completion containing a List of property keys. It performs the following steps when called:

  1. Let keys be a new empty List.
  2. If IsDetachedBuffer(O.[[ViewedArrayBuffer]]) is false, then
    1. For each integer i such that 0 ≤ i < O.[[ArrayLength]], in ascending order, do
      1. Append ! ToString(𝔽(i)) to keys.
  3. For each own property key P of O such that P is a String and P is not an integer index, in ascending chronological order of property creation, do
    1. Append P to keys.
  4. For each own property key P of O such that P is a Symbol, in ascending chronological order of property creation, do
    1. Append P to keys.
  5. Return keys.

10.4.5.8 IntegerIndexedObjectCreate ( prototype )

The abstract operation IntegerIndexedObjectCreate takes argument prototype (an Object) and returns an Integer-Indexed exotic object. It is used to specify the creation of new Integer-Indexed exotic objects. It performs the following steps when called:

  1. Let internalSlotsList be « [[Prototype]], [[Extensible]], [[ViewedArrayBuffer]], [[TypedArrayName]], [[ContentType]], [[ByteLength]], [[ByteOffset]], [[ArrayLength]] ».
  2. Let A be MakeBasicObject(internalSlotsList).
  3. Set A.[[GetOwnProperty]] as specified in 10.4.5.1.
  4. Set A.[[HasProperty]] as specified in 10.4.5.2.
  5. Set A.[[DefineOwnProperty]] as specified in 10.4.5.3.
  6. Set A.[[Get]] as specified in 10.4.5.4.
  7. Set A.[[Set]] as specified in 10.4.5.5.
  8. Set A.[[Delete]] as specified in 10.4.5.6.
  9. Set A.[[OwnPropertyKeys]] as specified in 10.4.5.7.
  10. Set A.[[Prototype]] to prototype.
  11. Return A.

10.4.5.9 IsValidIntegerIndex ( O, index )

The abstract operation IsValidIntegerIndex takes arguments O (an Integer-Indexed exotic object) and index (a Number) and returns a Boolean. It performs the following steps when called:

  1. If IsDetachedBuffer(O.[[ViewedArrayBuffer]]) is true, return false.
  2. If IsIntegralNumber(index) is false, return false.
  3. If index is -0𝔽, return false.
  4. If ℝ(index) < 0 or ℝ(index) ≥ O.[[ArrayLength]], return false.
  5. Return true.

10.4.5.10 IntegerIndexedElementGet ( O, index )

The abstract operation IntegerIndexedElementGet takes arguments O (an Integer-Indexed exotic object) and index (a Number) and returns a Number, a BigInt, or undefined. It performs the following steps when called:

  1. If IsValidIntegerIndex(O, index) is false, return undefined.
  2. Let offset be O.[[ByteOffset]].
  3. Let elementSize be TypedArrayElementSize(O).
  4. Let indexedPosition be (ℝ(index) × elementSize) offset.
  5. Let elementType be TypedArrayElementType(O).
  6. Return GetValueFromBuffer(O.[[ViewedArrayBuffer]], indexedPosition, elementType, true, Unordered).

10.4.5.11 IntegerIndexedElementSet ( O, index, value )

The abstract operation IntegerIndexedElementSet takes arguments O (an Integer-Indexed exotic object), index (a Number), and value (an ECMAScript language value) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

  1. If O.[[ContentType]] is BigInt, let numValue be ? ToBigInt(value).
  2. Otherwise, let numValue be ? ToNumber(value).
  3. If IsValidIntegerIndex(O, index) is true, then
    1. Let offset be O.[[ByteOffset]].
    2. Let elementSize be TypedArrayElementSize(O).
    3. Let indexedPosition be (ℝ(index) × elementSize) offset.
    4. Let elementType be TypedArrayElementType(O).
    5. Perform SetValueInBuffer(O.[[ViewedArrayBuffer]], indexedPosition, elementType, numValue, true, Unordered).
  4. Return unused.
 Note

This operation always appears to succeed, but it has no effect when attempting to write past the end of a TypedArray or to a TypedArray which is backed by a detached ArrayBuffer.

10.4.6 Module Namespace Exotic Objects

A module namespace exotic object is an exotic object that exposes the bindings exported from an ECMAScript Module (See 16.2.3). There is a one-to-one correspondence between the String-keyed own properties of a module namespace exotic object and the binding names exported by the Module. The exported bindings include any bindings that are indirectly exported using export * export items. Each String-valued own property key is the StringValue of the corresponding exported binding name. These are the only String-keyed properties of a module namespace exotic object. Each such property has the attributes { [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: false }. Module namespace exotic objects are not extensible.

An object is a module namespace exotic object if its [[GetPrototypeOf]], [[SetPrototypeOf]], [[IsExtensible]], [[PreventExtensions]], [[GetOwnProperty]], [[DefineOwnProperty]], [[HasProperty]], [[Get]], [[Set]], [[Delete]], and [[OwnPropertyKeys]] internal methods use the definitions in this section, and its other essential internal methods use the definitions found in 10.1. These methods are installed by ModuleNamespaceCreate.

Module namespace exotic objects have the internal slots defined in Table 32.

Table 32: Internal Slots of Module Namespace Exotic Objects
Internal Slot Type Description
[[Module]] a Module Record The Module Record whose exports this namespace exposes.
[[Exports]] a List of Strings A List whose elements are the String values of the exported names exposed as own properties of this object. The list is ordered as if an Array of those String values had been sorted using %Array.prototype.sort% using undefined as comparefn.

10.4.6.1 [[GetPrototypeOf]] ( )

The [[GetPrototypeOf]] internal method of a module namespace exotic object takes no arguments and returns a normal completion containing null. It performs the following steps when called:

  1. Return null.

10.4.6.2 [[SetPrototypeOf]] ( V )

The [[SetPrototypeOf]] internal method of a module namespace exotic object O takes argument V (an Object or null) and returns a normal completion containing a Boolean. It performs the following steps when called:

  1. Return ! SetImmutablePrototype(O, V).

10.4.6.3 [[IsExtensible]] ( )

The [[IsExtensible]] internal method of a module namespace exotic object takes no arguments and returns a normal completion containing false. It performs the following steps when called:

  1. Return false.

10.4.6.4 [[PreventExtensions]] ( )

The [[PreventExtensions]] internal method of a module namespace exotic object takes no arguments and returns a normal completion containing true. It performs the following steps when called:

  1. Return true.

10.4.6.5 [[GetOwnProperty]] ( P )

The [[GetOwnProperty]] internal method of a module namespace exotic object O takes argument P (a property key) and returns either a normal completion containing either a Property Descriptor or undefined, or a throw completion. It performs the following steps when called:

  1. If P is a Symbol, return OrdinaryGetOwnProperty(O, P).
  2. Let exports be O.[[Exports]].
  3. If exports does not contain P, return undefined.
  4. Let value be ? O.[[Get]](P, O).
  5. Return PropertyDescriptor { [[Value]]: value, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: false }.

10.4.6.6 [[DefineOwnProperty]] ( P, Desc )

The [[DefineOwnProperty]] internal method of a module namespace exotic object O takes arguments P (a property key) and Desc (a Property Descriptor) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

  1. If P is a Symbol, return ! OrdinaryDefineOwnProperty(O, P, Desc).
  2. Let current be ? O.[[GetOwnProperty]](P).
  3. If current is undefined, return false.
  4. If Desc has a [[Configurable]] field and Desc.[[Configurable]] is true, return false.
  5. If Desc has an [[Enumerable]] field and Desc.[[Enumerable]] is false, return false.
  6. If IsAccessorDescriptor(Desc) is true, return false.
  7. If Desc has a [[Writable]] field and Desc.[[Writable]] is false, return false.
  8. If Desc has a [[Value]] field, return SameValue(Desc.[[Value]], current.[[Value]]).
  9. Return true.

10.4.6.7 [[HasProperty]] ( P )

The [[HasProperty]] internal method of a module namespace exotic object O takes argument P (a property key) and returns a normal completion containing a Boolean. It performs the following steps when called:

  1. If P is a Symbol, return ! OrdinaryHasProperty(O, P).
  2. Let exports be O.[[Exports]].
  3. If exports contains P, return true.
  4. Return false.

10.4.6.8 [[Get]] ( P, Receiver )

The [[Get]] internal method of a module namespace exotic object O takes arguments P (a property key) and Receiver (an ECMAScript language value) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

  1. If P is a Symbol, then
    1. Return ! OrdinaryGet(O, P, Receiver).
  2. Let exports be O.[[Exports]].
  3. If exports does not contain P, return undefined.
  4. Let m be O.[[Module]].
  5. Let binding be m.ResolveExport(P).
  6. Assert: binding is a ResolvedBinding Record.
  7. Let targetModule be binding.[[Module]].
  8. Assert: targetModule is not undefined.
  9. If binding.[[BindingName]] is namespace, then
    1. Return GetModuleNamespace(targetModule).
  10. Let targetEnv be targetModule.[[Environment]].
  11. If targetEnv is empty, throw a ReferenceError exception.
  12. Return ? targetEnv.GetBindingValue(binding.[[BindingName]], true).
 Note

ResolveExport is side-effect free. Each time this operation is called with a specific exportName, resolveSet pair as arguments it must return the same result. An implementation might choose to pre-compute or cache the ResolveExport results for the [[Exports]] of each module namespace exotic object.

10.4.6.9 [[Set]] ( P, V, Receiver )

The [[Set]] internal method of a module namespace exotic object takes arguments P (a property key), V (an ECMAScript language value), and Receiver (an ECMAScript language value) and returns a normal completion containing false. It performs the following steps when called:

  1. Return false.

10.4.6.10 [[Delete]] ( P )

The [[Delete]] internal method of a module namespace exotic object O takes argument P (a property key) and returns a normal completion containing a Boolean. It performs the following steps when called:

  1. If P is a Symbol, then
    1. Return ! OrdinaryDelete(O, P).
  2. Let exports be O.[[Exports]].
  3. If exports contains P, return false.
  4. Return true.

10.4.6.11 [[OwnPropertyKeys]] ( )

The [[OwnPropertyKeys]] internal method of a module namespace exotic object O takes no arguments and returns a normal completion containing a List of property keys. It performs the following steps when called:

  1. Let exports be O.[[Exports]].
  2. Let symbolKeys be OrdinaryOwnPropertyKeys(O).
  3. Return the list-concatenation of exports and symbolKeys.

10.4.6.12 ModuleNamespaceCreate ( module, exports )

The abstract operation ModuleNamespaceCreate takes arguments module (a Module Record) and exports (a List of Strings) and returns a module namespace exotic object. It is used to specify the creation of new module namespace exotic objects. It performs the following steps when called:

  1. Assert: module.[[Namespace]] is empty.
  2. Let internalSlotsList be the internal slots listed in Table 32.
  3. Let M be MakeBasicObject(internalSlotsList).
  4. Set M's essential internal methods to the definitions specified in 10.4.6.
  5. Set M.[[Module]] to module.
  6. Let sortedExports be a List whose elements are the elements of exports ordered as if an Array of the same values had been sorted using %Array.prototype.sort% using undefined as comparefn.
  7. Set M.[[Exports]] to sortedExports.
  8. Create own properties of M corresponding to the definitions in 28.3.
  9. Set module.[[Namespace]] to M.
  10. Return M.

10.4.7 Immutable Prototype Exotic Objects

An immutable prototype exotic object is an exotic object that has a [[Prototype]] internal slot that will not change once it is initialized.

An object is an immutable prototype exotic object if its [[SetPrototypeOf]] internal method uses the following implementation. (Its other essential internal methods may use any implementation, depending on the specific immutable prototype exotic object in question.)

Note

Unlike other exotic objects, there is not a dedicated creation abstract operation provided for immutable prototype exotic objects. This is because they are only used by %Object.prototype% and by host environments, and in host environments, the relevant objects are potentially exotic in other ways and thus need their own dedicated creation operation.

10.4.7.1 [[SetPrototypeOf]] ( V )

The [[SetPrototypeOf]] internal method of an immutable prototype exotic object O takes argument V (an Object or null) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

  1. Return ? SetImmutablePrototype(O, V).

10.4.7.2 SetImmutablePrototype ( O, V )

The abstract operation SetImmutablePrototype takes arguments O (an Object) and V (an Object or null) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

  1. Let current be ? O.[[GetPrototypeOf]]().
  2. If SameValue(V, current) is true, return true.
  3. Return false.

10.5 Proxy Object Internal Methods and Internal Slots

A Proxy object is an exotic object whose essential internal methods are partially implemented using ECMAScript code. Every Proxy object has an internal slot called [[ProxyHandler]]. The value of [[ProxyHandler]] is an object, called the proxy's handler object, or null. Methods (see Table 33) of a handler object may be used to augment the implementation for one or more of the Proxy object's internal methods. Every Proxy object also has an internal slot called [[ProxyTarget]] whose value is either an object or the null value. This object is called the proxy's target object.

An object is a Proxy exotic object if its essential internal methods (including [[Call]] and [[Construct]], if applicable) use the definitions in this section. These internal methods are installed in ProxyCreate.

Table 33: Proxy Handler Methods
Internal Method Handler Method
[[GetPrototypeOf]] getPrototypeOf
[[SetPrototypeOf]] setPrototypeOf
[[IsExtensible]] isExtensible
[[PreventExtensions]] preventExtensions
[[GetOwnProperty]] getOwnPropertyDescriptor
[[DefineOwnProperty]] defineProperty
[[HasProperty]] has
[[Get]] get
[[Set]] set
[[Delete]] deleteProperty
[[OwnPropertyKeys]] ownKeys
[[Call]] apply
[[Construct]] construct

When a handler method is called to provide the implementation of a Proxy object internal method, the handler method is passed the proxy's target object as a parameter. A proxy's handler object does not necessarily have a method corresponding to every essential internal method. Invoking an internal method on the proxy results in the invocation of the corresponding internal method on the proxy's target object if the handler object does not have a method corresponding to the internal trap.

The [[ProxyHandler]] and [[ProxyTarget]] internal slots of a Proxy object are always initialized when the object is created and typically may not be modified. Some Proxy objects are created in a manner that permits them to be subsequently revoked. When a proxy is revoked, its [[ProxyHandler]] and [[ProxyTarget]] internal slots are set to null causing subsequent invocations of internal methods on that Proxy object to throw a TypeError exception.

Because Proxy objects permit the implementation of internal methods to be provided by arbitrary ECMAScript code, it is possible to define a Proxy object whose handler methods violates the invariants defined in 6.1.7.3. Some of the internal method invariants defined in 6.1.7.3 are essential integrity invariants. These invariants are explicitly enforced by the Proxy object internal methods specified in this section. An ECMAScript implementation must be robust in the presence of all possible invariant violations.

In the following algorithm descriptions, assume O is an ECMAScript Proxy object, P is a property key value, V is any ECMAScript language value and Desc is a Property Descriptor record.

10.5.1 [[GetPrototypeOf]] ( )

The [[GetPrototypeOf]] internal method of a Proxy exotic object O takes no arguments and returns either a normal completion containing either an Object or null, or a throw completion. It performs the following steps when called:

  1. Perform ? ValidateNonRevokedProxy(O).
  2. Let target be O.[[ProxyTarget]].
  3. Let handler be O.[[ProxyHandler]].
  4. Assert: handler is an Object.
  5. Let trap be ? GetMethod(handler, "getPrototypeOf").
  6. If trap is undefined, then
    1. Return ? target.[[GetPrototypeOf]]().
  7. Let handlerProto be ? Call(trap, handler, « target »).
  8. If handlerProto is not an Object and handlerProto is not null, throw a TypeError exception.
  9. Let extensibleTarget be ? IsExtensible(target).
  10. If extensibleTarget is true, return handlerProto.
  11. Let targetProto be ? target.[[GetPrototypeOf]]().
  12. If SameValue(handlerProto, targetProto) is false, throw a TypeError exception.
  13. Return handlerProto.
 Note

[[GetPrototypeOf]] for Proxy objects enforces the following invariants:

  • The result of [[GetPrototypeOf]] must be either an Object or null.
  • If the target object is not extensible, [[GetPrototypeOf]] applied to the Proxy object must return the same value as [[GetPrototypeOf]] applied to the Proxy object's target object.

10.5.2 [[SetPrototypeOf]] ( V )

The [[SetPrototypeOf]] internal method of a Proxy exotic object O takes argument V (an Object or null) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

  1. Perform ? ValidateNonRevokedProxy(O).
  2. Let target be O.[[ProxyTarget]].
  3. Let handler be O.[[ProxyHandler]].
  4. Assert: handler is an Object.
  5. Let trap be ? GetMethod(handler, "setPrototypeOf").
  6. If trap is undefined, then
    1. Return ? target.[[SetPrototypeOf]](V).
  7. Let booleanTrapResult be ToBoolean(? Call(trap, handler, « target, V »)).
  8. If booleanTrapResult is false, return false.
  9. Let extensibleTarget be ? IsExtensible(target).
  10. If extensibleTarget is true, return true.
  11. Let targetProto be ? target.[[GetPrototypeOf]]().
  12. If SameValue(V, targetProto) is false, throw a TypeError exception.
  13. Return true.
 Note

[[SetPrototypeOf]] for Proxy objects enforces the following invariants:

  • The result of [[SetPrototypeOf]] is a Boolean value.
  • If the target object is not extensible, the argument value must be the same as the result of [[GetPrototypeOf]] applied to target object.

10.5.3 [[IsExtensible]] ( )

The [[IsExtensible]] internal method of a Proxy exotic object O takes no arguments and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

  1. Perform ? ValidateNonRevokedProxy(O).
  2. Let target be O.[[ProxyTarget]].
  3. Let handler be O.[[ProxyHandler]].
  4. Assert: handler is an Object.
  5. Let trap be ? GetMethod(handler, "isExtensible").
  6. If trap is undefined, then
    1. Return ? IsExtensible(target).
  7. Let booleanTrapResult be ToBoolean(? Call(trap, handler, « target »)).
  8. Let targetResult be ? IsExtensible(target).
  9. If SameValue(booleanTrapResult, targetResult) is false, throw a TypeError exception.
  10. Return booleanTrapResult.
 Note

[[IsExtensible]] for Proxy objects enforces the following invariants:

  • The result of [[IsExtensible]] is a Boolean value.
  • [[IsExtensible]] applied to the Proxy object must return the same value as [[IsExtensible]] applied to the Proxy object's target object with the same argument.

10.5.4 [[PreventExtensions]] ( )

The [[PreventExtensions]] internal method of a Proxy exotic object O takes no arguments and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

  1. Perform ? ValidateNonRevokedProxy(O).
  2. Let target be O.[[ProxyTarget]].
  3. Let handler be O.[[ProxyHandler]].
  4. Assert: handler is an Object.
  5. Let trap be ? GetMethod(handler, "preventExtensions").
  6. If trap is undefined, then
    1. Return ? target.[[PreventExtensions]]().
  7. Let booleanTrapResult be ToBoolean(? Call(trap, handler, « target »)).
  8. If booleanTrapResult is true, then
    1. Let extensibleTarget be ? IsExtensible(target).
    2. If extensibleTarget is true, throw a TypeError exception.
  9. Return booleanTrapResult.
 Note

[[PreventExtensions]] for Proxy objects enforces the following invariants:

  • The result of [[PreventExtensions]] is a Boolean value.
  • [[PreventExtensions]] applied to the Proxy object only returns true if [[IsExtensible]] applied to the Proxy object's target object is false.

10.5.5 [[GetOwnProperty]] ( P )

The [[GetOwnProperty]] internal method of a Proxy exotic object O takes argument P (a property key) and returns either a normal completion containing either a Property Descriptor or undefined, or a throw completion. It performs the following steps when called:

  1. Perform ? ValidateNonRevokedProxy(O).
  2. Let target be O.[[ProxyTarget]].
  3. Let handler be O.[[ProxyHandler]].
  4. Assert: handler is an Object.
  5. Let trap be ? GetMethod(handler, "getOwnPropertyDescriptor").
  6. If trap is undefined, then
    1. Return ? target.[[GetOwnProperty]](P).
  7. Let trapResultObj be ? Call(trap, handler, « target, P »).
  8. If trapResultObj is not an Object and trapResultObj is not undefined, throw a TypeError exception.
  9. Let targetDesc be ? target.[[GetOwnProperty]](P).
  10. If trapResultObj is undefined, then
    1. If targetDesc is undefined, return undefined.
    2. If targetDesc.[[Configurable]] is false, throw a TypeError exception.
    3. Let extensibleTarget be ? IsExtensible(target).
    4. If extensibleTarget is false, throw a TypeError exception.
    5. Return undefined.
  11. Let extensibleTarget be ? IsExtensible(target).
  12. Let resultDesc be ? ToPropertyDescriptor(trapResultObj).
  13. Perform CompletePropertyDescriptor(resultDesc).
  14. Let valid be IsCompatiblePropertyDescriptor(extensibleTarget, resultDesc, targetDesc).
  15. If valid is false, throw a TypeError exception.
  16. If resultDesc.[[Configurable]] is false, then
    1. If targetDesc is undefined or targetDesc.[[Configurable]] is true, then
      1. Throw a TypeError exception.
    2. If resultDesc has a [[Writable]] field and resultDesc.[[Writable]] is false, then
      1. Assert: targetDesc has a [[Writable]] field.
      2. If targetDesc.[[Writable]] is true, throw a TypeError exception.
  17. Return resultDesc.
 Note

[[GetOwnProperty]] for Proxy objects enforces the following invariants:

  • The result of [[GetOwnProperty]] must be either an Object or undefined.
  • A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.
  • A property cannot be reported as non-existent, if it exists as an own property of a non-extensible target object.
  • A property cannot be reported as existent, if it does not exist as an own property of the target object and the target object is not extensible.
  • A property cannot be reported as non-configurable, unless it exists as a non-configurable own property of the target object.
  • A property cannot be reported as both non-configurable and non-writable, unless it exists as a non-configurable, non-writable own property of the target object.

10.5.6 [[DefineOwnProperty]] ( P, Desc )

The [[DefineOwnProperty]] internal method of a Proxy exotic object O takes arguments P (a property key) and Desc (a Property Descriptor) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

  1. Perform ? ValidateNonRevokedProxy(O).
  2. Let target be O.[[ProxyTarget]].
  3. Let handler be O.[[ProxyHandler]].
  4. Assert: handler is an Object.
  5. Let trap be ? GetMethod(handler, "defineProperty").
  6. If trap is undefined, then
    1. Return ? target.[[DefineOwnProperty]](P, Desc).
  7. Let descObj be FromPropertyDescriptor(Desc).
  8. Let booleanTrapResult be ToBoolean(? Call(trap, handler, « target, P, descObj »)).
  9. If booleanTrapResult is false, return false.
  10. Let targetDesc be ? target.[[GetOwnProperty]](P).
  11. Let extensibleTarget be ? IsExtensible(target).
  12. If Desc has a [[Configurable]] field and Desc.[[Configurable]] is false, then
    1. Let settingConfigFalse be true.
  13. Else, let settingConfigFalse be false.
  14. If targetDesc is undefined, then
    1. If extensibleTarget is false, throw a TypeError exception.
    2. If settingConfigFalse is true, throw a TypeError exception.
  15. Else,
    1. If IsCompatiblePropertyDescriptor(extensibleTarget, Desc, targetDesc) is false, throw a TypeError exception.
    2. If settingConfigFalse is true and targetDesc.[[Configurable]] is true, throw a TypeError exception.
    3. If IsDataDescriptor(targetDesc) is true, targetDesc.[[Configurable]] is false, and targetDesc.[[Writable]] is true, then
      1. If Desc has a [[Writable]] field and Desc.[[Writable]] is false, throw a TypeError exception.
  16. Return true.
 Note

[[DefineOwnProperty]] for Proxy objects enforces the following invariants:

  • The result of [[DefineOwnProperty]] is a Boolean value.
  • A property cannot be added, if the target object is not extensible.
  • A property cannot be non-configurable, unless there exists a corresponding non-configurable own property of the target object.
  • A non-configurable property cannot be non-writable, unless there exists a corresponding non-configurable, non-writable own property of the target object.
  • If a property has a corresponding target object property then applying the Property Descriptor of the property to the target object using [[DefineOwnProperty]] will not throw an exception.

10.5.7 [[HasProperty]] ( P )

The [[HasProperty]] internal method of a Proxy exotic object O takes argument P (a property key) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

  1. Perform ? ValidateNonRevokedProxy(O).
  2. Let target be O.[[ProxyTarget]].
  3. Let handler be O.[[ProxyHandler]].
  4. Assert: handler is an Object.
  5. Let trap be ? GetMethod(handler, "has").
  6. If trap is undefined, then
    1. Return ? target.[[HasProperty]](P).
  7. Let booleanTrapResult be ToBoolean(? Call(trap, handler, « target, P »)).
  8. If booleanTrapResult is false, then
    1. Let targetDesc be ? target.[[GetOwnProperty]](P).
    2. If targetDesc is not undefined, then
      1. If targetDesc.[[Configurable]] is false, throw a TypeError exception.
      2. Let extensibleTarget be ? IsExtensible(target).
      3. If extensibleTarget is false, throw a TypeError exception.
  9. Return booleanTrapResult.
 Note

[[HasProperty]] for Proxy objects enforces the following invariants:

  • The result of [[HasProperty]] is a Boolean value.
  • A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.
  • A property cannot be reported as non-existent, if it exists as an own property of the target object and the target object is not extensible.

10.5.8 [[Get]] ( P, Receiver )

The [[Get]] internal method of a Proxy exotic object O takes arguments P (a property key) and Receiver (an ECMAScript language value) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

  1. Perform ? ValidateNonRevokedProxy(O).
  2. Let target be O.[[ProxyTarget]].
  3. Let handler be O.[[ProxyHandler]].
  4. Assert: handler is an Object.
  5. Let trap be ? GetMethod(handler, "get").
  6. If trap is undefined, then
    1. Return ? target.[[Get]](P, Receiver).
  7. Let trapResult be ? Call(trap, handler, « target, P, Receiver »).
  8. Let targetDesc be ? target.[[GetOwnProperty]](P).
  9. If targetDesc is not undefined and targetDesc.[[Configurable]] is false, then
    1. If IsDataDescriptor(targetDesc) is true and targetDesc.[[Writable]] is false, then
      1. If SameValue(trapResult, targetDesc.[[Value]]) is false, throw a TypeError exception.
    2. If IsAccessorDescriptor(targetDesc) is true and targetDesc.[[Get]] is undefined, then
      1. If trapResult is not undefined, throw a TypeError exception.
  10. Return trapResult.
 Note

[[Get]] for Proxy objects enforces the following invariants:

  • The value reported for a property must be the same as the value of the corresponding target object property if the target object property is a non-writable, non-configurable own data property.
  • The value reported for a property must be undefined if the corresponding target object property is a non-configurable own accessor property that has undefined as its [[Get]] attribute.

10.5.9 [[Set]] ( P, V, Receiver )

The [[Set]] internal method of a Proxy exotic object O takes arguments P (a property key), V (an ECMAScript language value), and Receiver (an ECMAScript language value) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

  1. Perform ? ValidateNonRevokedProxy(O).
  2. Let target be O.[[ProxyTarget]].
  3. Let handler be O.[[ProxyHandler]].
  4. Assert: handler is an Object.
  5. Let trap be ? GetMethod(handler, "set").
  6. If trap is undefined, then
    1. Return ? target.[[Set]](P, V, Receiver).
  7. Let booleanTrapResult be ToBoolean(? Call(trap, handler, « target, P, V, Receiver »)).
  8. If booleanTrapResult is false, return false.
  9. Let targetDesc be ? target.[[GetOwnProperty]](P).
  10. If targetDesc is not undefined and targetDesc.[[Configurable]] is false, then
    1. If IsDataDescriptor(targetDesc) is true and targetDesc.[[Writable]] is false, then
      1. If SameValue(V, targetDesc.[[Value]]) is false, throw a TypeError exception.
    2. If IsAccessorDescriptor(targetDesc) is true, then
      1. If targetDesc.[[Set]] is undefined, throw a TypeError exception.
  11. Return true.
 Note

[[Set]] for Proxy objects enforces the following invariants:

  • The result of [[Set]] is a Boolean value.
  • Cannot change the value of a property to be different from the value of the corresponding target object property if the corresponding target object property is a non-writable, non-configurable own data property.
  • Cannot set the value of a property if the corresponding target object property is a non-configurable own accessor property that has undefined as its [[Set]] attribute.

10.5.10 [[Delete]] ( P )

The [[Delete]] internal method of a Proxy exotic object O takes argument P (a property key) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

  1. Perform ? ValidateNonRevokedProxy(O).
  2. Let target be O.[[ProxyTarget]].
  3. Let handler be O.[[ProxyHandler]].
  4. Assert: handler is an Object.
  5. Let trap be ? GetMethod(handler, "deleteProperty").
  6. If trap is undefined, then
    1. Return ? target.[[Delete]](P).
  7. Let booleanTrapResult be ToBoolean(? Call(trap, handler, « target, P »)).
  8. If booleanTrapResult is false, return false.
  9. Let targetDesc be ? target.[[GetOwnProperty]](P).
  10. If targetDesc is undefined, return true.
  11. If targetDesc.[[Configurable]] is false, throw a TypeError exception.
  12. Let extensibleTarget be ? IsExtensible(target).
  13. If extensibleTarget is false, throw a TypeError exception.
  14. Return true.
 Note

[[Delete]] for Proxy objects enforces the following invariants:

  • The result of [[Delete]] is a Boolean value.
  • A property cannot be reported as deleted, if it exists as a non-configurable own property of the target object.
  • A property cannot be reported as deleted, if it exists as an own property of the target object and the target object is non-extensible.

10.5.11 [[OwnPropertyKeys]] ( )

The [[OwnPropertyKeys]] internal method of a Proxy exotic object O takes no arguments and returns either a normal completion containing a List of property keys or a throw completion. It performs the following steps when called:

  1. Perform ? ValidateNonRevokedProxy(O).
  2. Let target be O.[[ProxyTarget]].
  3. Let handler be O.[[ProxyHandler]].
  4. Assert: handler is an Object.
  5. Let trap be ? GetMethod(handler, "ownKeys").
  6. If trap is undefined, then
    1. Return ? target.[[OwnPropertyKeys]]().
  7. Let trapResultArray be ? Call(trap, handler, « target »).
  8. Let trapResult be ? CreateListFromArrayLike(trapResultArray, « String, Symbol »).
  9. If trapResult contains any duplicate entries, throw a TypeError exception.
  10. Let extensibleTarget be ? IsExtensible(target).
  11. Let targetKeys be ? target.[[OwnPropertyKeys]]().
  12. Assert: targetKeys is a List of property keys.
  13. Assert: targetKeys contains no duplicate entries.
  14. Let targetConfigurableKeys be a new empty List.
  15. Let targetNonconfigurableKeys be a new empty List.
  16. For each element key of targetKeys, do
    1. Let desc be ? target.[[GetOwnProperty]](key).
    2. If desc is not undefined and desc.[[Configurable]] is false, then
      1. Append key to targetNonconfigurableKeys.
    3. Else,
      1. Append key to targetConfigurableKeys.
  17. If extensibleTarget is true and targetNonconfigurableKeys is empty, then
    1. Return trapResult.
  18. Let uncheckedResultKeys be a List whose elements are the elements of trapResult.
  19. For each element key of targetNonconfigurableKeys, do
    1. If uncheckedResultKeys does not contain key, throw a TypeError exception.
    2. Remove key from uncheckedResultKeys.
  20. If extensibleTarget is true, return trapResult.
  21. For each element key of targetConfigurableKeys, do
    1. If uncheckedResultKeys does not contain key, throw a TypeError exception.
    2. Remove key from uncheckedResultKeys.
  22. If uncheckedResultKeys is not empty, throw a TypeError exception.
  23. Return trapResult.
 Note

[[OwnPropertyKeys]] for Proxy objects enforces the following invariants:

  • The result of [[OwnPropertyKeys]] is a List.
  • The returned List contains no duplicate entries.
  • The Type of each result List element is either String or Symbol.
  • The result List must contain the keys of all non-configurable own properties of the target object.
  • If the target object is not extensible, then the result List must contain all the keys of the own properties of the target object and no other values.

10.5.12 [[Call]] ( thisArgument, argumentsList )

The [[Call]] internal method of a Proxy exotic object O takes arguments thisArgument (an ECMAScript language value) and argumentsList (a List of ECMAScript language values) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

  1. Perform ? ValidateNonRevokedProxy(O).
  2. Let target be O.[[ProxyTarget]].
  3. Let handler be O.[[ProxyHandler]].
  4. Assert: handler is an Object.
  5. Let trap be ? GetMethod(handler, "apply").
  6. If trap is undefined, then
    1. Return ? Call(target, thisArgument, argumentsList).
  7. Let argArray be CreateArrayFromList(argumentsList).
  8. Return ? Call(trap, handler, « target, thisArgument, argArray »).
 Note

A Proxy exotic object only has a [[Call]] internal method if the initial value of its [[ProxyTarget]] internal slot is an object that has a [[Call]] internal method.

10.5.13 [[Construct]] ( argumentsList, newTarget )

The [[Construct]] internal method of a Proxy exotic object O takes arguments argumentsList (a List of ECMAScript language values) and newTarget (a constructor) and returns either a normal completion containing an Object or a throw completion. It performs the following steps when called:

  1. Perform ? ValidateNonRevokedProxy(O).
  2. Let target be O.[[ProxyTarget]].
  3. Assert: IsConstructor(target) is true.
  4. Let handler be O.[[ProxyHandler]].
  5. Assert: handler is an Object.
  6. Let trap be ? GetMethod(handler, "construct").
  7. If trap is undefined, then
    1. Return ? Construct(target, argumentsList, newTarget).
  8. Let argArray be CreateArrayFromList(argumentsList).
  9. Let newObj be ? Call(trap, handler, « target, argArray, newTarget »).
  10. If newObj is not an Object, throw a TypeError exception.
  11. Return newObj.
 Note 1

A Proxy exotic object only has a [[Construct]] internal method if the initial value of its [[ProxyTarget]] internal slot is an object that has a [[Construct]] internal method.

 Note 2

[[Construct]] for Proxy objects enforces the following invariants:

  • The result of [[Construct]] must be an Object.

10.5.14 ValidateNonRevokedProxy ( proxy )

The abstract operation ValidateNonRevokedProxy takes argument proxy (a Proxy exotic object) and returns either a normal completion containing unused or a throw completion. It throws a TypeError exception if proxy has been revoked. It performs the following steps when called:

  1. If proxy.[[ProxyTarget]] is null, throw a TypeError exception.
  2. Assert: proxy.[[ProxyHandler]] is not null.
  3. Return unused.

10.5.15 ProxyCreate ( target, handler )

The abstract operation ProxyCreate takes arguments target (an ECMAScript language value) and handler (an ECMAScript language value) and returns either a normal completion containing a Proxy exotic object or a throw completion. It is used to specify the creation of new Proxy objects. It performs the following steps when called:

  1. If target is not an Object, throw a TypeError exception.
  2. If handler is not an Object, throw a TypeError exception.
  3. Let P be MakeBasicObject(« [[ProxyHandler]], [[ProxyTarget]] »).
  4. Set P's essential internal methods, except for [[Call]] and [[Construct]], to the definitions specified in 10.5.
  5. If IsCallable(target) is true, then
    1. Set P.[[Call]] as specified in 10.5.12.
    2. If IsConstructor(target) is true, then
      1. Set P.[[Construct]] as specified in 10.5.13.
  6. Set P.[[ProxyTarget]] to target.
  7. Set P.[[ProxyHandler]] to handler.
  8. Return P.

11 ECMAScript Language: Source Text

11.1 Source Text

Syntax

SourceCharacter :: any Unicode code point

ECMAScript source text is a sequence of Unicode code points. All Unicode code point values from U 0000 to U 10FFFF, including surrogate code points, may occur in ECMAScript source text where permitted by the ECMAScript grammars. The actual encodings used to store and interchange ECMAScript source text is not relevant to this specification. Regardless of the external source text encoding, a conforming ECMAScript implementation processes the source text as if it was an equivalent sequence of SourceCharacter values, each SourceCharacter being a Unicode code point. Conforming ECMAScript implementations are not required to perform any normalization of source text, or behave as though they were performing normalization of source text.

The components of a combining character sequence are treated as individual Unicode code points even though a user might think of the whole sequence as a single character.

Note

In string literals, regular expression literals, template literals and identifiers, any Unicode code point may also be expressed using Unicode escape sequences that explicitly express a code point's numeric value. Within a comment, such an escape sequence is effectively ignored as part of the comment.

ECMAScript differs from the Java programming language in the behaviour of Unicode escape sequences. In a Java program, if the Unicode escape sequence \u000A, for example, occurs within a single-line comment, it is interpreted as a line terminator (Unicode code point U 000A is LINE FEED (LF)) and therefore the next code point is not part of the comment. Similarly, if the Unicode escape sequence \u000A occurs within a string literal in a Java program, it is likewise interpreted as a line terminator, which is not allowed within a string literal—one must write \n instead of \u000A to cause a LINE FEED (LF) to be part of the String value of a string literal. In an ECMAScript program, a Unicode escape sequence occurring within a comment is never interpreted and therefore cannot contribute to termination of the comment. Similarly, a Unicode escape sequence occurring within a string literal in an ECMAScript program always contributes to the literal and is never interpreted as a line terminator or as a code point that might terminate the string literal.

11.1.1 Static Semantics: UTF16EncodeCodePoint ( cp )

The abstract operation UTF16EncodeCodePoint takes argument cp (a Unicode code point) and returns a String. It performs the following steps when called:

  1. Assert: 0 ≤ cp ≤ 0x10FFFF.
  2. If cp ≤ 0xFFFF, return the String value consisting of the code unit whose numeric value is cp.
  3. Let cu1 be the code unit whose numeric value is floor((cp - 0x10000) / 0x400) 0xD800.
  4. Let cu2 be the code unit whose numeric value is ((cp - 0x10000) modulo 0x400) 0xDC00.
  5. Return the string-concatenation of cu1 and cu2.

11.1.2 Static Semantics: CodePointsToString ( text )

The abstract operation CodePointsToString takes argument text (a sequence of Unicode code points) and returns a String. It converts text into a String value, as described in 6.1.4. It performs the following steps when called:

  1. Let result be the empty String.
  2. For each code point cp of text, do
    1. Set result to the string-concatenation of result and UTF16EncodeCodePoint(cp).
  3. Return result.

11.1.3 Static Semantics: UTF16SurrogatePairToCodePoint ( lead, trail )

The abstract operation UTF16SurrogatePairToCodePoint takes arguments lead (a code unit) and trail (a code unit) and returns a code point. Two code units that form a UTF-16 surrogate pair are converted to a code point. It performs the following steps when called:

  1. Assert: lead is a leading surrogate and trail is a trailing surrogate.
  2. Let cp be (lead - 0xD800) × 0x400 (trail - 0xDC00) 0x10000.
  3. Return the code point cp.

11.1.4 Static Semantics: CodePointAt ( string, position )

The abstract operation CodePointAt takes arguments string (a String) and position (a non-negative integer) and returns a Record with fields [[CodePoint]] (a code point), [[CodeUnitCount]] (a positive integer), and [[IsUnpairedSurrogate]] (a Boolean). It interprets string as a sequence of UTF-16 encoded code points, as described in 6.1.4, and reads from it a single code point starting with the code unit at index position. It performs the following steps when called:

  1. Let size be the length of string.
  2. Assert: position ≥ 0 and position < size.
  3. Let first be the code unit at index position within string.
  4. Let cp be the code point whose numeric value is the numeric value of first.
  5. If first is neither a leading surrogate nor a trailing surrogate, then
    1. Return the Record { [[CodePoint]]: cp, [[CodeUnitCount]]: 1, [[IsUnpairedSurrogate]]: false }.
  6. If first is a trailing surrogate or position 1 = size, then
    1. Return the Record { [[CodePoint]]: cp, [[CodeUnitCount]]: 1, [[IsUnpairedSurrogate]]: true }.
  7. Let second be the code unit at index position 1 within string.
  8. If second is not a trailing surrogate, then
    1. Return the Record { [[CodePoint]]: cp, [[CodeUnitCount]]: 1, [[IsUnpairedSurrogate]]: true }.
  9. Set cp to UTF16SurrogatePairToCodePoint(first, second).
  10. Return the Record { [[CodePoint]]: cp, [[CodeUnitCount]]: 2, [[IsUnpairedSurrogate]]: false }.

11.1.5 Static Semantics: StringToCodePoints ( string )

The abstract operation StringToCodePoints takes argument string (a String) and returns a List of code points. It returns the sequence of Unicode code points that results from interpreting string as UTF-16 encoded Unicode text as described in 6.1.4. It performs the following steps when called:

  1. Let codePoints be a new empty List.
  2. Let size be the length of string.
  3. Let position be 0.
  4. Repeat, while position < size,
    1. Let cp be CodePointAt(string, position).
    2. Append cp.[[CodePoint]] to codePoints.
    3. Set position to position cp.[[CodeUnitCount]].
  5. Return codePoints.

11.1.6 Static Semantics: ParseText ( sourceText, goalSymbol )

The abstract operation ParseText takes arguments sourceText (a sequence of Unicode code points) and goalSymbol (a nonterminal in one of the ECMAScript grammars) and returns a Parse Node or a non-empty List of SyntaxError objects. It performs the following steps when called:

  1. Attempt to parse sourceText using goalSymbol as the goal symbol, and analyse the parse result for any early error conditions. Parsing and early error detection may be interleaved in an implementation-defined manner.
  2. If the parse succeeded and no early errors were found, return the Parse Node (an instance of goalSymbol) at the root of the parse tree resulting from the parse.
  3. Otherwise, return a List of one or more SyntaxError objects representing the parsing errors and/or early errors. If more than one parsing error or early error is present, the number and ordering of error objects in the list is implementation-defined, but at least one must be present.
 Note 1

Consider a text that has an early error at a particular point, and also a syntax error at a later point. An implementation that does a parse pass followed by an early errors pass might report the syntax error and not proceed to the early errors pass. An implementation that interleaves the two activities might report the early error and not proceed to find the syntax error. A third implementation might report both errors. All of these behaviours are conformant.

 Note 2

See also clause 17.

11.2 Types of Source Code

There are four types of ECMAScript code:

Note 1

Function code is generally provided as the bodies of Function Definitions (15.2), Arrow Function Definitions (15.3), Method Definitions (15.4), Generator Function Definitions (15.5), Async Function Definitions (15.8), Async Generator Function Definitions (15.6), and Async Arrow Functions (15.9). Function code is also derived from the arguments to the Function constructor (20.2.1.1), the GeneratorFunction constructor (27.3.1.1), and the AsyncFunction constructor (27.7.1.1).

 Note 2

The practical effect of including the BindingIdentifier in function code is that the Early Errors for strict mode code are applied to a BindingIdentifier that is the name of a function whose body contains a "use strict" directive, even if the surrounding code is not strict mode code.

11.2.1 Directive Prologues and the Use Strict Directive

A Directive Prologue is the longest sequence of ExpressionStatements occurring as the initial StatementListItems or ModuleItems of a FunctionBody, a ScriptBody, or a ModuleBody and where each ExpressionStatement in the sequence consists entirely of a StringLiteral token followed by a semicolon. The semicolon may appear explicitly or may be inserted by automatic semicolon insertion (12.10). A Directive Prologue may be an empty sequence.

A Use Strict Directive is an ExpressionStatement in a Directive Prologue whose StringLiteral is either of the exact code point sequences "use strict" or 'use strict'. A Use Strict Directive may not contain an EscapeSequence or LineContinuation.

A Directive Prologue may contain more than one Use Strict Directive. However, an implementation may issue a warning if this occurs.

Note

The ExpressionStatements of a Directive Prologue are evaluated normally during evaluation of the containing production. Implementations may define implementation specific meanings for ExpressionStatements which are not a Use Strict Directive and which occur in a Directive Prologue. If an appropriate notification mechanism exists, an implementation should issue a warning if it encounters in a Directive Prologue an ExpressionStatement that is not a Use Strict Directive and which does not have a meaning defined by the implementation.

11.2.2 Strict Mode Code

An ECMAScript syntactic unit may be processed using either unrestricted or strict mode syntax and semantics (4.3.2). Code is interpreted as strict mode code in the following situations:

ECMAScript code that is not strict mode code is called non-strict code.

11.2.3 Non-ECMAScript Functions

An ECMAScript implementation may support the evaluation of function exotic objects whose evaluative behaviour is expressed in some host-defined form of executable code other than ECMAScript source text. Whether a function object is defined within ECMAScript code or is a built-in function is not observable from the perspective of ECMAScript code that calls or is called by such a function object.

12 ECMAScript Language: Lexical Grammar

The source text of an ECMAScript Script or Module is first converted into a sequence of input elements, which are tokens, line terminators, comments, or white space. The source text is scanned from left to right, repeatedly taking the longest possible sequence of code points as the next input element.

There are several situations where the identification of lexical input elements is sensitive to the syntactic grammar context that is consuming the input elements. This requires multiple goal symbols for the lexical grammar. The InputElementHashbangOrRegExp goal is used at the start of a Script or Module. The InputElementRegExpOrTemplateTail goal is used in syntactic grammar contexts where a RegularExpressionLiteral, a TemplateMiddle, or a TemplateTail is permitted. The InputElementRegExp goal symbol is used in all syntactic grammar contexts where a RegularExpressionLiteral is permitted but neither a TemplateMiddle, nor a TemplateTail is permitted. The InputElementTemplateTail goal is used in all syntactic grammar contexts where a TemplateMiddle or a TemplateTail is permitted but a RegularExpressionLiteral is not permitted. In all other contexts, InputElementDiv is used as the lexical goal symbol.

Note

The use of multiple lexical goals ensures that there are no lexical ambiguities that would affect automatic semicolon insertion. For example, there are no syntactic grammar contexts where both a leading division or division-assignment, and a leading RegularExpressionLiteral are permitted. This is not affected by semicolon insertion (see 12.10); in examples such as the following:

a = b
/hi/g.exec(c).map(d);

where the first non-whitespace, non-comment code point after a LineTerminator is U 002F (SOLIDUS) and the syntactic context allows division or division-assignment, no semicolon is inserted at the LineTerminator. That is, the above example is interpreted in the same way as:

a = b / hi / g.exec(c).map(d);

Syntax

InputElementDiv :: WhiteSpace LineTerminator Comment CommonToken DivPunctuator RightBracePunctuator InputElementRegExp :: WhiteSpace LineTerminator Comment CommonToken RightBracePunctuator RegularExpressionLiteral InputElementRegExpOrTemplateTail :: WhiteSpace LineTerminator Comment CommonToken RegularExpressionLiteral TemplateSubstitutionTail InputElementTemplateTail :: WhiteSpace LineTerminator Comment CommonToken DivPunctuator TemplateSubstitutionTail InputElementHashbangOrRegExp :: WhiteSpace LineTerminator Comment CommonToken HashbangComment RegularExpressionLiteral

12.1 Unicode Format-Control Characters

The Unicode format-control characters (i.e., the characters in category “Cf” in the Unicode Character Database such as LEFT-TO-RIGHT MARK or RIGHT-TO-LEFT MARK) are control codes used to control the formatting of a range of text in the absence of higher-level protocols for this (such as mark-up languages).

It is useful to allow format-control characters in source text to facilitate editing and display. All format control characters may be used within comments, and within string literals, template literals, and regular expression literals.

U 200C (ZERO WIDTH NON-JOINER) and U 200D (ZERO WIDTH JOINER) are format-control characters that are used to make necessary distinctions when forming words or phrases in certain languages. In ECMAScript source text these code points may also be used in an IdentifierName after the first character.

U FEFF (ZERO WIDTH NO-BREAK SPACE) is a format-control character used primarily at the start of a text to mark it as Unicode and to allow detection of the text's encoding and byte order. <ZWNBSP> characters intended for this purpose can sometimes also appear after the start of a text, for example as a result of concatenating files. In ECMAScript source text <ZWNBSP> code points are treated as white space characters (see 12.2).

The special treatment of certain format-control characters outside of comments, string literals, and regular expression literals is summarized in Table 34.

Table 34: Format-Control Code Point Usage
Code Point Name Abbreviation Usage
U 200C ZERO WIDTH NON-JOINER <ZWNJ> IdentifierPart
U 200D ZERO WIDTH JOINER <ZWJ> IdentifierPart
U FEFF ZERO WIDTH NO-BREAK SPACE <ZWNBSP> WhiteSpace

12.2 White Space

White space code points are used to improve source text readability and to separate tokens (indivisible lexical units) from each other, but are otherwise insignificant. White space code points may occur between any two tokens and at the start or end of input. White space code points may occur within a StringLiteral, a RegularExpressionLiteral, a Template, or a TemplateSubstitutionTail where they are considered significant code points forming part of a literal value. They may also occur within a Comment, but cannot appear within any other kind of token.

The ECMAScript white space code points are listed in Table 35.

Table 35: White Space Code Points
Code Points Name Abbreviation
U 0009 CHARACTER TABULATION <TAB>
U 000B LINE TABULATION <VT>
U 000C FORM FEED (FF) <FF>
U FEFF ZERO WIDTH NO-BREAK SPACE <ZWNBSP>
any code point in general category “Space_Separator” <USP>
Note 1

U 0020 (SPACE) and U 00A0 (NO-BREAK SPACE) code points are part of <USP>.

 Note 2

Other than for the code points listed in Table 35, ECMAScript WhiteSpace intentionally excludes all code points that have the Unicode “White_Space” property but which are not classified in general category “Space_Separator” (“Zs”).

Syntax

WhiteSpace :: <TAB> <VT> <FF> <ZWNBSP> <USP>

12.3 Line Terminators

Like white space code points, line terminator code points are used to improve source text readability and to separate tokens (indivisible lexical units) from each other. However, unlike white space code points, line terminators have some influence over the behaviour of the syntactic grammar. In general, line terminators may occur between any two tokens, but there are a few places where they are forbidden by the syntactic grammar. Line terminators also affect the process of automatic semicolon insertion (12.10). A line terminator cannot occur within any token except a StringLiteral, Template, or TemplateSubstitutionTail. <LF> and <CR> line terminators cannot occur within a StringLiteral token except as part of a LineContinuation.

A line terminator can occur within a MultiLineComment but cannot occur within a SingleLineComment.

Line terminators are included in the set of white space code points that are matched by the \s class in regular expressions.

The ECMAScript line terminator code points are listed in Table 36.

Table 36: Line Terminator Code Points
Code Point Unicode Name Abbreviation
U 000A LINE FEED (LF) <LF>
U 000D CARRIAGE RETURN (CR) <CR>
U 2028 LINE SEPARATOR <LS>
U 2029 PARAGRAPH SEPARATOR <PS>

Only the Unicode code points in Table 36 are treated as line terminators. Other new line or line breaking Unicode code points are not treated as line terminators but are treated as white space if they meet the requirements listed in Table 35. The sequence <CR><LF> is commonly used as a line terminator. It should be considered a single SourceCharacter for the purpose of reporting line numbers.

Syntax

LineTerminator :: <LF> <CR> <LS> <PS> LineTerminatorSequence :: <LF> <CR> [lookahead ≠ <LF>] <LS> <PS> <CR> <LF>

12.4 Comments

Comments can be either single or multi-line. Multi-line comments cannot nest.

Because a single-line comment can contain any Unicode code point except a LineTerminator code point, and because of the general rule that a token is always as long as possible, a single-line comment always consists of all code points from the // marker to the end of the line. However, the LineTerminator at the end of the line is not considered to be part of the single-line comment; it is recognized separately by the lexical grammar and becomes part of the stream of input elements for the syntactic grammar. This point is very important, because it implies that the presence or absence of single-line comments does not affect the process of automatic semicolon insertion (see 12.10).

Comments behave like white space and are discarded except that, if a MultiLineComment contains a line terminator code point, then the entire comment is considered to be a LineTerminator for purposes of parsing by the syntactic grammar.

Syntax

Comment :: MultiLineComment SingleLineComment MultiLineComment :: /* MultiLineCommentCharsopt */ MultiLineCommentChars :: MultiLineNotAsteriskChar MultiLineCommentCharsopt * PostAsteriskCommentCharsopt PostAsteriskCommentChars :: MultiLineNotForwardSlashOrAsteriskChar MultiLineCommentCharsopt * PostAsteriskCommentCharsopt MultiLineNotAsteriskChar :: SourceCharacter but not * MultiLineNotForwardSlashOrAsteriskChar :: SourceCharacter but not one of / or * SingleLineComment :: // SingleLineCommentCharsopt SingleLineCommentChars :: SingleLineCommentChar SingleLineCommentCharsopt SingleLineCommentChar :: SourceCharacter but not LineTerminator

A number of productions in this section are given alternative definitions in section B.1.1

12.5 Hashbang Comments

Hashbang Comments are location-sensitive and like other types of comments are discarded from the stream of input elements for the syntactic grammar.

Syntax

HashbangComment :: #! SingleLineCommentCharsopt

12.6 Tokens

Syntax

CommonToken :: IdentifierName PrivateIdentifier Punctuator NumericLiteral StringLiteral Template Note

The DivPunctuator, RegularExpressionLiteral, RightBracePunctuator, and TemplateSubstitutionTail productions derive additional tokens that are not included in the CommonToken production.

12.7 Names and Keywords

IdentifierName and ReservedWord are tokens that are interpreted according to the Default Identifier Syntax given in Unicode Standard Annex #31, Identifier and Pattern Syntax, with some small modifications. ReservedWord is an enumerated subset of IdentifierName. The syntactic grammar defines Identifier as an IdentifierName that is not a ReservedWord. The Unicode identifier grammar is based on character properties specified by the Unicode Standard. The Unicode code points in the specified categories in the latest version of the Unicode Standard must be treated as in those categories by all conforming ECMAScript implementations. ECMAScript implementations may recognize identifier code points defined in later editions of the Unicode Standard.

Note 1

This standard specifies specific code point additions: U 0024 (DOLLAR SIGN) and U 005F (LOW LINE) are permitted anywhere in an IdentifierName, and the code points U 200C (ZERO WIDTH NON-JOINER) and U 200D (ZERO WIDTH JOINER) are permitted anywhere after the first code point of an IdentifierName.

Syntax

PrivateIdentifier :: # IdentifierName IdentifierName :: IdentifierStart IdentifierName IdentifierPart IdentifierStart :: IdentifierStartChar \ UnicodeEscapeSequence IdentifierPart :: IdentifierPartChar \ UnicodeEscapeSequence IdentifierStartChar :: UnicodeIDStart $ _ IdentifierPartChar :: UnicodeIDContinue $ <ZWNJ> <ZWJ> AsciiLetter :: one of a b c d e f g h i j k l m n o p q r s t u v w x y z A B C D E F G H I J K L M N O P Q R S T U V W X Y Z UnicodeIDStart :: any Unicode code point with the Unicode property “ID_Start” UnicodeIDContinue :: any Unicode code point with the Unicode property “ID_Continue”

The definitions of the nonterminal UnicodeEscapeSequence is given in 12.9.4.

Note 2

The nonterminal IdentifierPart derives _ via UnicodeIDContinue.

 Note 3

The sets of code points with Unicode properties “ID_Start” and “ID_Continue” include, respectively, the code points with Unicode properties “Other_ID_Start” and “Other_ID_Continue”.

12.7.1 Identifier Names

Unicode escape sequences are permitted in an IdentifierName, where they contribute a single Unicode code point equal to the IdentifierCodePoint of the UnicodeEscapeSequence. The \ preceding the UnicodeEscapeSequence does not contribute any code points. A UnicodeEscapeSequence cannot be used to contribute a code point to an IdentifierName that would otherwise be invalid. In other words, if a \ UnicodeEscapeSequence sequence were replaced by the SourceCharacter it contributes, the result must still be a valid IdentifierName that has the exact same sequence of SourceCharacter elements as the original IdentifierName. All interpretations of IdentifierName within this specification are based upon their actual code points regardless of whether or not an escape sequence was used to contribute any particular code point.

Two IdentifierNames that are canonically equivalent according to the Unicode Standard are not equal unless, after replacement of each UnicodeEscapeSequence, they are represented by the exact same sequence of code points.

12.7.1.1 Static Semantics: Early Errors

IdentifierStart :: \ UnicodeEscapeSequence IdentifierPart :: \ UnicodeEscapeSequence

12.7.1.2 Static Semantics: IdentifierCodePoints

The syntax-directed operation IdentifierCodePoints takes no arguments and returns a List of code points. It is defined piecewise over the following productions:

IdentifierName :: IdentifierStart
  1. Let cp be IdentifierCodePoint of IdentifierStart.
  2. Return « cp ».
 IdentifierName :: IdentifierName IdentifierPart
  1. Let cps be IdentifierCodePoints of the derived IdentifierName.
  2. Let cp be IdentifierCodePoint of IdentifierPart.
  3. Return the list-concatenation of cps and « cp ».

12.7.1.3 Static Semantics: IdentifierCodePoint

The syntax-directed operation IdentifierCodePoint takes no arguments and returns a code point. It is defined piecewise over the following productions:

IdentifierStart :: IdentifierStartChar
  1. Return the code point matched by IdentifierStartChar.
 IdentifierPart :: IdentifierPartChar
  1. Return the code point matched by IdentifierPartChar.
 UnicodeEscapeSequence :: u Hex4Digits
  1. Return the code point whose numeric value is the MV of Hex4Digits.
 UnicodeEscapeSequence :: u{ CodePoint }
  1. Return the code point whose numeric value is the MV of CodePoint.

12.7.2 Keywords and Reserved Words

A keyword is a token that matches IdentifierName, but also has a syntactic use; that is, it appears literally, in a fixed width font, in some syntactic production. The keywords of ECMAScript include if, while, async, await, and many others.

A reserved word is an IdentifierName that cannot be used as an identifier. Many keywords are reserved words, but some are not, and some are reserved only in certain contexts. if and while are reserved words. await is reserved only inside async functions and modules. async is not reserved; it can be used as a variable name or statement label without restriction.

This specification uses a combination of grammatical productions and early error rules to specify which names are valid identifiers and which are reserved words. All tokens in the ReservedWord list below, except for await and yield, are unconditionally reserved. Exceptions for await and yield are specified in 13.1, using parameterized syntactic productions. Lastly, several early error rules restrict the set of valid identifiers. See 13.1.1, 14.3.1.1, 14.7.5.1, and 15.7.1. In summary, there are five categories of identifier names:

  • Those that are always allowed as identifiers, and are not keywords, such as Math, window, toString, and _;

  • Those that are never allowed as identifiers, namely the ReservedWords listed below except await and yield;

  • Those that are contextually allowed as identifiers, namely await and yield;

  • Those that are contextually disallowed as identifiers, in strict mode code: let, static, implements, interface, package, private, protected, and public;

  • Those that are always allowed as identifiers, but also appear as keywords within certain syntactic productions, at places where Identifier is not allowed: as, async, from, get, meta, of, set, and target.

The term conditional keyword, or contextual keyword, is sometimes used to refer to the keywords that fall in the last three categories, and thus can be used as identifiers in some contexts and as keywords in others.

Syntax

ReservedWord :: one of await break case catch class const continue debugger default delete do else enum export extends false finally for function if import in instanceof new null return super switch this throw true try typeof var void while with yield Note 1

Per 5.1.5, keywords in the grammar match literal sequences of specific SourceCharacter elements. A code point in a keyword cannot be expressed by a \ UnicodeEscapeSequence.

An IdentifierName can contain \ UnicodeEscapeSequences, but it is not possible to declare a variable named "else" by spelling it els\u{65}. The early error rules in 13.1.1 rule out identifiers with the same StringValue as a reserved word.

 Note 2

enum is not currently used as a keyword in this specification. It is a future reserved word, set aside for use as a keyword in future language extensions.

Similarly, implements, interface, package, private, protected, and public are future reserved words in strict mode code.

 Note 3

The names arguments and eval are not keywords, but they are subject to some restrictions in strict mode code. See 13.1.1, 8.6.4, 15.2.1, 15.5.1, 15.6.1, and 15.8.1.

12.8 Punctuators

Syntax

Punctuator :: OptionalChainingPunctuator OtherPunctuator OptionalChainingPunctuator :: ?. [lookahead ∉ DecimalDigit] OtherPunctuator :: one of { ( ) [ ] . ... ; , < > <= >= == != === !== - * % ** -- << >> >>> & | ^ ! ~ && || ?? ? : = = -= *= %= **= <<= >>= >>>= &= |= ^= &&= ||= ??= => DivPunctuator :: / /= RightBracePunctuator :: }

12.9 Literals

12.9.1 Null Literals

Syntax

NullLiteral :: null

12.9.2 Boolean Literals

Syntax

BooleanLiteral :: true false

12.9.3 Numeric Literals

Syntax

NumericLiteralSeparator :: _ NumericLiteral :: DecimalLiteral DecimalBigIntegerLiteral NonDecimalIntegerLiteral[ Sep] NonDecimalIntegerLiteral[ Sep] BigIntLiteralSuffix LegacyOctalIntegerLiteral DecimalBigIntegerLiteral :: 0 BigIntLiteralSuffix NonZeroDigit DecimalDigits[ Sep]opt BigIntLiteralSuffix NonZeroDigit NumericLiteralSeparator DecimalDigits[ Sep] BigIntLiteralSuffix NonDecimalIntegerLiteral[Sep] :: BinaryIntegerLiteral[?Sep] OctalIntegerLiteral[?Sep] HexIntegerLiteral[?Sep] BigIntLiteralSuffix :: n DecimalLiteral :: DecimalIntegerLiteral . DecimalDigits[ Sep]opt ExponentPart[ Sep]opt . DecimalDigits[ Sep] ExponentPart[ Sep]opt DecimalIntegerLiteral ExponentPart[ Sep]opt DecimalIntegerLiteral :: 0 NonZeroDigit NonZeroDigit NumericLiteralSeparatoropt DecimalDigits[ Sep] NonOctalDecimalIntegerLiteral DecimalDigits[Sep] :: DecimalDigit DecimalDigits[?Sep] DecimalDigit [ Sep] DecimalDigits[ Sep] NumericLiteralSeparator DecimalDigit DecimalDigit :: one of 0 1 2 3 4 5 6 7 8 9 NonZeroDigit :: one of 1 2 3 4 5 6 7 8 9 ExponentPart[Sep] :: ExponentIndicator SignedInteger[?Sep] ExponentIndicator :: one of e E SignedInteger[Sep] :: DecimalDigits[?Sep] DecimalDigits[?Sep] - DecimalDigits[?Sep] BinaryIntegerLiteral[Sep] :: 0b BinaryDigits[?Sep] 0B BinaryDigits[?Sep] BinaryDigits[Sep] :: BinaryDigit BinaryDigits[?Sep] BinaryDigit [ Sep] BinaryDigits[ Sep] NumericLiteralSeparator BinaryDigit BinaryDigit :: one of 0 1 OctalIntegerLiteral[Sep] :: 0o OctalDigits[?Sep] 0O OctalDigits[?Sep] OctalDigits[Sep] :: OctalDigit OctalDigits[?Sep] OctalDigit [ Sep] OctalDigits[ Sep] NumericLiteralSeparator OctalDigit LegacyOctalIntegerLiteral :: 0 OctalDigit LegacyOctalIntegerLiteral OctalDigit NonOctalDecimalIntegerLiteral :: 0 NonOctalDigit LegacyOctalLikeDecimalIntegerLiteral NonOctalDigit NonOctalDecimalIntegerLiteral DecimalDigit LegacyOctalLikeDecimalIntegerLiteral :: 0 OctalDigit LegacyOctalLikeDecimalIntegerLiteral OctalDigit OctalDigit :: one of 0 1 2 3 4 5 6 7 NonOctalDigit :: one of 8 9 HexIntegerLiteral[Sep] :: 0x HexDigits[?Sep] 0X HexDigits[?Sep] HexDigits[Sep] :: HexDigit HexDigits[?Sep] HexDigit [ Sep] HexDigits[ Sep] NumericLiteralSeparator HexDigit HexDigit :: one of 0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F

The SourceCharacter immediately following a NumericLiteral must not be an IdentifierStart or DecimalDigit.

Note

For example: 3in is an error and not the two input elements 3 and in.

12.9.3.1 Static Semantics: Early Errors

NumericLiteral :: LegacyOctalIntegerLiteral DecimalIntegerLiteral :: NonOctalDecimalIntegerLiteral Note
In non-strict code, this syntax is Legacy.

12.9.3.2 Static Semantics: MV

A numeric literal stands for a value of the Number type or the BigInt type.

12.9.3.3 Static Semantics: NumericValue

The syntax-directed operation NumericValue takes no arguments and returns a Number or a BigInt. It is defined piecewise over the following productions:

NumericLiteral :: DecimalLiteral
  1. Return RoundMVResult(MV of DecimalLiteral).
 NumericLiteral :: NonDecimalIntegerLiteral
  1. Return 𝔽(MV of NonDecimalIntegerLiteral).
 NumericLiteral :: LegacyOctalIntegerLiteral
  1. Return 𝔽(MV of LegacyOctalIntegerLiteral).
 NumericLiteral :: NonDecimalIntegerLiteral BigIntLiteralSuffix
  1. Return the BigInt value that represents the MV of NonDecimalIntegerLiteral.
 DecimalBigIntegerLiteral :: 0 BigIntLiteralSuffix
  1. Return 0ℤ.
 DecimalBigIntegerLiteral :: NonZeroDigit BigIntLiteralSuffix
  1. Return the BigInt value that represents the MV of NonZeroDigit.
 DecimalBigIntegerLiteral :: NonZeroDigit DecimalDigits BigIntLiteralSuffix NonZeroDigit NumericLiteralSeparator DecimalDigits BigIntLiteralSuffix
  1. Let n be the number of code points in DecimalDigits, excluding all occurrences of NumericLiteralSeparator.
  2. Let mv be (the MV of NonZeroDigit × 10n) plus the MV of DecimalDigits.
  3. Return ℤ(mv).

12.9.4 String Literals

Note 1

A string literal is 0 or more Unicode code points enclosed in single or double quotes. Unicode code points may also be represented by an escape sequence. All code points may appear literally in a string literal except for the closing quote code points, U 005C (REVERSE SOLIDUS), U 000D (CARRIAGE RETURN), and U 000A (LINE FEED). Any code points may appear in the form of an escape sequence. String literals evaluate to ECMAScript String values. When generating these String values Unicode code points are UTF-16 encoded as defined in 11.1.1. Code points belonging to the Basic Multilingual Plane are encoded as a single code unit element of the string. All other code points are encoded as two code unit elements of the string.

Syntax

StringLiteral :: " DoubleStringCharactersopt " ' SingleStringCharactersopt ' DoubleStringCharacters :: DoubleStringCharacter DoubleStringCharactersopt SingleStringCharacters :: SingleStringCharacter SingleStringCharactersopt DoubleStringCharacter :: SourceCharacter but not one of " or \ or LineTerminator <LS> <PS> \ EscapeSequence LineContinuation SingleStringCharacter :: SourceCharacter but not one of ' or \ or LineTerminator <LS> <PS> \ EscapeSequence LineContinuation LineContinuation :: \ LineTerminatorSequence EscapeSequence :: CharacterEscapeSequence 0 [lookahead ∉ DecimalDigit] LegacyOctalEscapeSequence NonOctalDecimalEscapeSequence HexEscapeSequence UnicodeEscapeSequence CharacterEscapeSequence :: SingleEscapeCharacter NonEscapeCharacter SingleEscapeCharacter :: one of ' " \ b f n r t v NonEscapeCharacter :: SourceCharacter but not one of EscapeCharacter or LineTerminator EscapeCharacter :: SingleEscapeCharacter DecimalDigit x u LegacyOctalEscapeSequence :: 0 [lookahead ∈ { 8, 9 }] NonZeroOctalDigit [lookahead ∉ OctalDigit] ZeroToThree OctalDigit [lookahead ∉ OctalDigit] FourToSeven OctalDigit ZeroToThree OctalDigit OctalDigit NonZeroOctalDigit :: OctalDigit but not 0 ZeroToThree :: one of 0 1 2 3 FourToSeven :: one of 4 5 6 7 NonOctalDecimalEscapeSequence :: one of 8 9 HexEscapeSequence :: x HexDigit HexDigit UnicodeEscapeSequence :: u Hex4Digits u{ CodePoint } Hex4Digits :: HexDigit HexDigit HexDigit HexDigit

The definition of the nonterminal HexDigit is given in 12.9.3. SourceCharacter is defined in 11.1.

Note 2

<LF> and <CR> cannot appear in a string literal, except as part of a LineContinuation to produce the empty code points sequence. The proper way to include either in the String value of a string literal is to use an escape sequence such as \n or \u000A.

12.9.4.1 Static Semantics: Early Errors

EscapeSequence :: LegacyOctalEscapeSequence NonOctalDecimalEscapeSequence Note 1
In non-strict code, this syntax is Legacy.
 Note 2

It is possible for string literals to precede a Use Strict Directive that places the enclosing code in strict mode, and implementations must take care to enforce the above rules for such literals. For example, the following source text contains a Syntax Error:

function invalid() { "\7"; "use strict"; }

12.9.4.2 Static Semantics: SV

The syntax-directed operation SV takes no arguments and returns a String.

A string literal stands for a value of the String type. SV produces String values for string literals through recursive application on the various parts of the string literal. As part of this process, some Unicode code points within the string literal are interpreted as having a mathematical value, as described below or in 12.9.3.

Table 37: String Single Character Escape Sequences
Escape Sequence Code Unit Value Unicode Character Name Symbol
\b 0x0008 BACKSPACE <BS>
\t 0x0009 CHARACTER TABULATION <HT>
\n 0x000A LINE FEED (LF) <LF>
\v 0x000B LINE TABULATION <VT>
\f 0x000C FORM FEED (FF) <FF>
\r 0x000D CARRIAGE RETURN (CR) <CR>
\" 0x0022 QUOTATION MARK "
\' 0x0027 APOSTROPHE '
\\ 0x005C REVERSE SOLIDUS \

12.9.4.3 Static Semantics: MV

12.9.5 Regular Expression Literals

Note 1

A regular expression literal is an input element that is converted to a RegExp object (see 22.2) each time the literal is evaluated. Two regular expression literals in a program evaluate to regular expression objects that never compare as === to each other even if the two literals' contents are identical. A RegExp object may also be created at runtime by new RegExp or calling the RegExp constructor as a function (see 22.2.4).

The productions below describe the syntax for a regular expression literal and are used by the input element scanner to find the end of the regular expression literal. The source text comprising the RegularExpressionBody and the RegularExpressionFlags are subsequently parsed again using the more stringent ECMAScript Regular Expression grammar (22.2.1).

An implementation may extend the ECMAScript Regular Expression grammar defined in 22.2.1, but it must not extend the RegularExpressionBody and RegularExpressionFlags productions defined below or the productions used by these productions.

Syntax

RegularExpressionLiteral :: / RegularExpressionBody / RegularExpressionFlags RegularExpressionBody :: RegularExpressionFirstChar RegularExpressionChars RegularExpressionChars :: [empty] RegularExpressionChars RegularExpressionChar RegularExpressionFirstChar :: RegularExpressionNonTerminator but not one of * or \ or / or [ RegularExpressionBackslashSequence RegularExpressionClass RegularExpressionChar :: RegularExpressionNonTerminator but not one of \ or / or [ RegularExpressionBackslashSequence RegularExpressionClass RegularExpressionBackslashSequence :: \ RegularExpressionNonTerminator RegularExpressionNonTerminator :: SourceCharacter but not LineTerminator RegularExpressionClass :: [ RegularExpressionClassChars ] RegularExpressionClassChars :: [empty] RegularExpressionClassChars RegularExpressionClassChar RegularExpressionClassChar :: RegularExpressionNonTerminator but not one of ] or \ RegularExpressionBackslashSequence RegularExpressionFlags :: [empty] RegularExpressionFlags IdentifierPartChar Note 2

Regular expression literals may not be empty; instead of representing an empty regular expression literal, the code unit sequence // starts a single-line comment. To specify an empty regular expression, use: /(?:)/.

12.9.5.1 Static Semantics: BodyText

The syntax-directed operation BodyText takes no arguments and returns source text. It is defined piecewise over the following productions:

RegularExpressionLiteral :: / RegularExpressionBody / RegularExpressionFlags
  1. Return the source text that was recognized as RegularExpressionBody.

12.9.5.2 Static Semantics: FlagText

The syntax-directed operation FlagText takes no arguments and returns source text. It is defined piecewise over the following productions:

RegularExpressionLiteral :: / RegularExpressionBody / RegularExpressionFlags
  1. Return the source text that was recognized as RegularExpressionFlags.

12.9.6 Template Literal Lexical Components

Syntax

Template :: NoSubstitutionTemplate TemplateHead NoSubstitutionTemplate :: ` TemplateCharactersopt ` TemplateHead :: ` TemplateCharactersopt ${ TemplateSubstitutionTail :: TemplateMiddle TemplateTail TemplateMiddle :: } TemplateCharactersopt ${ TemplateTail :: } TemplateCharactersopt ` TemplateCharacters :: TemplateCharacter TemplateCharactersopt TemplateCharacter :: $ [lookahead ≠ {] \ TemplateEscapeSequence \ NotEscapeSequence LineContinuation LineTerminatorSequence SourceCharacter but not one of ` or \ or $ or LineTerminator TemplateEscapeSequence :: CharacterEscapeSequence 0 [lookahead ∉ DecimalDigit] HexEscapeSequence UnicodeEscapeSequence NotEscapeSequence :: 0 DecimalDigit DecimalDigit but not 0 x [lookahead ∉ HexDigit] x HexDigit [lookahead ∉ HexDigit] u [lookahead ∉ HexDigit] [lookahead ≠ {] u HexDigit [lookahead ∉ HexDigit] u HexDigit HexDigit [lookahead ∉ HexDigit] u HexDigit HexDigit HexDigit [lookahead ∉ HexDigit] u { [lookahead ∉ HexDigit] u { NotCodePoint [lookahead ∉ HexDigit] u { CodePoint [lookahead ∉ HexDigit] [lookahead ≠ }] NotCodePoint :: HexDigits[~Sep] but only if MV of HexDigits > 0x10FFFF CodePoint :: HexDigits[~Sep] but only if MV of HexDigits ≤ 0x10FFFF Note

TemplateSubstitutionTail is used by the InputElementTemplateTail alternative lexical goal.

12.9.6.1 Static Semantics: TV

The syntax-directed operation TV takes no arguments and returns a String or undefined. A template literal component is interpreted by TV as a value of the String type. TV is used to construct the indexed components of a template object (colloquially, the template values). In TV, escape sequences are replaced by the UTF-16 code unit(s) of the Unicode code point represented by the escape sequence.

12.9.6.2 Static Semantics: TRV

The syntax-directed operation TRV takes no arguments and returns a String. A template literal component is interpreted by TRV as a value of the String type. TRV is used to construct the raw components of a template object (colloquially, the template raw values). TRV is similar to TV with the difference being that in TRV, escape sequences are interpreted as they appear in the literal.

Note

TV excludes the code units of LineContinuation while TRV includes them. <CR><LF> and <CR> LineTerminatorSequences are normalized to <LF> for both TV and TRV. An explicit TemplateEscapeSequence is needed to include a <CR> or <CR><LF> sequence.

12.10 Automatic Semicolon Insertion

Most ECMAScript statements and declarations must be terminated with a semicolon. Such semicolons may always appear explicitly in the source text. For convenience, however, such semicolons may be omitted from the source text in certain situations. These situations are described by saying that semicolons are automatically inserted into the source code token stream in those situations.

12.10.1 Rules of Automatic Semicolon Insertion

In the following rules, “token” means the actual recognized lexical token determined using the current lexical goal symbol as described in clause 12.

There are three basic rules of semicolon insertion:

  1. When, as the source text is parsed from left to right, a token (called the offending token) is encountered that is not allowed by any production of the grammar, then a semicolon is automatically inserted before the offending token if one or more of the following conditions is true:

    • The offending token is separated from the previous token by at least one LineTerminator.
    • The offending token is }.
    • The previous token is ) and the inserted semicolon would then be parsed as the terminating semicolon of a do-while statement (14.7.2).
  2. When, as the source text is parsed from left to right, the end of the input stream of tokens is encountered and the parser is unable to parse the input token stream as a single instance of the goal nonterminal, then a semicolon is automatically inserted at the end of the input stream.
  3. When, as the source text is parsed from left to right, a token is encountered that is allowed by some production of the grammar, but the production is a restricted production and the token would be the first token for a terminal or nonterminal immediately following the annotation “[no LineTerminator here]” within the restricted production (and therefore such a token is called a restricted token), and the restricted token is separated from the previous token by at least one LineTerminator, then a semicolon is automatically inserted before the restricted token.

However, there is an additional overriding condition on the preceding rules: a semicolon is never inserted automatically if the semicolon would then be parsed as an empty statement or if that semicolon would become one of the two semicolons in the header of a for statement (see 14.7.4).

Note

The following are the only restricted productions in the grammar:

UpdateExpression[Yield, Await] : LeftHandSideExpression[?Yield, ?Await] [no LineTerminator here] LeftHandSideExpression[?Yield, ?Await] [no LineTerminator here] -- ContinueStatement[Yield, Await] : continue ; continue [no LineTerminator here] LabelIdentifier[?Yield, ?Await] ; BreakStatement[Yield, Await] : break ; break [no LineTerminator here] LabelIdentifier[?Yield, ?Await] ; ReturnStatement[Yield, Await] : return ; return [no LineTerminator here] Expression[ In, ?Yield, ?Await] ; ThrowStatement[Yield, Await] : throw [no LineTerminator here] Expression[ In, ?Yield, ?Await] ; YieldExpression[In, Await] : yield yield [no LineTerminator here] AssignmentExpression[?In, Yield, ?Await] yield [no LineTerminator here] * AssignmentExpression[?In, Yield, ?Await] ArrowFunction[In, Yield, Await] : ArrowParameters[?Yield, ?Await] [no LineTerminator here] => ConciseBody[?In] AsyncFunctionDeclaration[Yield, Await, Default] : async [no LineTerminator here] function BindingIdentifier[?Yield, ?Await] ( FormalParameters[~Yield, Await] ) { AsyncFunctionBody } [ Default] async [no LineTerminator here] function ( FormalParameters[~Yield, Await] ) { AsyncFunctionBody } AsyncFunctionExpression : async [no LineTerminator here] function BindingIdentifier[~Yield, Await]opt ( FormalParameters[~Yield, Await] ) { AsyncFunctionBody } AsyncMethod[Yield, Await] : async [no LineTerminator here] ClassElementName[?Yield, ?Await] ( UniqueFormalParameters[~Yield, Await] ) { AsyncFunctionBody } AsyncGeneratorDeclaration[Yield, Await, Default] : async [no LineTerminator here] function * BindingIdentifier[?Yield, ?Await] ( FormalParameters[ Yield, Await] ) { AsyncGeneratorBody } [ Default] async [no LineTerminator here] function * ( FormalParameters[ Yield, Await] ) { AsyncGeneratorBody } AsyncGeneratorExpression : async [no LineTerminator here] function * BindingIdentifier[ Yield, Await]opt ( FormalParameters[ Yield, Await] ) { AsyncGeneratorBody } AsyncGeneratorMethod[Yield, Await] : async [no LineTerminator here] * ClassElementName[?Yield, ?Await] ( UniqueFormalParameters[ Yield, Await] ) { AsyncGeneratorBody } AsyncArrowFunction[In, Yield, Await] : async [no LineTerminator here] AsyncArrowBindingIdentifier[?Yield] [no LineTerminator here] => AsyncConciseBody[?In] CoverCallExpressionAndAsyncArrowHead[?Yield, ?Await] [no LineTerminator here] => AsyncConciseBody[?In] AsyncArrowHead : async [no LineTerminator here] ArrowFormalParameters[~Yield, Await]

The practical effect of these restricted productions is as follows:

  • When a or -- token is encountered where the parser would treat it as a postfix operator, and at least one LineTerminator occurred between the preceding token and the or -- token, then a semicolon is automatically inserted before the or -- token.
  • When a continue, break, return, throw, or yield token is encountered and a LineTerminator is encountered before the next token, a semicolon is automatically inserted after the continue, break, return, throw, or yield token.
  • When arrow function parameter(s) are followed by a LineTerminator before a => token, a semicolon is automatically inserted and the punctuator causes a syntax error.
  • When an async token is followed by a LineTerminator before a function or IdentifierName or ( token, a semicolon is automatically inserted and the async token is not treated as part of the same expression or class element as the following tokens.
  • When an async token is followed by a LineTerminator before a * token, a semicolon is automatically inserted and the punctuator causes a syntax error.

The resulting practical advice to ECMAScript programmers is:

  • A postfix or -- operator should be on the same line as its operand.
  • An Expression in a return or throw statement or an AssignmentExpression in a yield expression should start on the same line as the return, throw, or yield token.
  • A LabelIdentifier in a break or continue statement should be on the same line as the break or continue token.
  • The end of an arrow function's parameter(s) and its => should be on the same line.
  • The async token preceding an asynchronous function or method should be on the same line as the immediately following token.

12.10.2 Examples of Automatic Semicolon Insertion

This section is non-normative.

The source

{ 1 2 } 3

is not a valid sentence in the ECMAScript grammar, even with the automatic semicolon insertion rules. In contrast, the source

{ 1
2 } 3

is also not a valid ECMAScript sentence, but is transformed by automatic semicolon insertion into the following:

{ 1
;2 ;} 3;

which is a valid ECMAScript sentence.

The source

for (a; b
)

is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion because the semicolon is needed for the header of a for statement. Automatic semicolon insertion never inserts one of the two semicolons in the header of a for statement.

The source

return
a   b

is transformed by automatic semicolon insertion into the following:

return;
a   b;
Note 1

The expression a b is not treated as a value to be returned by the return statement, because a LineTerminator separates it from the token return.

The source

a = b
  c

is transformed by automatic semicolon insertion into the following:

a = b;
  c;
Note 2

The token is not treated as a postfix operator applying to the variable b, because a LineTerminator occurs between b and .

The source

if (a > b)
else c = d

is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion before the else token, even though no production of the grammar applies at that point, because an automatically inserted semicolon would then be parsed as an empty statement.

The source

a = b   c
(d   e).print()

is not transformed by automatic semicolon insertion, because the parenthesized expression that begins the second line can be interpreted as an argument list for a function call:

a = b   c(d   e).print()

In the circumstance that an assignment statement must begin with a left parenthesis, it is a good idea for the programmer to provide an explicit semicolon at the end of the preceding statement rather than to rely on automatic semicolon insertion.

12.10.3 Interesting Cases of Automatic Semicolon Insertion

This section is non-normative.

ECMAScript programs can be written in a style with very few semicolons by relying on automatic semicolon insertion. As described above, semicolons are not inserted at every newline, and automatic semicolon insertion can depend on multiple tokens across line terminators.

As new syntactic features are added to ECMAScript, additional grammar productions could be added that cause lines relying on automatic semicolon insertion preceding them to change grammar productions when parsed.

For the purposes of this section, a case of automatic semicolon insertion is considered interesting if it is a place where a semicolon may or may not be inserted, depending on the source text which precedes it. The rest of this section describes a number of interesting cases of automatic semicolon insertion in this version of ECMAScript.

12.10.3.1 Interesting Cases of Automatic Semicolon Insertion in Statement Lists

In a StatementList, many StatementListItems end in semicolons, which may be omitted using automatic semicolon insertion. As a consequence of the rules above, at the end of a line ending an expression, a semicolon is required if the following line begins with any of the following:

  • An opening parenthesis ((). Without a semicolon, the two lines together are treated as a CallExpression.
  • An opening square bracket ([). Without a semicolon, the two lines together are treated as property access, rather than an ArrayLiteral or ArrayAssignmentPattern.
  • A template literal (`). Without a semicolon, the two lines together are interpreted as a tagged Template (13.3.11), with the previous expression as the MemberExpression.
  • Unary or -. Without a semicolon, the two lines together are interpreted as a usage of the corresponding binary operator.
  • A RegExp literal. Without a semicolon, the two lines together may be parsed instead as the / MultiplicativeOperator, for example if the RegExp has flags.

12.10.3.2 Cases of Automatic Semicolon Insertion and “[no LineTerminator here]”

This section is non-normative.

ECMAScript contains grammar productions which include “[no LineTerminator here]”. These productions are sometimes a means to have optional operands in the grammar. Introducing a LineTerminator in these locations would change the grammar production of a source text by using the grammar production without the optional operand.

The rest of this section describes a number of productions using “[no LineTerminator here]” in this version of ECMAScript.

12.10.3.2.1 List of Grammar Productions with Optional Operands and “[no LineTerminator here]”

13 ECMAScript Language: Expressions

13.1 Identifiers

Syntax

IdentifierReference[Yield, Await] : Identifier [~Yield] yield [~Await] await BindingIdentifier[Yield, Await] : Identifier yield await LabelIdentifier[Yield, Await] : Identifier [~Yield] yield [~Await] await Identifier : IdentifierName but not ReservedWord Note

yield and await are permitted as BindingIdentifier in the grammar, and prohibited with static semantics below, to prohibit automatic semicolon insertion in cases such as

let
await 0;

13.1.1 Static Semantics: Early Errors

BindingIdentifier : Identifier IdentifierReference : yield BindingIdentifier : yield LabelIdentifier : yield IdentifierReference : await BindingIdentifier : await LabelIdentifier : await BindingIdentifier[Yield, Await] : yield
  • It is a Syntax Error if this production has a [Yield] parameter.
BindingIdentifier[Yield, Await] : await
  • It is a Syntax Error if this production has an [Await] parameter.
IdentifierReference[Yield, Await] : Identifier BindingIdentifier[Yield, Await] : Identifier LabelIdentifier[Yield, Await] : Identifier
  • It is a Syntax Error if this production has a [Yield] parameter and StringValue of Identifier is "yield".
  • It is a Syntax Error if this production has an [Await] parameter and StringValue of Identifier is "await".
Identifier : IdentifierName but not ReservedWord Note

StringValue of IdentifierName normalizes any Unicode escape sequences in IdentifierName hence such escapes cannot be used to write an Identifier whose code point sequence is the same as a ReservedWord.

13.1.2 Static Semantics: StringValue

The syntax-directed operation StringValue takes no arguments and returns a String. It is defined piecewise over the following productions:

IdentifierName :: IdentifierStart IdentifierName IdentifierPart
  1. Let idTextUnescaped be IdentifierCodePoints of IdentifierName.
  2. Return CodePointsToString(idTextUnescaped).
 IdentifierReference : yield BindingIdentifier : yield LabelIdentifier : yield
  1. Return "yield".
 IdentifierReference : await BindingIdentifier : await LabelIdentifier : await
  1. Return "await".
 Identifier : IdentifierName but not ReservedWord
  1. Return the StringValue of IdentifierName.
 PrivateIdentifier :: # IdentifierName
  1. Return the string-concatenation of 0x0023 (NUMBER SIGN) and the StringValue of IdentifierName.
 ModuleExportName : StringLiteral
  1. Return the SV of StringLiteral.

13.1.3 Runtime Semantics: Evaluation

IdentifierReference : Identifier
  1. Return ? ResolveBinding(StringValue of Identifier).
 IdentifierReference : yield
  1. Return ? ResolveBinding("yield").
 IdentifierReference : await
  1. Return ? ResolveBinding("await").
 Note 1

The result of evaluating an IdentifierReference is always a value of type Reference.

 Note 2

In non-strict code, the keyword yield may be used as an identifier. Evaluating the IdentifierReference resolves the binding of yield as if it was an Identifier. Early Error restriction ensures that such an evaluation only can occur for non-strict code.

13.2 Primary Expression

Syntax

PrimaryExpression[Yield, Await] : this IdentifierReference[?Yield, ?Await] Literal ArrayLiteral[?Yield, ?Await] ObjectLiteral[?Yield, ?Await] FunctionExpression ClassExpression[?Yield, ?Await] GeneratorExpression AsyncFunctionExpression AsyncGeneratorExpression RegularExpressionLiteral TemplateLiteral[?Yield, ?Await, ~Tagged] CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await] CoverParenthesizedExpressionAndArrowParameterList[Yield, Await] : ( Expression[ In, ?Yield, ?Await] ) ( Expression[ In, ?Yield, ?Await] , ) ( ) ( ... BindingIdentifier[?Yield, ?Await] ) ( ... BindingPattern[?Yield, ?Await] ) ( Expression[ In, ?Yield, ?Await] , ... BindingIdentifier[?Yield, ?Await] ) ( Expression[ In, ?Yield, ?Await] , ... BindingPattern[?Yield, ?Await] )

Supplemental Syntax

When processing an instance of the production
PrimaryExpression[Yield, Await] : CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await]
the interpretation of CoverParenthesizedExpressionAndArrowParameterList is refined using the following grammar:

ParenthesizedExpression[Yield, Await] : ( Expression[ In, ?Yield, ?Await] )

13.2.1 The this Keyword

13.2.1.1 Runtime Semantics: Evaluation

PrimaryExpression : this
  1. Return ? ResolveThisBinding().

13.2.2 Identifier Reference

See 13.1 for IdentifierReference.

13.2.3 Literals

Syntax

Literal : NullLiteral BooleanLiteral NumericLiteral StringLiteral

13.2.3.1 Runtime Semantics: Evaluation

Literal : NullLiteral
  1. Return null.
 Literal : BooleanLiteral
  1. If BooleanLiteral is the token false, return false.
  2. If BooleanLiteral is the token true, return true.
 Literal : NumericLiteral
  1. Return the NumericValue of NumericLiteral as defined in 12.9.3.
 Literal : StringLiteral
  1. Return the SV of StringLiteral as defined in 12.9.4.2.

13.2.4 Array Initializer

Note

An ArrayLiteral is an expression describing the initialization of an Array, using a list, of zero or more expressions each of which represents an array element, enclosed in square brackets. The elements need not be literals; they are evaluated each time the array initializer is evaluated.

Array elements may be elided at the beginning, middle or end of the element list. Whenever a comma in the element list is not preceded by an AssignmentExpression (i.e., a comma at the beginning or after another comma), the missing array element contributes to the length of the Array and increases the index of subsequent elements. Elided array elements are not defined. If an element is elided at the end of an array, that element does not contribute to the length of the Array.

Syntax

ArrayLiteral[Yield, Await] : [ Elisionopt ] [ ElementList[?Yield, ?Await] ] [ ElementList[?Yield, ?Await] , Elisionopt ] ElementList[Yield, Await] : Elisionopt AssignmentExpression[ In, ?Yield, ?Await] Elisionopt SpreadElement[?Yield, ?Await] ElementList[?Yield, ?Await] , Elisionopt AssignmentExpression[ In, ?Yield, ?Await] ElementList[?Yield, ?Await] , Elisionopt SpreadElement[?Yield, ?Await] Elision : , Elision , SpreadElement[Yield, Await] : ... AssignmentExpression[ In, ?Yield, ?Await]

13.2.4.1 Runtime Semantics: ArrayAccumulation

The syntax-directed operation ArrayAccumulation takes arguments array (an Array) and nextIndex (an integer) and returns either a normal completion containing an integer or an abrupt completion. It is defined piecewise over the following productions:

Elision : ,
  1. Let len be nextIndex 1.
  2. Perform ? Set(array, "length", 𝔽(len), true).
  3. NOTE: The above step throws if len exceeds 232-1.
  4. Return len.
 Elision : Elision ,
  1. Return ? ArrayAccumulation of Elision with arguments array and (nextIndex 1).
 ElementList : Elisionopt AssignmentExpression
  1. If Elision is present, then
    1. Set nextIndex to ? ArrayAccumulation of Elision with arguments array and nextIndex.
  2. Let initResult be ? Evaluation of AssignmentExpression.
  3. Let initValue be ? GetValue(initResult).
  4. Perform ! CreateDataPropertyOrThrow(array, ! ToString(𝔽(nextIndex)), initValue).
  5. Return nextIndex 1.
 ElementList : Elisionopt SpreadElement
  1. If Elision is present, then
    1. Set nextIndex to ? ArrayAccumulation of Elision with arguments array and nextIndex.
  2. Return ? ArrayAccumulation of SpreadElement with arguments array and nextIndex.
 ElementList : ElementList , Elisionopt AssignmentExpression
  1. Set nextIndex to ? ArrayAccumulation of ElementList with arguments array and nextIndex.
  2. If Elision is present, then
    1. Set nextIndex to ? ArrayAccumulation of Elision with arguments array and nextIndex.
  3. Let initResult be ? Evaluation of AssignmentExpression.
  4. Let initValue be ? GetValue(initResult).
  5. Perform ! CreateDataPropertyOrThrow(array, ! ToString(𝔽(nextIndex)), initValue).
  6. Return nextIndex 1.
 ElementList : ElementList , Elisionopt SpreadElement
  1. Set nextIndex to ? ArrayAccumulation of ElementList with arguments array and nextIndex.
  2. If Elision is present, then
    1. Set nextIndex to ? ArrayAccumulation of Elision with arguments array and nextIndex.
  3. Return ? ArrayAccumulation of SpreadElement with arguments array and nextIndex.
 SpreadElement : ... AssignmentExpression
  1. Let spreadRef be ? Evaluation of AssignmentExpression.
  2. Let spreadObj be ? GetValue(spreadRef).
  3. Let iteratorRecord be ? GetIterator(spreadObj, sync).
  4. Repeat,
    1. Let next be ? IteratorStep(iteratorRecord).
    2. If next is false, return nextIndex.
    3. Let nextValue be ? IteratorValue(next).
    4. Perform ! CreateDataPropertyOrThrow(array, ! ToString(𝔽(nextIndex)), nextValue).
    5. Set nextIndex to nextIndex 1.
 Note

CreateDataPropertyOrThrow is used to ensure that own properties are defined for the array even if the standard built-in Array prototype object has been modified in a manner that would preclude the creation of new own properties using [[Set]].

13.2.4.2 Runtime Semantics: Evaluation

ArrayLiteral : [ Elisionopt ]
  1. Let array be ! ArrayCreate(0).
  2. If Elision is present, then
    1. Perform ? ArrayAccumulation of Elision with arguments array and 0.
  3. Return array.
 ArrayLiteral : [ ElementList ]
  1. Let array be ! ArrayCreate(0).
  2. Perform ? ArrayAccumulation of ElementList with arguments array and 0.
  3. Return array.
 ArrayLiteral : [ ElementList , Elisionopt ]
  1. Let array be ! ArrayCreate(0).
  2. Let nextIndex be ? ArrayAccumulation of ElementList with arguments array and 0.
  3. If Elision is present, then
    1. Perform ? ArrayAccumulation of Elision with arguments array and nextIndex.
  4. Return array.

13.2.5 Object Initializer

Note 1

An object initializer is an expression describing the initialization of an Object, written in a form resembling a literal. It is a list of zero or more pairs of property keys and associated values, enclosed in curly brackets. The values need not be literals; they are evaluated each time the object initializer is evaluated.

Syntax

ObjectLiteral[Yield, Await] : { } { PropertyDefinitionList[?Yield, ?Await] } { PropertyDefinitionList[?Yield, ?Await] , } PropertyDefinitionList[Yield, Await] : PropertyDefinition[?Yield, ?Await] PropertyDefinitionList[?Yield, ?Await] , PropertyDefinition[?Yield, ?Await] PropertyDefinition[Yield, Await] : IdentifierReference[?Yield, ?Await] CoverInitializedName[?Yield, ?Await] PropertyName[?Yield, ?Await] : AssignmentExpression[ In, ?Yield, ?Await] MethodDefinition[?Yield, ?Await] ... AssignmentExpression[ In, ?Yield, ?Await] PropertyName[Yield, Await] : LiteralPropertyName ComputedPropertyName[?Yield, ?Await] LiteralPropertyName : IdentifierName StringLiteral NumericLiteral ComputedPropertyName[Yield, Await] : [ AssignmentExpression[ In, ?Yield, ?Await] ] CoverInitializedName[Yield, Await] : IdentifierReference[?Yield, ?Await] Initializer[ In, ?Yield, ?Await] Initializer[In, Yield, Await] : = AssignmentExpression[?In, ?Yield, ?Await] Note 2

MethodDefinition is defined in 15.4.

 Note 3

In certain contexts, ObjectLiteral is used as a cover grammar for a more restricted secondary grammar. The CoverInitializedName production is necessary to fully cover these secondary grammars. However, use of this production results in an early Syntax Error in normal contexts where an actual ObjectLiteral is expected.

13.2.5.1 Static Semantics: Early Errors

PropertyDefinition : MethodDefinition

In addition to describing an actual object initializer the ObjectLiteral productions are also used as a cover grammar for ObjectAssignmentPattern and may be recognized as part of a CoverParenthesizedExpressionAndArrowParameterList. When ObjectLiteral appears in a context where ObjectAssignmentPattern is required the following Early Error rules are not applied. In addition, they are not applied when initially parsing a CoverParenthesizedExpressionAndArrowParameterList or CoverCallExpressionAndAsyncArrowHead.

PropertyDefinition : CoverInitializedName
  • It is a Syntax Error if any source text is matched by this production.
Note 1

This production exists so that ObjectLiteral can serve as a cover grammar for ObjectAssignmentPattern. It cannot occur in an actual object initializer.

 ObjectLiteral : { PropertyDefinitionList } { PropertyDefinitionList , } Note 2

The List returned by PropertyNameList does not include property names defined using a ComputedPropertyName.

13.2.5.2 Static Semantics: IsComputedPropertyKey

The syntax-directed operation IsComputedPropertyKey takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

PropertyName : LiteralPropertyName
  1. Return false.
 PropertyName : ComputedPropertyName
  1. Return true.

13.2.5.3 Static Semantics: PropertyNameList

The syntax-directed operation PropertyNameList takes no arguments and returns a List of Strings. It is defined piecewise over the following productions:

PropertyDefinitionList : PropertyDefinition
  1. Let propName be PropName of PropertyDefinition.
  2. If propName is empty, return a new empty List.
  3. Return « propName ».
 PropertyDefinitionList : PropertyDefinitionList , PropertyDefinition
  1. Let list be PropertyNameList of PropertyDefinitionList.
  2. Let propName be PropName of PropertyDefinition.
  3. If propName is empty, return list.
  4. Return the list-concatenation of list and « propName ».

13.2.5.4 Runtime Semantics: Evaluation

ObjectLiteral : { }
  1. Return OrdinaryObjectCreate(%Object.prototype%).
 ObjectLiteral : { PropertyDefinitionList } { PropertyDefinitionList , }
  1. Let obj be OrdinaryObjectCreate(%Object.prototype%).
  2. Perform ? PropertyDefinitionEvaluation of PropertyDefinitionList with argument obj.
  3. Return obj.
 LiteralPropertyName : IdentifierName
  1. Return StringValue of IdentifierName.
 LiteralPropertyName : StringLiteral
  1. Return the SV of StringLiteral.
 LiteralPropertyName : NumericLiteral
  1. Let nbr be the NumericValue of NumericLiteral.
  2. Return ! ToString(nbr).
 ComputedPropertyName : [ AssignmentExpression ]
  1. Let exprValue be ? Evaluation of AssignmentExpression.
  2. Let propName be ? GetValue(exprValue).
  3. Return ? ToPropertyKey(propName).

13.2.5.5 Runtime Semantics: PropertyDefinitionEvaluation

The syntax-directed operation PropertyDefinitionEvaluation takes argument object (an Object) and returns either a normal completion containing unused or an abrupt completion. It is defined piecewise over the following productions:

PropertyDefinitionList : PropertyDefinitionList , PropertyDefinition
  1. Perform ? PropertyDefinitionEvaluation of PropertyDefinitionList with argument object.
  2. Perform ? PropertyDefinitionEvaluation of PropertyDefinition with argument object.
  3. Return unused.
 PropertyDefinition : ... AssignmentExpression
  1. Let exprValue be ? Evaluation of AssignmentExpression.
  2. Let fromValue be ? GetValue(exprValue).
  3. Let excludedNames be a new empty List.
  4. Perform ? CopyDataProperties(object, fromValue, excludedNames).
  5. Return unused.
 PropertyDefinition : IdentifierReference
  1. Let propName be StringValue of IdentifierReference.
  2. Let exprValue be ? Evaluation of IdentifierReference.
  3. Let propValue be ? GetValue(exprValue).
  4. Assert: object is an ordinary, extensible object with no non-configurable properties.
  5. Perform ! CreateDataPropertyOrThrow(object, propName, propValue).
  6. Return unused.
 PropertyDefinition : PropertyName : AssignmentExpression
  1. Let propKey be ? Evaluation of PropertyName.
  2. If this PropertyDefinition is contained within a Script that is being evaluated for JSON.parse (see step 7 of JSON.parse), then
    1. Let isProtoSetter be false.
  3. Else if propKey is "__proto__" and IsComputedPropertyKey of PropertyName is false, then
    1. Let isProtoSetter be true.
  4. Else,
    1. Let isProtoSetter be false.
  5. If IsAnonymousFunctionDefinition(AssignmentExpression) is true and isProtoSetter is false, then
    1. Let propValue be ? NamedEvaluation of AssignmentExpression with argument propKey.
  6. Else,
    1. Let exprValueRef be ? Evaluation of AssignmentExpression.
    2. Let propValue be ? GetValue(exprValueRef).
  7. If isProtoSetter is true, then
    1. If propValue is an Object or propValue is null, then
      1. Perform ! object.[[SetPrototypeOf]](propValue).
    2. Return unused.
  8. Assert: object is an ordinary, extensible object with no non-configurable properties.
  9. Perform ! CreateDataPropertyOrThrow(object, propKey, propValue).
  10. Return unused.
 PropertyDefinition : MethodDefinition
  1. Perform ? MethodDefinitionEvaluation of MethodDefinition with arguments object and true.
  2. Return unused.

13.2.6 Function Defining Expressions

See 15.2 for PrimaryExpression : FunctionExpression .

See 15.5 for PrimaryExpression : GeneratorExpression .

See 15.7 for PrimaryExpression : ClassExpression .

See 15.8 for PrimaryExpression : AsyncFunctionExpression .

See 15.6 for PrimaryExpression : AsyncGeneratorExpression .

13.2.7 Regular Expression Literals

Syntax

See 12.9.5.

13.2.7.1 Static Semantics: Early Errors

PrimaryExpression : RegularExpressionLiteral

13.2.7.2 Static Semantics: IsValidRegularExpressionLiteral ( literal )

The abstract operation IsValidRegularExpressionLiteral takes argument literal (a RegularExpressionLiteral Parse Node) and returns a Boolean. It determines if its argument is a valid regular expression literal. It performs the following steps when called:

  1. Let flags be FlagText of literal.
  2. If flags contains any code points other than d, g, i, m, s, u, or y, or if flags contains any code point more than once, return false.
  3. If flags contains u, let u be true; else let u be false.
  4. Let patternText be BodyText of literal.
  5. If u is false, then
    1. Let stringValue be CodePointsToString(patternText).
    2. Set patternText to the sequence of code points resulting from interpreting each of the 16-bit elements of stringValue as a Unicode BMP code point. UTF-16 decoding is not applied to the elements.
  6. Let parseResult be ParsePattern(patternText, u).
  7. If parseResult is a Parse Node, return true; else return false.

13.2.7.3 Runtime Semantics: Evaluation

PrimaryExpression : RegularExpressionLiteral
  1. Let pattern be CodePointsToString(BodyText of RegularExpressionLiteral).
  2. Let flags be CodePointsToString(FlagText of RegularExpressionLiteral).
  3. Return ! RegExpCreate(pattern, flags).

13.2.8 Template Literals

Syntax

TemplateLiteral[Yield, Await, Tagged] : NoSubstitutionTemplate SubstitutionTemplate[?Yield, ?Await, ?Tagged] SubstitutionTemplate[Yield, Await, Tagged] : TemplateHead Expression[ In, ?Yield, ?Await] TemplateSpans[?Yield, ?Await, ?Tagged] TemplateSpans[Yield, Await, Tagged] : TemplateTail TemplateMiddleList[?Yield, ?Await, ?Tagged] TemplateTail TemplateMiddleList[Yield, Await, Tagged] : TemplateMiddle Expression[ In, ?Yield, ?Await] TemplateMiddleList[?Yield, ?Await, ?Tagged] TemplateMiddle Expression[ In, ?Yield, ?Await]

13.2.8.1 Static Semantics: Early Errors

TemplateLiteral[Yield, Await, Tagged] : NoSubstitutionTemplate TemplateLiteral[Yield, Await, Tagged] : SubstitutionTemplate[?Yield, ?Await, ?Tagged]
  • It is a Syntax Error if the number of elements in the result of TemplateStrings of TemplateLiteral with argument false is greater than or equal to 232.
SubstitutionTemplate[Yield, Await, Tagged] : TemplateHead Expression[ In, ?Yield, ?Await] TemplateSpans[?Yield, ?Await, ?Tagged] TemplateSpans[Yield, Await, Tagged] : TemplateTail TemplateMiddleList[Yield, Await, Tagged] : TemplateMiddle Expression[ In, ?Yield, ?Await] TemplateMiddleList[?Yield, ?Await, ?Tagged] TemplateMiddle Expression[ In, ?Yield, ?Await]

13.2.8.2 Static Semantics: TemplateStrings

The syntax-directed operation TemplateStrings takes argument raw (a Boolean) and returns a List of Strings. It is defined piecewise over the following productions:

TemplateLiteral : NoSubstitutionTemplate
  1. Return « TemplateString(NoSubstitutionTemplate, raw) ».
 SubstitutionTemplate : TemplateHead Expression TemplateSpans
  1. Let head be « TemplateString(TemplateHead, raw) ».
  2. Let tail be TemplateStrings of TemplateSpans with argument raw.
  3. Return the list-concatenation of head and tail.
 TemplateSpans : TemplateTail
  1. Return « TemplateString(TemplateTail, raw) ».
 TemplateSpans : TemplateMiddleList TemplateTail
  1. Let middle be TemplateStrings of TemplateMiddleList with argument raw.
  2. Let tail be « TemplateString(TemplateTail, raw) ».
  3. Return the list-concatenation of middle and tail.
 TemplateMiddleList : TemplateMiddle Expression
  1. Return « TemplateString(TemplateMiddle, raw) ».
 TemplateMiddleList : TemplateMiddleList TemplateMiddle Expression
  1. Let front be TemplateStrings of TemplateMiddleList with argument raw.
  2. Let last be « TemplateString(TemplateMiddle, raw) ».
  3. Return the list-concatenation of front and last.

13.2.8.3 Static Semantics: TemplateString ( templateToken, raw )

The abstract operation TemplateString takes arguments templateToken (a NoSubstitutionTemplate Parse Node, a TemplateHead Parse Node, a TemplateMiddle Parse Node, or a TemplateTail Parse Node) and raw (a Boolean) and returns a String. It performs the following steps when called:

  1. If raw is true, then
    1. Let string be the TRV of templateToken.
  2. Else,
    1. Let string be the TV of templateToken.
  3. Return string.

13.2.8.4 GetTemplateObject ( templateLiteral )

The abstract operation GetTemplateObject takes argument templateLiteral (a Parse Node) and returns an Array. It performs the following steps when called:

  1. Let realm be the current Realm Record.
  2. Let templateRegistry be realm.[[TemplateMap]].
  3. For each element e of templateRegistry, do
    1. If e.[[Site]] is the same Parse Node as templateLiteral, then
      1. Return e.[[Array]].
  4. Let rawStrings be TemplateStrings of templateLiteral with argument true.
  5. Let cookedStrings be TemplateStrings of templateLiteral with argument false.
  6. Let count be the number of elements in the List cookedStrings.
  7. Assert: count ≤ 232 - 1.
  8. Let template be ! ArrayCreate(count).
  9. Let rawObj be ! ArrayCreate(count).
  10. Let index be 0.
  11. Repeat, while index < count,
    1. Let prop be ! ToString(𝔽(index)).
    2. Let cookedValue be cookedStrings[index].
    3. Perform ! DefinePropertyOrThrow(template, prop, PropertyDescriptor { [[Value]]: cookedValue, [[Writable]]: false, [[Enumerable]]: true, [[Configurable]]: false }).
    4. Let rawValue be the String value rawStrings[index].
    5. Perform ! DefinePropertyOrThrow(rawObj, prop, PropertyDescriptor { [[Value]]: rawValue, [[Writable]]: false, [[Enumerable]]: true, [[Configurable]]: false }).
    6. Set index to index 1.
  12. Perform ! SetIntegrityLevel(rawObj, frozen).
  13. Perform ! DefinePropertyOrThrow(template, "raw", PropertyDescriptor { [[Value]]: rawObj, [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }).
  14. Perform ! SetIntegrityLevel(template, frozen).
  15. Append the Record { [[Site]]: templateLiteral, [[Array]]: template } to realm.[[TemplateMap]].
  16. Return template.
 Note 1

The creation of a template object cannot result in an abrupt completion.

 Note 2

Each TemplateLiteral in the program code of a realm is associated with a unique template object that is used in the evaluation of tagged Templates (13.2.8.6). The template objects are frozen and the same template object is used each time a specific tagged Template is evaluated. Whether template objects are created lazily upon first evaluation of the TemplateLiteral or eagerly prior to first evaluation is an implementation choice that is not observable to ECMAScript code.

 Note 3

Future editions of this specification may define additional non-enumerable properties of template objects.

13.2.8.5 Runtime Semantics: SubstitutionEvaluation

The syntax-directed operation SubstitutionEvaluation takes no arguments and returns either a normal completion containing a List of ECMAScript language values or an abrupt completion. It is defined piecewise over the following productions:

TemplateSpans : TemplateTail
  1. Return a new empty List.
 TemplateSpans : TemplateMiddleList TemplateTail
  1. Return ? SubstitutionEvaluation of TemplateMiddleList.
 TemplateMiddleList : TemplateMiddle Expression
  1. Let subRef be ? Evaluation of Expression.
  2. Let sub be ? GetValue(subRef).
  3. Return « sub ».
 TemplateMiddleList : TemplateMiddleList TemplateMiddle Expression
  1. Let preceding be ? SubstitutionEvaluation of TemplateMiddleList.
  2. Let nextRef be ? Evaluation of Expression.
  3. Let next be ? GetValue(nextRef).
  4. Return the list-concatenation of preceding and « next ».

13.2.8.6 Runtime Semantics: Evaluation

TemplateLiteral : NoSubstitutionTemplate
  1. Return the TV of NoSubstitutionTemplate as defined in 12.9.6.
 SubstitutionTemplate : TemplateHead Expression TemplateSpans
  1. Let head be the TV of TemplateHead as defined in 12.9.6.
  2. Let subRef be ? Evaluation of Expression.
  3. Let sub be ? GetValue(subRef).
  4. Let middle be ? ToString(sub).
  5. Let tail be ? Evaluation of TemplateSpans.
  6. Return the string-concatenation of head, middle, and tail.
 Note 1

The string conversion semantics applied to the Expression value are like String.prototype.concat rather than the operator.

 TemplateSpans : TemplateTail
  1. Return the TV of TemplateTail as defined in 12.9.6.
 TemplateSpans : TemplateMiddleList TemplateTail
  1. Let head be ? Evaluation of TemplateMiddleList.
  2. Let tail be the TV of TemplateTail as defined in 12.9.6.
  3. Return the string-concatenation of head and tail.
 TemplateMiddleList : TemplateMiddle Expression
  1. Let head be the TV of TemplateMiddle as defined in 12.9.6.
  2. Let subRef be ? Evaluation of Expression.
  3. Let sub be ? GetValue(subRef).
  4. Let middle be ? ToString(sub).
  5. Return the string-concatenation of head and middle.
 Note 2

The string conversion semantics applied to the Expression value are like String.prototype.concat rather than the operator.

 TemplateMiddleList : TemplateMiddleList TemplateMiddle Expression
  1. Let rest be ? Evaluation of TemplateMiddleList.
  2. Let middle be the TV of TemplateMiddle as defined in 12.9.6.
  3. Let subRef be ? Evaluation of Expression.
  4. Let sub be ? GetValue(subRef).
  5. Let last be ? ToString(sub).
  6. Return the string-concatenation of rest, middle, and last.
 Note 3

The string conversion semantics applied to the Expression value are like String.prototype.concat rather than the operator.

13.2.9 The Grouping Operator

13.2.9.1 Static Semantics: Early Errors

PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList

13.2.9.2 Runtime Semantics: Evaluation

PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList
  1. Let expr be the ParenthesizedExpression that is covered by CoverParenthesizedExpressionAndArrowParameterList.
  2. Return ? Evaluation of expr.
 ParenthesizedExpression : ( Expression )
  1. Return ? Evaluation of Expression. This may be of type Reference.
 Note

This algorithm does not apply GetValue to Evaluation of Expression. The principal motivation for this is so that operators such as delete and typeof may be applied to parenthesized expressions.

13.3 Left-Hand-Side Expressions

Syntax

MemberExpression[Yield, Await] : PrimaryExpression[?Yield, ?Await] MemberExpression[?Yield, ?Await] [ Expression[ In, ?Yield, ?Await] ] MemberExpression[?Yield, ?Await] . IdentifierName MemberExpression[?Yield, ?Await] TemplateLiteral[?Yield, ?Await, Tagged] SuperProperty[?Yield, ?Await] MetaProperty new MemberExpression[?Yield, ?Await] Arguments[?Yield, ?Await] MemberExpression[?Yield, ?Await] . PrivateIdentifier SuperProperty[Yield, Await] : super [ Expression[ In, ?Yield, ?Await] ] super . IdentifierName MetaProperty : NewTarget ImportMeta NewTarget : new . target ImportMeta : import . meta NewExpression[Yield, Await] : MemberExpression[?Yield, ?Await] new NewExpression[?Yield, ?Await] CallExpression[Yield, Await] : CoverCallExpressionAndAsyncArrowHead[?Yield, ?Await] SuperCall[?Yield, ?Await] ImportCall[?Yield, ?Await] CallExpression[?Yield, ?Await] Arguments[?Yield, ?Await] CallExpression[?Yield, ?Await] [ Expression[ In, ?Yield, ?Await] ] CallExpression[?Yield, ?Await] . IdentifierName CallExpression[?Yield, ?Await] TemplateLiteral[?Yield, ?Await, Tagged] CallExpression[?Yield, ?Await] . PrivateIdentifier SuperCall[Yield, Await] : super Arguments[?Yield, ?Await] ImportCall[Yield, Await] : import ( AssignmentExpression[ In, ?Yield, ?Await] ) Arguments[Yield, Await] : ( ) ( ArgumentList[?Yield, ?Await] ) ( ArgumentList[?Yield, ?Await] , ) ArgumentList[Yield, Await] : AssignmentExpression[ In, ?Yield, ?Await] ... AssignmentExpression[ In, ?Yield, ?Await] ArgumentList[?Yield, ?Await] , AssignmentExpression[ In, ?Yield, ?Await] ArgumentList[?Yield, ?Await] , ... AssignmentExpression[ In, ?Yield, ?Await] OptionalExpression[Yield, Await] : MemberExpression[?Yield, ?Await] OptionalChain[?Yield, ?Await] CallExpression[?Yield, ?Await] OptionalChain[?Yield, ?Await] OptionalExpression[?Yield, ?Await] OptionalChain[?Yield, ?Await] OptionalChain[Yield, Await] : ?. Arguments[?Yield, ?Await] ?. [ Expression[ In, ?Yield, ?Await] ] ?. IdentifierName ?. TemplateLiteral[?Yield, ?Await, Tagged] ?. PrivateIdentifier OptionalChain[?Yield, ?Await] Arguments[?Yield, ?Await] OptionalChain[?Yield, ?Await] [ Expression[ In, ?Yield, ?Await] ] OptionalChain[?Yield, ?Await] . IdentifierName OptionalChain[?Yield, ?Await] TemplateLiteral[?Yield, ?Await, Tagged] OptionalChain[?Yield, ?Await] . PrivateIdentifier LeftHandSideExpression[Yield, Await] : NewExpression[?Yield, ?Await] CallExpression[?Yield, ?Await] OptionalExpression[?Yield, ?Await]

Supplemental Syntax

When processing an instance of the production
CallExpression : CoverCallExpressionAndAsyncArrowHead
the interpretation of CoverCallExpressionAndAsyncArrowHead is refined using the following grammar:

CallMemberExpression[Yield, Await] : MemberExpression[?Yield, ?Await] Arguments[?Yield, ?Await]

13.3.1 Static Semantics

13.3.1.1 Static Semantics: Early Errors

OptionalChain : ?. TemplateLiteral OptionalChain TemplateLiteral
  • It is a Syntax Error if any source text is matched by this production.
Note

This production exists in order to prevent automatic semicolon insertion rules (12.10) from being applied to the following code:

a?.b
`c`

so that it would be interpreted as two valid statements. The purpose is to maintain consistency with similar code without optional chaining:

a.b
`c`

which is a valid statement and where automatic semicolon insertion does not apply.

 ImportMeta : import . meta

13.3.2 Property Accessors

Note

Properties are accessed by name, using either the dot notation:

MemberExpression . IdentifierName
CallExpression . IdentifierName

or the bracket notation:

MemberExpression [ Expression ]
CallExpression [ Expression ]

The dot notation is explained by the following syntactic conversion:

MemberExpression . IdentifierName

is identical in its behaviour to

MemberExpression [ <identifier-name-string> ]

and similarly

CallExpression . IdentifierName

is identical in its behaviour to

CallExpression [ <identifier-name-string> ]

where <identifier-name-string> is the result of evaluating StringValue of IdentifierName.

13.3.2.1 Runtime Semantics: Evaluation

MemberExpression : MemberExpression [ Expression ]
  1. Let baseReference be ? Evaluation of MemberExpression.
  2. Let baseValue be ? GetValue(baseReference).
  3. If the source text matched by this MemberExpression is strict mode code, let strict be true; else let strict be false.
  4. Return ? EvaluatePropertyAccessWithExpressionKey(baseValue, Expression, strict).
 MemberExpression : MemberExpression . IdentifierName
  1. Let baseReference be ? Evaluation of MemberExpression.
  2. Let baseValue be ? GetValue(baseReference).
  3. If the source text matched by this MemberExpression is strict mode code, let strict be true; else let strict be false.
  4. Return EvaluatePropertyAccessWithIdentifierKey(baseValue, IdentifierName, strict).
 MemberExpression : MemberExpression . PrivateIdentifier
  1. Let baseReference be ? Evaluation of MemberExpression.
  2. Let baseValue be ? GetValue(baseReference).
  3. Let fieldNameString be the StringValue of PrivateIdentifier.
  4. Return MakePrivateReference(baseValue, fieldNameString).
 CallExpression : CallExpression [ Expression ]
  1. Let baseReference be ? Evaluation of CallExpression.
  2. Let baseValue be ? GetValue(baseReference).
  3. If the source text matched by this CallExpression is strict mode code, let strict be true; else let strict be false.
  4. Return ? EvaluatePropertyAccessWithExpressionKey(baseValue, Expression, strict).
 CallExpression : CallExpression . IdentifierName
  1. Let baseReference be ? Evaluation of CallExpression.
  2. Let baseValue be ? GetValue(baseReference).
  3. If the source text matched by this CallExpression is strict mode code, let strict be true; else let strict be false.
  4. Return EvaluatePropertyAccessWithIdentifierKey(baseValue, IdentifierName, strict).
 CallExpression : CallExpression . PrivateIdentifier
  1. Let baseReference be ? Evaluation of CallExpression.
  2. Let baseValue be ? GetValue(baseReference).
  3. Let fieldNameString be the StringValue of PrivateIdentifier.
  4. Return MakePrivateReference(baseValue, fieldNameString).

13.3.3 EvaluatePropertyAccessWithExpressionKey ( baseValue, expression, strict )

The abstract operation EvaluatePropertyAccessWithExpressionKey takes arguments baseValue (an ECMAScript language value), expression (a Parse Node), and strict (a Boolean) and returns either a normal completion containing a Reference Record or an abrupt completion. It performs the following steps when called:

  1. Let propertyNameReference be ? Evaluation of expression.
  2. Let propertyNameValue be ? GetValue(propertyNameReference).
  3. Let propertyKey be ? ToPropertyKey(propertyNameValue).
  4. Return the Reference Record { [[Base]]: baseValue, [[ReferencedName]]: propertyKey, [[Strict]]: strict, [[ThisValue]]: empty }.

13.3.4 EvaluatePropertyAccessWithIdentifierKey ( baseValue, identifierName, strict )

The abstract operation EvaluatePropertyAccessWithIdentifierKey takes arguments baseValue (an ECMAScript language value), identifierName (an IdentifierName Parse Node), and strict (a Boolean) and returns a Reference Record. It performs the following steps when called:

  1. Let propertyNameString be StringValue of identifierName.
  2. Return the Reference Record { [[Base]]: baseValue, [[ReferencedName]]: propertyNameString, [[Strict]]: strict, [[ThisValue]]: empty }.

13.3.5 The new Operator

13.3.5.1 Runtime Semantics: Evaluation

NewExpression : new NewExpression
  1. Return ? EvaluateNew(NewExpression, empty).
 MemberExpression : new MemberExpression Arguments
  1. Return ? EvaluateNew(MemberExpression, Arguments).

13.3.5.1.1 EvaluateNew ( constructExpr, arguments )

The abstract operation EvaluateNew takes arguments constructExpr (a NewExpression Parse Node or a MemberExpression Parse Node) and arguments (empty or an Arguments Parse Node) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It performs the following steps when called:

  1. Let ref be ? Evaluation of constructExpr.
  2. Let constructor be ? GetValue(ref).
  3. If arguments is empty, let argList be a new empty List.
  4. Else,
    1. Let argList be ? ArgumentListEvaluation of arguments.
  5. If IsConstructor(constructor) is false, throw a TypeError exception.
  6. Return ? Construct(constructor, argList).

13.3.6 Function Calls

13.3.6.1 Runtime Semantics: Evaluation

CallExpression : CoverCallExpressionAndAsyncArrowHead
  1. Let expr be the CallMemberExpression that is covered by CoverCallExpressionAndAsyncArrowHead.
  2. Let memberExpr be the MemberExpression of expr.
  3. Let arguments be the Arguments of expr.
  4. Let ref be ? Evaluation of memberExpr.
  5. Let func be ? GetValue(ref).
  6. If ref is a Reference Record, IsPropertyReference(ref) is false, and ref.[[ReferencedName]] is "eval", then
    1. If SameValue(func, %eval%) is true, then
      1. Let argList be ? ArgumentListEvaluation of arguments.
      2. If argList has no elements, return undefined.
      3. Let evalArg be the first element of argList.
      4. If the source text matched by this CallExpression is strict mode code, let strictCaller be true. Otherwise let strictCaller be false.
      5. Return ? PerformEval(evalArg, strictCaller, true).
  7. Let thisCall be this CallExpression.
  8. Let tailCall be IsInTailPosition(thisCall).
  9. Return ? EvaluateCall(func, ref, arguments, tailCall).

A CallExpression evaluation that executes step 6.a.v is a direct eval.

CallExpression : CallExpression Arguments
  1. Let ref be ? Evaluation of CallExpression.
  2. Let func be ? GetValue(ref).
  3. Let thisCall be this CallExpression.
  4. Let tailCall be IsInTailPosition(thisCall).
  5. Return ? EvaluateCall(func, ref, Arguments, tailCall).

13.3.6.2 EvaluateCall ( func, ref, arguments, tailPosition )

The abstract operation EvaluateCall takes arguments func (an ECMAScript language value), ref (an ECMAScript language value or a Reference Record), arguments (a Parse Node), and tailPosition (a Boolean) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It performs the following steps when called:

  1. If ref is a Reference Record, then
    1. If IsPropertyReference(ref) is true, then
      1. Let thisValue be GetThisValue(ref).
    2. Else,
      1. Let refEnv be ref.[[Base]].
      2. Assert: refEnv is an Environment Record.
      3. Let thisValue be refEnv.WithBaseObject().
  2. Else,
    1. Let thisValue be undefined.
  3. Let argList be ? ArgumentListEvaluation of arguments.
  4. If func is not an Object, throw a TypeError exception.
  5. If IsCallable(func) is false, throw a TypeError exception.
  6. If tailPosition is true, perform PrepareForTailCall().
  7. Return ? Call(func, thisValue, argList).

13.3.7 The super Keyword

13.3.7.1 Runtime Semantics: Evaluation

SuperProperty : super [ Expression ]
  1. Let env be GetThisEnvironment().
  2. Let actualThis be ? env.GetThisBinding().
  3. Let propertyNameReference be ? Evaluation of Expression.
  4. Let propertyNameValue be ? GetValue(propertyNameReference).
  5. Let propertyKey be ? ToPropertyKey(propertyNameValue).
  6. If the source text matched by this SuperProperty is strict mode code, let strict be true; else let strict be false.
  7. Return ? MakeSuperPropertyReference(actualThis, propertyKey, strict).
 SuperProperty : super . IdentifierName
  1. Let env be GetThisEnvironment().
  2. Let actualThis be ? env.GetThisBinding().
  3. Let propertyKey be StringValue of IdentifierName.
  4. If the source text matched by this SuperProperty is strict mode code, let strict be true; else let strict be false.
  5. Return ? MakeSuperPropertyReference(actualThis, propertyKey, strict).
 SuperCall : super Arguments
  1. Let newTarget be GetNewTarget().
  2. Assert: newTarget is an Object.
  3. Let func be GetSuperConstructor().
  4. Let argList be ? ArgumentListEvaluation of Arguments.
  5. If IsConstructor(func) is false, throw a TypeError exception.
  6. Let result be ? Construct(func, argList, newTarget).
  7. Let thisER be GetThisEnvironment().
  8. Perform ? thisER.BindThisValue(result).
  9. Let F be thisER.[[FunctionObject]].
  10. Assert: F is an ECMAScript function object.
  11. Perform ? InitializeInstanceElements(result, F).
  12. Return result.

13.3.7.2 GetSuperConstructor ( )

The abstract operation GetSuperConstructor takes no arguments and returns an ECMAScript language value. It performs the following steps when called:

  1. Let envRec be GetThisEnvironment().
  2. Assert: envRec is a Function Environment Record.
  3. Let activeFunction be envRec.[[FunctionObject]].
  4. Assert: activeFunction is an ECMAScript function object.
  5. Let superConstructor be ! activeFunction.[[GetPrototypeOf]]().
  6. Return superConstructor.

13.3.7.3 MakeSuperPropertyReference ( actualThis, propertyKey, strict )

The abstract operation MakeSuperPropertyReference takes arguments actualThis (an ECMAScript language value), propertyKey (a property key), and strict (a Boolean) and returns either a normal completion containing a Super Reference Record or a throw completion. It performs the following steps when called:

  1. Let env be GetThisEnvironment().
  2. Assert: env.HasSuperBinding() is true.
  3. Let baseValue be ? env.GetSuperBase().
  4. Return the Reference Record { [[Base]]: baseValue, [[ReferencedName]]: propertyKey, [[Strict]]: strict, [[ThisValue]]: actualThis }.

13.3.8 Argument Lists

Note

The evaluation of an argument list produces a List of values.

13.3.8.1 Runtime Semantics: ArgumentListEvaluation

The syntax-directed operation ArgumentListEvaluation takes no arguments and returns either a normal completion containing a List of ECMAScript language values or an abrupt completion. It is defined piecewise over the following productions:

Arguments : ( )
  1. Return a new empty List.
 ArgumentList : AssignmentExpression
  1. Let ref be ? Evaluation of AssignmentExpression.
  2. Let arg be ? GetValue(ref).
  3. Return « arg ».
 ArgumentList : ... AssignmentExpression
  1. Let list be a new empty List.
  2. Let spreadRef be ? Evaluation of AssignmentExpression.
  3. Let spreadObj be ? GetValue(spreadRef).
  4. Let iteratorRecord be ? GetIterator(spreadObj, sync).
  5. Repeat,
    1. Let next be ? IteratorStep(iteratorRecord).
    2. If next is false, return list.
    3. Let nextArg be ? IteratorValue(next).
    4. Append nextArg to list.
 ArgumentList : ArgumentList , AssignmentExpression
  1. Let precedingArgs be ? ArgumentListEvaluation of ArgumentList.
  2. Let ref be ? Evaluation of AssignmentExpression.
  3. Let arg be ? GetValue(ref).
  4. Return the list-concatenation of precedingArgs and « arg ».
 ArgumentList : ArgumentList , ... AssignmentExpression
  1. Let precedingArgs be ? ArgumentListEvaluation of ArgumentList.
  2. Let spreadRef be ? Evaluation of AssignmentExpression.
  3. Let iteratorRecord be ? GetIterator(? GetValue(spreadRef), sync).
  4. Repeat,
    1. Let next be ? IteratorStep(iteratorRecord).
    2. If next is false, return precedingArgs.
    3. Let nextArg be ? IteratorValue(next).
    4. Append nextArg to precedingArgs.
 TemplateLiteral : NoSubstitutionTemplate
  1. Let templateLiteral be this TemplateLiteral.
  2. Let siteObj be GetTemplateObject(templateLiteral).
  3. Return « siteObj ».
 TemplateLiteral : SubstitutionTemplate
  1. Let templateLiteral be this TemplateLiteral.
  2. Let siteObj be GetTemplateObject(templateLiteral).
  3. Let remaining be ? ArgumentListEvaluation of SubstitutionTemplate.
  4. Return the list-concatenation of « siteObj » and remaining.
 SubstitutionTemplate : TemplateHead Expression TemplateSpans
  1. Let firstSubRef be ? Evaluation of Expression.
  2. Let firstSub be ? GetValue(firstSubRef).
  3. Let restSub be ? SubstitutionEvaluation of TemplateSpans.
  4. Assert: restSub is a possibly empty List.
  5. Return the list-concatenation of « firstSub » and restSub.

13.3.9 Optional Chains

Note
An optional chain is a chain of one or more property accesses and function calls, the first of which begins with the token ?..

13.3.9.1 Runtime Semantics: Evaluation

OptionalExpression : MemberExpression OptionalChain
  1. Let baseReference be ? Evaluation of MemberExpression.
  2. Let baseValue be ? GetValue(baseReference).
  3. If baseValue is either undefined or null, then
    1. Return undefined.
  4. Return ? ChainEvaluation of OptionalChain with arguments baseValue and baseReference.
 OptionalExpression : CallExpression OptionalChain
  1. Let baseReference be ? Evaluation of CallExpression.
  2. Let baseValue be ? GetValue(baseReference).
  3. If baseValue is either undefined or null, then
    1. Return undefined.
  4. Return ? ChainEvaluation of OptionalChain with arguments baseValue and baseReference.
 OptionalExpression : OptionalExpression OptionalChain
  1. Let baseReference be ? Evaluation of OptionalExpression.
  2. Let baseValue be ? GetValue(baseReference).
  3. If baseValue is either undefined or null, then
    1. Return undefined.
  4. Return ? ChainEvaluation of OptionalChain with arguments baseValue and baseReference.

13.3.9.2 Runtime Semantics: ChainEvaluation

The syntax-directed operation ChainEvaluation takes arguments baseValue (an ECMAScript language value) and baseReference (an ECMAScript language value or a Reference Record) and returns either a normal completion containing either an ECMAScript language value or a Reference Record, or an abrupt completion. It is defined piecewise over the following productions:

OptionalChain : ?. Arguments
  1. Let thisChain be this OptionalChain.
  2. Let tailCall be IsInTailPosition(thisChain).
  3. Return ? EvaluateCall(baseValue, baseReference, Arguments, tailCall).
 OptionalChain : ?. [ Expression ]
  1. If the source text matched by this OptionalChain is strict mode code, let strict be true; else let strict be false.
  2. Return ? EvaluatePropertyAccessWithExpressionKey(baseValue, Expression, strict).
 OptionalChain : ?. IdentifierName
  1. If the source text matched by this OptionalChain is strict mode code, let strict be true; else let strict be false.
  2. Return EvaluatePropertyAccessWithIdentifierKey(baseValue, IdentifierName, strict).
 OptionalChain : ?. PrivateIdentifier
  1. Let fieldNameString be the StringValue of PrivateIdentifier.
  2. Return MakePrivateReference(baseValue, fieldNameString).
 OptionalChain : OptionalChain Arguments
  1. Let optionalChain be OptionalChain.
  2. Let newReference be ? ChainEvaluation of optionalChain with arguments baseValue and baseReference.
  3. Let newValue be ? GetValue(newReference).
  4. Let thisChain be this OptionalChain.
  5. Let tailCall be IsInTailPosition(thisChain).
  6. Return ? EvaluateCall(newValue, newReference, Arguments, tailCall).
 OptionalChain : OptionalChain [ Expression ]
  1. Let optionalChain be OptionalChain.
  2. Let newReference be ? ChainEvaluation of optionalChain with arguments baseValue and baseReference.
  3. Let newValue be ? GetValue(newReference).
  4. If the source text matched by this OptionalChain is strict mode code, let strict be true; else let strict be false.
  5. Return ? EvaluatePropertyAccessWithExpressionKey(newValue, Expression, strict).
 OptionalChain : OptionalChain . IdentifierName
  1. Let optionalChain be OptionalChain.
  2. Let newReference be ? ChainEvaluation of optionalChain with arguments baseValue and baseReference.
  3. Let newValue be ? GetValue(newReference).
  4. If the source text matched by this OptionalChain is strict mode code, let strict be true; else let strict be false.
  5. Return EvaluatePropertyAccessWithIdentifierKey(newValue, IdentifierName, strict).
 OptionalChain : OptionalChain . PrivateIdentifier
  1. Let optionalChain be OptionalChain.
  2. Let newReference be ? ChainEvaluation of optionalChain with arguments baseValue and baseReference.
  3. Let newValue be ? GetValue(newReference).
  4. Let fieldNameString be the StringValue of PrivateIdentifier.
  5. Return MakePrivateReference(newValue, fieldNameString).

13.3.10 Import Calls

13.3.10.1 Runtime Semantics: Evaluation

ImportCall : import ( AssignmentExpression )
  1. Let referrer be GetActiveScriptOrModule().
  2. If referrer is null, set referrer to the current Realm Record.
  3. Let argRef be ? Evaluation of AssignmentExpression.
  4. Let specifier be ? GetValue(argRef).
  5. Let promiseCapability be ! NewPromiseCapability(%Promise%).
  6. Let specifierString be Completion(ToString(specifier)).
  7. IfAbruptRejectPromise(specifierString, promiseCapability).
  8. Perform HostLoadImportedModule(referrer, specifierString, empty, promiseCapability).
  9. Return promiseCapability.[[Promise]].

13.3.10.1.1 ContinueDynamicImport ( promiseCapability, moduleCompletion )

The abstract operation ContinueDynamicImport takes arguments promiseCapability (a PromiseCapability Record) and moduleCompletion (either a normal completion containing a Module Record or a throw completion) and returns unused. It completes the process of a dynamic import originally started by an import() call, resolving or rejecting the promise returned by that call as appropriate. It performs the following steps when called:

  1. If moduleCompletion is an abrupt completion, then
    1. Perform ! Call(promiseCapability.[[Reject]], undefined, « moduleCompletion.[[Value]] »).
    2. Return unused.
  2. Let module be moduleCompletion.[[Value]].
  3. Let loadPromise be module.LoadRequestedModules().
  4. Let rejectedClosure be a new Abstract Closure with parameters (reason) that captures promiseCapability and performs the following steps when called:
    1. Perform ! Call(promiseCapability.[[Reject]], undefined, « reason »).
    2. Return unused.
  5. Let onRejected be CreateBuiltinFunction(rejectedClosure, 1, "", « »).
  6. Let linkAndEvaluateClosure be a new Abstract Closure with no parameters that captures module, promiseCapability, and onRejected and performs the following steps when called:
    1. Let link be Completion(module.Link()).
    2. If link is an abrupt completion, then
      1. Perform ! Call(promiseCapability.[[Reject]], undefined, « link.[[Value]] »).
      2. Return unused.
    3. Let evaluatePromise be module.Evaluate().
    4. Let fulfilledClosure be a new Abstract Closure with no parameters that captures module and promiseCapability and performs the following steps when called:
      1. Let namespace be GetModuleNamespace(module).
      2. Perform ! Call(promiseCapability.[[Resolve]], undefined, « namespace »).
      3. Return unused.
    5. Let onFulfilled be CreateBuiltinFunction(fulfilledClosure, 0, "", « »).
    6. Perform PerformPromiseThen(evaluatePromise, onFulfilled, onRejected).
    7. Return unused.
  7. Let linkAndEvaluate be CreateBuiltinFunction(linkAndEvaluateClosure, 0, "", « »).
  8. Perform PerformPromiseThen(loadPromise, linkAndEvaluate, onRejected).
  9. Return unused.

13.3.11 Tagged Templates

Note

A tagged template is a function call where the arguments of the call are derived from a TemplateLiteral (13.2.8). The actual arguments include a template object (13.2.8.4) and the values produced by evaluating the expressions embedded within the TemplateLiteral.

13.3.11.1 Runtime Semantics: Evaluation

MemberExpression : MemberExpression TemplateLiteral
  1. Let tagRef be ? Evaluation of MemberExpression.
  2. Let tagFunc be ? GetValue(tagRef).
  3. Let thisCall be this MemberExpression.
  4. Let tailCall be IsInTailPosition(thisCall).
  5. Return ? EvaluateCall(tagFunc, tagRef, TemplateLiteral, tailCall).
 CallExpression : CallExpression TemplateLiteral
  1. Let tagRef be ? Evaluation of CallExpression.
  2. Let tagFunc be ? GetValue(tagRef).
  3. Let thisCall be this CallExpression.
  4. Let tailCall be IsInTailPosition(thisCall).
  5. Return ? EvaluateCall(tagFunc, tagRef, TemplateLiteral, tailCall).

13.3.12 Meta Properties

13.3.12.1 Runtime Semantics: Evaluation

NewTarget : new . target
  1. Return GetNewTarget().
 ImportMeta : import . meta
  1. Let module be GetActiveScriptOrModule().
  2. Assert: module is a Source Text Module Record.
  3. Let importMeta be module.[[ImportMeta]].
  4. If importMeta is empty, then
    1. Set importMeta to OrdinaryObjectCreate(null).
    2. Let importMetaValues be HostGetImportMetaProperties(module).
    3. For each Record { [[Key]], [[Value]] } p of importMetaValues, do
      1. Perform ! CreateDataPropertyOrThrow(importMeta, p.[[Key]], p.[[Value]]).
    4. Perform HostFinalizeImportMeta(importMeta, module).
    5. Set module.[[ImportMeta]] to importMeta.
    6. Return importMeta.
  5. Else,
    1. Assert: importMeta is an Object.
    2. Return importMeta.

13.3.12.1.1 HostGetImportMetaProperties ( moduleRecord )

The host-defined abstract operation HostGetImportMetaProperties takes argument moduleRecord (a Module Record) and returns a List of Records with fields [[Key]] (a property key) and [[Value]] (an ECMAScript language value). It allows hosts to provide property keys and values for the object returned from import.meta.

An implementation of HostGetImportMetaProperties must conform to the following requirements:

The default implementation of HostGetImportMetaProperties is to return a new empty List.

13.3.12.1.2 HostFinalizeImportMeta ( importMeta, moduleRecord )

The host-defined abstract operation HostFinalizeImportMeta takes arguments importMeta (an Object) and moduleRecord (a Module Record) and returns unused. It allows hosts to perform any extraordinary operations to prepare the object returned from import.meta.

Most hosts will be able to simply define HostGetImportMetaProperties, and leave HostFinalizeImportMeta with its default behaviour. However, HostFinalizeImportMeta provides an "escape hatch" for hosts which need to directly manipulate the object before it is exposed to ECMAScript code.

An implementation of HostFinalizeImportMeta must conform to the following requirements:

  • It must return unused.

The default implementation of HostFinalizeImportMeta is to return unused.

13.4 Update Expressions

Syntax

UpdateExpression[Yield, Await] : LeftHandSideExpression[?Yield, ?Await] LeftHandSideExpression[?Yield, ?Await] [no LineTerminator here] LeftHandSideExpression[?Yield, ?Await] [no LineTerminator here] -- UnaryExpression[?Yield, ?Await] -- UnaryExpression[?Yield, ?Await]

13.4.1 Static Semantics: Early Errors

UpdateExpression : LeftHandSideExpression LeftHandSideExpression -- UpdateExpression : UnaryExpression -- UnaryExpression

13.4.2 Postfix Increment Operator

13.4.2.1 Runtime Semantics: Evaluation

UpdateExpression : LeftHandSideExpression
  1. Let lhs be ? Evaluation of LeftHandSideExpression.
  2. Let oldValue be ? ToNumeric(? GetValue(lhs)).
  3. If oldValue is a Number, then
    1. Let newValue be Number::add(oldValue, 1𝔽).
  4. Else,
    1. Assert: oldValue is a BigInt.
    2. Let newValue be BigInt::add(oldValue, 1ℤ).
  5. Perform ? PutValue(lhs, newValue).
  6. Return oldValue.

13.4.3 Postfix Decrement Operator

13.4.3.1 Runtime Semantics: Evaluation

UpdateExpression : LeftHandSideExpression --
  1. Let lhs be ? Evaluation of LeftHandSideExpression.
  2. Let oldValue be ? ToNumeric(? GetValue(lhs)).
  3. If oldValue is a Number, then
    1. Let newValue be Number::subtract(oldValue, 1𝔽).
  4. Else,
    1. Assert: oldValue is a BigInt.
    2. Let newValue be BigInt::subtract(oldValue, 1ℤ).
  5. Perform ? PutValue(lhs, newValue).
  6. Return oldValue.

13.4.4 Prefix Increment Operator

13.4.4.1 Runtime Semantics: Evaluation

UpdateExpression : UnaryExpression
  1. Let expr be ? Evaluation of UnaryExpression.
  2. Let oldValue be ? ToNumeric(? GetValue(expr)).
  3. If oldValue is a Number, then
    1. Let newValue be Number::add(oldValue, 1𝔽).
  4. Else,
    1. Assert: oldValue is a BigInt.
    2. Let newValue be BigInt::add(oldValue, 1ℤ).
  5. Perform ? PutValue(expr, newValue).
  6. Return newValue.

13.4.5 Prefix Decrement Operator

13.4.5.1 Runtime Semantics: Evaluation

UpdateExpression : -- UnaryExpression
  1. Let expr be ? Evaluation of UnaryExpression.
  2. Let oldValue be ? ToNumeric(? GetValue(expr)).
  3. If oldValue is a Number, then
    1. Let newValue be Number::subtract(oldValue, 1𝔽).
  4. Else,
    1. Assert: oldValue is a BigInt.
    2. Let newValue be BigInt::subtract(oldValue, 1ℤ).
  5. Perform ? PutValue(expr, newValue).
  6. Return newValue.

13.5 Unary Operators

Syntax

UnaryExpression[Yield, Await] : UpdateExpression[?Yield, ?Await] delete UnaryExpression[?Yield, ?Await] void UnaryExpression[?Yield, ?Await] typeof UnaryExpression[?Yield, ?Await] UnaryExpression[?Yield, ?Await] - UnaryExpression[?Yield, ?Await] ~ UnaryExpression[?Yield, ?Await] ! UnaryExpression[?Yield, ?Await] [ Await] AwaitExpression[?Yield]

13.5.1 The delete Operator

13.5.1.1 Static Semantics: Early Errors

UnaryExpression : delete UnaryExpression Note

The last rule means that expressions such as delete (((foo))) produce early errors because of recursive application of the first rule.

13.5.1.2 Runtime Semantics: Evaluation

UnaryExpression : delete UnaryExpression
  1. Let ref be ? Evaluation of UnaryExpression.
  2. If ref is not a Reference Record, return true.
  3. If IsUnresolvableReference(ref) is true, then
    1. Assert: ref.[[Strict]] is false.
    2. Return true.
  4. If IsPropertyReference(ref) is true, then
    1. Assert: IsPrivateReference(ref) is false.
    2. If IsSuperReference(ref) is true, throw a ReferenceError exception.
    3. Let baseObj be ? ToObject(ref.[[Base]]).
    4. Let deleteStatus be ? baseObj.[[Delete]](ref.[[ReferencedName]]).
    5. If deleteStatus is false and ref.[[Strict]] is true, throw a TypeError exception.
    6. Return deleteStatus.
  5. Else,
    1. Let base be ref.[[Base]].
    2. Assert: base is an Environment Record.
    3. Return ? base.DeleteBinding(ref.[[ReferencedName]]).
 Note 1

When a delete operator occurs within strict mode code, a SyntaxError exception is thrown if its UnaryExpression is a direct reference to a variable, function argument, or function name. In addition, if a delete operator occurs within strict mode code and the property to be deleted has the attribute { [[Configurable]]: false } (or otherwise cannot be deleted), a TypeError exception is thrown.

 Note 2

The object that may be created in step 4.c is not accessible outside of the above abstract operation and the ordinary object [[Delete]] internal method. An implementation might choose to avoid the actual creation of that object.

13.5.2 The void Operator

13.5.2.1 Runtime Semantics: Evaluation

UnaryExpression : void UnaryExpression
  1. Let expr be ? Evaluation of UnaryExpression.
  2. Perform ? GetValue(expr).
  3. Return undefined.
 Note

GetValue must be called even though its value is not used because it may have observable side-effects.

13.5.3 The typeof Operator

13.5.3.1 Runtime Semantics: Evaluation

UnaryExpression : typeof UnaryExpression
  1. Let val be ? Evaluation of UnaryExpression.
  2. If val is a Reference Record, then
    1. If IsUnresolvableReference(val) is true, return "undefined".
  3. Set val to ? GetValue(val).
  4. If val is undefined, return "undefined".
  5. If val is null, return "object".
  6. If val is a String, return "string".
  7. If val is a Symbol, return "symbol".
  8. If val is a Boolean, return "boolean".
  9. If val is a Number, return "number".
  10. If val is a BigInt, return "bigint".
  11. Assert: val is an Object.
  12. NOTE: This step is replaced in section B.3.6.3.
  13. If val has a [[Call]] internal slot, return "function".
  14. Return "object".

13.5.4 Unary Operator

Note

The unary operator converts its operand to Number type.

13.5.4.1 Runtime Semantics: Evaluation

UnaryExpression : UnaryExpression
  1. Let expr be ? Evaluation of UnaryExpression.
  2. Return ? ToNumber(? GetValue(expr)).

13.5.5 Unary - Operator

Note

The unary - operator converts its operand to a numeric value and then negates it. Negating 0𝔽 produces -0𝔽, and negating -0𝔽 produces 0𝔽.

13.5.5.1 Runtime Semantics: Evaluation

UnaryExpression : - UnaryExpression
  1. Let expr be ? Evaluation of UnaryExpression.
  2. Let oldValue be ? ToNumeric(? GetValue(expr)).
  3. If oldValue is a Number, then
    1. Return Number::unaryMinus(oldValue).
  4. Else,
    1. Assert: oldValue is a BigInt.
    2. Return BigInt::unaryMinus(oldValue).

13.5.6 Bitwise NOT Operator ( ~ )

13.5.6.1 Runtime Semantics: Evaluation

UnaryExpression : ~ UnaryExpression
  1. Let expr be ? Evaluation of UnaryExpression.
  2. Let oldValue be ? ToNumeric(? GetValue(expr)).
  3. If oldValue is a Number, then
    1. Return Number::bitwiseNOT(oldValue).
  4. Else,
    1. Assert: oldValue is a BigInt.
    2. Return BigInt::bitwiseNOT(oldValue).

13.5.7 Logical NOT Operator ( ! )

13.5.7.1 Runtime Semantics: Evaluation

UnaryExpression : ! UnaryExpression
  1. Let expr be ? Evaluation of UnaryExpression.
  2. Let oldValue be ToBoolean(? GetValue(expr)).
  3. If oldValue is true, return false.
  4. Return true.

13.6 Exponentiation Operator

Syntax

ExponentiationExpression[Yield, Await] : UnaryExpression[?Yield, ?Await] UpdateExpression[?Yield, ?Await] ** ExponentiationExpression[?Yield, ?Await]

13.6.1 Runtime Semantics: Evaluation

ExponentiationExpression : UpdateExpression ** ExponentiationExpression
  1. Return ? EvaluateStringOrNumericBinaryExpression(UpdateExpression, **, ExponentiationExpression).

13.7 Multiplicative Operators

Syntax

MultiplicativeExpression[Yield, Await] : ExponentiationExpression[?Yield, ?Await] MultiplicativeExpression[?Yield, ?Await] MultiplicativeOperator ExponentiationExpression[?Yield, ?Await] MultiplicativeOperator : one of * / % Note
  • The * operator performs multiplication, producing the product of its operands.
  • The / operator performs division, producing the quotient of its operands.
  • The % operator yields the remainder of its operands from an implied division.

13.7.1 Runtime Semantics: Evaluation

MultiplicativeExpression : MultiplicativeExpression MultiplicativeOperator ExponentiationExpression
  1. Let opText be the source text matched by MultiplicativeOperator.
  2. Return ? EvaluateStringOrNumericBinaryExpression(MultiplicativeExpression, opText, ExponentiationExpression).

13.8 Additive Operators

Syntax

AdditiveExpression[Yield, Await] : MultiplicativeExpression[?Yield, ?Await] AdditiveExpression[?Yield, ?Await] MultiplicativeExpression[?Yield, ?Await] AdditiveExpression[?Yield, ?Await] - MultiplicativeExpression[?Yield, ?Await]

13.8.1 The Addition Operator ( )

Note

The addition operator either performs string concatenation or numeric addition.

13.8.1.1 Runtime Semantics: Evaluation

AdditiveExpression : AdditiveExpression MultiplicativeExpression
  1. Return ? EvaluateStringOrNumericBinaryExpression(AdditiveExpression, , MultiplicativeExpression).

13.8.2 The Subtraction Operator ( - )

Note

The - operator performs subtraction, producing the difference of its operands.

13.8.2.1 Runtime Semantics: Evaluation

AdditiveExpression : AdditiveExpression - MultiplicativeExpression
  1. Return ? EvaluateStringOrNumericBinaryExpression(AdditiveExpression, -, MultiplicativeExpression).

13.9 Bitwise Shift Operators

Syntax

ShiftExpression[Yield, Await] : AdditiveExpression[?Yield, ?Await] ShiftExpression[?Yield, ?Await] << AdditiveExpression[?Yield, ?Await] ShiftExpression[?Yield, ?Await] >> AdditiveExpression[?Yield, ?Await] ShiftExpression[?Yield, ?Await] >>> AdditiveExpression[?Yield, ?Await]

13.9.1 The Left Shift Operator ( << )

Note

Performs a bitwise left shift operation on the left operand by the amount specified by the right operand.

13.9.1.1 Runtime Semantics: Evaluation

ShiftExpression : ShiftExpression << AdditiveExpression
  1. Return ? EvaluateStringOrNumericBinaryExpression(ShiftExpression, <<, AdditiveExpression).

13.9.2 The Signed Right Shift Operator ( >> )

Note

Performs a sign-filling bitwise right shift operation on the left operand by the amount specified by the right operand.

13.9.2.1 Runtime Semantics: Evaluation

ShiftExpression : ShiftExpression >> AdditiveExpression
  1. Return ? EvaluateStringOrNumericBinaryExpression(ShiftExpression, >>, AdditiveExpression).

13.9.3 The Unsigned Right Shift Operator ( >>> )

Note

Performs a zero-filling bitwise right shift operation on the left operand by the amount specified by the right operand.

13.9.3.1 Runtime Semantics: Evaluation

ShiftExpression : ShiftExpression >>> AdditiveExpression
  1. Return ? EvaluateStringOrNumericBinaryExpression(ShiftExpression, >>>, AdditiveExpression).

13.10 Relational Operators

Note 1

The result of evaluating a relational operator is always of type Boolean, reflecting whether the relationship named by the operator holds between its two operands.

Syntax

RelationalExpression[In, Yield, Await] : ShiftExpression[?Yield, ?Await] RelationalExpression[?In, ?Yield, ?Await] < ShiftExpression[?Yield, ?Await] RelationalExpression[?In, ?Yield, ?Await] > ShiftExpression[?Yield, ?Await] RelationalExpression[?In, ?Yield, ?Await] <= ShiftExpression[?Yield, ?Await] RelationalExpression[?In, ?Yield, ?Await] >= ShiftExpression[?Yield, ?Await] RelationalExpression[?In, ?Yield, ?Await] instanceof ShiftExpression[?Yield, ?Await] [ In] RelationalExpression[ In, ?Yield, ?Await] in ShiftExpression[?Yield, ?Await] [ In] PrivateIdentifier in ShiftExpression[?Yield, ?Await] Note 2

The [In] grammar parameter is needed to avoid confusing the in operator in a relational expression with the in operator in a for statement.

13.10.1 Runtime Semantics: Evaluation

RelationalExpression : RelationalExpression < ShiftExpression
  1. Let lref be ? Evaluation of RelationalExpression.
  2. Let lval be ? GetValue(lref).
  3. Let rref be ? Evaluation of ShiftExpression.
  4. Let rval be ? GetValue(rref).
  5. Let r be ? IsLessThan(lval, rval, true).
  6. If r is undefined, return false. Otherwise, return r.
 RelationalExpression : RelationalExpression > ShiftExpression
  1. Let lref be ? Evaluation of RelationalExpression.
  2. Let lval be ? GetValue(lref).
  3. Let rref be ? Evaluation of ShiftExpression.
  4. Let rval be ? GetValue(rref).
  5. Let r be ? IsLessThan(rval, lval, false).
  6. If r is undefined, return false. Otherwise, return r.
 RelationalExpression : RelationalExpression <= ShiftExpression
  1. Let lref be ? Evaluation of RelationalExpression.
  2. Let lval be ? GetValue(lref).
  3. Let rref be ? Evaluation of ShiftExpression.
  4. Let rval be ? GetValue(rref).
  5. Let r be ? IsLessThan(rval, lval, false).
  6. If r is either true or undefined, return false. Otherwise, return true.
 RelationalExpression : RelationalExpression >= ShiftExpression
  1. Let lref be ? Evaluation of RelationalExpression.
  2. Let lval be ? GetValue(lref).
  3. Let rref be ? Evaluation of ShiftExpression.
  4. Let rval be ? GetValue(rref).
  5. Let r be ? IsLessThan(lval, rval, true).
  6. If r is either true or undefined, return false. Otherwise, return true.
 RelationalExpression : RelationalExpression instanceof ShiftExpression
  1. Let lref be ? Evaluation of RelationalExpression.
  2. Let lval be ? GetValue(lref).
  3. Let rref be ? Evaluation of ShiftExpression.
  4. Let rval be ? GetValue(rref).
  5. Return ? InstanceofOperator(lval, rval).
 RelationalExpression : RelationalExpression in ShiftExpression
  1. Let lref be ? Evaluation of RelationalExpression.
  2. Let lval be ? GetValue(lref).
  3. Let rref be ? Evaluation of ShiftExpression.
  4. Let rval be ? GetValue(rref).
  5. If rval is not an Object, throw a TypeError exception.
  6. Return ? HasProperty(rval, ? ToPropertyKey(lval)).
 RelationalExpression : PrivateIdentifier in ShiftExpression
  1. Let privateIdentifier be the StringValue of PrivateIdentifier.
  2. Let rref be ? Evaluation of ShiftExpression.
  3. Let rval be ? GetValue(rref).
  4. If rval is not an Object, throw a TypeError exception.
  5. Let privateEnv be the running execution context's PrivateEnvironment.
  6. Let privateName be ResolvePrivateIdentifier(privateEnv, privateIdentifier).
  7. If PrivateElementFind(rval, privateName) is not empty, return true.
  8. Return false.

13.10.2 InstanceofOperator ( V, target )

The abstract operation InstanceofOperator takes arguments V (an ECMAScript language value) and target (an ECMAScript language value) and returns either a normal completion containing a Boolean or a throw completion. It implements the generic algorithm for determining if V is an instance of target either by consulting target's @@hasInstance method or, if absent, determining whether the value of target's "prototype" property is present in V's prototype chain. It performs the following steps when called:

  1. If target is not an Object, throw a TypeError exception.
  2. Let instOfHandler be ? GetMethod(target, @@hasInstance).
  3. If instOfHandler is not undefined, then
    1. Return ToBoolean(? Call(instOfHandler, target, « V »)).
  4. If IsCallable(target) is false, throw a TypeError exception.
  5. Return ? OrdinaryHasInstance(target, V).
 Note

Steps 4 and 5 provide compatibility with previous editions of ECMAScript that did not use a @@hasInstance method to define the instanceof operator semantics. If an object does not define or inherit @@hasInstance it uses the default instanceof semantics.

13.11 Equality Operators

Note

The result of evaluating an equality operator is always of type Boolean, reflecting whether the relationship named by the operator holds between its two operands.

Syntax

EqualityExpression[In, Yield, Await] : RelationalExpression[?In, ?Yield, ?Await] EqualityExpression[?In, ?Yield, ?Await] == RelationalExpression[?In, ?Yield, ?Await] EqualityExpression[?In, ?Yield, ?Await] != RelationalExpression[?In, ?Yield, ?Await] EqualityExpression[?In, ?Yield, ?Await] === RelationalExpression[?In, ?Yield, ?Await] EqualityExpression[?In, ?Yield, ?Await] !== RelationalExpression[?In, ?Yield, ?Await]

13.11.1 Runtime Semantics: Evaluation

EqualityExpression : EqualityExpression == RelationalExpression
  1. Let lref be ? Evaluation of EqualityExpression.
  2. Let lval be ? GetValue(lref).
  3. Let rref be ? Evaluation of RelationalExpression.
  4. Let rval be ? GetValue(rref).
  5. Return ? IsLooselyEqual(rval, lval).
 EqualityExpression : EqualityExpression != RelationalExpression
  1. Let lref be ? Evaluation of EqualityExpression.
  2. Let lval be ? GetValue(lref).
  3. Let rref be ? Evaluation of RelationalExpression.
  4. Let rval be ? GetValue(rref).
  5. Let r be ? IsLooselyEqual(rval, lval).
  6. If r is true, return false. Otherwise, return true.
 EqualityExpression : EqualityExpression === RelationalExpression
  1. Let lref be ? Evaluation of EqualityExpression.
  2. Let lval be ? GetValue(lref).
  3. Let rref be ? Evaluation of RelationalExpression.
  4. Let rval be ? GetValue(rref).
  5. Return IsStrictlyEqual(rval, lval).
 EqualityExpression : EqualityExpression !== RelationalExpression
  1. Let lref be ? Evaluation of EqualityExpression.
  2. Let lval be ? GetValue(lref).
  3. Let rref be ? Evaluation of RelationalExpression.
  4. Let rval be ? GetValue(rref).
  5. Let r be IsStrictlyEqual(rval, lval).
  6. If r is true, return false. Otherwise, return true.
 Note 1

Given the above definition of equality:

  • String comparison can be forced by: `${a}` == `${b}`.
  • Numeric comparison can be forced by: a == b.
  • Boolean comparison can be forced by: !a == !b.
Note 2

The equality operators maintain the following invariants:

  • A != B is equivalent to !(A == B).
  • A == B is equivalent to B == A, except in the order of evaluation of A and B.
Note 3

The equality operator is not always transitive. For example, there might be two distinct String objects, each representing the same String value; each String object would be considered equal to the String value by the == operator, but the two String objects would not be equal to each other. For example:

  • new String("a") == "a" and "a" == new String("a") are both true.
  • new String("a") == new String("a") is false.
Note 4

Comparison of Strings uses a simple equality test on sequences of code unit values. There is no attempt to use the more complex, semantically oriented definitions of character or string equality and collating order defined in the Unicode specification. Therefore Strings values that are canonically equal according to the Unicode Standard could test as unequal. In effect this algorithm assumes that both Strings are already in normalized form.

13.12 Binary Bitwise Operators

Syntax

BitwiseANDExpression[In, Yield, Await] : EqualityExpression[?In, ?Yield, ?Await] BitwiseANDExpression[?In, ?Yield, ?Await] & EqualityExpression[?In, ?Yield, ?Await] BitwiseXORExpression[In, Yield, Await] : BitwiseANDExpression[?In, ?Yield, ?Await] BitwiseXORExpression[?In, ?Yield, ?Await] ^ BitwiseANDExpression[?In, ?Yield, ?Await] BitwiseORExpression[In, Yield, Await] : BitwiseXORExpression[?In, ?Yield, ?Await] BitwiseORExpression[?In, ?Yield, ?Await] | BitwiseXORExpression[?In, ?Yield, ?Await]

13.12.1 Runtime Semantics: Evaluation

BitwiseANDExpression : BitwiseANDExpression & EqualityExpression
  1. Return ? EvaluateStringOrNumericBinaryExpression(BitwiseANDExpression, &, EqualityExpression).
 BitwiseXORExpression : BitwiseXORExpression ^ BitwiseANDExpression
  1. Return ? EvaluateStringOrNumericBinaryExpression(BitwiseXORExpression, ^, BitwiseANDExpression).
 BitwiseORExpression : BitwiseORExpression | BitwiseXORExpression
  1. Return ? EvaluateStringOrNumericBinaryExpression(BitwiseORExpression, |, BitwiseXORExpression).

13.13 Binary Logical Operators

Syntax

LogicalANDExpression[In, Yield, Await] : BitwiseORExpression[?In, ?Yield, ?Await] LogicalANDExpression[?In, ?Yield, ?Await] && BitwiseORExpression[?In, ?Yield, ?Await] LogicalORExpression[In, Yield, Await] : LogicalANDExpression[?In, ?Yield, ?Await] LogicalORExpression[?In, ?Yield, ?Await] || LogicalANDExpression[?In, ?Yield, ?Await] CoalesceExpression[In, Yield, Await] : CoalesceExpressionHead[?In, ?Yield, ?Await] ?? BitwiseORExpression[?In, ?Yield, ?Await] CoalesceExpressionHead[In, Yield, Await] : CoalesceExpression[?In, ?Yield, ?Await] BitwiseORExpression[?In, ?Yield, ?Await] ShortCircuitExpression[In, Yield, Await] : LogicalORExpression[?In, ?Yield, ?Await] CoalesceExpression[?In, ?Yield, ?Await] Note

The value produced by a && or || operator is not necessarily of type Boolean. The value produced will always be the value of one of the two operand expressions.

13.13.1 Runtime Semantics: Evaluation

LogicalANDExpression : LogicalANDExpression && BitwiseORExpression
  1. Let lref be ? Evaluation of LogicalANDExpression.
  2. Let lval be ? GetValue(lref).
  3. Let lbool be ToBoolean(lval).
  4. If lbool is false, return lval.
  5. Let rref be ? Evaluation of BitwiseORExpression.
  6. Return ? GetValue(rref).
 LogicalORExpression : LogicalORExpression || LogicalANDExpression
  1. Let lref be ? Evaluation of LogicalORExpression.
  2. Let lval be ? GetValue(lref).
  3. Let lbool be ToBoolean(lval).
  4. If lbool is true, return lval.
  5. Let rref be ? Evaluation of LogicalANDExpression.
  6. Return ? GetValue(rref).
 CoalesceExpression : CoalesceExpressionHead ?? BitwiseORExpression
  1. Let lref be ? Evaluation of CoalesceExpressionHead.
  2. Let lval be ? GetValue(lref).
  3. If lval is either undefined or null, then
    1. Let rref be ? Evaluation of BitwiseORExpression.
    2. Return ? GetValue(rref).
  4. Otherwise, return lval.

13.14 Conditional Operator ( ? : )

Syntax

ConditionalExpression[In, Yield, Await] : ShortCircuitExpression[?In, ?Yield, ?Await] ShortCircuitExpression[?In, ?Yield, ?Await] ? AssignmentExpression[ In, ?Yield, ?Await] : AssignmentExpression[?In, ?Yield, ?Await] Note

The grammar for a ConditionalExpression in ECMAScript is slightly different from that in C and Java, which each allow the second subexpression to be an Expression but restrict the third expression to be a ConditionalExpression. The motivation for this difference in ECMAScript is to allow an assignment expression to be governed by either arm of a conditional and to eliminate the confusing and fairly useless case of a comma expression as the centre expression.

13.14.1 Runtime Semantics: Evaluation

ConditionalExpression : ShortCircuitExpression ? AssignmentExpression : AssignmentExpression
  1. Let lref be ? Evaluation of ShortCircuitExpression.
  2. Let lval be ToBoolean(? GetValue(lref)).
  3. If lval is true, then
    1. Let trueRef be ? Evaluation of the first AssignmentExpression.
    2. Return ? GetValue(trueRef).
  4. Else,
    1. Let falseRef be ? Evaluation of the second AssignmentExpression.
    2. Return ? GetValue(falseRef).

13.15 Assignment Operators

Syntax

AssignmentExpression[In, Yield, Await] : ConditionalExpression[?In, ?Yield, ?Await] [ Yield] YieldExpression[?In, ?Await] ArrowFunction[?In, ?Yield, ?Await] AsyncArrowFunction[?In, ?Yield, ?Await] LeftHandSideExpression[?Yield, ?Await] = AssignmentExpression[?In, ?Yield, ?Await] LeftHandSideExpression[?Yield, ?Await] AssignmentOperator AssignmentExpression[?In, ?Yield, ?Await] LeftHandSideExpression[?Yield, ?Await] &&= AssignmentExpression[?In, ?Yield, ?Await] LeftHandSideExpression[?Yield, ?Await] ||= AssignmentExpression[?In, ?Yield, ?Await] LeftHandSideExpression[?Yield, ?Await] ??= AssignmentExpression[?In, ?Yield, ?Await] AssignmentOperator : one of *= /= %= = -= <<= >>= >>>= &= ^= |= **=

13.15.1 Static Semantics: Early Errors

AssignmentExpression : LeftHandSideExpression = AssignmentExpression

If LeftHandSideExpression is either an ObjectLiteral or an ArrayLiteral, the following Early Error rules are applied:

If LeftHandSideExpression is neither an ObjectLiteral nor an ArrayLiteral, the following Early Error rule is applied:

AssignmentExpression : LeftHandSideExpression AssignmentOperator AssignmentExpression LeftHandSideExpression &&= AssignmentExpression LeftHandSideExpression ||= AssignmentExpression LeftHandSideExpression ??= AssignmentExpression

13.15.2 Runtime Semantics: Evaluation

AssignmentExpression : LeftHandSideExpression = AssignmentExpression
  1. If LeftHandSideExpression is neither an ObjectLiteral nor an ArrayLiteral, then
    1. Let lref be ? Evaluation of LeftHandSideExpression.
    2. If IsAnonymousFunctionDefinition(AssignmentExpression) and IsIdentifierRef of LeftHandSideExpression are both true, then
      1. Let rval be ? NamedEvaluation of AssignmentExpression with argument lref.[[ReferencedName]].
    3. Else,
      1. Let rref be ? Evaluation of AssignmentExpression.
      2. Let rval be ? GetValue(rref).
    4. Perform ? PutValue(lref, rval).
    5. Return rval.
  2. Let assignmentPattern be the AssignmentPattern that is covered by LeftHandSideExpression.
  3. Let rref be ? Evaluation of AssignmentExpression.
  4. Let rval be ? GetValue(rref).
  5. Perform ? DestructuringAssignmentEvaluation of assignmentPattern with argument rval.
  6. Return rval.
 AssignmentExpression : LeftHandSideExpression AssignmentOperator AssignmentExpression
  1. Let lref be ? Evaluation of LeftHandSideExpression.
  2. Let lval be ? GetValue(lref).
  3. Let rref be ? Evaluation of AssignmentExpression.
  4. Let rval be ? GetValue(rref).
  5. Let assignmentOpText be the source text matched by AssignmentOperator.
  6. Let opText be the sequence of Unicode code points associated with assignmentOpText in the following table:
    assignmentOpText opText
    **= **
    *= *
    /= /
    %= %
    =
    -= -
    <<= <<
    >>= >>
    >>>= >>>
    &= &
    ^= ^
    |= |
  7. Let r be ? ApplyStringOrNumericBinaryOperator(lval, opText, rval).
  8. Perform ? PutValue(lref, r).
  9. Return r.
 AssignmentExpression : LeftHandSideExpression &&= AssignmentExpression
  1. Let lref be ? Evaluation of LeftHandSideExpression.
  2. Let lval be ? GetValue(lref).
  3. Let lbool be ToBoolean(lval).
  4. If lbool is false, return lval.
  5. If IsAnonymousFunctionDefinition(AssignmentExpression) is true and IsIdentifierRef of LeftHandSideExpression is true, then
    1. Let rval be ? NamedEvaluation of AssignmentExpression with argument lref.[[ReferencedName]].
  6. Else,
    1. Let rref be ? Evaluation of AssignmentExpression.
    2. Let rval be ? GetValue(rref).
  7. Perform ? PutValue(lref, rval).
  8. Return rval.
 AssignmentExpression : LeftHandSideExpression ||= AssignmentExpression
  1. Let lref be ? Evaluation of LeftHandSideExpression.
  2. Let lval be ? GetValue(lref).
  3. Let lbool be ToBoolean(lval).
  4. If lbool is true, return lval.
  5. If IsAnonymousFunctionDefinition(AssignmentExpression) is true and IsIdentifierRef of LeftHandSideExpression is true, then
    1. Let rval be ? NamedEvaluation of AssignmentExpression with argument lref.[[ReferencedName]].
  6. Else,
    1. Let rref be ? Evaluation of AssignmentExpression.
    2. Let rval be ? GetValue(rref).
  7. Perform ? PutValue(lref, rval).
  8. Return rval.
 AssignmentExpression : LeftHandSideExpression ??= AssignmentExpression
  1. Let lref be ? Evaluation of LeftHandSideExpression.
  2. Let lval be ? GetValue(lref).
  3. If lval is neither undefined nor null, return lval.
  4. If IsAnonymousFunctionDefinition(AssignmentExpression) is true and IsIdentifierRef of LeftHandSideExpression is true, then
    1. Let rval be ? NamedEvaluation of AssignmentExpression with argument lref.[[ReferencedName]].
  5. Else,
    1. Let rref be ? Evaluation of AssignmentExpression.
    2. Let rval be ? GetValue(rref).
  6. Perform ? PutValue(lref, rval).
  7. Return rval.
 Note

When this expression occurs within strict mode code, it is a runtime error if lref in step 1.d, 2, 2, 2, 2 is an unresolvable reference. If it is, a ReferenceError exception is thrown. Additionally, it is a runtime error if the lref in step 8, 7, 7, 6 is a reference to a data property with the attribute value { [[Writable]]: false }, to an accessor property with the attribute value { [[Set]]: undefined }, or to a non-existent property of an object for which the IsExtensible predicate returns the value false. In these cases a TypeError exception is thrown.

13.15.3 ApplyStringOrNumericBinaryOperator ( lval, opText, rval )

The abstract operation ApplyStringOrNumericBinaryOperator takes arguments lval (an ECMAScript language value), opText (**, *, /, %, , -, <<, >>, >>>, &, ^, or |), and rval (an ECMAScript language value) and returns either a normal completion containing either a String, a BigInt, or a Number, or a throw completion. It performs the following steps when called:

  1. If opText is , then
    1. Let lprim be ? ToPrimitive(lval).
    2. Let rprim be ? ToPrimitive(rval).
    3. If lprim is a String or rprim is a String, then
      1. Let lstr be ? ToString(lprim).
      2. Let rstr be ? ToString(rprim).
      3. Return the string-concatenation of lstr and rstr.
    4. Set lval to lprim.
    5. Set rval to rprim.
  2. NOTE: At this point, it must be a numeric operation.
  3. Let lnum be ? ToNumeric(lval).
  4. Let rnum be ? ToNumeric(rval).
  5. If Type(lnum) is not Type(rnum), throw a TypeError exception.
  6. If lnum is a BigInt, then
    1. If opText is **, return ? BigInt::exponentiate(lnum, rnum).
    2. If opText is /, return ? BigInt::divide(lnum, rnum).
    3. If opText is %, return ? BigInt::remainder(lnum, rnum).
    4. If opText is >>>, return ? BigInt::unsignedRightShift(lnum, rnum).
  7. Let operation be the abstract operation associated with opText and Type(lnum) in the following table:
    opText Type(lnum) operation
    ** Number Number::exponentiate
    * Number Number::multiply
    * BigInt BigInt::multiply
    / Number Number::divide
    % Number Number::remainder
    Number Number::add
    BigInt BigInt::add
    - Number Number::subtract
    - BigInt BigInt::subtract
    << Number Number::leftShift
    << BigInt BigInt::leftShift
    >> Number Number::signedRightShift
    >> BigInt BigInt::signedRightShift
    >>> Number Number::unsignedRightShift
    & Number Number::bitwiseAND
    & BigInt BigInt::bitwiseAND
    ^ Number Number::bitwiseXOR
    ^ BigInt BigInt::bitwiseXOR
    | Number Number::bitwiseOR
    | BigInt BigInt::bitwiseOR
  8. Return operation(lnum, rnum).
 Note 1

No hint is provided in the calls to ToPrimitive in steps 1.a and 1.b. All standard objects except Dates handle the absence of a hint as if number were given; Dates handle the absence of a hint as if string were given. Exotic objects may handle the absence of a hint in some other manner.

 Note 2

Step 1.c differs from step 3 of the IsLessThan algorithm, by using the logical-or operation instead of the logical-and operation.

13.15.4 EvaluateStringOrNumericBinaryExpression ( leftOperand, opText, rightOperand )

The abstract operation EvaluateStringOrNumericBinaryExpression takes arguments leftOperand (a Parse Node), opText (a sequence of Unicode code points), and rightOperand (a Parse Node) and returns either a normal completion containing either a String, a BigInt, or a Number, or an abrupt completion. It performs the following steps when called:

  1. Let lref be ? Evaluation of leftOperand.
  2. Let lval be ? GetValue(lref).
  3. Let rref be ? Evaluation of rightOperand.
  4. Let rval be ? GetValue(rref).
  5. Return ? ApplyStringOrNumericBinaryOperator(lval, opText, rval).

13.15.5 Destructuring Assignment

Supplemental Syntax

In certain circumstances when processing an instance of the production
AssignmentExpression : LeftHandSideExpression = AssignmentExpression
the interpretation of LeftHandSideExpression is refined using the following grammar:

AssignmentPattern[Yield, Await] : ObjectAssignmentPattern[?Yield, ?Await] ArrayAssignmentPattern[?Yield, ?Await] ObjectAssignmentPattern[Yield, Await] : { } { AssignmentRestProperty[?Yield, ?Await] } { AssignmentPropertyList[?Yield, ?Await] } { AssignmentPropertyList[?Yield, ?Await] , AssignmentRestProperty[?Yield, ?Await]opt } ArrayAssignmentPattern[Yield, Await] : [ Elisionopt AssignmentRestElement[?Yield, ?Await]opt ] [ AssignmentElementList[?Yield, ?Await] ] [ AssignmentElementList[?Yield, ?Await] , Elisionopt AssignmentRestElement[?Yield, ?Await]opt ] AssignmentRestProperty[Yield, Await] : ... DestructuringAssignmentTarget[?Yield, ?Await] AssignmentPropertyList[Yield, Await] : AssignmentProperty[?Yield, ?Await] AssignmentPropertyList[?Yield, ?Await] , AssignmentProperty[?Yield, ?Await] AssignmentElementList[Yield, Await] : AssignmentElisionElement[?Yield, ?Await] AssignmentElementList[?Yield, ?Await] , AssignmentElisionElement[?Yield, ?Await] AssignmentElisionElement[Yield, Await] : Elisionopt AssignmentElement[?Yield, ?Await] AssignmentProperty[Yield, Await] : IdentifierReference[?Yield, ?Await] Initializer[ In, ?Yield, ?Await]opt PropertyName[?Yield, ?Await] : AssignmentElement[?Yield, ?Await] AssignmentElement[Yield, Await] : DestructuringAssignmentTarget[?Yield, ?Await] Initializer[ In, ?Yield, ?Await]opt AssignmentRestElement[Yield, Await] : ... DestructuringAssignmentTarget[?Yield, ?Await] DestructuringAssignmentTarget[Yield, Await] : LeftHandSideExpression[?Yield, ?Await]

13.15.5.1 Static Semantics: Early Errors

AssignmentProperty : IdentifierReference Initializeropt AssignmentRestProperty : ... DestructuringAssignmentTarget DestructuringAssignmentTarget : LeftHandSideExpression

If LeftHandSideExpression is either an ObjectLiteral or an ArrayLiteral, the following Early Error rules are applied:

If LeftHandSideExpression is neither an ObjectLiteral nor an ArrayLiteral, the following Early Error rule is applied:

13.15.5.2 Runtime Semantics: DestructuringAssignmentEvaluation

The syntax-directed operation DestructuringAssignmentEvaluation takes argument value (an ECMAScript language value) and returns either a normal completion containing unused or an abrupt completion. It is defined piecewise over the following productions:

ObjectAssignmentPattern : { }
  1. Perform ? RequireObjectCoercible(value).
  2. Return unused.
 ObjectAssignmentPattern : { AssignmentPropertyList } { AssignmentPropertyList , }
  1. Perform ? RequireObjectCoercible(value).
  2. Perform ? PropertyDestructuringAssignmentEvaluation of AssignmentPropertyList with argument value.
  3. Return unused.
 ObjectAssignmentPattern : { AssignmentRestProperty }
  1. Perform ? RequireObjectCoercible(value).
  2. Let excludedNames be a new empty List.
  3. Return ? RestDestructuringAssignmentEvaluation of AssignmentRestProperty with arguments value and excludedNames.
 ObjectAssignmentPattern : { AssignmentPropertyList , AssignmentRestProperty }
  1. Perform ? RequireObjectCoercible(value).
  2. Let excludedNames be ? PropertyDestructuringAssignmentEvaluation of AssignmentPropertyList with argument value.
  3. Return ? RestDestructuringAssignmentEvaluation of AssignmentRestProperty with arguments value and excludedNames.
 ArrayAssignmentPattern : [ ]
  1. Let iteratorRecord be ? GetIterator(value, sync).
  2. Return ? IteratorClose(iteratorRecord, NormalCompletion(unused)).
 ArrayAssignmentPattern : [ Elision ]
  1. Let iteratorRecord be ? GetIterator(value, sync).
  2. Let result be Completion(IteratorDestructuringAssignmentEvaluation of Elision with argument iteratorRecord).
  3. If iteratorRecord.[[Done]] is false, return ? IteratorClose(iteratorRecord, result).
  4. Return result.
 ArrayAssignmentPattern : [ Elisionopt AssignmentRestElement ]
  1. Let iteratorRecord be ? GetIterator(value, sync).
  2. If Elision is present, then
    1. Let status be Completion(IteratorDestructuringAssignmentEvaluation of Elision with argument iteratorRecord).
    2. If status is an abrupt completion, then
      1. Assert: iteratorRecord.[[Done]] is true.
      2. Return ? status.
  3. Let result be Completion(IteratorDestructuringAssignmentEvaluation of AssignmentRestElement with argument iteratorRecord).
  4. If iteratorRecord.[[Done]] is false, return ? IteratorClose(iteratorRecord, result).
  5. Return result.
 ArrayAssignmentPattern : [ AssignmentElementList ]
  1. Let iteratorRecord be ? GetIterator(value, sync).
  2. Let result be Completion(IteratorDestructuringAssignmentEvaluation of AssignmentElementList with argument iteratorRecord).
  3. If iteratorRecord.[[Done]] is false, return ? IteratorClose(iteratorRecord, result).
  4. Return result.
 ArrayAssignmentPattern : [ AssignmentElementList , Elisionopt AssignmentRestElementopt ]
  1. Let iteratorRecord be ? GetIterator(value, sync).
  2. Let status be Completion(IteratorDestructuringAssignmentEvaluation of AssignmentElementList with argument iteratorRecord).
  3. If status is an abrupt completion, then
    1. If iteratorRecord.[[Done]] is false, return ? IteratorClose(iteratorRecord, status).
    2. Return ? status.
  4. If Elision is present, then
    1. Set status to Completion(IteratorDestructuringAssignmentEvaluation of Elision with argument iteratorRecord).
    2. If status is an abrupt completion, then
      1. Assert: iteratorRecord.[[Done]] is true.
      2. Return ? status.
  5. If AssignmentRestElement is present, then
    1. Set status to Completion(IteratorDestructuringAssignmentEvaluation of AssignmentRestElement with argument iteratorRecord).
  6. If iteratorRecord.[[Done]] is false, return ? IteratorClose(iteratorRecord, status).
  7. Return ? status.

13.15.5.3 Runtime Semantics: PropertyDestructuringAssignmentEvaluation

The syntax-directed operation PropertyDestructuringAssignmentEvaluation takes argument value (an ECMAScript language value) and returns either a normal completion containing a List of property keys or an abrupt completion. It collects a list of all destructured property keys. It is defined piecewise over the following productions:

AssignmentPropertyList : AssignmentPropertyList , AssignmentProperty
  1. Let propertyNames be ? PropertyDestructuringAssignmentEvaluation of AssignmentPropertyList with argument value.
  2. Let nextNames be ? PropertyDestructuringAssignmentEvaluation of AssignmentProperty with argument value.
  3. Return the list-concatenation of propertyNames and nextNames.
 AssignmentProperty : IdentifierReference Initializeropt
  1. Let P be StringValue of IdentifierReference.
  2. Let lref be ? ResolveBinding(P).
  3. Let v be ? GetV(value, P).
  4. If Initializeropt is present and v is undefined, then
    1. If IsAnonymousFunctionDefinition(Initializer) is true, then
      1. Set v to ? NamedEvaluation of Initializer with argument P.
    2. Else,
      1. Let defaultValue be ? Evaluation of Initializer.
      2. Set v to ? GetValue(defaultValue).
  5. Perform ? PutValue(lref, v).
  6. Return « P ».
 AssignmentProperty : PropertyName : AssignmentElement
  1. Let name be ? Evaluation of PropertyName.
  2. Perform ? KeyedDestructuringAssignmentEvaluation of AssignmentElement with arguments value and name.
  3. Return « name ».

13.15.5.4 Runtime Semantics: RestDestructuringAssignmentEvaluation

The syntax-directed operation RestDestructuringAssignmentEvaluation takes arguments value (an ECMAScript language value) and excludedNames (a List of property keys) and returns either a normal completion containing unused or an abrupt completion. It is defined piecewise over the following productions:

AssignmentRestProperty : ... DestructuringAssignmentTarget
  1. Let lref be ? Evaluation of DestructuringAssignmentTarget.
  2. Let restObj be OrdinaryObjectCreate(%Object.prototype%).
  3. Perform ? CopyDataProperties(restObj, value, excludedNames).
  4. Return ? PutValue(lref, restObj).

13.15.5.5 Runtime Semantics: IteratorDestructuringAssignmentEvaluation

The syntax-directed operation IteratorDestructuringAssignmentEvaluation takes argument iteratorRecord (an Iterator Record) and returns either a normal completion containing unused or an abrupt completion. It is defined piecewise over the following productions:

AssignmentElementList : AssignmentElisionElement
  1. Return ? IteratorDestructuringAssignmentEvaluation of AssignmentElisionElement with argument iteratorRecord.
 AssignmentElementList : AssignmentElementList , AssignmentElisionElement
  1. Perform ? IteratorDestructuringAssignmentEvaluation of AssignmentElementList with argument iteratorRecord.
  2. Return ? IteratorDestructuringAssignmentEvaluation of AssignmentElisionElement with argument iteratorRecord.
 AssignmentElisionElement : AssignmentElement
  1. Return ? IteratorDestructuringAssignmentEvaluation of AssignmentElement with argument iteratorRecord.
 AssignmentElisionElement : Elision AssignmentElement
  1. Perform ? IteratorDestructuringAssignmentEvaluation of Elision with argument iteratorRecord.
  2. Return ? IteratorDestructuringAssignmentEvaluation of AssignmentElement with argument iteratorRecord.
 Elision : ,
  1. If iteratorRecord.[[Done]] is false, then
    1. Let next be Completion(IteratorStep(iteratorRecord)).
    2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
    3. ReturnIfAbrupt(next).
    4. If next is false, set iteratorRecord.[[Done]] to true.
  2. Return unused.
 Elision : Elision ,
  1. Perform ? IteratorDestructuringAssignmentEvaluation of Elision with argument iteratorRecord.
  2. If iteratorRecord.[[Done]] is false, then
    1. Let next be Completion(IteratorStep(iteratorRecord)).
    2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
    3. ReturnIfAbrupt(next).
    4. If next is false, set iteratorRecord.[[Done]] to true.
  3. Return unused.
 AssignmentElement : DestructuringAssignmentTarget Initializeropt
  1. If DestructuringAssignmentTarget is neither an ObjectLiteral nor an ArrayLiteral, then
    1. Let lref be ? Evaluation of DestructuringAssignmentTarget.
  2. If iteratorRecord.[[Done]] is false, then
    1. Let next be Completion(IteratorStep(iteratorRecord)).
    2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
    3. ReturnIfAbrupt(next).
    4. If next is false, set iteratorRecord.[[Done]] to true.
    5. Else,
      1. Let value be Completion(IteratorValue(next)).
      2. If value is an abrupt completion, set iteratorRecord.[[Done]] to true.
      3. ReturnIfAbrupt(value).
  3. If iteratorRecord.[[Done]] is true, let value be undefined.
  4. If Initializer is present and value is undefined, then
    1. If IsAnonymousFunctionDefinition(Initializer) is true and IsIdentifierRef of DestructuringAssignmentTarget is true, then
      1. Let v be ? NamedEvaluation of Initializer with argument lref.[[ReferencedName]].
    2. Else,
      1. Let defaultValue be ? Evaluation of Initializer.
      2. Let v be ? GetValue(defaultValue).
  5. Else, let v be value.
  6. If DestructuringAssignmentTarget is either an ObjectLiteral or an ArrayLiteral, then
    1. Let nestedAssignmentPattern be the AssignmentPattern that is covered by DestructuringAssignmentTarget.
    2. Return ? DestructuringAssignmentEvaluation of nestedAssignmentPattern with argument v.
  7. Return ? PutValue(lref, v).
 Note

Left to right evaluation order is maintained by evaluating a DestructuringAssignmentTarget that is not a destructuring pattern prior to accessing the iterator or evaluating the Initializer.

 AssignmentRestElement : ... DestructuringAssignmentTarget
  1. If DestructuringAssignmentTarget is neither an ObjectLiteral nor an ArrayLiteral, then
    1. Let lref be ? Evaluation of DestructuringAssignmentTarget.
  2. Let A be ! ArrayCreate(0).
  3. Let n be 0.
  4. Repeat, while iteratorRecord.[[Done]] is false,
    1. Let next be Completion(IteratorStep(iteratorRecord)).
    2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
    3. ReturnIfAbrupt(next).
    4. If next is false, set iteratorRecord.[[Done]] to true.
    5. Else,
      1. Let nextValue be Completion(IteratorValue(next)).
      2. If nextValue is an abrupt completion, set iteratorRecord.[[Done]] to true.
      3. ReturnIfAbrupt(nextValue).
      4. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(n)), nextValue).
      5. Set n to n 1.
  5. If DestructuringAssignmentTarget is neither an ObjectLiteral nor an ArrayLiteral, then
    1. Return ? PutValue(lref, A).
  6. Let nestedAssignmentPattern be the AssignmentPattern that is covered by DestructuringAssignmentTarget.
  7. Return ? DestructuringAssignmentEvaluation of nestedAssignmentPattern with argument A.

13.15.5.6 Runtime Semantics: KeyedDestructuringAssignmentEvaluation

The syntax-directed operation KeyedDestructuringAssignmentEvaluation takes arguments value (an ECMAScript language value) and propertyName (a property key) and returns either a normal completion containing unused or an abrupt completion. It is defined piecewise over the following productions:

AssignmentElement : DestructuringAssignmentTarget Initializeropt
  1. If DestructuringAssignmentTarget is neither an ObjectLiteral nor an ArrayLiteral, then
    1. Let lref be ? Evaluation of DestructuringAssignmentTarget.
  2. Let v be ? GetV(value, propertyName).
  3. If Initializer is present and v is undefined, then
    1. If IsAnonymousFunctionDefinition(Initializer) and IsIdentifierRef of DestructuringAssignmentTarget are both true, then
      1. Let rhsValue be ? NamedEvaluation of Initializer with argument lref.[[ReferencedName]].
    2. Else,
      1. Let defaultValue be ? Evaluation of Initializer.
      2. Let rhsValue be ? GetValue(defaultValue).
  4. Else, let rhsValue be v.
  5. If DestructuringAssignmentTarget is either an ObjectLiteral or an ArrayLiteral, then
    1. Let assignmentPattern be the AssignmentPattern that is covered by DestructuringAssignmentTarget.
    2. Return ? DestructuringAssignmentEvaluation of assignmentPattern with argument rhsValue.
  6. Return ? PutValue(lref, rhsValue).

13.16 Comma Operator ( , )

Syntax

Expression[In, Yield, Await] : AssignmentExpression[?In, ?Yield, ?Await] Expression[?In, ?Yield, ?Await] , AssignmentExpression[?In, ?Yield, ?Await]

13.16.1 Runtime Semantics: Evaluation

Expression : Expression , AssignmentExpression
  1. Let lref be ? Evaluation of Expression.
  2. Perform ? GetValue(lref).
  3. Let rref be ? Evaluation of AssignmentExpression.
  4. Return ? GetValue(rref).
 Note

GetValue must be called even though its value is not used because it may have observable side-effects.

14 ECMAScript Language: Statements and Declarations

Syntax

Statement[Yield, Await, Return] : BlockStatement[?Yield, ?Await, ?Return] VariableStatement[?Yield, ?Await] EmptyStatement ExpressionStatement[?Yield, ?Await] IfStatement[?Yield, ?Await, ?Return] BreakableStatement[?Yield, ?Await, ?Return] ContinueStatement[?Yield, ?Await] BreakStatement[?Yield, ?Await] [ Return] ReturnStatement[?Yield, ?Await] WithStatement[?Yield, ?Await, ?Return] LabelledStatement[?Yield, ?Await, ?Return] ThrowStatement[?Yield, ?Await] TryStatement[?Yield, ?Await, ?Return] DebuggerStatement Declaration[Yield, Await] : HoistableDeclaration[?Yield, ?Await, ~Default] ClassDeclaration[?Yield, ?Await, ~Default] LexicalDeclaration[ In, ?Yield, ?Await] HoistableDeclaration[Yield, Await, Default] : FunctionDeclaration[?Yield, ?Await, ?Default] GeneratorDeclaration[?Yield, ?Await, ?Default] AsyncFunctionDeclaration[?Yield, ?Await, ?Default] AsyncGeneratorDeclaration[?Yield, ?Await, ?Default] BreakableStatement[Yield, Await, Return] : IterationStatement[?Yield, ?Await, ?Return] SwitchStatement[?Yield, ?Await, ?Return]

14.1 Statement Semantics

14.1.1 Runtime Semantics: Evaluation

HoistableDeclaration : GeneratorDeclaration AsyncFunctionDeclaration AsyncGeneratorDeclaration
  1. Return empty.
 HoistableDeclaration : FunctionDeclaration
  1. Return ? Evaluation of FunctionDeclaration.
 BreakableStatement : IterationStatement SwitchStatement
  1. Let newLabelSet be a new empty List.
  2. Return ? LabelledEvaluation of this BreakableStatement with argument newLabelSet.

14.2 Block

Syntax

BlockStatement[Yield, Await, Return] : Block[?Yield, ?Await, ?Return] Block[Yield, Await, Return] : { StatementList[?Yield, ?Await, ?Return]opt } StatementList[Yield, Await, Return] : StatementListItem[?Yield, ?Await, ?Return] StatementList[?Yield, ?Await, ?Return] StatementListItem[?Yield, ?Await, ?Return] StatementListItem[Yield, Await, Return] : Statement[?Yield, ?Await, ?Return] Declaration[?Yield, ?Await]

14.2.1 Static Semantics: Early Errors

Block : { StatementList }

14.2.2 Runtime Semantics: Evaluation

Block : { }
  1. Return empty.
 Block : { StatementList }
  1. Let oldEnv be the running execution context's LexicalEnvironment.
  2. Let blockEnv be NewDeclarativeEnvironment(oldEnv).
  3. Perform BlockDeclarationInstantiation(StatementList, blockEnv).
  4. Set the running execution context's LexicalEnvironment to blockEnv.
  5. Let blockValue be Completion(Evaluation of StatementList).
  6. Set the running execution context's LexicalEnvironment to oldEnv.
  7. Return ? blockValue.
 Note 1

No matter how control leaves the Block the LexicalEnvironment is always restored to its former state.

 StatementList : StatementList StatementListItem
  1. Let sl be ? Evaluation of StatementList.
  2. Let s be Completion(Evaluation of StatementListItem).
  3. Return ? UpdateEmpty(s, sl).
 Note 2

The value of a StatementList is the value of the last value-producing item in the StatementList. For example, the following calls to the eval function all return the value 1:

eval("1;;;;;")
eval("1;{}")
eval("1;var a;")

14.2.3 BlockDeclarationInstantiation ( code, env )

The abstract operation BlockDeclarationInstantiation takes arguments code (a Parse Node) and env (a Declarative Environment Record) and returns unused. code is the Parse Node corresponding to the body of the block. env is the Environment Record in which bindings are to be created.

Note

When a Block or CaseBlock is evaluated a new Declarative Environment Record is created and bindings for each block scoped variable, constant, function, or class declared in the block are instantiated in the Environment Record.

It performs the following steps when called:

  1. Let declarations be the LexicallyScopedDeclarations of code.
  2. Let privateEnv be the running execution context's PrivateEnvironment.
  3. For each element d of declarations, do
    1. For each element dn of the BoundNames of d, do
      1. If IsConstantDeclaration of d is true, then
        1. Perform ! env.CreateImmutableBinding(dn, true).
      2. Else,
        1. Perform ! env.CreateMutableBinding(dn, false). NOTE: This step is replaced in section B.3.2.6.
    2. If d is either a FunctionDeclaration, a GeneratorDeclaration, an AsyncFunctionDeclaration, or an AsyncGeneratorDeclaration, then
      1. Let fn be the sole element of the BoundNames of d.
      2. Let fo be InstantiateFunctionObject of d with arguments env and privateEnv.
      3. Perform ! env.InitializeBinding(fn, fo). NOTE: This step is replaced in section B.3.2.6.
  4. Return unused.

14.3 Declarations and the Variable Statement

14.3.1 Let and Const Declarations

Note

let and const declarations define variables that are scoped to the running execution context's LexicalEnvironment. The variables are created when their containing Environment Record is instantiated but may not be accessed in any way until the variable's LexicalBinding is evaluated. A variable defined by a LexicalBinding with an Initializer is assigned the value of its Initializer's AssignmentExpression when the LexicalBinding is evaluated, not when the variable is created. If a LexicalBinding in a let declaration does not have an Initializer the variable is assigned the value undefined when the LexicalBinding is evaluated.

Syntax

LexicalDeclaration[In, Yield, Await] : LetOrConst BindingList[?In, ?Yield, ?Await] ; LetOrConst : let const BindingList[In, Yield, Await] : LexicalBinding[?In, ?Yield, ?Await] BindingList[?In, ?Yield, ?Await] , LexicalBinding[?In, ?Yield, ?Await] LexicalBinding[In, Yield, Await] : BindingIdentifier[?Yield, ?Await] Initializer[?In, ?Yield, ?Await]opt BindingPattern[?Yield, ?Await] Initializer[?In, ?Yield, ?Await]

14.3.1.1 Static Semantics: Early Errors

LexicalDeclaration : LetOrConst BindingList ; LexicalBinding : BindingIdentifier Initializeropt

14.3.1.2 Runtime Semantics: Evaluation

LexicalDeclaration : LetOrConst BindingList ;
  1. Perform ? Evaluation of BindingList.
  2. Return empty.
 BindingList : BindingList , LexicalBinding
  1. Perform ? Evaluation of BindingList.
  2. Return ? Evaluation of LexicalBinding.
 LexicalBinding : BindingIdentifier
  1. Let lhs be ! ResolveBinding(StringValue of BindingIdentifier).
  2. Perform ! InitializeReferencedBinding(lhs, undefined).
  3. Return empty.
 Note

A static semantics rule ensures that this form of LexicalBinding never occurs in a const declaration.

 LexicalBinding : BindingIdentifier Initializer
  1. Let bindingId be StringValue of BindingIdentifier.
  2. Let lhs be ! ResolveBinding(bindingId).
  3. If IsAnonymousFunctionDefinition(Initializer) is true, then
    1. Let value be ? NamedEvaluation of Initializer with argument bindingId.
  4. Else,
    1. Let rhs be ? Evaluation of Initializer.
    2. Let value be ? GetValue(rhs).
  5. Perform ! InitializeReferencedBinding(lhs, value).
  6. Return empty.
 LexicalBinding : BindingPattern Initializer
  1. Let rhs be ? Evaluation of Initializer.
  2. Let value be ? GetValue(rhs).
  3. Let env be the running execution context's LexicalEnvironment.
  4. Return ? BindingInitialization of BindingPattern with arguments value and env.

14.3.2 Variable Statement

Note

A var statement declares variables that are scoped to the running execution context's VariableEnvironment. Var variables are created when their containing Environment Record is instantiated and are initialized to undefined when created. Within the scope of any VariableEnvironment a common BindingIdentifier may appear in more than one VariableDeclaration but those declarations collectively define only one variable. A variable defined by a VariableDeclaration with an Initializer is assigned the value of its Initializer's AssignmentExpression when the VariableDeclaration is executed, not when the variable is created.

Syntax

VariableStatement[Yield, Await] : var VariableDeclarationList[ In, ?Yield, ?Await] ; VariableDeclarationList[In, Yield, Await] : VariableDeclaration[?In, ?Yield, ?Await] VariableDeclarationList[?In, ?Yield, ?Await] , VariableDeclaration[?In, ?Yield, ?Await] VariableDeclaration[In, Yield, Await] : BindingIdentifier[?Yield, ?Await] Initializer[?In, ?Yield, ?Await]opt BindingPattern[?Yield, ?Await] Initializer[?In, ?Yield, ?Await]

14.3.2.1 Runtime Semantics: Evaluation

VariableStatement : var VariableDeclarationList ;
  1. Perform ? Evaluation of VariableDeclarationList.
  2. Return empty.
 VariableDeclarationList : VariableDeclarationList , VariableDeclaration
  1. Perform ? Evaluation of VariableDeclarationList.
  2. Return ? Evaluation of VariableDeclaration.
 VariableDeclaration : BindingIdentifier
  1. Return empty.
 VariableDeclaration : BindingIdentifier Initializer
  1. Let bindingId be StringValue of BindingIdentifier.
  2. Let lhs be ? ResolveBinding(bindingId).
  3. If IsAnonymousFunctionDefinition(Initializer) is true, then
    1. Let value be ? NamedEvaluation of Initializer with argument bindingId.
  4. Else,
    1. Let rhs be ? Evaluation of Initializer.
    2. Let value be ? GetValue(rhs).
  5. Perform ? PutValue(lhs, value).
  6. Return empty.
 Note

If a VariableDeclaration is nested within a with statement and the BindingIdentifier in the VariableDeclaration is the same as a property name of the binding object of the with statement's Object Environment Record, then step 5 will assign value to the property instead of assigning to the VariableEnvironment binding of the Identifier.

 VariableDeclaration : BindingPattern Initializer
  1. Let rhs be ? Evaluation of Initializer.
  2. Let rval be ? GetValue(rhs).
  3. Return ? BindingInitialization of BindingPattern with arguments rval and undefined.

14.3.3 Destructuring Binding Patterns

Syntax

BindingPattern[Yield, Await] : ObjectBindingPattern[?Yield, ?Await] ArrayBindingPattern[?Yield, ?Await] ObjectBindingPattern[Yield, Await] : { } { BindingRestProperty[?Yield, ?Await] } { BindingPropertyList[?Yield, ?Await] } { BindingPropertyList[?Yield, ?Await] , BindingRestProperty[?Yield, ?Await]opt } ArrayBindingPattern[Yield, Await] : [ Elisionopt BindingRestElement[?Yield, ?Await]opt ] [ BindingElementList[?Yield, ?Await] ] [ BindingElementList[?Yield, ?Await] , Elisionopt BindingRestElement[?Yield, ?Await]opt ] BindingRestProperty[Yield, Await] : ... BindingIdentifier[?Yield, ?Await] BindingPropertyList[Yield, Await] : BindingProperty[?Yield, ?Await] BindingPropertyList[?Yield, ?Await] , BindingProperty[?Yield, ?Await] BindingElementList[Yield, Await] : BindingElisionElement[?Yield, ?Await] BindingElementList[?Yield, ?Await] , BindingElisionElement[?Yield, ?Await] BindingElisionElement[Yield, Await] : Elisionopt BindingElement[?Yield, ?Await] BindingProperty[Yield, Await] : SingleNameBinding[?Yield, ?Await] PropertyName[?Yield, ?Await] : BindingElement[?Yield, ?Await] BindingElement[Yield, Await] : SingleNameBinding[?Yield, ?Await] BindingPattern[?Yield, ?Await] Initializer[ In, ?Yield, ?Await]opt SingleNameBinding[Yield, Await] : BindingIdentifier[?Yield, ?Await] Initializer[ In, ?Yield, ?Await]opt BindingRestElement[Yield, Await] : ... BindingIdentifier[?Yield, ?Await] ... BindingPattern[?Yield, ?Await]

14.3.3.1 Runtime Semantics: PropertyBindingInitialization

The syntax-directed operation PropertyBindingInitialization takes arguments value (an ECMAScript language value) and environment (an Environment Record or undefined) and returns either a normal completion containing a List of property keys or an abrupt completion. It collects a list of all bound property names. It is defined piecewise over the following productions:

BindingPropertyList : BindingPropertyList , BindingProperty
  1. Let boundNames be ? PropertyBindingInitialization of BindingPropertyList with arguments value and environment.
  2. Let nextNames be ? PropertyBindingInitialization of BindingProperty with arguments value and environment.
  3. Return the list-concatenation of boundNames and nextNames.
 BindingProperty : SingleNameBinding
  1. Let name be the sole element of the BoundNames of SingleNameBinding.
  2. Perform ? KeyedBindingInitialization of SingleNameBinding with arguments value, environment, and name.
  3. Return « name ».
 BindingProperty : PropertyName : BindingElement
  1. Let P be ? Evaluation of PropertyName.
  2. Perform ? KeyedBindingInitialization of BindingElement with arguments value, environment, and P.
  3. Return « P ».

14.3.3.2 Runtime Semantics: RestBindingInitialization

The syntax-directed operation RestBindingInitialization takes arguments value (an ECMAScript language value), environment (an Environment Record or undefined), and excludedNames (a List of property keys) and returns either a normal completion containing unused or an abrupt completion. It is defined piecewise over the following productions:

BindingRestProperty : ... BindingIdentifier
  1. Let lhs be ? ResolveBinding(StringValue of BindingIdentifier, environment).
  2. Let restObj be OrdinaryObjectCreate(%Object.prototype%).
  3. Perform ? CopyDataProperties(restObj, value, excludedNames).
  4. If environment is undefined, return ? PutValue(lhs, restObj).
  5. Return ? InitializeReferencedBinding(lhs, restObj).

14.3.3.3 Runtime Semantics: KeyedBindingInitialization

The syntax-directed operation KeyedBindingInitialization takes arguments value (an ECMAScript language value), environment (an Environment Record or undefined), and propertyName (a property key) and returns either a normal completion containing unused or an abrupt completion.

Note

When undefined is passed for environment it indicates that a PutValue operation should be used to assign the initialization value. This is the case for formal parameter lists of non-strict functions. In that case the formal parameter bindings are preinitialized in order to deal with the possibility of multiple parameters with the same name.

It is defined piecewise over the following productions:

BindingElement : BindingPattern Initializeropt
  1. Let v be ? GetV(value, propertyName).
  2. If Initializer is present and v is undefined, then
    1. Let defaultValue be ? Evaluation of Initializer.
    2. Set v to ? GetValue(defaultValue).
  3. Return ? BindingInitialization of BindingPattern with arguments v and environment.
 SingleNameBinding : BindingIdentifier Initializeropt
  1. Let bindingId be StringValue of BindingIdentifier.
  2. Let lhs be ? ResolveBinding(bindingId, environment).
  3. Let v be ? GetV(value, propertyName).
  4. If Initializer is present and v is undefined, then
    1. If IsAnonymousFunctionDefinition(Initializer) is true, then
      1. Set v to ? NamedEvaluation of Initializer with argument bindingId.
    2. Else,
      1. Let defaultValue be ? Evaluation of Initializer.
      2. Set v to ? GetValue(defaultValue).
  5. If environment is undefined, return ? PutValue(lhs, v).
  6. Return ? InitializeReferencedBinding(lhs, v).

14.4 Empty Statement

Syntax

EmptyStatement : ;

14.4.1 Runtime Semantics: Evaluation

EmptyStatement : ;
  1. Return empty.

14.5 Expression Statement

Syntax

ExpressionStatement[Yield, Await] : [lookahead ∉ { {, function, async [no LineTerminator here] function, class, let [ }] Expression[ In, ?Yield, ?Await] ; Note

An ExpressionStatement cannot start with a U 007B (LEFT CURLY BRACKET) because that might make it ambiguous with a Block. An ExpressionStatement cannot start with the function or class keywords because that would make it ambiguous with a FunctionDeclaration, a GeneratorDeclaration, or a ClassDeclaration. An ExpressionStatement cannot start with async function because that would make it ambiguous with an AsyncFunctionDeclaration or a AsyncGeneratorDeclaration. An ExpressionStatement cannot start with the two token sequence let [ because that would make it ambiguous with a let LexicalDeclaration whose first LexicalBinding was an ArrayBindingPattern.

14.5.1 Runtime Semantics: Evaluation

ExpressionStatement : Expression ;
  1. Let exprRef be ? Evaluation of Expression.
  2. Return ? GetValue(exprRef).

14.6 The if Statement

Syntax

IfStatement[Yield, Await, Return] : if ( Expression[ In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] else Statement[?Yield, ?Await, ?Return] if ( Expression[ In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] [lookahead ≠ else] Note
The lookahead-restriction [lookahead ≠ else] resolves the classic "dangling else" problem in the usual way. That is, when the choice of associated if is otherwise ambiguous, the else is associated with the nearest (innermost) of the candidate ifs

14.6.1 Static Semantics: Early Errors

IfStatement : if ( Expression ) Statement else Statement IfStatement : if ( Expression ) Statement Note

It is only necessary to apply this rule if the extension specified in B.3.1 is implemented.

14.6.2 Runtime Semantics: Evaluation

IfStatement : if ( Expression ) Statement else Statement
  1. Let exprRef be ? Evaluation of Expression.
  2. Let exprValue be ToBoolean(? GetValue(exprRef)).
  3. If exprValue is true, then
    1. Let stmtCompletion be Completion(Evaluation of the first Statement).
  4. Else,
    1. Let stmtCompletion be Completion(Evaluation of the second Statement).
  5. Return ? UpdateEmpty(stmtCompletion, undefined).
 IfStatement : if ( Expression ) Statement
  1. Let exprRef be ? Evaluation of Expression.
  2. Let exprValue be ToBoolean(? GetValue(exprRef)).
  3. If exprValue is false, then
    1. Return undefined.
  4. Else,
    1. Let stmtCompletion be Completion(Evaluation of Statement).
    2. Return ? UpdateEmpty(stmtCompletion, undefined).

14.7 Iteration Statements

Syntax

IterationStatement[Yield, Await, Return] : DoWhileStatement[?Yield, ?Await, ?Return] WhileStatement[?Yield, ?Await, ?Return] ForStatement[?Yield, ?Await, ?Return] ForInOfStatement[?Yield, ?Await, ?Return]

14.7.1 Semantics

14.7.1.1 LoopContinues ( completion, labelSet )

The abstract operation LoopContinues takes arguments completion (a Completion Record) and labelSet (a List of Strings) and returns a Boolean. It performs the following steps when called:

  1. If completion.[[Type]] is normal, return true.
  2. If completion.[[Type]] is not continue, return false.
  3. If completion.[[Target]] is empty, return true.
  4. If labelSet contains completion.[[Target]], return true.
  5. Return false.
 Note

Within the Statement part of an IterationStatement a ContinueStatement may be used to begin a new iteration.

14.7.1.2 Runtime Semantics: LoopEvaluation

The syntax-directed operation LoopEvaluation takes argument labelSet (a List of Strings) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It is defined piecewise over the following productions:

IterationStatement : DoWhileStatement
  1. Return ? DoWhileLoopEvaluation of DoWhileStatement with argument labelSet.
 IterationStatement : WhileStatement
  1. Return ? WhileLoopEvaluation of WhileStatement with argument labelSet.
 IterationStatement : ForStatement
  1. Return ? ForLoopEvaluation of ForStatement with argument labelSet.
 IterationStatement : ForInOfStatement
  1. Return ? ForInOfLoopEvaluation of ForInOfStatement with argument labelSet.

14.7.2 The do-while Statement

Syntax

DoWhileStatement[Yield, Await, Return] : do Statement[?Yield, ?Await, ?Return] while ( Expression[ In, ?Yield, ?Await] ) ;

14.7.2.1 Static Semantics: Early Errors

DoWhileStatement : do Statement while ( Expression ) ; Note

It is only necessary to apply this rule if the extension specified in B.3.1 is implemented.

14.7.2.2 Runtime Semantics: DoWhileLoopEvaluation

The syntax-directed operation DoWhileLoopEvaluation takes argument labelSet (a List of Strings) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It is defined piecewise over the following productions:

DoWhileStatement : do Statement while ( Expression ) ;
  1. Let V be undefined.
  2. Repeat,
    1. Let stmtResult be Completion(Evaluation of Statement).
    2. If LoopContinues(stmtResult, labelSet) is false, return ? UpdateEmpty(stmtResult, V).
    3. If stmtResult.[[Value]] is not empty, set V to stmtResult.[[Value]].
    4. Let exprRef be ? Evaluation of Expression.
    5. Let exprValue be ? GetValue(exprRef).
    6. If ToBoolean(exprValue) is false, return V.

14.7.3 The while Statement

Syntax

WhileStatement[Yield, Await, Return] : while ( Expression[ In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return]

14.7.3.1 Static Semantics: Early Errors

WhileStatement : while ( Expression ) Statement Note

It is only necessary to apply this rule if the extension specified in B.3.1 is implemented.

14.7.3.2 Runtime Semantics: WhileLoopEvaluation

The syntax-directed operation WhileLoopEvaluation takes argument labelSet (a List of Strings) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It is defined piecewise over the following productions:

WhileStatement : while ( Expression ) Statement
  1. Let V be undefined.
  2. Repeat,
    1. Let exprRef be ? Evaluation of Expression.
    2. Let exprValue be ? GetValue(exprRef).
    3. If ToBoolean(exprValue) is false, return V.
    4. Let stmtResult be Completion(Evaluation of Statement).
    5. If LoopContinues(stmtResult, labelSet) is false, return ? UpdateEmpty(stmtResult, V).
    6. If stmtResult.[[Value]] is not empty, set V to stmtResult.[[Value]].

14.7.4 The for Statement

Syntax

ForStatement[Yield, Await, Return] : for ( [lookahead ≠ let [] Expression[~In, ?Yield, ?Await]opt ; Expression[ In, ?Yield, ?Await]opt ; Expression[ In, ?Yield, ?Await]opt ) Statement[?Yield, ?Await, ?Return] for ( var VariableDeclarationList[~In, ?Yield, ?Await] ; Expression[ In, ?Yield, ?Await]opt ; Expression[ In, ?Yield, ?Await]opt ) Statement[?Yield, ?Await, ?Return] for ( LexicalDeclaration[~In, ?Yield, ?Await] Expression[ In, ?Yield, ?Await]opt ; Expression[ In, ?Yield, ?Await]opt ) Statement[?Yield, ?Await, ?Return]

14.7.4.1 Static Semantics: Early Errors

ForStatement : for ( Expressionopt ; Expressionopt ; Expressionopt ) Statement for ( var VariableDeclarationList ; Expressionopt ; Expressionopt ) Statement for ( LexicalDeclaration Expressionopt ; Expressionopt ) Statement Note

It is only necessary to apply this rule if the extension specified in B.3.1 is implemented.

 ForStatement : for ( LexicalDeclaration Expressionopt ; Expressionopt ) Statement

14.7.4.2 Runtime Semantics: ForLoopEvaluation

The syntax-directed operation ForLoopEvaluation takes argument labelSet (a List of Strings) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It is defined piecewise over the following productions:

ForStatement : for ( Expressionopt ; Expressionopt ; Expressionopt ) Statement
  1. If the first Expression is present, then
    1. Let exprRef be ? Evaluation of the first Expression.
    2. Perform ? GetValue(exprRef).
  2. If the second Expression is present, let test be the second Expression; otherwise, let test be empty.
  3. If the third Expression is present, let increment be the third Expression; otherwise, let increment be empty.
  4. Return ? ForBodyEvaluation(test, increment, Statement, « », labelSet).
 ForStatement : for ( var VariableDeclarationList ; Expressionopt ; Expressionopt ) Statement
  1. Perform ? Evaluation of VariableDeclarationList.
  2. If the first Expression is present, let test be the first Expression; otherwise, let test be empty.
  3. If the second Expression is present, let increment be the second Expression; otherwise, let increment be empty.
  4. Return ? ForBodyEvaluation(test, increment, Statement, « », labelSet).
 ForStatement : for ( LexicalDeclaration Expressionopt ; Expressionopt ) Statement
  1. Let oldEnv be the running execution context's LexicalEnvironment.
  2. Let loopEnv be NewDeclarativeEnvironment(oldEnv).
  3. Let isConst be IsConstantDeclaration of LexicalDeclaration.
  4. Let boundNames be the BoundNames of LexicalDeclaration.
  5. For each element dn of boundNames, do
    1. If isConst is true, then
      1. Perform ! loopEnv.CreateImmutableBinding(dn, true).
    2. Else,
      1. Perform ! loopEnv.CreateMutableBinding(dn, false).
  6. Set the running execution context's LexicalEnvironment to loopEnv.
  7. Let forDcl be Completion(Evaluation of LexicalDeclaration).
  8. If forDcl is an abrupt completion, then
    1. Set the running execution context's LexicalEnvironment to oldEnv.
    2. Return ? forDcl.
  9. If isConst is false, let perIterationLets be boundNames; otherwise let perIterationLets be a new empty List.
  10. If the first Expression is present, let test be the first Expression; otherwise, let test be empty.
  11. If the second Expression is present, let increment be the second Expression; otherwise, let increment be empty.
  12. Let bodyResult be Completion(ForBodyEvaluation(test, increment, Statement, perIterationLets, labelSet)).
  13. Set the running execution context's LexicalEnvironment to oldEnv.
  14. Return ? bodyResult.

14.7.4.3 ForBodyEvaluation ( test, increment, stmt, perIterationBindings, labelSet )

The abstract operation ForBodyEvaluation takes arguments test (an Expression Parse Node or empty), increment (an Expression Parse Node or empty), stmt (a Statement Parse Node), perIterationBindings (a List of Strings), and labelSet (a List of Strings) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It performs the following steps when called:

  1. Let V be undefined.
  2. Perform ? CreatePerIterationEnvironment(perIterationBindings).
  3. Repeat,
    1. If test is not empty, then
      1. Let testRef be ? Evaluation of test.
      2. Let testValue be ? GetValue(testRef).
      3. If ToBoolean(testValue) is false, return V.
    2. Let result be Completion(Evaluation of stmt).
    3. If LoopContinues(result, labelSet) is false, return ? UpdateEmpty(result, V).
    4. If result.[[Value]] is not empty, set V to result.[[Value]].
    5. Perform ? CreatePerIterationEnvironment(perIterationBindings).
    6. If increment is not empty, then
      1. Let incRef be ? Evaluation of increment.
      2. Perform ? GetValue(incRef).

14.7.4.4 CreatePerIterationEnvironment ( perIterationBindings )

The abstract operation CreatePerIterationEnvironment takes argument perIterationBindings (a List of Strings) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

  1. If perIterationBindings has any elements, then
    1. Let lastIterationEnv be the running execution context's LexicalEnvironment.
    2. Let outer be lastIterationEnv.[[OuterEnv]].
    3. Assert: outer is not null.
    4. Let thisIterationEnv be NewDeclarativeEnvironment(outer).
    5. For each element bn of perIterationBindings, do
      1. Perform ! thisIterationEnv.CreateMutableBinding(bn, false).
      2. Let lastValue be ? lastIterationEnv.GetBindingValue(bn, true).
      3. Perform ! thisIterationEnv.InitializeBinding(bn, lastValue).
    6. Set the running execution context's LexicalEnvironment to thisIterationEnv.
  2. Return unused.

14.7.5 The for-in, for-of, and for-await-of Statements

Syntax

ForInOfStatement[Yield, Await, Return] : for ( [lookahead ≠ let [] LeftHandSideExpression[?Yield, ?Await] in Expression[ In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] for ( var ForBinding[?Yield, ?Await] in Expression[ In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] for ( ForDeclaration[?Yield, ?Await] in Expression[ In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] for ( [lookahead ∉ { let, async of }] LeftHandSideExpression[?Yield, ?Await] of AssignmentExpression[ In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] for ( var ForBinding[?Yield, ?Await] of AssignmentExpression[ In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] for ( ForDeclaration[?Yield, ?Await] of AssignmentExpression[ In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] [ Await] for await ( [lookahead ≠ let] LeftHandSideExpression[?Yield, ?Await] of AssignmentExpression[ In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] [ Await] for await ( var ForBinding[?Yield, ?Await] of AssignmentExpression[ In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] [ Await] for await ( ForDeclaration[?Yield, ?Await] of AssignmentExpression[ In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] ForDeclaration[Yield, Await] : LetOrConst ForBinding[?Yield, ?Await] ForBinding[Yield, Await] : BindingIdentifier[?Yield, ?Await] BindingPattern[?Yield, ?Await] Note

This section is extended by Annex B.3.5.

14.7.5.1 Static Semantics: Early Errors

ForInOfStatement : for ( LeftHandSideExpression in Expression ) Statement for ( var ForBinding in Expression ) Statement for ( ForDeclaration in Expression ) Statement for ( LeftHandSideExpression of AssignmentExpression ) Statement for ( var ForBinding of AssignmentExpression ) Statement for ( ForDeclaration of AssignmentExpression ) Statement for await ( LeftHandSideExpression of AssignmentExpression ) Statement for await ( var ForBinding of AssignmentExpression ) Statement for await ( ForDeclaration of AssignmentExpression ) Statement Note

It is only necessary to apply this rule if the extension specified in B.3.1 is implemented.

 ForInOfStatement : for ( LeftHandSideExpression in Expression ) Statement for ( LeftHandSideExpression of AssignmentExpression ) Statement for await ( LeftHandSideExpression of AssignmentExpression ) Statement

If LeftHandSideExpression is either an ObjectLiteral or an ArrayLiteral, the following Early Error rules are applied:

If LeftHandSideExpression is neither an ObjectLiteral nor an ArrayLiteral, the following Early Error rule is applied:

ForInOfStatement : for ( ForDeclaration in Expression ) Statement for ( ForDeclaration of AssignmentExpression ) Statement for await ( ForDeclaration of AssignmentExpression ) Statement

14.7.5.2 Static Semantics: IsDestructuring

The syntax-directed operation IsDestructuring takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

MemberExpression : PrimaryExpression
  1. If PrimaryExpression is either an ObjectLiteral or an ArrayLiteral, return true.
  2. Return false.
 MemberExpression : MemberExpression [ Expression ] MemberExpression . IdentifierName MemberExpression TemplateLiteral SuperProperty MetaProperty new MemberExpression Arguments MemberExpression . PrivateIdentifier NewExpression : new NewExpression LeftHandSideExpression : CallExpression OptionalExpression
  1. Return false.
 ForDeclaration : LetOrConst ForBinding
  1. Return IsDestructuring of ForBinding.
 ForBinding : BindingIdentifier
  1. Return false.
 ForBinding : BindingPattern
  1. Return true.
 Note

This section is extended by Annex B.3.5.

14.7.5.3 Runtime Semantics: ForDeclarationBindingInitialization

The syntax-directed operation ForDeclarationBindingInitialization takes arguments value (an ECMAScript language value) and environment (an Environment Record or undefined) and returns either a normal completion containing unused or an abrupt completion.

Note

undefined is passed for environment to indicate that a PutValue operation should be used to assign the initialization value. This is the case for var statements and the formal parameter lists of some non-strict functions (see 10.2.11). In those cases a lexical binding is hoisted and preinitialized prior to evaluation of its initializer.

It is defined piecewise over the following productions:

ForDeclaration : LetOrConst ForBinding
  1. Return ? BindingInitialization of ForBinding with arguments value and environment.

14.7.5.4 Runtime Semantics: ForDeclarationBindingInstantiation

The syntax-directed operation ForDeclarationBindingInstantiation takes argument environment (a Declarative Environment Record) and returns unused. It is defined piecewise over the following productions:

ForDeclaration : LetOrConst ForBinding
  1. For each element name of the BoundNames of ForBinding, do
    1. If IsConstantDeclaration of LetOrConst is true, then
      1. Perform ! environment.CreateImmutableBinding(name, true).
    2. Else,
      1. Perform ! environment.CreateMutableBinding(name, false).
  2. Return unused.

14.7.5.5 Runtime Semantics: ForInOfLoopEvaluation

The syntax-directed operation ForInOfLoopEvaluation takes argument labelSet (a List of Strings) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It is defined piecewise over the following productions:

ForInOfStatement : for ( LeftHandSideExpression in Expression ) Statement
  1. Let keyResult be ? ForIn/OfHeadEvaluation(« », Expression, enumerate).
  2. Return ? ForIn/OfBodyEvaluation(LeftHandSideExpression, Statement, keyResult, enumerate, assignment, labelSet).
 ForInOfStatement : for ( var ForBinding in Expression ) Statement
  1. Let keyResult be ? ForIn/OfHeadEvaluation(« », Expression, enumerate).
  2. Return ? ForIn/OfBodyEvaluation(ForBinding, Statement, keyResult, enumerate, varBinding, labelSet).
 ForInOfStatement : for ( ForDeclaration in Expression ) Statement
  1. Let keyResult be ? ForIn/OfHeadEvaluation(BoundNames of ForDeclaration, Expression, enumerate).
  2. Return ? ForIn/OfBodyEvaluation(ForDeclaration, Statement, keyResult, enumerate, lexicalBinding, labelSet).
 ForInOfStatement : for ( LeftHandSideExpression of AssignmentExpression ) Statement
  1. Let keyResult be ? ForIn/OfHeadEvaluation(« », AssignmentExpression, iterate).
  2. Return ? ForIn/OfBodyEvaluation(LeftHandSideExpression, Statement, keyResult, iterate, assignment, labelSet).
 ForInOfStatement : for ( var ForBinding of AssignmentExpression ) Statement
  1. Let keyResult be ? ForIn/OfHeadEvaluation(« », AssignmentExpression, iterate).
  2. Return ? ForIn/OfBodyEvaluation(ForBinding, Statement, keyResult, iterate, varBinding, labelSet).
 ForInOfStatement : for ( ForDeclaration of AssignmentExpression ) Statement
  1. Let keyResult be ? ForIn/OfHeadEvaluation(BoundNames of ForDeclaration, AssignmentExpression, iterate).
  2. Return ? ForIn/OfBodyEvaluation(ForDeclaration, Statement, keyResult, iterate, lexicalBinding, labelSet).
 ForInOfStatement : for await ( LeftHandSideExpression of AssignmentExpression ) Statement
  1. Let keyResult be ? ForIn/OfHeadEvaluation(« », AssignmentExpression, async-iterate).
  2. Return ? ForIn/OfBodyEvaluation(LeftHandSideExpression, Statement, keyResult, iterate, assignment, labelSet, async).
 ForInOfStatement : for await ( var ForBinding of AssignmentExpression ) Statement
  1. Let keyResult be ? ForIn/OfHeadEvaluation(« », AssignmentExpression, async-iterate).
  2. Return ? ForIn/OfBodyEvaluation(ForBinding, Statement, keyResult, iterate, varBinding, labelSet, async).
 ForInOfStatement : for await ( ForDeclaration of AssignmentExpression ) Statement
  1. Let keyResult be ? ForIn/OfHeadEvaluation(BoundNames of ForDeclaration, AssignmentExpression, async-iterate).
  2. Return ? ForIn/OfBodyEvaluation(ForDeclaration, Statement, keyResult, iterate, lexicalBinding, labelSet, async).
 Note

This section is extended by Annex B.3.5.

14.7.5.6 ForIn/OfHeadEvaluation ( uninitializedBoundNames, expr, iterationKind )

The abstract operation ForIn/OfHeadEvaluation takes arguments uninitializedBoundNames (a List of Strings), expr (an Expression Parse Node or an AssignmentExpression Parse Node), and iterationKind (enumerate, iterate, or async-iterate) and returns either a normal completion containing an Iterator Record or an abrupt completion. It performs the following steps when called:

  1. Let oldEnv be the running execution context's LexicalEnvironment.
  2. If uninitializedBoundNames is not empty, then
    1. Assert: uninitializedBoundNames has no duplicate entries.
    2. Let newEnv be NewDeclarativeEnvironment(oldEnv).
    3. For each String name of uninitializedBoundNames, do
      1. Perform ! newEnv.CreateMutableBinding(name, false).
    4. Set the running execution context's LexicalEnvironment to newEnv.
  3. Let exprRef be Completion(Evaluation of expr).
  4. Set the running execution context's LexicalEnvironment to oldEnv.
  5. Let exprValue be ? GetValue(? exprRef).
  6. If iterationKind is enumerate, then
    1. If exprValue is either undefined or null, then
      1. Return Completion Record { [[Type]]: break, [[Value]]: empty, [[Target]]: empty }.
    2. Let obj be ! ToObject(exprValue).
    3. Let iterator be EnumerateObjectProperties(obj).
    4. Let nextMethod be ! GetV(iterator, "next").
    5. Return the Iterator Record { [[Iterator]]: iterator, [[NextMethod]]: nextMethod, [[Done]]: false }.
  7. Else,
    1. Assert: iterationKind is either iterate or async-iterate.
    2. If iterationKind is async-iterate, let iteratorKind be async.
    3. Else, let iteratorKind be sync.
    4. Return ? GetIterator(exprValue, iteratorKind).

14.7.5.7 ForIn/OfBodyEvaluation ( lhs, stmt, iteratorRecord, iterationKind, lhsKind, labelSet [ , iteratorKind ] )

The abstract operation ForIn/OfBodyEvaluation takes arguments lhs (a Parse Node), stmt (a Statement Parse Node), iteratorRecord (an Iterator Record), iterationKind (enumerate or iterate), lhsKind (assignment, varBinding, or lexicalBinding), and labelSet (a List of Strings) and optional argument iteratorKind (sync or async) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It performs the following steps when called:

  1. If iteratorKind is not present, set iteratorKind to sync.
  2. Let oldEnv be the running execution context's LexicalEnvironment.
  3. Let V be undefined.
  4. Let destructuring be IsDestructuring of lhs.
  5. If destructuring is true and lhsKind is assignment, then
    1. Assert: lhs is a LeftHandSideExpression.
    2. Let assignmentPattern be the AssignmentPattern that is covered by lhs.
  6. Repeat,
    1. Let nextResult be ? Call(iteratorRecord.[[NextMethod]], iteratorRecord.[[Iterator]]).
    2. If iteratorKind is async, set nextResult to ? Await(nextResult).
    3. If nextResult is not an Object, throw a TypeError exception.
    4. Let done be ? IteratorComplete(nextResult).
    5. If done is true, return V.
    6. Let nextValue be ? IteratorValue(nextResult).
    7. If lhsKind is either assignment or varBinding, then
      1. If destructuring is true, then
        1. If lhsKind is assignment, then
          1. Let status be Completion(DestructuringAssignmentEvaluation of assignmentPattern with argument nextValue).
        2. Else,
          1. Assert: lhsKind is varBinding.
          2. Assert: lhs is a ForBinding.
          3. Let status be Completion(BindingInitialization of lhs with arguments nextValue and undefined).
      2. Else,
        1. Let lhsRef be Completion(Evaluation of lhs). (It may be evaluated repeatedly.)
        2. If lhsRef is an abrupt completion, then
          1. Let status be lhsRef.
        3. Else,
          1. Let status be Completion(PutValue(lhsRef.[[Value]], nextValue)).
    8. Else,
      1. Assert: lhsKind is lexicalBinding.
      2. Assert: lhs is a ForDeclaration.
      3. Let iterationEnv be NewDeclarativeEnvironment(oldEnv).
      4. Perform ForDeclarationBindingInstantiation of lhs with argument iterationEnv.
      5. Set the running execution context's LexicalEnvironment to iterationEnv.
      6. If destructuring is true, then
        1. Let status be Completion(ForDeclarationBindingInitialization of lhs with arguments nextValue and iterationEnv).
      7. Else,
        1. Assert: lhs binds a single name.
        2. Let lhsName be the sole element of BoundNames of lhs.
        3. Let lhsRef be ! ResolveBinding(lhsName).
        4. Let status be Completion(InitializeReferencedBinding(lhsRef, nextValue)).
    9. If status is an abrupt completion, then
      1. Set the running execution context's LexicalEnvironment to oldEnv.
      2. If iteratorKind is async, return ? AsyncIteratorClose(iteratorRecord, status).
      3. If iterationKind is enumerate, then
        1. Return ? status.
      4. Else,
        1. Assert: iterationKind is iterate.
        2. Return ? IteratorClose(iteratorRecord, status).
    10. Let result be Completion(Evaluation of stmt).
    11. Set the running execution context's LexicalEnvironment to oldEnv.
    12. If LoopContinues(result, labelSet) is false, then
      1. If iterationKind is enumerate, then
        1. Return ? UpdateEmpty(result, V).
      2. Else,
        1. Assert: iterationKind is iterate.
        2. Set status to Completion(UpdateEmpty(result, V)).
        3. If iteratorKind is async, return ? AsyncIteratorClose(iteratorRecord, status).
        4. Return ? IteratorClose(iteratorRecord, status).
    13. If result.[[Value]] is not empty, set V to result.[[Value]].

14.7.5.8 Runtime Semantics: Evaluation

BindingIdentifier : Identifier yield await
  1. Let bindingId be StringValue of BindingIdentifier.
  2. Return ? ResolveBinding(bindingId).

14.7.5.9 EnumerateObjectProperties ( O )

The abstract operation EnumerateObjectProperties takes argument O (an Object) and returns an Iterator. It performs the following steps when called:

  1. Return an Iterator object (27.1.1.2) whose next method iterates over all the String-valued keys of enumerable properties of O. The iterator object is never directly accessible to ECMAScript code. The mechanics and order of enumerating the properties is not specified but must conform to the rules specified below.

The iterator's throw and return methods are null and are never invoked. The iterator's next method processes object properties to determine whether the property key should be returned as an iterator value. Returned property keys do not include keys that are Symbols. Properties of the target object may be deleted during enumeration. A property that is deleted before it is processed by the iterator's next method is ignored. If new properties are added to the target object during enumeration, the newly added properties are not guaranteed to be processed in the active enumeration. A property name will be returned by the iterator's next method at most once in any enumeration.

Enumerating the properties of the target object includes enumerating properties of its prototype, and the prototype of the prototype, and so on, recursively; but a property of a prototype is not processed if it has the same name as a property that has already been processed by the iterator's next method. The values of [[Enumerable]] attributes are not considered when determining if a property of a prototype object has already been processed. The enumerable property names of prototype objects must be obtained by invoking EnumerateObjectProperties passing the prototype object as the argument. EnumerateObjectProperties must obtain the own property keys of the target object by calling its [[OwnPropertyKeys]] internal method. Property attributes of the target object must be obtained by calling its [[GetOwnProperty]] internal method.

In addition, if neither O nor any object in its prototype chain is a Proxy exotic object, Integer-Indexed exotic object, module namespace exotic object, or implementation provided exotic object, then the iterator must behave as would the iterator given by CreateForInIterator(O) until one of the following occurs:

  • the value of the [[Prototype]] internal slot of O or an object in its prototype chain changes,
  • a property is removed from O or an object in its prototype chain,
  • a property is added to an object in O's prototype chain, or
  • the value of the [[Enumerable]] attribute of a property of O or an object in its prototype chain changes.
Note 1

ECMAScript implementations are not required to implement the algorithm in 14.7.5.10.2.1 directly. They may choose any implementation whose behaviour will not deviate from that algorithm unless one of the constraints in the previous paragraph is violated.

The following is an informative definition of an ECMAScript generator function that conforms to these rules:

function* EnumerateObjectProperties(obj) {
  const visited = new Set();
  for (const key of Reflect.ownKeys(obj)) {
    if (typeof key === "symbol") continue;
    const desc = Reflect.getOwnPropertyDescriptor(obj, key);
    if (desc) {
      visited.add(key);
      if (desc.enumerable) yield key;
    }
  }
  const proto = Reflect.getPrototypeOf(obj);
  if (proto === null) return;
  for (const protoKey of EnumerateObjectProperties(proto)) {
    if (!visited.has(protoKey)) yield protoKey;
  }
}
 Note 2
The list of exotic objects for which implementations are not required to match CreateForInIterator was chosen because implementations historically differed in behaviour for those cases, and agreed in all others.

14.7.5.10 For-In Iterator Objects

A For-In Iterator is an object that represents a specific iteration over some specific object. For-In Iterator objects are never directly accessible to ECMAScript code; they exist solely to illustrate the behaviour of EnumerateObjectProperties.

14.7.5.10.1 CreateForInIterator ( object )

The abstract operation CreateForInIterator takes argument object (an Object) and returns a For-In Iterator. It is used to create a For-In Iterator object which iterates over the own and inherited enumerable string properties of object in a specific order. It performs the following steps when called:

  1. Let iterator be OrdinaryObjectCreate(%ForInIteratorPrototype%, « [[Object]], [[ObjectWasVisited]], [[VisitedKeys]], [[RemainingKeys]] »).
  2. Set iterator.[[Object]] to object.
  3. Set iterator.[[ObjectWasVisited]] to false.
  4. Set iterator.[[VisitedKeys]] to a new empty List.
  5. Set iterator.[[RemainingKeys]] to a new empty List.
  6. Return iterator.

14.7.5.10.2 The %ForInIteratorPrototype% Object

The %ForInIteratorPrototype% object:

  • has properties that are inherited by all For-In Iterator Objects.
  • is an ordinary object.
  • has a [[Prototype]] internal slot whose value is %IteratorPrototype%.
  • is never directly accessible to ECMAScript code.
  • has the following properties:

14.7.5.10.2.1 %ForInIteratorPrototype%.next ( )

  1. Let O be the this value.
  2. Assert: O is an Object.
  3. Assert: O has all of the internal slots of a For-In Iterator Instance (14.7.5.10.3).
  4. Let object be O.[[Object]].
  5. Repeat,
    1. If O.[[ObjectWasVisited]] is false, then
      1. Let keys be ? object.[[OwnPropertyKeys]]().
      2. For each element key of keys, do
        1. If key is a String, then
          1. Append key to O.[[RemainingKeys]].
      3. Set O.[[ObjectWasVisited]] to true.
    2. Repeat, while O.[[RemainingKeys]] is not empty,
      1. Let r be the first element of O.[[RemainingKeys]].
      2. Remove the first element from O.[[RemainingKeys]].
      3. If there does not exist an element v of O.[[VisitedKeys]] such that SameValue(r, v) is true, then
        1. Let desc be ? object.[[GetOwnProperty]](r).
        2. If desc is not undefined, then
          1. Append r to O.[[VisitedKeys]].
          2. If desc.[[Enumerable]] is true, return CreateIterResultObject(r, false).
    3. Set object to ? object.[[GetPrototypeOf]]().
    4. Set O.[[Object]] to object.
    5. Set O.[[ObjectWasVisited]] to false.
    6. If object is null, return CreateIterResultObject(undefined, true).

14.7.5.10.3 Properties of For-In Iterator Instances

For-In Iterator instances are ordinary objects that inherit properties from the %ForInIteratorPrototype% intrinsic object. For-In Iterator instances are initially created with the internal slots listed in Table 38.

Table 38: Internal Slots of For-In Iterator Instances
Internal Slot Type Description
[[Object]] an Object The Object value whose properties are being iterated.
[[ObjectWasVisited]] a Boolean true if the iterator has invoked [[OwnPropertyKeys]] on [[Object]], false otherwise.
[[VisitedKeys]] a List of Strings The values that have been emitted by this iterator thus far.
[[RemainingKeys]] a List of Strings The values remaining to be emitted for the current object, before iterating the properties of its prototype (if its prototype is not null).

14.8 The continue Statement

Syntax

ContinueStatement[Yield, Await] : continue ; continue [no LineTerminator here] LabelIdentifier[?Yield, ?Await] ;

14.8.1 Static Semantics: Early Errors

ContinueStatement : continue ; continue LabelIdentifier ;
  • It is a Syntax Error if this ContinueStatement is not nested, directly or indirectly (but not crossing function or static initialization block boundaries), within an IterationStatement.

14.8.2 Runtime Semantics: Evaluation

ContinueStatement : continue ;
  1. Return Completion Record { [[Type]]: continue, [[Value]]: empty, [[Target]]: empty }.
 ContinueStatement : continue LabelIdentifier ;
  1. Let label be the StringValue of LabelIdentifier.
  2. Return Completion Record { [[Type]]: continue, [[Value]]: empty, [[Target]]: label }.

14.9 The break Statement

Syntax

BreakStatement[Yield, Await] : break ; break [no LineTerminator here] LabelIdentifier[?Yield, ?Await] ;

14.9.1 Static Semantics: Early Errors

BreakStatement : break ;

14.9.2 Runtime Semantics: Evaluation

BreakStatement : break ;
  1. Return Completion Record { [[Type]]: break, [[Value]]: empty, [[Target]]: empty }.
 BreakStatement : break LabelIdentifier ;
  1. Let label be the StringValue of LabelIdentifier.
  2. Return Completion Record { [[Type]]: break, [[Value]]: empty, [[Target]]: label }.

14.10 The return Statement

Syntax

ReturnStatement[Yield, Await] : return ; return [no LineTerminator here] Expression[ In, ?Yield, ?Await] ; Note

A return statement causes a function to cease execution and, in most cases, returns a value to the caller. If Expression is omitted, the return value is undefined. Otherwise, the return value is the value of Expression. A return statement may not actually return a value to the caller depending on surrounding context. For example, in a try block, a return statement's Completion Record may be replaced with another Completion Record during evaluation of the finally block.

14.10.1 Runtime Semantics: Evaluation

ReturnStatement : return ;
  1. Return Completion Record { [[Type]]: return, [[Value]]: undefined, [[Target]]: empty }.
 ReturnStatement : return Expression ;
  1. Let exprRef be ? Evaluation of Expression.
  2. Let exprValue be ? GetValue(exprRef).
  3. If GetGeneratorKind() is async, set exprValue to ? Await(exprValue).
  4. Return Completion Record { [[Type]]: return, [[Value]]: exprValue, [[Target]]: empty }.
Legacy

14.11 The with Statement

Note 1

Use of the Legacy with statement is discouraged in new ECMAScript code. Consider alternatives that are permitted in both strict mode code and non-strict code, such as destructuring assignment.

Syntax

WithStatement[Yield, Await, Return] : with ( Expression[ In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] Note 2

The with statement adds an Object Environment Record for a computed object to the lexical environment of the running execution context. It then executes a statement using this augmented lexical environment. Finally, it restores the original lexical environment.

14.11.1 Static Semantics: Early Errors

WithStatement : with ( Expression ) Statement Note

It is only necessary to apply the second rule if the extension specified in B.3.1 is implemented.

14.11.2 Runtime Semantics: Evaluation

WithStatement : with ( Expression ) Statement
  1. Let val be ? Evaluation of Expression.
  2. Let obj be ? ToObject(? GetValue(val)).
  3. Let oldEnv be the running execution context's LexicalEnvironment.
  4. Let newEnv be NewObjectEnvironment(obj, true, oldEnv).
  5. Set the running execution context's LexicalEnvironment to newEnv.
  6. Let C be Completion(Evaluation of Statement).
  7. Set the running execution context's LexicalEnvironment to oldEnv.
  8. Return ? UpdateEmpty(C, undefined).
 Note

No matter how control leaves the embedded Statement, whether normally or by some form of abrupt completion or exception, the LexicalEnvironment is always restored to its former state.

14.12 The switch Statement

Syntax

SwitchStatement[Yield, Await, Return] : switch ( Expression[ In, ?Yield, ?Await] ) CaseBlock[?Yield, ?Await, ?Return] CaseBlock[Yield, Await, Return] : { CaseClauses[?Yield, ?Await, ?Return]opt } { CaseClauses[?Yield, ?Await, ?Return]opt DefaultClause[?Yield, ?Await, ?Return] CaseClauses[?Yield, ?Await, ?Return]opt } CaseClauses[Yield, Await, Return] : CaseClause[?Yield, ?Await, ?Return] CaseClauses[?Yield, ?Await, ?Return] CaseClause[?Yield, ?Await, ?Return] CaseClause[Yield, Await, Return] : case Expression[ In, ?Yield, ?Await] : StatementList[?Yield, ?Await, ?Return]opt DefaultClause[Yield, Await, Return] : default : StatementList[?Yield, ?Await, ?Return]opt

14.12.1 Static Semantics: Early Errors

SwitchStatement : switch ( Expression ) CaseBlock

14.12.2 Runtime Semantics: CaseBlockEvaluation

The syntax-directed operation CaseBlockEvaluation takes argument input (an ECMAScript language value) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It is defined piecewise over the following productions:

CaseBlock : { }
  1. Return undefined.
 CaseBlock : { CaseClauses }
  1. Let V be undefined.
  2. Let A be the List of CaseClause items in CaseClauses, in source text order.
  3. Let found be false.
  4. For each CaseClause C of A, do
    1. If found is false, then
      1. Set found to ? CaseClauseIsSelected(C, input).
    2. If found is true, then
      1. Let R be Completion(Evaluation of C).
      2. If R.[[Value]] is not empty, set V to R.[[Value]].
      3. If R is an abrupt completion, return ? UpdateEmpty(R, V).
  5. Return V.
 CaseBlock : { CaseClausesopt DefaultClause CaseClausesopt }
  1. Let V be undefined.
  2. If the first CaseClauses is present, then
    1. Let A be the List of CaseClause items in the first CaseClauses, in source text order.
  3. Else,
    1. Let A be a new empty List.
  4. Let found be false.
  5. For each CaseClause C of A, do
    1. If found is false, then
      1. Set found to ? CaseClauseIsSelected(C, input).
    2. If found is true, then
      1. Let R be Completion(Evaluation of C).
      2. If R.[[Value]] is not empty, set V to R.[[Value]].
      3. If R is an abrupt completion, return ? UpdateEmpty(R, V).
  6. Let foundInB be false.
  7. If the second CaseClauses is present, then
    1. Let B be the List of CaseClause items in the second CaseClauses, in source text order.
  8. Else,
    1. Let B be a new empty List.
  9. If found is false, then
    1. For each CaseClause C of B, do
      1. If foundInB is false, then
        1. Set foundInB to ? CaseClauseIsSelected(C, input).
      2. If foundInB is true, then
        1. Let R be Completion(Evaluation of CaseClause C).
        2. If R.[[Value]] is not empty, set V to R.[[Value]].
        3. If R is an abrupt completion, return ? UpdateEmpty(R, V).
  10. If foundInB is true, return V.
  11. Let R be Completion(Evaluation of DefaultClause).
  12. If R.[[Value]] is not empty, set V to R.[[Value]].
  13. If R is an abrupt completion, return ? UpdateEmpty(R, V).
  14. NOTE: The following is another complete iteration of the second CaseClauses.
  15. For each CaseClause C of B, do
    1. Let R be Completion(Evaluation of CaseClause C).
    2. If R.[[Value]] is not empty, set V to R.[[Value]].
    3. If R is an abrupt completion, return ? UpdateEmpty(R, V).
  16. Return V.

14.12.3 CaseClauseIsSelected ( C, input )

The abstract operation CaseClauseIsSelected takes arguments C (a CaseClause Parse Node) and input (an ECMAScript language value) and returns either a normal completion containing a Boolean or an abrupt completion. It determines whether C matches input. It performs the following steps when called:

  1. Assert: C is an instance of the production CaseClause : case Expression : StatementListopt .
  2. Let exprRef be ? Evaluation of the Expression of C.
  3. Let clauseSelector be ? GetValue(exprRef).
  4. Return IsStrictlyEqual(input, clauseSelector).
 Note

This operation does not execute C's StatementList (if any). The CaseBlock algorithm uses its return value to determine which StatementList to start executing.

14.12.4 Runtime Semantics: Evaluation

SwitchStatement : switch ( Expression ) CaseBlock
  1. Let exprRef be ? Evaluation of Expression.
  2. Let switchValue be ? GetValue(exprRef).
  3. Let oldEnv be the running execution context's LexicalEnvironment.
  4. Let blockEnv be NewDeclarativeEnvironment(oldEnv).
  5. Perform BlockDeclarationInstantiation(CaseBlock, blockEnv).
  6. Set the running execution context's LexicalEnvironment to blockEnv.
  7. Let R be Completion(CaseBlockEvaluation of CaseBlock with argument switchValue).
  8. Set the running execution context's LexicalEnvironment to oldEnv.
  9. Return R.
 Note

No matter how control leaves the SwitchStatement the LexicalEnvironment is always restored to its former state.

 CaseClause : case Expression :
  1. Return empty.
 CaseClause : case Expression : StatementList
  1. Return ? Evaluation of StatementList.
 DefaultClause : default :
  1. Return empty.
 DefaultClause : default : StatementList
  1. Return ? Evaluation of StatementList.

14.13 Labelled Statements

Syntax

LabelledStatement[Yield, Await, Return] : LabelIdentifier[?Yield, ?Await] : LabelledItem[?Yield, ?Await, ?Return] LabelledItem[Yield, Await, Return] : Statement[?Yield, ?Await, ?Return] FunctionDeclaration[?Yield, ?Await, ~Default] Note

A Statement may be prefixed by a label. Labelled statements are only used in conjunction with labelled break and continue statements. ECMAScript has no goto statement. A Statement can be part of a LabelledStatement, which itself can be part of a LabelledStatement, and so on. The labels introduced this way are collectively referred to as the “current label set” when describing the semantics of individual statements.

14.13.1 Static Semantics: Early Errors

LabelledItem : FunctionDeclaration
  • It is a Syntax Error if any source text is matched by this production.
Note

An alternative definition for this rule is provided in B.3.1.

14.13.2 Static Semantics: IsLabelledFunction ( stmt )

The abstract operation IsLabelledFunction takes argument stmt (a Statement Parse Node) and returns a Boolean. It performs the following steps when called:

  1. If stmt is not a LabelledStatement, return false.
  2. Let item be the LabelledItem of stmt.
  3. If item is LabelledItem : FunctionDeclaration , return true.
  4. Let subStmt be the Statement of item.
  5. Return IsLabelledFunction(subStmt).

14.13.3 Runtime Semantics: Evaluation

LabelledStatement : LabelIdentifier : LabelledItem
  1. Return ? LabelledEvaluation of this LabelledStatement with argument « ».

14.13.4 Runtime Semantics: LabelledEvaluation

The syntax-directed operation LabelledEvaluation takes argument labelSet (a List of Strings) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It is defined piecewise over the following productions:

BreakableStatement : IterationStatement
  1. Let stmtResult be Completion(LoopEvaluation of IterationStatement with argument labelSet).
  2. If stmtResult.[[Type]] is break, then
    1. If stmtResult.[[Target]] is empty, then
      1. If stmtResult.[[Value]] is empty, set stmtResult to NormalCompletion(undefined).
      2. Else, set stmtResult to NormalCompletion(stmtResult.[[Value]]).
  3. Return ? stmtResult.
 BreakableStatement : SwitchStatement
  1. Let stmtResult be Completion(Evaluation of SwitchStatement).
  2. If stmtResult.[[Type]] is break, then
    1. If stmtResult.[[Target]] is empty, then
      1. If stmtResult.[[Value]] is empty, set stmtResult to NormalCompletion(undefined).
      2. Else, set stmtResult to NormalCompletion(stmtResult.[[Value]]).
  3. Return ? stmtResult.
 Note 1

A BreakableStatement is one that can be exited via an unlabelled BreakStatement.

 LabelledStatement : LabelIdentifier : LabelledItem
  1. Let label be the StringValue of LabelIdentifier.
  2. Let newLabelSet be the list-concatenation of labelSet and « label ».
  3. Let stmtResult be Completion(LabelledEvaluation of LabelledItem with argument newLabelSet).
  4. If stmtResult.[[Type]] is break and SameValue(stmtResult.[[Target]], label) is true, then
    1. Set stmtResult to NormalCompletion(stmtResult.[[Value]]).
  5. Return ? stmtResult.
 LabelledItem : FunctionDeclaration
  1. Return ? Evaluation of FunctionDeclaration.
 Statement : BlockStatement VariableStatement EmptyStatement ExpressionStatement IfStatement ContinueStatement BreakStatement ReturnStatement WithStatement ThrowStatement TryStatement DebuggerStatement
  1. Return ? Evaluation of Statement.
 Note 2

The only two productions of Statement which have special semantics for LabelledEvaluation are BreakableStatement and LabelledStatement.

14.14 The throw Statement

Syntax

ThrowStatement[Yield, Await] : throw [no LineTerminator here] Expression[ In, ?Yield, ?Await] ;

14.14.1 Runtime Semantics: Evaluation

ThrowStatement : throw Expression ;
  1. Let exprRef be ? Evaluation of Expression.
  2. Let exprValue be ? GetValue(exprRef).
  3. Return ThrowCompletion(exprValue).

14.15 The try Statement

Syntax

TryStatement[Yield, Await, Return] : try Block[?Yield, ?Await, ?Return] Catch[?Yield, ?Await, ?Return] try Block[?Yield, ?Await, ?Return] Finally[?Yield, ?Await, ?Return] try Block[?Yield, ?Await, ?Return] Catch[?Yield, ?Await, ?Return] Finally[?Yield, ?Await, ?Return] Catch[Yield, Await, Return] : catch ( CatchParameter[?Yield, ?Await] ) Block[?Yield, ?Await, ?Return] catch Block[?Yield, ?Await, ?Return] Finally[Yield, Await, Return] : finally Block[?Yield, ?Await, ?Return] CatchParameter[Yield, Await] : BindingIdentifier[?Yield, ?Await] BindingPattern[?Yield, ?Await] Note

The try statement encloses a block of code in which an exceptional condition can occur, such as a runtime error or a throw statement. The catch clause provides the exception-handling code. When a catch clause catches an exception, its CatchParameter is bound to that exception.

14.15.1 Static Semantics: Early Errors

Catch : catch ( CatchParameter ) Block Note

An alternative static semantics for this production is given in B.3.4.

14.15.2 Runtime Semantics: CatchClauseEvaluation

The syntax-directed operation CatchClauseEvaluation takes argument thrownValue (an ECMAScript language value) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It is defined piecewise over the following productions:

Catch : catch ( CatchParameter ) Block
  1. Let oldEnv be the running execution context's LexicalEnvironment.
  2. Let catchEnv be NewDeclarativeEnvironment(oldEnv).
  3. For each element argName of the BoundNames of CatchParameter, do
    1. Perform ! catchEnv.CreateMutableBinding(argName, false).
  4. Set the running execution context's LexicalEnvironment to catchEnv.
  5. Let status be Completion(BindingInitialization of CatchParameter with arguments thrownValue and catchEnv).
  6. If status is an abrupt completion, then
    1. Set the running execution context's LexicalEnvironment to oldEnv.
    2. Return ? status.
  7. Let B be Completion(Evaluation of Block).
  8. Set the running execution context's LexicalEnvironment to oldEnv.
  9. Return ? B.
 Catch : catch Block
  1. Return ? Evaluation of Block.
 Note

No matter how control leaves the Block the LexicalEnvironment is always restored to its former state.

14.15.3 Runtime Semantics: Evaluation

TryStatement : try Block Catch
  1. Let B be Completion(Evaluation of Block).
  2. If B.[[Type]] is throw, let C be Completion(CatchClauseEvaluation of Catch with argument B.[[Value]]).
  3. Else, let C be B.
  4. Return ? UpdateEmpty(C, undefined).
 TryStatement : try Block Finally
  1. Let B be Completion(Evaluation of Block).
  2. Let F be Completion(Evaluation of Finally).
  3. If F.[[Type]] is normal, set F to B.
  4. Return ? UpdateEmpty(F, undefined).
 TryStatement : try Block Catch Finally
  1. Let B be Completion(Evaluation of Block).
  2. If B.[[Type]] is throw, let C be Completion(CatchClauseEvaluation of Catch with argument B.[[Value]]).
  3. Else, let C be B.
  4. Let F be Completion(Evaluation of Finally).
  5. If F.[[Type]] is normal, set F to C.
  6. Return ? UpdateEmpty(F, undefined).

14.16 The debugger Statement

Syntax

DebuggerStatement : debugger ;

14.16.1 Runtime Semantics: Evaluation

Note

Evaluating a DebuggerStatement may allow an implementation to cause a breakpoint when run under a debugger. If a debugger is not present or active this statement has no observable effect.

 DebuggerStatement : debugger ;
  1. If an implementation-defined debugging facility is available and enabled, then
    1. Perform an implementation-defined debugging action.
    2. Return a new implementation-defined Completion Record.
  2. Else,
    1. Return empty.

15 ECMAScript Language: Functions and Classes

Note

Various ECMAScript language elements cause the creation of ECMAScript function objects (10.2). Evaluation of such functions starts with the execution of their [[Call]] internal method (10.2.1).

15.1 Parameter Lists

Syntax

UniqueFormalParameters[Yield, Await] : FormalParameters[?Yield, ?Await] FormalParameters[Yield, Await] : [empty] FunctionRestParameter[?Yield, ?Await] FormalParameterList[?Yield, ?Await] FormalParameterList[?Yield, ?Await] , FormalParameterList[?Yield, ?Await] , FunctionRestParameter[?Yield, ?Await] FormalParameterList[Yield, Await] : FormalParameter[?Yield, ?Await] FormalParameterList[?Yield, ?Await] , FormalParameter[?Yield, ?Await] FunctionRestParameter[Yield, Await] : BindingRestElement[?Yield, ?Await] FormalParameter[Yield, Await] : BindingElement[?Yield, ?Await]

15.1.1 Static Semantics: Early Errors

UniqueFormalParameters : FormalParameters FormalParameters : FormalParameterList Note

Multiple occurrences of the same BindingIdentifier in a FormalParameterList is only allowed for functions which have simple parameter lists and which are not defined in strict mode code.

15.1.2 Static Semantics: ContainsExpression

The syntax-directed operation ContainsExpression takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

ObjectBindingPattern : { } { BindingRestProperty }
  1. Return false.
 ObjectBindingPattern : { BindingPropertyList , BindingRestProperty }
  1. Return ContainsExpression of BindingPropertyList.
 ArrayBindingPattern : [ Elisionopt ]
  1. Return false.
 ArrayBindingPattern : [ Elisionopt BindingRestElement ]
  1. Return ContainsExpression of BindingRestElement.
 ArrayBindingPattern : [ BindingElementList , Elisionopt ]
  1. Return ContainsExpression of BindingElementList.
 ArrayBindingPattern : [ BindingElementList , Elisionopt BindingRestElement ]
  1. Let has be ContainsExpression of BindingElementList.
  2. If has is true, return true.
  3. Return ContainsExpression of BindingRestElement.
 BindingPropertyList : BindingPropertyList , BindingProperty
  1. Let has be ContainsExpression of BindingPropertyList.
  2. If has is true, return true.
  3. Return ContainsExpression of BindingProperty.
 BindingElementList : BindingElementList , BindingElisionElement
  1. Let has be ContainsExpression of BindingElementList.
  2. If has is true, return true.
  3. Return ContainsExpression of BindingElisionElement.
 BindingElisionElement : Elisionopt BindingElement
  1. Return ContainsExpression of BindingElement.
 BindingProperty : PropertyName : BindingElement
  1. Let has be IsComputedPropertyKey of PropertyName.
  2. If has is true, return true.
  3. Return ContainsExpression of BindingElement.
 BindingElement : BindingPattern Initializer
  1. Return true.
 SingleNameBinding : BindingIdentifier
  1. Return false.
 SingleNameBinding : BindingIdentifier Initializer
  1. Return true.
 BindingRestElement : ... BindingIdentifier
  1. Return false.
 BindingRestElement : ... BindingPattern
  1. Return ContainsExpression of BindingPattern.
 FormalParameters : [empty]
  1. Return false.
 FormalParameters : FormalParameterList , FunctionRestParameter
  1. If ContainsExpression of FormalParameterList is true, return true.
  2. Return ContainsExpression of FunctionRestParameter.
 FormalParameterList : FormalParameterList , FormalParameter
  1. If ContainsExpression of FormalParameterList is true, return true.
  2. Return ContainsExpression of FormalParameter.
 ArrowParameters : BindingIdentifier
  1. Return false.
 ArrowParameters : CoverParenthesizedExpressionAndArrowParameterList
  1. Let formals be the ArrowFormalParameters that is covered by CoverParenthesizedExpressionAndArrowParameterList.
  2. Return ContainsExpression of formals.
 AsyncArrowBindingIdentifier : BindingIdentifier
  1. Return false.

15.1.3 Static Semantics: IsSimpleParameterList

The syntax-directed operation IsSimpleParameterList takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

BindingElement : BindingPattern
  1. Return false.
 BindingElement : BindingPattern Initializer
  1. Return false.
 SingleNameBinding : BindingIdentifier
  1. Return true.
 SingleNameBinding : BindingIdentifier Initializer
  1. Return false.
 FormalParameters : [empty]
  1. Return true.
 FormalParameters : FunctionRestParameter
  1. Return false.
 FormalParameters : FormalParameterList , FunctionRestParameter
  1. Return false.
 FormalParameterList : FormalParameterList , FormalParameter
  1. If IsSimpleParameterList of FormalParameterList is false, return false.
  2. Return IsSimpleParameterList of FormalParameter.
 FormalParameter : BindingElement
  1. Return IsSimpleParameterList of BindingElement.
 ArrowParameters : BindingIdentifier
  1. Return true.
 ArrowParameters : CoverParenthesizedExpressionAndArrowParameterList
  1. Let formals be the ArrowFormalParameters that is covered by CoverParenthesizedExpressionAndArrowParameterList.
  2. Return IsSimpleParameterList of formals.
 AsyncArrowBindingIdentifier : BindingIdentifier
  1. Return true.
 CoverCallExpressionAndAsyncArrowHead : MemberExpression Arguments
  1. Let head be the AsyncArrowHead that is covered by CoverCallExpressionAndAsyncArrowHead.
  2. Return IsSimpleParameterList of head.

15.1.4 Static Semantics: HasInitializer

The syntax-directed operation HasInitializer takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

BindingElement : BindingPattern
  1. Return false.
 BindingElement : BindingPattern Initializer
  1. Return true.
 SingleNameBinding : BindingIdentifier
  1. Return false.
 SingleNameBinding : BindingIdentifier Initializer
  1. Return true.
 FormalParameterList : FormalParameterList , FormalParameter
  1. If HasInitializer of FormalParameterList is true, return true.
  2. Return HasInitializer of FormalParameter.

15.1.5 Static Semantics: ExpectedArgumentCount

The syntax-directed operation ExpectedArgumentCount takes no arguments and returns an integer. It is defined piecewise over the following productions:

FormalParameters : [empty] FunctionRestParameter
  1. Return 0.
 FormalParameters : FormalParameterList , FunctionRestParameter
  1. Return ExpectedArgumentCount of FormalParameterList.
 Note

The ExpectedArgumentCount of a FormalParameterList is the number of FormalParameters to the left of either the rest parameter or the first FormalParameter with an Initializer. A FormalParameter without an initializer is allowed after the first parameter with an initializer but such parameters are considered to be optional with undefined as their default value.

 FormalParameterList : FormalParameter
  1. If HasInitializer of FormalParameter is true, return 0.
  2. Return 1.
 FormalParameterList : FormalParameterList , FormalParameter
  1. Let count be ExpectedArgumentCount of FormalParameterList.
  2. If HasInitializer of FormalParameterList is true or HasInitializer of FormalParameter is true, return count.
  3. Return count 1.
 ArrowParameters : BindingIdentifier
  1. Return 1.
 ArrowParameters : CoverParenthesizedExpressionAndArrowParameterList
  1. Let formals be the ArrowFormalParameters that is covered by CoverParenthesizedExpressionAndArrowParameterList.
  2. Return ExpectedArgumentCount of formals.
 PropertySetParameterList : FormalParameter
  1. If HasInitializer of FormalParameter is true, return 0.
  2. Return 1.
 AsyncArrowBindingIdentifier : BindingIdentifier
  1. Return 1.

15.2 Function Definitions

Syntax

FunctionDeclaration[Yield, Await, Default] : function BindingIdentifier[?Yield, ?Await] ( FormalParameters[~Yield, ~Await] ) { FunctionBody[~Yield, ~Await] } [ Default] function ( FormalParameters[~Yield, ~Await] ) { FunctionBody[~Yield, ~Await] } FunctionExpression : function BindingIdentifier[~Yield, ~Await]opt ( FormalParameters[~Yield, ~Await] ) { FunctionBody[~Yield, ~Await] } FunctionBody[Yield, Await] : FunctionStatementList[?Yield, ?Await] FunctionStatementList[Yield, Await] : StatementList[?Yield, ?Await, Return]opt

15.2.1 Static Semantics: Early Errors

FunctionDeclaration : function BindingIdentifier ( FormalParameters ) { FunctionBody } function ( FormalParameters ) { FunctionBody } FunctionExpression : function BindingIdentifieropt ( FormalParameters ) { FunctionBody } Note

The LexicallyDeclaredNames of a FunctionBody does not include identifiers bound using var or function declarations.

 FunctionBody : FunctionStatementList

15.2.2 Static Semantics: FunctionBodyContainsUseStrict

The syntax-directed operation FunctionBodyContainsUseStrict takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

FunctionBody : FunctionStatementList
  1. If the Directive Prologue of FunctionBody contains a Use Strict Directive, return true; otherwise, return false.

15.2.3 Runtime Semantics: EvaluateFunctionBody

The syntax-directed operation EvaluateFunctionBody takes arguments functionObject (a function object) and argumentsList (a List of ECMAScript language values) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It is defined piecewise over the following productions:

FunctionBody : FunctionStatementList
  1. Perform ? FunctionDeclarationInstantiation(functionObject, argumentsList).
  2. Return ? Evaluation of FunctionStatementList.

15.2.4 Runtime Semantics: InstantiateOrdinaryFunctionObject

The syntax-directed operation InstantiateOrdinaryFunctionObject takes arguments env (an Environment Record) and privateEnv (a PrivateEnvironment Record or null) and returns a function object. It is defined piecewise over the following productions:

FunctionDeclaration : function BindingIdentifier ( FormalParameters ) { FunctionBody }
  1. Let name be StringValue of BindingIdentifier.
  2. Let sourceText be the source text matched by FunctionDeclaration.
  3. Let F be OrdinaryFunctionCreate(%Function.prototype%, sourceText, FormalParameters, FunctionBody, non-lexical-this, env, privateEnv).
  4. Perform SetFunctionName(F, name).
  5. Perform MakeConstructor(F).
  6. Return F.
 FunctionDeclaration : function ( FormalParameters ) { FunctionBody }
  1. Let sourceText be the source text matched by FunctionDeclaration.
  2. Let F be OrdinaryFunctionCreate(%Function.prototype%, sourceText, FormalParameters, FunctionBody, non-lexical-this, env, privateEnv).
  3. Perform SetFunctionName(F, "default").
  4. Perform MakeConstructor(F).
  5. Return F.
 Note

An anonymous FunctionDeclaration can only occur as part of an export default declaration, and its function code is therefore always strict mode code.

15.2.5 Runtime Semantics: InstantiateOrdinaryFunctionExpression

The syntax-directed operation InstantiateOrdinaryFunctionExpression takes optional argument name (a property key or a Private Name) and returns a function object. It is defined piecewise over the following productions:

FunctionExpression : function ( FormalParameters ) { FunctionBody }
  1. If name is not present, set name to "".
  2. Let env be the LexicalEnvironment of the running execution context.
  3. Let privateEnv be the running execution context's PrivateEnvironment.
  4. Let sourceText be the source text matched by FunctionExpression.
  5. Let closure be OrdinaryFunctionCreate(%Function.prototype%, sourceText, FormalParameters, FunctionBody, non-lexical-this, env, privateEnv).
  6. Perform SetFunctionName(closure, name).
  7. Perform MakeConstructor(closure).
  8. Return closure.
 FunctionExpression : function BindingIdentifier ( FormalParameters ) { FunctionBody }
  1. Assert: name is not present.
  2. Set name to StringValue of BindingIdentifier.
  3. Let outerEnv be the running execution context's LexicalEnvironment.
  4. Let funcEnv be NewDeclarativeEnvironment(outerEnv).
  5. Perform ! funcEnv.CreateImmutableBinding(name, false).
  6. Let privateEnv be the running execution context's PrivateEnvironment.
  7. Let sourceText be the source text matched by FunctionExpression.
  8. Let closure be OrdinaryFunctionCreate(%Function.prototype%, sourceText, FormalParameters, FunctionBody, non-lexical-this, funcEnv, privateEnv).
  9. Perform SetFunctionName(closure, name).
  10. Perform MakeConstructor(closure).
  11. Perform ! funcEnv.InitializeBinding(name, closure).
  12. Return closure.
 Note

The BindingIdentifier in a FunctionExpression can be referenced from inside the FunctionExpression's FunctionBody to allow the function to call itself recursively. However, unlike in a FunctionDeclaration, the BindingIdentifier in a FunctionExpression cannot be referenced from and does not affect the scope enclosing the FunctionExpression.

15.2.6 Runtime Semantics: Evaluation

FunctionDeclaration : function BindingIdentifier ( FormalParameters ) { FunctionBody }
  1. Return empty.
 Note 1

An alternative semantics is provided in B.3.2.

 FunctionDeclaration : function ( FormalParameters ) { FunctionBody }
  1. Return empty.
 FunctionExpression : function BindingIdentifieropt ( FormalParameters ) { FunctionBody }
  1. Return InstantiateOrdinaryFunctionExpression of FunctionExpression.
 Note 2

A "prototype" property is automatically created for every function defined using a FunctionDeclaration or FunctionExpression, to allow for the possibility that the function will be used as a constructor.

 FunctionStatementList : [empty]
  1. Return undefined.

15.3 Arrow Function Definitions

Syntax

ArrowFunction[In, Yield, Await] : ArrowParameters[?Yield, ?Await] [no LineTerminator here] => ConciseBody[?In] ArrowParameters[Yield, Await] : BindingIdentifier[?Yield, ?Await] CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await] ConciseBody[In] : [lookahead ≠ {] ExpressionBody[?In, ~Await] { FunctionBody[~Yield, ~Await] } ExpressionBody[In, Await] : AssignmentExpression[?In, ~Yield, ?Await]

Supplemental Syntax

When processing an instance of the production
ArrowParameters[Yield, Await] : CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await]
the interpretation of CoverParenthesizedExpressionAndArrowParameterList is refined using the following grammar:

ArrowFormalParameters[Yield, Await] : ( UniqueFormalParameters[?Yield, ?Await] )

15.3.1 Static Semantics: Early Errors

ArrowFunction : ArrowParameters => ConciseBody ArrowParameters : CoverParenthesizedExpressionAndArrowParameterList

15.3.2 Static Semantics: ConciseBodyContainsUseStrict

The syntax-directed operation ConciseBodyContainsUseStrict takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

ConciseBody : ExpressionBody
  1. Return false.
 ConciseBody : { FunctionBody }
  1. Return FunctionBodyContainsUseStrict of FunctionBody.

15.3.3 Runtime Semantics: EvaluateConciseBody

The syntax-directed operation EvaluateConciseBody takes arguments functionObject (a function object) and argumentsList (a List of ECMAScript language values) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It is defined piecewise over the following productions:

ConciseBody : ExpressionBody
  1. Perform ? FunctionDeclarationInstantiation(functionObject, argumentsList).
  2. Return ? Evaluation of ExpressionBody.

15.3.4 Runtime Semantics: InstantiateArrowFunctionExpression

The syntax-directed operation InstantiateArrowFunctionExpression takes optional argument name (a property key or a Private Name) and returns a function object. It is defined piecewise over the following productions:

ArrowFunction : ArrowParameters => ConciseBody
  1. If name is not present, set name to "".
  2. Let env be the LexicalEnvironment of the running execution context.
  3. Let privateEnv be the running execution context's PrivateEnvironment.
  4. Let sourceText be the source text matched by ArrowFunction.
  5. Let closure be OrdinaryFunctionCreate(%Function.prototype%, sourceText, ArrowParameters, ConciseBody, lexical-this, env, privateEnv).
  6. Perform SetFunctionName(closure, name).
  7. Return closure.
 Note

An ArrowFunction does not define local bindings for arguments, super, this, or new.target. Any reference to arguments, super, this, or new.target within an ArrowFunction must resolve to a binding in a lexically enclosing environment. Typically this will be the Function Environment of an immediately enclosing function. Even though an ArrowFunction may contain references to super, the function object created in step 5 is not made into a method by performing MakeMethod. An ArrowFunction that references super is always contained within a non-ArrowFunction and the necessary state to implement super is accessible via the env that is captured by the function object of the ArrowFunction.

15.3.5 Runtime Semantics: Evaluation

ArrowFunction : ArrowParameters => ConciseBody
  1. Return InstantiateArrowFunctionExpression of ArrowFunction.
 ExpressionBody : AssignmentExpression
  1. Let exprRef be ? Evaluation of AssignmentExpression.
  2. Let exprValue be ? GetValue(exprRef).
  3. Return Completion Record { [[Type]]: return, [[Value]]: exprValue, [[Target]]: empty }.

15.4 Method Definitions

Syntax

MethodDefinition[Yield, Await] : ClassElementName[?Yield, ?Await] ( UniqueFormalParameters[~Yield, ~Await] ) { FunctionBody[~Yield, ~Await] } GeneratorMethod[?Yield, ?Await] AsyncMethod[?Yield, ?Await] AsyncGeneratorMethod[?Yield, ?Await] get ClassElementName[?Yield, ?Await] ( ) { FunctionBody[~Yield, ~Await] } set ClassElementName[?Yield, ?Await] ( PropertySetParameterList ) { FunctionBody[~Yield, ~Await] } PropertySetParameterList : FormalParameter[~Yield, ~Await]

15.4.1 Static Semantics: Early Errors

MethodDefinition : ClassElementName ( UniqueFormalParameters ) { FunctionBody } MethodDefinition : set ClassElementName ( PropertySetParameterList ) { FunctionBody }

15.4.2 Static Semantics: HasDirectSuper

The syntax-directed operation HasDirectSuper takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

MethodDefinition : ClassElementName ( UniqueFormalParameters ) { FunctionBody }
  1. If UniqueFormalParameters Contains SuperCall is true, return true.
  2. Return FunctionBody Contains SuperCall.
 MethodDefinition : get ClassElementName ( ) { FunctionBody }
  1. Return FunctionBody Contains SuperCall.
 MethodDefinition : set ClassElementName ( PropertySetParameterList ) { FunctionBody }
  1. If PropertySetParameterList Contains SuperCall is true, return true.
  2. Return FunctionBody Contains SuperCall.
 GeneratorMethod : * ClassElementName ( UniqueFormalParameters ) { GeneratorBody }
  1. If UniqueFormalParameters Contains SuperCall is true, return true.
  2. Return GeneratorBody Contains SuperCall.
 AsyncGeneratorMethod : async * ClassElementName ( UniqueFormalParameters ) { AsyncGeneratorBody }
  1. If UniqueFormalParameters Contains SuperCall is true, return true.
  2. Return AsyncGeneratorBody Contains SuperCall.
 AsyncMethod : async ClassElementName ( UniqueFormalParameters ) { AsyncFunctionBody }
  1. If UniqueFormalParameters Contains SuperCall is true, return true.
  2. Return AsyncFunctionBody Contains SuperCall.

15.4.3 Static Semantics: SpecialMethod

The syntax-directed operation SpecialMethod takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

MethodDefinition : ClassElementName ( UniqueFormalParameters ) { FunctionBody }
  1. Return false.
 MethodDefinition : GeneratorMethod AsyncMethod AsyncGeneratorMethod get ClassElementName ( ) { FunctionBody } set ClassElementName ( PropertySetParameterList ) { FunctionBody }
  1. Return true.

15.4.4 Runtime Semantics: DefineMethod

The syntax-directed operation DefineMethod takes argument object (an Object) and optional argument functionPrototype (an Object) and returns either a normal completion containing a Record with fields [[Key]] (a property key) and [[Closure]] (a function object) or an abrupt completion. It is defined piecewise over the following productions:

MethodDefinition : ClassElementName ( UniqueFormalParameters ) { FunctionBody }
  1. Let propKey be ? Evaluation of ClassElementName.
  2. Let env be the running execution context's LexicalEnvironment.
  3. Let privateEnv be the running execution context's PrivateEnvironment.
  4. If functionPrototype is present, then
    1. Let prototype be functionPrototype.
  5. Else,
    1. Let prototype be %Function.prototype%.
  6. Let sourceText be the source text matched by MethodDefinition.
  7. Let closure be OrdinaryFunctionCreate(prototype, sourceText, UniqueFormalParameters, FunctionBody, non-lexical-this, env, privateEnv).
  8. Perform MakeMethod(closure, object).
  9. Return the Record { [[Key]]: propKey, [[Closure]]: closure }.

15.4.5 Runtime Semantics: MethodDefinitionEvaluation

The syntax-directed operation MethodDefinitionEvaluation takes arguments object (an Object) and enumerable (a Boolean) and returns either a normal completion containing either a PrivateElement or unused, or an abrupt completion. It is defined piecewise over the following productions:

MethodDefinition : ClassElementName ( UniqueFormalParameters ) { FunctionBody }
  1. Let methodDef be ? DefineMethod of MethodDefinition with argument object.
  2. Perform SetFunctionName(methodDef.[[Closure]], methodDef.[[Key]]).
  3. Return DefineMethodProperty(object, methodDef.[[Key]], methodDef.[[Closure]], enumerable).
 MethodDefinition : get ClassElementName ( ) { FunctionBody }
  1. Let propKey be ? Evaluation of ClassElementName.
  2. Let env be the running execution context's LexicalEnvironment.
  3. Let privateEnv be the running execution context's PrivateEnvironment.
  4. Let sourceText be the source text matched by MethodDefinition.
  5. Let formalParameterList be an instance of the production FormalParameters : [empty] .
  6. Let closure be OrdinaryFunctionCreate(%Function.prototype%, sourceText, formalParameterList, FunctionBody, non-lexical-this, env, privateEnv).
  7. Perform MakeMethod(closure, object).
  8. Perform SetFunctionName(closure, propKey, "get").
  9. If propKey is a Private Name, then
    1. Return PrivateElement { [[Key]]: propKey, [[Kind]]: accessor, [[Get]]: closure, [[Set]]: undefined }.
  10. Else,
    1. Let desc be the PropertyDescriptor { [[Get]]: closure, [[Enumerable]]: enumerable, [[Configurable]]: true }.
    2. Perform ? DefinePropertyOrThrow(object, propKey, desc).
    3. Return unused.
 MethodDefinition : set ClassElementName ( PropertySetParameterList ) { FunctionBody }
  1. Let propKey be ? Evaluation of ClassElementName.
  2. Let env be the running execution context's LexicalEnvironment.
  3. Let privateEnv be the running execution context's PrivateEnvironment.
  4. Let sourceText be the source text matched by MethodDefinition.
  5. Let closure be OrdinaryFunctionCreate(%Function.prototype%, sourceText, PropertySetParameterList, FunctionBody, non-lexical-this, env, privateEnv).
  6. Perform MakeMethod(closure, object).
  7. Perform SetFunctionName(closure, propKey, "set").
  8. If propKey is a Private Name, then
    1. Return PrivateElement { [[Key]]: propKey, [[Kind]]: accessor, [[Get]]: undefined, [[Set]]: closure }.
  9. Else,
    1. Let desc be the PropertyDescriptor { [[Set]]: closure, [[Enumerable]]: enumerable, [[Configurable]]: true }.
    2. Perform ? DefinePropertyOrThrow(object, propKey, desc).
    3. Return unused.
 GeneratorMethod : * ClassElementName ( UniqueFormalParameters ) { GeneratorBody }
  1. Let propKey be ? Evaluation of ClassElementName.
  2. Let env be the running execution context's LexicalEnvironment.
  3. Let privateEnv be the running execution context's PrivateEnvironment.
  4. Let sourceText be the source text matched by GeneratorMethod.
  5. Let closure be OrdinaryFunctionCreate(%GeneratorFunction.prototype%, sourceText, UniqueFormalParameters, GeneratorBody, non-lexical-this, env, privateEnv).
  6. Perform MakeMethod(closure, object).
  7. Perform SetFunctionName(closure, propKey).
  8. Let prototype be OrdinaryObjectCreate(%GeneratorFunction.prototype.prototype%).
  9. Perform ! DefinePropertyOrThrow(closure, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
  10. Return DefineMethodProperty(object, propKey, closure, enumerable).
 AsyncGeneratorMethod : async * ClassElementName ( UniqueFormalParameters ) { AsyncGeneratorBody }
  1. Let propKey be ? Evaluation of ClassElementName.
  2. Let env be the running execution context's LexicalEnvironment.
  3. Let privateEnv be the running execution context's PrivateEnvironment.
  4. Let sourceText be the source text matched by AsyncGeneratorMethod.
  5. Let closure be OrdinaryFunctionCreate(%AsyncGeneratorFunction.prototype%, sourceText, UniqueFormalParameters, AsyncGeneratorBody, non-lexical-this, env, privateEnv).
  6. Perform MakeMethod(closure, object).
  7. Perform SetFunctionName(closure, propKey).
  8. Let prototype be OrdinaryObjectCreate(%AsyncGeneratorFunction.prototype.prototype%).
  9. Perform ! DefinePropertyOrThrow(closure, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
  10. Return DefineMethodProperty(object, propKey, closure, enumerable).
 AsyncMethod : async ClassElementName ( UniqueFormalParameters ) { AsyncFunctionBody }
  1. Let propKey be ? Evaluation of ClassElementName.
  2. Let env be the LexicalEnvironment of the running execution context.
  3. Let privateEnv be the running execution context's PrivateEnvironment.
  4. Let sourceText be the source text matched by AsyncMethod.
  5. Let closure be OrdinaryFunctionCreate(%AsyncFunction.prototype%, sourceText, UniqueFormalParameters, AsyncFunctionBody, non-lexical-this, env, privateEnv).
  6. Perform MakeMethod(closure, object).
  7. Perform SetFunctionName(closure, propKey).
  8. Return DefineMethodProperty(object, propKey, closure, enumerable).

15.5 Generator Function Definitions

Syntax

GeneratorDeclaration[Yield, Await, Default] : function * BindingIdentifier[?Yield, ?Await] ( FormalParameters[ Yield, ~Await] ) { GeneratorBody } [ Default] function * ( FormalParameters[ Yield, ~Await] ) { GeneratorBody } GeneratorExpression : function * BindingIdentifier[ Yield, ~Await]opt ( FormalParameters[ Yield, ~Await] ) { GeneratorBody } GeneratorMethod[Yield, Await] : * ClassElementName[?Yield, ?Await] ( UniqueFormalParameters[ Yield, ~Await] ) { GeneratorBody } GeneratorBody : FunctionBody[ Yield, ~Await] YieldExpression[In, Await] : yield yield [no LineTerminator here] AssignmentExpression[?In, Yield, ?Await] yield [no LineTerminator here] * AssignmentExpression[?In, Yield, ?Await] Note 1

The syntactic context immediately following yield requires use of the InputElementRegExpOrTemplateTail lexical goal.

 Note 2

YieldExpression cannot be used within the FormalParameters of a generator function because any expressions that are part of FormalParameters are evaluated before the resulting Generator is in a resumable state.

 Note 3

Abstract operations relating to Generators are defined in 27.5.3.

15.5.1 Static Semantics: Early Errors

GeneratorMethod : * ClassElementName ( UniqueFormalParameters ) { GeneratorBody } GeneratorDeclaration : function * BindingIdentifier ( FormalParameters ) { GeneratorBody } function * ( FormalParameters ) { GeneratorBody } GeneratorExpression : function * BindingIdentifieropt ( FormalParameters ) { GeneratorBody }

15.5.2 Runtime Semantics: EvaluateGeneratorBody

The syntax-directed operation EvaluateGeneratorBody takes arguments functionObject (a function object) and argumentsList (a List of ECMAScript language values) and returns a throw completion or a return completion. It is defined piecewise over the following productions:

GeneratorBody : FunctionBody
  1. Perform ? FunctionDeclarationInstantiation(functionObject, argumentsList).
  2. Let G be ? OrdinaryCreateFromConstructor(functionObject, "%GeneratorFunction.prototype.prototype%", « [[GeneratorState]], [[GeneratorContext]], [[GeneratorBrand]] »).
  3. Set G.[[GeneratorBrand]] to empty.
  4. Perform GeneratorStart(G, FunctionBody).
  5. Return Completion Record { [[Type]]: return, [[Value]]: G, [[Target]]: empty }.

15.5.3 Runtime Semantics: InstantiateGeneratorFunctionObject

The syntax-directed operation InstantiateGeneratorFunctionObject takes arguments env (an Environment Record) and privateEnv (a PrivateEnvironment Record or null) and returns a function object. It is defined piecewise over the following productions:

GeneratorDeclaration : function * BindingIdentifier ( FormalParameters ) { GeneratorBody }
  1. Let name be StringValue of BindingIdentifier.
  2. Let sourceText be the source text matched by GeneratorDeclaration.
  3. Let F be OrdinaryFunctionCreate(%GeneratorFunction.prototype%, sourceText, FormalParameters, GeneratorBody, non-lexical-this, env, privateEnv).
  4. Perform SetFunctionName(F, name).
  5. Let prototype be OrdinaryObjectCreate(%GeneratorFunction.prototype.prototype%).
  6. Perform ! DefinePropertyOrThrow(F, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
  7. Return F.
 GeneratorDeclaration : function * ( FormalParameters ) { GeneratorBody }
  1. Let sourceText be the source text matched by GeneratorDeclaration.
  2. Let F be OrdinaryFunctionCreate(%GeneratorFunction.prototype%, sourceText, FormalParameters, GeneratorBody, non-lexical-this, env, privateEnv).
  3. Perform SetFunctionName(F, "default").
  4. Let prototype be OrdinaryObjectCreate(%GeneratorFunction.prototype.prototype%).
  5. Perform ! DefinePropertyOrThrow(F, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
  6. Return F.
 Note

An anonymous GeneratorDeclaration can only occur as part of an export default declaration, and its function code is therefore always strict mode code.

15.5.4 Runtime Semantics: InstantiateGeneratorFunctionExpression

The syntax-directed operation InstantiateGeneratorFunctionExpression takes optional argument name (a property key or a Private Name) and returns a function object. It is defined piecewise over the following productions:

GeneratorExpression : function * ( FormalParameters ) { GeneratorBody }
  1. If name is not present, set name to "".
  2. Let env be the LexicalEnvironment of the running execution context.
  3. Let privateEnv be the running execution context's PrivateEnvironment.
  4. Let sourceText be the source text matched by GeneratorExpression.
  5. Let closure be OrdinaryFunctionCreate(%GeneratorFunction.prototype%, sourceText, FormalParameters, GeneratorBody, non-lexical-this, env, privateEnv).
  6. Perform SetFunctionName(closure, name).
  7. Let prototype be OrdinaryObjectCreate(%GeneratorFunction.prototype.prototype%).
  8. Perform ! DefinePropertyOrThrow(closure, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
  9. Return closure.
 GeneratorExpression : function * BindingIdentifier ( FormalParameters ) { GeneratorBody }
  1. Assert: name is not present.
  2. Set name to StringValue of BindingIdentifier.
  3. Let outerEnv be the running execution context's LexicalEnvironment.
  4. Let funcEnv be NewDeclarativeEnvironment(outerEnv).
  5. Perform ! funcEnv.CreateImmutableBinding(name, false).
  6. Let privateEnv be the running execution context's PrivateEnvironment.
  7. Let sourceText be the source text matched by GeneratorExpression.
  8. Let closure be OrdinaryFunctionCreate(%GeneratorFunction.prototype%, sourceText, FormalParameters, GeneratorBody, non-lexical-this, funcEnv, privateEnv).
  9. Perform SetFunctionName(closure, name).
  10. Let prototype be OrdinaryObjectCreate(%GeneratorFunction.prototype.prototype%).
  11. Perform ! DefinePropertyOrThrow(closure, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
  12. Perform ! funcEnv.InitializeBinding(name, closure).
  13. Return closure.
 Note

The BindingIdentifier in a GeneratorExpression can be referenced from inside the GeneratorExpression's FunctionBody to allow the generator code to call itself recursively. However, unlike in a GeneratorDeclaration, the BindingIdentifier in a GeneratorExpression cannot be referenced from and does not affect the scope enclosing the GeneratorExpression.

15.5.5 Runtime Semantics: Evaluation

GeneratorExpression : function * BindingIdentifieropt ( FormalParameters ) { GeneratorBody }
  1. Return InstantiateGeneratorFunctionExpression of GeneratorExpression.
 YieldExpression : yield
  1. Return ? Yield(undefined).
 YieldExpression : yield AssignmentExpression
  1. Let exprRef be ? Evaluation of AssignmentExpression.
  2. Let value be ? GetValue(exprRef).
  3. Return ? Yield(value).
 YieldExpression : yield * AssignmentExpression
  1. Let generatorKind be GetGeneratorKind().
  2. Let exprRef be ? Evaluation of AssignmentExpression.
  3. Let value be ? GetValue(exprRef).
  4. Let iteratorRecord be ? GetIterator(value, generatorKind).
  5. Let iterator be iteratorRecord.[[Iterator]].
  6. Let received be NormalCompletion(undefined).
  7. Repeat,
    1. If received.[[Type]] is normal, then
      1. Let innerResult be ? Call(iteratorRecord.[[NextMethod]], iteratorRecord.[[Iterator]], « received.[[Value]] »).
      2. If generatorKind is async, set innerResult to ? Await(innerResult).
      3. If innerResult is not an Object, throw a TypeError exception.
      4. Let done be ? IteratorComplete(innerResult).
      5. If done is true, then
        1. Return ? IteratorValue(innerResult).
      6. If generatorKind is async, set received to Completion(AsyncGeneratorYield(? IteratorValue(innerResult))).
      7. Else, set received to Completion(GeneratorYield(innerResult)).
    2. Else if received.[[Type]] is throw, then
      1. Let throw be ? GetMethod(iterator, "throw").
      2. If throw is not undefined, then
        1. Let innerResult be ? Call(throw, iterator, « received.[[Value]] »).
        2. If generatorKind is async, set innerResult to ? Await(innerResult).
        3. NOTE: Exceptions from the inner iterator throw method are propagated. Normal completions from an inner throw method are processed similarly to an inner next.
        4. If innerResult is not an Object, throw a TypeError exception.
        5. Let done be ? IteratorComplete(innerResult).
        6. If done is true, then
          1. Return ? IteratorValue(innerResult).
        7. If generatorKind is async, set received to Completion(AsyncGeneratorYield(? IteratorValue(innerResult))).
        8. Else, set received to Completion(GeneratorYield(innerResult)).
      3. Else,
        1. NOTE: If iterator does not have a throw method, this throw is going to terminate the yield* loop. But first we need to give iterator a chance to clean up.
        2. Let closeCompletion be Completion Record { [[Type]]: normal, [[Value]]: empty, [[Target]]: empty }.
        3. If generatorKind is async, perform ? AsyncIteratorClose(iteratorRecord, closeCompletion).
        4. Else, perform ? IteratorClose(iteratorRecord, closeCompletion).
        5. NOTE: The next step throws a TypeError to indicate that there was a yield* protocol violation: iterator does not have a throw method.
        6. Throw a TypeError exception.
    3. Else,
      1. Assert: received.[[Type]] is return.
      2. Let return be ? GetMethod(iterator, "return").
      3. If return is undefined, then
        1. Let value be received.[[Value]].
        2. If generatorKind is async, then
          1. Set value to ? Await(value).
        3. Return Completion Record { [[Type]]: return, [[Value]]: value, [[Target]]: empty }.
      4. Let innerReturnResult be ? Call(return, iterator, « received.[[Value]] »).
      5. If generatorKind is async, set innerReturnResult to ? Await(innerReturnResult).
      6. If innerReturnResult is not an Object, throw a TypeError exception.
      7. Let done be ? IteratorComplete(innerReturnResult).
      8. If done is true, then
        1. Let value be ? IteratorValue(innerReturnResult).
        2. Return Completion Record { [[Type]]: return, [[Value]]: value, [[Target]]: empty }.
      9. If generatorKind is async, set received to Completion(AsyncGeneratorYield(? IteratorValue(innerReturnResult))).
      10. Else, set received to Completion(GeneratorYield(innerReturnResult)).

15.6 Async Generator Function Definitions

Syntax

AsyncGeneratorDeclaration[Yield, Await, Default] : async [no LineTerminator here] function * BindingIdentifier[?Yield, ?Await] ( FormalParameters[ Yield, Await] ) { AsyncGeneratorBody } [ Default] async [no LineTerminator here] function * ( FormalParameters[ Yield, Await] ) { AsyncGeneratorBody } AsyncGeneratorExpression : async [no LineTerminator here] function * BindingIdentifier[ Yield, Await]opt ( FormalParameters[ Yield, Await] ) { AsyncGeneratorBody } AsyncGeneratorMethod[Yield, Await] : async [no LineTerminator here] * ClassElementName[?Yield, ?Await] ( UniqueFormalParameters[ Yield, Await] ) { AsyncGeneratorBody } AsyncGeneratorBody : FunctionBody[ Yield, Await] Note 1

YieldExpression and AwaitExpression cannot be used within the FormalParameters of an async generator function because any expressions that are part of FormalParameters are evaluated before the resulting AsyncGenerator is in a resumable state.

 Note 2

Abstract operations relating to AsyncGenerators are defined in 27.6.3.

15.6.1 Static Semantics: Early Errors

AsyncGeneratorMethod : async * ClassElementName ( UniqueFormalParameters ) { AsyncGeneratorBody } AsyncGeneratorDeclaration : async function * BindingIdentifier ( FormalParameters ) { AsyncGeneratorBody } async function * ( FormalParameters ) { AsyncGeneratorBody } AsyncGeneratorExpression : async function * BindingIdentifieropt ( FormalParameters ) { AsyncGeneratorBody }

15.6.2 Runtime Semantics: EvaluateAsyncGeneratorBody

The syntax-directed operation EvaluateAsyncGeneratorBody takes arguments functionObject (a function object) and argumentsList (a List of ECMAScript language values) and returns a throw completion or a return completion. It is defined piecewise over the following productions:

AsyncGeneratorBody : FunctionBody
  1. Perform ? FunctionDeclarationInstantiation(functionObject, argumentsList).
  2. Let generator be ? OrdinaryCreateFromConstructor(functionObject, "%AsyncGeneratorFunction.prototype.prototype%", « [[AsyncGeneratorState]], [[AsyncGeneratorContext]], [[AsyncGeneratorQueue]], [[GeneratorBrand]] »).
  3. Set generator.[[GeneratorBrand]] to empty.
  4. Perform AsyncGeneratorStart(generator, FunctionBody).
  5. Return Completion Record { [[Type]]: return, [[Value]]: generator, [[Target]]: empty }.

15.6.3 Runtime Semantics: InstantiateAsyncGeneratorFunctionObject

The syntax-directed operation InstantiateAsyncGeneratorFunctionObject takes arguments env (an Environment Record) and privateEnv (a PrivateEnvironment Record or null) and returns a function object. It is defined piecewise over the following productions:

AsyncGeneratorDeclaration : async function * BindingIdentifier ( FormalParameters ) { AsyncGeneratorBody }
  1. Let name be StringValue of BindingIdentifier.
  2. Let sourceText be the source text matched by AsyncGeneratorDeclaration.
  3. Let F be OrdinaryFunctionCreate(%AsyncGeneratorFunction.prototype%, sourceText, FormalParameters, AsyncGeneratorBody, non-lexical-this, env, privateEnv).
  4. Perform SetFunctionName(F, name).
  5. Let prototype be OrdinaryObjectCreate(%AsyncGeneratorFunction.prototype.prototype%).
  6. Perform ! DefinePropertyOrThrow(F, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
  7. Return F.
 AsyncGeneratorDeclaration : async function * ( FormalParameters ) { AsyncGeneratorBody }
  1. Let sourceText be the source text matched by AsyncGeneratorDeclaration.
  2. Let F be OrdinaryFunctionCreate(%AsyncGeneratorFunction.prototype%, sourceText, FormalParameters, AsyncGeneratorBody, non-lexical-this, env, privateEnv).
  3. Perform SetFunctionName(F, "default").
  4. Let prototype be OrdinaryObjectCreate(%AsyncGeneratorFunction.prototype.prototype%).
  5. Perform ! DefinePropertyOrThrow(F, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
  6. Return F.
 Note

An anonymous AsyncGeneratorDeclaration can only occur as part of an export default declaration.

15.6.4 Runtime Semantics: InstantiateAsyncGeneratorFunctionExpression

The syntax-directed operation InstantiateAsyncGeneratorFunctionExpression takes optional argument name (a property key or a Private Name) and returns a function object. It is defined piecewise over the following productions:

AsyncGeneratorExpression : async function * ( FormalParameters ) { AsyncGeneratorBody }
  1. If name is not present, set name to "".
  2. Let env be the LexicalEnvironment of the running execution context.
  3. Let privateEnv be the running execution context's PrivateEnvironment.
  4. Let sourceText be the source text matched by AsyncGeneratorExpression.
  5. Let closure be OrdinaryFunctionCreate(%AsyncGeneratorFunction.prototype%, sourceText, FormalParameters, AsyncGeneratorBody, non-lexical-this, env, privateEnv).
  6. Perform SetFunctionName(closure, name).
  7. Let prototype be OrdinaryObjectCreate(%AsyncGeneratorFunction.prototype.prototype%).
  8. Perform ! DefinePropertyOrThrow(closure, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
  9. Return closure.
 AsyncGeneratorExpression : async function * BindingIdentifier ( FormalParameters ) { AsyncGeneratorBody }
  1. Assert: name is not present.
  2. Set name to StringValue of BindingIdentifier.
  3. Let outerEnv be the running execution context's LexicalEnvironment.
  4. Let funcEnv be NewDeclarativeEnvironment(outerEnv).
  5. Perform ! funcEnv.CreateImmutableBinding(name, false).
  6. Let privateEnv be the running execution context's PrivateEnvironment.
  7. Let sourceText be the source text matched by AsyncGeneratorExpression.
  8. Let closure be OrdinaryFunctionCreate(%AsyncGeneratorFunction.prototype%, sourceText, FormalParameters, AsyncGeneratorBody, non-lexical-this, funcEnv, privateEnv).
  9. Perform SetFunctionName(closure, name).
  10. Let prototype be OrdinaryObjectCreate(%AsyncGeneratorFunction.prototype.prototype%).
  11. Perform ! DefinePropertyOrThrow(closure, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
  12. Perform ! funcEnv.InitializeBinding(name, closure).
  13. Return closure.
 Note

The BindingIdentifier in an AsyncGeneratorExpression can be referenced from inside the AsyncGeneratorExpression's AsyncGeneratorBody to allow the generator code to call itself recursively. However, unlike in an AsyncGeneratorDeclaration, the BindingIdentifier in an AsyncGeneratorExpression cannot be referenced from and does not affect the scope enclosing the AsyncGeneratorExpression.

15.6.5 Runtime Semantics: Evaluation

AsyncGeneratorExpression : async function * BindingIdentifieropt ( FormalParameters ) { AsyncGeneratorBody }
  1. Return InstantiateAsyncGeneratorFunctionExpression of AsyncGeneratorExpression.

15.7 Class Definitions

Syntax

ClassDeclaration[Yield, Await, Default] : class BindingIdentifier[?Yield, ?Await] ClassTail[?Yield, ?Await] [ Default] class ClassTail[?Yield, ?Await] ClassExpression[Yield, Await] : class BindingIdentifier[?Yield, ?Await]opt ClassTail[?Yield, ?Await] ClassTail[Yield, Await] : ClassHeritage[?Yield, ?Await]opt { ClassBody[?Yield, ?Await]opt } ClassHeritage[Yield, Await] : extends LeftHandSideExpression[?Yield, ?Await] ClassBody[Yield, Await] : ClassElementList[?Yield, ?Await] ClassElementList[Yield, Await] : ClassElement[?Yield, ?Await] ClassElementList[?Yield, ?Await] ClassElement[?Yield, ?Await] ClassElement[Yield, Await] : MethodDefinition[?Yield, ?Await] static MethodDefinition[?Yield, ?Await] FieldDefinition[?Yield, ?Await] ; static FieldDefinition[?Yield, ?Await] ; ClassStaticBlock ; FieldDefinition[Yield, Await] : ClassElementName[?Yield, ?Await] Initializer[ In, ?Yield, ?Await]opt ClassElementName[Yield, Await] : PropertyName[?Yield, ?Await] PrivateIdentifier ClassStaticBlock : static { ClassStaticBlockBody } ClassStaticBlockBody : ClassStaticBlockStatementList ClassStaticBlockStatementList : StatementList[~Yield, Await, ~Return]opt Note

A class definition is always strict mode code.

15.7.1 Static Semantics: Early Errors

ClassTail : ClassHeritageopt { ClassBody } ClassBody : ClassElementList ClassElement : MethodDefinition ClassElement : static MethodDefinition ClassElement : FieldDefinition ; ClassElement : static FieldDefinition ; FieldDefinition : ClassElementName Initializeropt ClassElementName : PrivateIdentifier ClassStaticBlockBody : ClassStaticBlockStatementList

15.7.2 Static Semantics: ClassElementKind

The syntax-directed operation ClassElementKind takes no arguments and returns ConstructorMethod, NonConstructorMethod, or empty. It is defined piecewise over the following productions:

ClassElement : MethodDefinition
  1. If PropName of MethodDefinition is "constructor", return ConstructorMethod.
  2. Return NonConstructorMethod.
 ClassElement : static MethodDefinition FieldDefinition ; static FieldDefinition ;
  1. Return NonConstructorMethod.
 ClassElement : ClassStaticBlock
  1. Return NonConstructorMethod.
 ClassElement : ;
  1. Return empty.

15.7.3 Static Semantics: ConstructorMethod

The syntax-directed operation ConstructorMethod takes no arguments and returns a ClassElement Parse Node or empty. It is defined piecewise over the following productions:

ClassElementList : ClassElement
  1. If ClassElementKind of ClassElement is ConstructorMethod, return ClassElement.
  2. Return empty.
 ClassElementList : ClassElementList ClassElement
  1. Let head be ConstructorMethod of ClassElementList.
  2. If head is not empty, return head.
  3. If ClassElementKind of ClassElement is ConstructorMethod, return ClassElement.
  4. Return empty.
 Note

Early Error rules ensure that there is only one method definition named "constructor" and that it is not an accessor property or generator definition.

15.7.4 Static Semantics: IsStatic

The syntax-directed operation IsStatic takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

ClassElement : MethodDefinition
  1. Return false.
 ClassElement : static MethodDefinition
  1. Return true.
 ClassElement : FieldDefinition ;
  1. Return false.
 ClassElement : static FieldDefinition ;
  1. Return true.
 ClassElement : ClassStaticBlock
  1. Return true.
 ClassElement : ;
  1. Return false.

15.7.5 Static Semantics: NonConstructorElements

The syntax-directed operation NonConstructorElements takes no arguments and returns a List of ClassElement Parse Nodes. It is defined piecewise over the following productions:

ClassElementList : ClassElement
  1. If ClassElementKind of ClassElement is NonConstructorMethod, then
    1. Return « ClassElement ».
  2. Return a new empty List.
 ClassElementList : ClassElementList ClassElement
  1. Let list be NonConstructorElements of ClassElementList.
  2. If ClassElementKind of ClassElement is NonConstructorMethod, then
    1. Append ClassElement to the end of list.
  3. Return list.

15.7.6 Static Semantics: PrototypePropertyNameList

The syntax-directed operation PrototypePropertyNameList takes no arguments and returns a List of property keys. It is defined piecewise over the following productions:

ClassElementList : ClassElement
  1. Let propName be PropName of ClassElement.
  2. If propName is empty, return a new empty List.
  3. If IsStatic of ClassElement is true, return a new empty List.
  4. Return « propName ».
 ClassElementList : ClassElementList ClassElement
  1. Let list be PrototypePropertyNameList of ClassElementList.
  2. Let propName be PropName of ClassElement.
  3. If propName is empty, return list.
  4. If IsStatic of ClassElement is true, return list.
  5. Return the list-concatenation of list and « propName ».

15.7.7 Static Semantics: AllPrivateIdentifiersValid

The syntax-directed operation AllPrivateIdentifiersValid takes argument names (a List of Strings) and returns a Boolean.

Every grammar production alternative in this specification which is not listed below implicitly has the following default definition of AllPrivateIdentifiersValid:

  1. For each child node child of this Parse Node, do
    1. If child is an instance of a nonterminal, then
      1. If AllPrivateIdentifiersValid of child with argument names is false, return false.
  2. Return true.
 MemberExpression : MemberExpression . PrivateIdentifier
  1. If names contains the StringValue of PrivateIdentifier, then
    1. Return AllPrivateIdentifiersValid of MemberExpression with argument names.
  2. Return false.
 CallExpression : CallExpression . PrivateIdentifier
  1. If names contains the StringValue of PrivateIdentifier, then
    1. Return AllPrivateIdentifiersValid of CallExpression with argument names.
  2. Return false.
 OptionalChain : ?. PrivateIdentifier
  1. If names contains the StringValue of PrivateIdentifier, return true.
  2. Return false.
 OptionalChain : OptionalChain . PrivateIdentifier
  1. If names contains the StringValue of PrivateIdentifier, then
    1. Return AllPrivateIdentifiersValid of OptionalChain with argument names.
  2. Return false.
 ClassBody : ClassElementList
  1. Let newNames be the list-concatenation of names and PrivateBoundIdentifiers of ClassBody.
  2. Return AllPrivateIdentifiersValid of ClassElementList with argument newNames.
 RelationalExpression : PrivateIdentifier in ShiftExpression
  1. If names contains the StringValue of PrivateIdentifier, then
    1. Return AllPrivateIdentifiersValid of ShiftExpression with argument names.
  2. Return false.

15.7.8 Static Semantics: PrivateBoundIdentifiers

The syntax-directed operation PrivateBoundIdentifiers takes no arguments and returns a List of Strings. It is defined piecewise over the following productions:

FieldDefinition : ClassElementName Initializeropt
  1. Return PrivateBoundIdentifiers of ClassElementName.
 ClassElementName : PrivateIdentifier
  1. Return a List whose sole element is the StringValue of PrivateIdentifier.
 ClassElementName : PropertyName ClassElement : ClassStaticBlock ;
  1. Return a new empty List.
 ClassElementList : ClassElementList ClassElement
  1. Let names1 be PrivateBoundIdentifiers of ClassElementList.
  2. Let names2 be PrivateBoundIdentifiers of ClassElement.
  3. Return the list-concatenation of names1 and names2.
 MethodDefinition : ClassElementName ( UniqueFormalParameters ) { FunctionBody } get ClassElementName ( ) { FunctionBody } set ClassElementName ( PropertySetParameterList ) { FunctionBody } GeneratorMethod : * ClassElementName ( UniqueFormalParameters ) { GeneratorBody } AsyncMethod : async ClassElementName ( UniqueFormalParameters ) { AsyncFunctionBody } AsyncGeneratorMethod : async * ClassElementName ( UniqueFormalParameters ) { AsyncGeneratorBody }
  1. Return PrivateBoundIdentifiers of ClassElementName.

15.7.9 Static Semantics: ContainsArguments

The syntax-directed operation ContainsArguments takes no arguments and returns a Boolean.

Every grammar production alternative in this specification which is not listed below implicitly has the following default definition of ContainsArguments:

  1. For each child node child of this Parse Node, do
    1. If child is an instance of a nonterminal, then
      1. If ContainsArguments of child is true, return true.
  2. Return false.
 IdentifierReference : Identifier
  1. If the StringValue of Identifier is "arguments", return true.
  2. Return false.
 FunctionDeclaration : function BindingIdentifier ( FormalParameters ) { FunctionBody } function ( FormalParameters ) { FunctionBody } FunctionExpression : function BindingIdentifieropt ( FormalParameters ) { FunctionBody } GeneratorDeclaration : function * BindingIdentifier ( FormalParameters ) { GeneratorBody } function * ( FormalParameters ) { GeneratorBody } GeneratorExpression : function * BindingIdentifieropt ( FormalParameters ) { GeneratorBody } AsyncGeneratorDeclaration : async function * BindingIdentifier ( FormalParameters ) { AsyncGeneratorBody } async function * ( FormalParameters ) { AsyncGeneratorBody } AsyncGeneratorExpression : async function * BindingIdentifieropt ( FormalParameters ) { AsyncGeneratorBody } AsyncFunctionDeclaration : async function BindingIdentifier ( FormalParameters ) { AsyncFunctionBody } async function ( FormalParameters ) { AsyncFunctionBody } AsyncFunctionExpression : async function BindingIdentifieropt ( FormalParameters ) { AsyncFunctionBody }
  1. Return false.
 MethodDefinition : ClassElementName ( UniqueFormalParameters ) { FunctionBody } get ClassElementName ( ) { FunctionBody } set ClassElementName ( PropertySetParameterList ) { FunctionBody } GeneratorMethod : * ClassElementName ( UniqueFormalParameters ) { GeneratorBody } AsyncGeneratorMethod : async * ClassElementName ( UniqueFormalParameters ) { AsyncGeneratorBody } AsyncMethod : async ClassElementName ( UniqueFormalParameters ) { AsyncFunctionBody }
  1. Return ContainsArguments of ClassElementName.

15.7.10 Runtime Semantics: ClassFieldDefinitionEvaluation

The syntax-directed operation ClassFieldDefinitionEvaluation takes argument homeObject (an Object) and returns either a normal completion containing a ClassFieldDefinition Record or an abrupt completion. It is defined piecewise over the following productions:

FieldDefinition : ClassElementName Initializeropt
  1. Let name be ? Evaluation of ClassElementName.
  2. If Initializeropt is present, then
    1. Let formalParameterList be an instance of the production FormalParameters : [empty] .
    2. Let env be the LexicalEnvironment of the running execution context.
    3. Let privateEnv be the running execution context's PrivateEnvironment.
    4. Let sourceText be the empty sequence of Unicode code points.
    5. Let initializer be OrdinaryFunctionCreate(%Function.prototype%, sourceText, formalParameterList, Initializer, non-lexical-this, env, privateEnv).
    6. Perform MakeMethod(initializer, homeObject).
    7. Set initializer.[[ClassFieldInitializerName]] to name.
  3. Else,
    1. Let initializer be empty.
  4. Return the ClassFieldDefinition Record { [[Name]]: name, [[Initializer]]: initializer }.
 Note
The function created for initializer is never directly accessible to ECMAScript code.

15.7.11 Runtime Semantics: ClassStaticBlockDefinitionEvaluation

The syntax-directed operation ClassStaticBlockDefinitionEvaluation takes argument homeObject (an Object) and returns a ClassStaticBlockDefinition Record. It is defined piecewise over the following productions:

ClassStaticBlock : static { ClassStaticBlockBody }
  1. Let lex be the running execution context's LexicalEnvironment.
  2. Let privateEnv be the running execution context's PrivateEnvironment.
  3. Let sourceText be the empty sequence of Unicode code points.
  4. Let formalParameters be an instance of the production FormalParameters : [empty] .
  5. Let bodyFunction be OrdinaryFunctionCreate(%Function.prototype%, sourceText, formalParameters, ClassStaticBlockBody, non-lexical-this, lex, privateEnv).
  6. Perform MakeMethod(bodyFunction, homeObject).
  7. Return the ClassStaticBlockDefinition Record { [[BodyFunction]]: bodyFunction }.
 Note
The function bodyFunction is never directly accessible to ECMAScript code.

15.7.12 Runtime Semantics: EvaluateClassStaticBlockBody

The syntax-directed operation EvaluateClassStaticBlockBody takes argument functionObject (a function object) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It is defined piecewise over the following productions:

ClassStaticBlockBody : ClassStaticBlockStatementList
  1. Perform ? FunctionDeclarationInstantiation(functionObject, « »).
  2. Return ? Evaluation of ClassStaticBlockStatementList.

15.7.13 Runtime Semantics: ClassElementEvaluation

The syntax-directed operation ClassElementEvaluation takes argument object (an Object) and returns either a normal completion containing either a ClassFieldDefinition Record, a ClassStaticBlockDefinition Record, a PrivateElement, or unused, or an abrupt completion. It is defined piecewise over the following productions:

ClassElement : FieldDefinition ; static FieldDefinition ;
  1. Return ? ClassFieldDefinitionEvaluation of FieldDefinition with argument object.
 ClassElement : MethodDefinition static MethodDefinition
  1. Return ? MethodDefinitionEvaluation of MethodDefinition with arguments object and false.
 ClassElement : ClassStaticBlock
  1. Return ClassStaticBlockDefinitionEvaluation of ClassStaticBlock with argument object.
 ClassElement : ;
  1. Return unused.

15.7.14 Runtime Semantics: ClassDefinitionEvaluation

The syntax-directed operation ClassDefinitionEvaluation takes arguments classBinding (a String or undefined) and className (a property key or a Private Name) and returns either a normal completion containing a function object or an abrupt completion.

Note

For ease of specification, private methods and accessors are included alongside private fields in the [[PrivateElements]] slot of class instances. However, any given object has either all or none of the private methods and accessors defined by a given class. This feature has been designed so that implementations may choose to implement private methods and accessors using a strategy which does not require tracking each method or accessor individually.

For example, an implementation could directly associate instance private methods with their corresponding Private Name and track, for each object, which class constructors have run with that object as their this value. Looking up an instance private method on an object then consists of checking that the class constructor which defines the method has been used to initialize the object, then returning the method associated with the Private Name.

This differs from private fields: because field initializers can throw during class instantiation, an individual object may have some proper subset of the private fields of a given class, and so private fields must in general be tracked individually.

It is defined piecewise over the following productions:

ClassTail : ClassHeritageopt { ClassBodyopt }
  1. Let env be the LexicalEnvironment of the running execution context.
  2. Let classEnv be NewDeclarativeEnvironment(env).
  3. If classBinding is not undefined, then
    1. Perform ! classEnv.CreateImmutableBinding(classBinding, true).
  4. Let outerPrivateEnvironment be the running execution context's PrivateEnvironment.
  5. Let classPrivateEnvironment be NewPrivateEnvironment(outerPrivateEnvironment).
  6. If ClassBodyopt is present, then
    1. For each String dn of the PrivateBoundIdentifiers of ClassBodyopt, do
      1. If classPrivateEnvironment.[[Names]] contains a Private Name pn such that pn.[[Description]] is dn, then
        1. Assert: This is only possible for getter/setter pairs.
      2. Else,
        1. Let name be a new Private Name whose [[Description]] is dn.
        2. Append name to classPrivateEnvironment.[[Names]].
  7. If ClassHeritageopt is not present, then
    1. Let protoParent be %Object.prototype%.
    2. Let constructorParent be %Function.prototype%.
  8. Else,
    1. Set the running execution context's LexicalEnvironment to classEnv.
    2. NOTE: The running execution context's PrivateEnvironment is outerPrivateEnvironment when evaluating ClassHeritage.
    3. Let superclassRef be Completion(Evaluation of ClassHeritage).
    4. Set the running execution context's LexicalEnvironment to env.
    5. Let superclass be ? GetValue(? superclassRef).
    6. If superclass is null, then
      1. Let protoParent be null.
      2. Let constructorParent be %Function.prototype%.
    7. Else if IsConstructor(superclass) is false, throw a TypeError exception.
    8. Else,
      1. Let protoParent be ? Get(superclass, "prototype").
      2. If protoParent is not an Object and protoParent is not null, throw a TypeError exception.
      3. Let constructorParent be superclass.
  9. Let proto be OrdinaryObjectCreate(protoParent).
  10. If ClassBodyopt is not present, let constructor be empty.
  11. Else, let constructor be ConstructorMethod of ClassBody.
  12. Set the running execution context's LexicalEnvironment to classEnv.
  13. Set the running execution context's PrivateEnvironment to classPrivateEnvironment.
  14. If constructor is empty, then
    1. Let defaultConstructor be a new Abstract Closure with no parameters that captures nothing and performs the following steps when called:
      1. Let args be the List of arguments that was passed to this function by [[Call]] or [[Construct]].
      2. If NewTarget is undefined, throw a TypeError exception.
      3. Let F be the active function object.
      4. If F.[[ConstructorKind]] is derived, then
        1. NOTE: This branch behaves similarly to constructor(...args) { super(...args); }. The most notable distinction is that while the aforementioned ECMAScript source text observably calls the @@iterator method on %Array.prototype%, this function does not.
        2. Let func be ! F.[[GetPrototypeOf]]().
        3. If IsConstructor(func) is false, throw a TypeError exception.
        4. Let result be ? Construct(func, args, NewTarget).
      5. Else,
        1. NOTE: This branch behaves similarly to constructor() {}.
        2. Let result be ? OrdinaryCreateFromConstructor(NewTarget, "%Object.prototype%").
      6. Perform ? InitializeInstanceElements(result, F).
      7. Return result.
    2. Let F be CreateBuiltinFunction(defaultConstructor, 0, className, « [[ConstructorKind]], [[SourceText]] », the current Realm Record, constructorParent).
  15. Else,
    1. Let constructorInfo be ! DefineMethod of constructor with arguments proto and constructorParent.
    2. Let F be constructorInfo.[[Closure]].
    3. Perform MakeClassConstructor(F).
    4. Perform SetFunctionName(F, className).
  16. Perform MakeConstructor(F, false, proto).
  17. If ClassHeritageopt is present, set F.[[ConstructorKind]] to derived.
  18. Perform CreateMethodProperty(proto, "constructor", F).
  19. If ClassBodyopt is not present, let elements be a new empty List.
  20. Else, let elements be NonConstructorElements of ClassBody.
  21. Let instancePrivateMethods be a new empty List.
  22. Let staticPrivateMethods be a new empty List.
  23. Let instanceFields be a new empty List.
  24. Let staticElements be a new empty List.
  25. For each ClassElement e of elements, do
    1. If IsStatic of e is false, then
      1. Let element be Completion(ClassElementEvaluation of e with argument proto).
    2. Else,
      1. Let element be Completion(ClassElementEvaluation of e with argument F).
    3. If element is an abrupt completion, then
      1. Set the running execution context's LexicalEnvironment to env.
      2. Set the running execution context's PrivateEnvironment to outerPrivateEnvironment.
      3. Return ? element.
    4. Set element to element.[[Value]].
    5. If element is a PrivateElement, then
      1. Assert: element.[[Kind]] is either method or accessor.
      2. If IsStatic of e is false, let container be instancePrivateMethods.
      3. Else, let container be staticPrivateMethods.
      4. If container contains a PrivateElement pe such that pe.[[Key]] is element.[[Key]], then
        1. Assert: element.[[Kind]] and pe.[[Kind]] are both accessor.
        2. If element.[[Get]] is undefined, then
          1. Let combined be PrivateElement { [[Key]]: element.[[Key]], [[Kind]]: accessor, [[Get]]: pe.[[Get]], [[Set]]: element.[[Set]] }.
        3. Else,
          1. Let combined be PrivateElement { [[Key]]: element.[[Key]], [[Kind]]: accessor, [[Get]]: element.[[Get]], [[Set]]: pe.[[Set]] }.
        4. Replace pe in container with combined.
      5. Else,
        1. Append element to container.
    6. Else if element is a ClassFieldDefinition Record, then
      1. If IsStatic of e is false, append element to instanceFields.
      2. Else, append element to staticElements.
    7. Else if element is a ClassStaticBlockDefinition Record, then
      1. Append element to staticElements.
  26. Set the running execution context's LexicalEnvironment to env.
  27. If classBinding is not undefined, then
    1. Perform ! classEnv.InitializeBinding(classBinding, F).
  28. Set F.[[PrivateMethods]] to instancePrivateMethods.
  29. Set F.[[Fields]] to instanceFields.
  30. For each PrivateElement method of staticPrivateMethods, do
    1. Perform ! PrivateMethodOrAccessorAdd(F, method).
  31. For each element elementRecord of staticElements, do
    1. If elementRecord is a ClassFieldDefinition Record, then
      1. Let result be Completion(DefineField(F, elementRecord)).
    2. Else,
      1. Assert: elementRecord is a ClassStaticBlockDefinition Record.
      2. Let result be Completion(Call(elementRecord.[[BodyFunction]], F)).
    3. If result is an abrupt completion, then
      1. Set the running execution context's PrivateEnvironment to outerPrivateEnvironment.
      2. Return ? result.
  32. Set the running execution context's PrivateEnvironment to outerPrivateEnvironment.
  33. Return F.

15.7.15 Runtime Semantics: BindingClassDeclarationEvaluation

The syntax-directed operation BindingClassDeclarationEvaluation takes no arguments and returns either a normal completion containing a function object or an abrupt completion. It is defined piecewise over the following productions:

ClassDeclaration : class BindingIdentifier ClassTail
  1. Let className be StringValue of BindingIdentifier.
  2. Let value be ? ClassDefinitionEvaluation of ClassTail with arguments className and className.
  3. Set value.[[SourceText]] to the source text matched by ClassDeclaration.
  4. Let env be the running execution context's LexicalEnvironment.
  5. Perform ? InitializeBoundName(className, value, env).
  6. Return value.
 ClassDeclaration : class ClassTail
  1. Let value be ? ClassDefinitionEvaluation of ClassTail with arguments undefined and "default".
  2. Set value.[[SourceText]] to the source text matched by ClassDeclaration.
  3. Return value.
 Note

ClassDeclaration : class ClassTail only occurs as part of an ExportDeclaration and establishing its binding is handled as part of the evaluation action for that production. See 16.2.3.7.

15.7.16 Runtime Semantics: Evaluation

ClassDeclaration : class BindingIdentifier ClassTail
  1. Perform ? BindingClassDeclarationEvaluation of this ClassDeclaration.
  2. Return empty.
 Note

ClassDeclaration : class ClassTail only occurs as part of an ExportDeclaration and is never directly evaluated.

 ClassExpression : class ClassTail
  1. Let value be ? ClassDefinitionEvaluation of ClassTail with arguments undefined and "".
  2. Set value.[[SourceText]] to the source text matched by ClassExpression.
  3. Return value.
 ClassExpression : class BindingIdentifier ClassTail
  1. Let className be StringValue of BindingIdentifier.
  2. Let value be ? ClassDefinitionEvaluation of ClassTail with arguments className and className.
  3. Set value.[[SourceText]] to the source text matched by ClassExpression.
  4. Return value.
 ClassElementName : PrivateIdentifier
  1. Let privateIdentifier be StringValue of PrivateIdentifier.
  2. Let privateEnvRec be the running execution context's PrivateEnvironment.
  3. Let names be privateEnvRec.[[Names]].
  4. Assert: Exactly one element of names is a Private Name whose [[Description]] is privateIdentifier.
  5. Let privateName be the Private Name in names whose [[Description]] is privateIdentifier.
  6. Return privateName.
 ClassStaticBlockStatementList : [empty]
  1. Return undefined.

15.8 Async Function Definitions

Syntax

AsyncFunctionDeclaration[Yield, Await, Default] : async [no LineTerminator here] function BindingIdentifier[?Yield, ?Await] ( FormalParameters[~Yield, Await] ) { AsyncFunctionBody } [ Default] async [no LineTerminator here] function ( FormalParameters[~Yield, Await] ) { AsyncFunctionBody } AsyncFunctionExpression : async [no LineTerminator here] function BindingIdentifier[~Yield, Await]opt ( FormalParameters[~Yield, Await] ) { AsyncFunctionBody } AsyncMethod[Yield, Await] : async [no LineTerminator here] ClassElementName[?Yield, ?Await] ( UniqueFormalParameters[~Yield, Await] ) { AsyncFunctionBody } AsyncFunctionBody : FunctionBody[~Yield, Await] AwaitExpression[Yield] : await UnaryExpression[?Yield, Await] Note 1

await is parsed as a keyword of an AwaitExpression when the [Await] parameter is present. The [Await] parameter is present in the top level of the following contexts, although the parameter may be absent in some contexts depending on the nonterminals, such as FunctionBody:

When Script is the syntactic goal symbol, await may be parsed as an identifier when the [Await] parameter is absent. This includes the following contexts:

 Note 2

Unlike YieldExpression, it is a Syntax Error to omit the operand of an AwaitExpression. You must await something.

15.8.1 Static Semantics: Early Errors

AsyncMethod : async ClassElementName ( UniqueFormalParameters ) { AsyncFunctionBody } AsyncFunctionDeclaration : async function BindingIdentifier ( FormalParameters ) { AsyncFunctionBody } async function ( FormalParameters ) { AsyncFunctionBody } AsyncFunctionExpression : async function BindingIdentifieropt ( FormalParameters ) { AsyncFunctionBody }

15.8.2 Runtime Semantics: InstantiateAsyncFunctionObject

The syntax-directed operation InstantiateAsyncFunctionObject takes arguments env (an Environment Record) and privateEnv (a PrivateEnvironment Record or null) and returns a function object. It is defined piecewise over the following productions:

AsyncFunctionDeclaration : async function BindingIdentifier ( FormalParameters ) { AsyncFunctionBody }
  1. Let name be StringValue of BindingIdentifier.
  2. Let sourceText be the source text matched by AsyncFunctionDeclaration.
  3. Let F be OrdinaryFunctionCreate(%AsyncFunction.prototype%, sourceText, FormalParameters, AsyncFunctionBody, non-lexical-this, env, privateEnv).
  4. Perform SetFunctionName(F, name).
  5. Return F.
 AsyncFunctionDeclaration : async function ( FormalParameters ) { AsyncFunctionBody }
  1. Let sourceText be the source text matched by AsyncFunctionDeclaration.
  2. Let F be OrdinaryFunctionCreate(%AsyncFunction.prototype%, sourceText, FormalParameters, AsyncFunctionBody, non-lexical-this, env, privateEnv).
  3. Perform SetFunctionName(F, "default").
  4. Return F.

15.8.3 Runtime Semantics: InstantiateAsyncFunctionExpression

The syntax-directed operation InstantiateAsyncFunctionExpression takes optional argument name (a property key or a Private Name) and returns a function object. It is defined piecewise over the following productions:

AsyncFunctionExpression : async function ( FormalParameters ) { AsyncFunctionBody }
  1. If name is not present, set name to "".
  2. Let env be the LexicalEnvironment of the running execution context.
  3. Let privateEnv be the running execution context's PrivateEnvironment.
  4. Let sourceText be the source text matched by AsyncFunctionExpression.
  5. Let closure be OrdinaryFunctionCreate(%AsyncFunction.prototype%, sourceText, FormalParameters, AsyncFunctionBody, non-lexical-this, env, privateEnv).
  6. Perform SetFunctionName(closure, name).
  7. Return closure.
 AsyncFunctionExpression : async function BindingIdentifier ( FormalParameters ) { AsyncFunctionBody }
  1. Assert: name is not present.
  2. Set name to StringValue of BindingIdentifier.
  3. Let outerEnv be the LexicalEnvironment of the running execution context.
  4. Let funcEnv be NewDeclarativeEnvironment(outerEnv).
  5. Perform ! funcEnv.CreateImmutableBinding(name, false).
  6. Let privateEnv be the running execution context's PrivateEnvironment.
  7. Let sourceText be the source text matched by AsyncFunctionExpression.
  8. Let closure be OrdinaryFunctionCreate(%AsyncFunction.prototype%, sourceText, FormalParameters, AsyncFunctionBody, non-lexical-this, funcEnv, privateEnv).
  9. Perform SetFunctionName(closure, name).
  10. Perform ! funcEnv.InitializeBinding(name, closure).
  11. Return closure.
 Note

The BindingIdentifier in an AsyncFunctionExpression can be referenced from inside the AsyncFunctionExpression's AsyncFunctionBody to allow the function to call itself recursively. However, unlike in a FunctionDeclaration, the BindingIdentifier in a AsyncFunctionExpression cannot be referenced from and does not affect the scope enclosing the AsyncFunctionExpression.

15.8.4 Runtime Semantics: EvaluateAsyncFunctionBody

The syntax-directed operation EvaluateAsyncFunctionBody takes arguments functionObject (a function object) and argumentsList (a List of ECMAScript language values) and returns a return completion. It is defined piecewise over the following productions:

AsyncFunctionBody : FunctionBody
  1. Let promiseCapability be ! NewPromiseCapability(%Promise%).
  2. Let declResult be Completion(FunctionDeclarationInstantiation(functionObject, argumentsList)).
  3. If declResult is an abrupt completion, then
    1. Perform ! Call(promiseCapability.[[Reject]], undefined, « declResult.[[Value]] »).
  4. Else,
    1. Perform AsyncFunctionStart(promiseCapability, FunctionBody).
  5. Return Completion Record { [[Type]]: return, [[Value]]: promiseCapability.[[Promise]], [[Target]]: empty }.

15.8.5 Runtime Semantics: Evaluation

AsyncFunctionExpression : async function BindingIdentifieropt ( FormalParameters ) { AsyncFunctionBody }
  1. Return InstantiateAsyncFunctionExpression of AsyncFunctionExpression.
 AwaitExpression : await UnaryExpression
  1. Let exprRef be ? Evaluation of UnaryExpression.
  2. Let value be ? GetValue(exprRef).
  3. Return ? Await(value).

15.9 Async Arrow Function Definitions

Syntax

AsyncArrowFunction[In, Yield, Await] : async [no LineTerminator here] AsyncArrowBindingIdentifier[?Yield] [no LineTerminator here] => AsyncConciseBody[?In] CoverCallExpressionAndAsyncArrowHead[?Yield, ?Await] [no LineTerminator here] => AsyncConciseBody[?In] AsyncConciseBody[In] : [lookahead ≠ {] ExpressionBody[?In, Await] { AsyncFunctionBody } AsyncArrowBindingIdentifier[Yield] : BindingIdentifier[?Yield, Await] CoverCallExpressionAndAsyncArrowHead[Yield, Await] : MemberExpression[?Yield, ?Await] Arguments[?Yield, ?Await]

Supplemental Syntax

When processing an instance of the production
AsyncArrowFunction : CoverCallExpressionAndAsyncArrowHead => AsyncConciseBody
the interpretation of CoverCallExpressionAndAsyncArrowHead is refined using the following grammar:

AsyncArrowHead : async [no LineTerminator here] ArrowFormalParameters[~Yield, Await]

15.9.1 Static Semantics: Early Errors

AsyncArrowFunction : async AsyncArrowBindingIdentifier => AsyncConciseBody AsyncArrowFunction : CoverCallExpressionAndAsyncArrowHead => AsyncConciseBody

15.9.2 Static Semantics: AsyncConciseBodyContainsUseStrict

The syntax-directed operation AsyncConciseBodyContainsUseStrict takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

AsyncConciseBody : ExpressionBody
  1. Return false.
 AsyncConciseBody : { AsyncFunctionBody }
  1. Return FunctionBodyContainsUseStrict of AsyncFunctionBody.

15.9.3 Runtime Semantics: EvaluateAsyncConciseBody

The syntax-directed operation EvaluateAsyncConciseBody takes arguments functionObject (a function object) and argumentsList (a List of ECMAScript language values) and returns a return completion. It is defined piecewise over the following productions:

AsyncConciseBody : ExpressionBody
  1. Let promiseCapability be ! NewPromiseCapability(%Promise%).
  2. Let declResult be Completion(FunctionDeclarationInstantiation(functionObject, argumentsList)).
  3. If declResult is an abrupt completion, then
    1. Perform ! Call(promiseCapability.[[Reject]], undefined, « declResult.[[Value]] »).
  4. Else,
    1. Perform AsyncFunctionStart(promiseCapability, ExpressionBody).
  5. Return Completion Record { [[Type]]: return, [[Value]]: promiseCapability.[[Promise]], [[Target]]: empty }.

15.9.4 Runtime Semantics: InstantiateAsyncArrowFunctionExpression

The syntax-directed operation InstantiateAsyncArrowFunctionExpression takes optional argument name (a property key or a Private Name) and returns a function object. It is defined piecewise over the following productions:

AsyncArrowFunction : async AsyncArrowBindingIdentifier => AsyncConciseBody
  1. If name is not present, set name to "".
  2. Let env be the LexicalEnvironment of the running execution context.
  3. Let privateEnv be the running execution context's PrivateEnvironment.
  4. Let sourceText be the source text matched by AsyncArrowFunction.
  5. Let parameters be AsyncArrowBindingIdentifier.
  6. Let closure be OrdinaryFunctionCreate(%AsyncFunction.prototype%, sourceText, parameters, AsyncConciseBody, lexical-this, env, privateEnv).
  7. Perform SetFunctionName(closure, name).
  8. Return closure.
 AsyncArrowFunction : CoverCallExpressionAndAsyncArrowHead => AsyncConciseBody
  1. If name is not present, set name to "".
  2. Let env be the LexicalEnvironment of the running execution context.
  3. Let privateEnv be the running execution context's PrivateEnvironment.
  4. Let sourceText be the source text matched by AsyncArrowFunction.
  5. Let head be the AsyncArrowHead that is covered by CoverCallExpressionAndAsyncArrowHead.
  6. Let parameters be the ArrowFormalParameters of head.
  7. Let closure be OrdinaryFunctionCreate(%AsyncFunction.prototype%, sourceText, parameters, AsyncConciseBody, lexical-this, env, privateEnv).
  8. Perform SetFunctionName(closure, name).
  9. Return closure.

15.9.5 Runtime Semantics: Evaluation

AsyncArrowFunction : async AsyncArrowBindingIdentifier => AsyncConciseBody CoverCallExpressionAndAsyncArrowHead => AsyncConciseBody
  1. Return InstantiateAsyncArrowFunctionExpression of AsyncArrowFunction.

15.10 Tail Position Calls

15.10.1 Static Semantics: IsInTailPosition ( call )

The abstract operation IsInTailPosition takes argument call (a CallExpression Parse Node, a MemberExpression Parse Node, or an OptionalChain Parse Node) and returns a Boolean. It performs the following steps when called:

  1. If the source text matched by call is non-strict code, return false.
  2. If call is not contained within a FunctionBody, a ConciseBody, or an AsyncConciseBody, return false.
  3. Let body be the FunctionBody, ConciseBody, or AsyncConciseBody that most closely contains call.
  4. If body is the FunctionBody of a GeneratorBody, return false.
  5. If body is the FunctionBody of an AsyncFunctionBody, return false.
  6. If body is the FunctionBody of an AsyncGeneratorBody, return false.
  7. If body is an AsyncConciseBody, return false.
  8. Return the result of HasCallInTailPosition of body with argument call.
 Note

Tail Position calls are only defined in strict mode code because of a common non-standard language extension (see 10.2.4) that enables observation of the chain of caller contexts.

15.10.2 Static Semantics: HasCallInTailPosition

The syntax-directed operation HasCallInTailPosition takes argument call (a CallExpression Parse Node, a MemberExpression Parse Node, or an OptionalChain Parse Node) and returns a Boolean.

Note 1

call is a Parse Node that represents a specific range of source text. When the following algorithms compare call to another Parse Node, it is a test of whether they represent the same source text.

 Note 2

A potential tail position call that is immediately followed by return GetValue of the call result is also a possible tail position call. A function call cannot return a Reference Record, so such a GetValue operation will always return the same value as the actual function call result.

It is defined piecewise over the following productions:

StatementList : StatementList StatementListItem
  1. Let has be HasCallInTailPosition of StatementList with argument call.
  2. If has is true, return true.
  3. Return HasCallInTailPosition of StatementListItem with argument call.
 FunctionStatementList : [empty] StatementListItem : Declaration Statement : VariableStatement EmptyStatement ExpressionStatement ContinueStatement BreakStatement ThrowStatement DebuggerStatement Block : { } ReturnStatement : return ; LabelledItem : FunctionDeclaration ForInOfStatement : for ( LeftHandSideExpression of AssignmentExpression ) Statement for ( var ForBinding of AssignmentExpression ) Statement for ( ForDeclaration of AssignmentExpression ) Statement CaseBlock : { }
  1. Return false.
 IfStatement : if ( Expression ) Statement else Statement
  1. Let has be HasCallInTailPosition of the first Statement with argument call.
  2. If has is true, return true.
  3. Return HasCallInTailPosition of the second Statement with argument call.
 IfStatement : if ( Expression ) Statement DoWhileStatement : do Statement while ( Expression ) ; WhileStatement : while ( Expression ) Statement ForStatement : for ( Expressionopt ; Expressionopt ; Expressionopt ) Statement for ( var VariableDeclarationList ; Expressionopt ; Expressionopt ) Statement for ( LexicalDeclaration Expressionopt ; Expressionopt ) Statement ForInOfStatement : for ( LeftHandSideExpression in Expression ) Statement for ( var ForBinding in Expression ) Statement for ( ForDeclaration in Expression ) Statement WithStatement : with ( Expression ) Statement
  1. Return HasCallInTailPosition of Statement with argument call.
 LabelledStatement : LabelIdentifier : LabelledItem
  1. Return HasCallInTailPosition of LabelledItem with argument call.
 ReturnStatement : return Expression ;
  1. Return HasCallInTailPosition of Expression with argument call.
 SwitchStatement : switch ( Expression ) CaseBlock
  1. Return HasCallInTailPosition of CaseBlock with argument call.
 CaseBlock : { CaseClausesopt DefaultClause CaseClausesopt }
  1. Let has be false.
  2. If the first CaseClauses is present, let has be HasCallInTailPosition of the first CaseClauses with argument call.
  3. If has is true, return true.
  4. Let has be HasCallInTailPosition of DefaultClause with argument call.
  5. If has is true, return true.
  6. If the second CaseClauses is present, let has be HasCallInTailPosition of the second CaseClauses with argument call.
  7. Return has.
 CaseClauses : CaseClauses CaseClause
  1. Let has be HasCallInTailPosition of CaseClauses with argument call.
  2. If has is true, return true.
  3. Return HasCallInTailPosition of CaseClause with argument call.
 CaseClause : case Expression : StatementListopt DefaultClause : default : StatementListopt
  1. If StatementList is present, return HasCallInTailPosition of StatementList with argument call.
  2. Return false.
 TryStatement : try Block Catch
  1. Return HasCallInTailPosition of Catch with argument call.
 TryStatement : try Block Finally try Block Catch Finally
  1. Return HasCallInTailPosition of Finally with argument call.
 Catch : catch ( CatchParameter ) Block
  1. Return HasCallInTailPosition of Block with argument call.
 AssignmentExpression : YieldExpression ArrowFunction AsyncArrowFunction LeftHandSideExpression = AssignmentExpression LeftHandSideExpression AssignmentOperator AssignmentExpression LeftHandSideExpression &&= AssignmentExpression LeftHandSideExpression ||= AssignmentExpression LeftHandSideExpression ??= AssignmentExpression BitwiseANDExpression : BitwiseANDExpression & EqualityExpression BitwiseXORExpression : BitwiseXORExpression ^ BitwiseANDExpression BitwiseORExpression : BitwiseORExpression | BitwiseXORExpression EqualityExpression : EqualityExpression == RelationalExpression EqualityExpression != RelationalExpression EqualityExpression === RelationalExpression EqualityExpression !== RelationalExpression RelationalExpression : RelationalExpression < ShiftExpression RelationalExpression > ShiftExpression RelationalExpression <= ShiftExpression RelationalExpression >= ShiftExpression RelationalExpression instanceof ShiftExpression RelationalExpression in ShiftExpression PrivateIdentifier in ShiftExpression ShiftExpression : ShiftExpression << AdditiveExpression ShiftExpression >> AdditiveExpression ShiftExpression >>> AdditiveExpression AdditiveExpression : AdditiveExpression MultiplicativeExpression AdditiveExpression - MultiplicativeExpression MultiplicativeExpression : MultiplicativeExpression MultiplicativeOperator ExponentiationExpression ExponentiationExpression : UpdateExpression ** ExponentiationExpression UpdateExpression : LeftHandSideExpression LeftHandSideExpression -- UnaryExpression -- UnaryExpression UnaryExpression : delete UnaryExpression void UnaryExpression typeof UnaryExpression UnaryExpression - UnaryExpression ~ UnaryExpression ! UnaryExpression AwaitExpression CallExpression : SuperCall CallExpression [ Expression ] CallExpression . IdentifierName CallExpression . PrivateIdentifier NewExpression : new NewExpression MemberExpression : MemberExpression [ Expression ] MemberExpression . IdentifierName SuperProperty MetaProperty new MemberExpression Arguments MemberExpression . PrivateIdentifier PrimaryExpression : this IdentifierReference Literal ArrayLiteral ObjectLiteral FunctionExpression ClassExpression GeneratorExpression AsyncFunctionExpression AsyncGeneratorExpression RegularExpressionLiteral TemplateLiteral
  1. Return false.
 Expression : AssignmentExpression Expression , AssignmentExpression
  1. Return HasCallInTailPosition of AssignmentExpression with argument call.
 ConditionalExpression : ShortCircuitExpression ? AssignmentExpression : AssignmentExpression
  1. Let has be HasCallInTailPosition of the first AssignmentExpression with argument call.
  2. If has is true, return true.
  3. Return HasCallInTailPosition of the second AssignmentExpression with argument call.
 LogicalANDExpression : LogicalANDExpression && BitwiseORExpression
  1. Return HasCallInTailPosition of BitwiseORExpression with argument call.
 LogicalORExpression : LogicalORExpression || LogicalANDExpression
  1. Return HasCallInTailPosition of LogicalANDExpression with argument call.
 CoalesceExpression : CoalesceExpressionHead ?? BitwiseORExpression
  1. Return HasCallInTailPosition of BitwiseORExpression with argument call.
 CallExpression : CoverCallExpressionAndAsyncArrowHead CallExpression Arguments CallExpression TemplateLiteral
  1. If this CallExpression is call, return true.
  2. Return false.
 OptionalExpression : MemberExpression OptionalChain CallExpression OptionalChain OptionalExpression OptionalChain
  1. Return HasCallInTailPosition of OptionalChain with argument call.
 OptionalChain : ?. [ Expression ] ?. IdentifierName ?. PrivateIdentifier OptionalChain [ Expression ] OptionalChain . IdentifierName OptionalChain . PrivateIdentifier
  1. Return false.
 OptionalChain : ?. Arguments OptionalChain Arguments
  1. If this OptionalChain is call, return true.
  2. Return false.
 MemberExpression : MemberExpression TemplateLiteral
  1. If this MemberExpression is call, return true.
  2. Return false.
 PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList
  1. Let expr be the ParenthesizedExpression that is covered by CoverParenthesizedExpressionAndArrowParameterList.
  2. Return HasCallInTailPosition of expr with argument call.
 ParenthesizedExpression : ( Expression )
  1. Return HasCallInTailPosition of Expression with argument call.

15.10.3 PrepareForTailCall ( )

The abstract operation PrepareForTailCall takes no arguments and returns unused. It performs the following steps when called:

  1. Assert: The current execution context will not subsequently be used for the evaluation of any ECMAScript code or built-in functions. The invocation of Call subsequent to the invocation of this abstract operation will create and push a new execution context before performing any such evaluation.
  2. Discard all resources associated with the current execution context.
  3. Return unused.

A tail position call must either release any transient internal resources associated with the currently executing function execution context before invoking the target function or reuse those resources in support of the target function.

Note

For example, a tail position call should only grow an implementation's activation record stack by the amount that the size of the target function's activation record exceeds the size of the calling function's activation record. If the target function's activation record is smaller, then the total size of the stack should decrease.

16 ECMAScript Language: Scripts and Modules

16.1 Scripts

Syntax

Script : ScriptBodyopt ScriptBody : StatementList[~Yield, ~Await, ~Return]

16.1.1 Static Semantics: Early Errors

Script : ScriptBody ScriptBody : StatementList

16.1.2 Static Semantics: IsStrict

The syntax-directed operation IsStrict takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

Script : ScriptBodyopt
  1. If ScriptBody is present and the Directive Prologue of ScriptBody contains a Use Strict Directive, return true; otherwise, return false.

16.1.3 Runtime Semantics: Evaluation

Script : [empty]
  1. Return undefined.

16.1.4 Script Records

A Script Record encapsulates information about a script being evaluated. Each script record contains the fields listed in Table 39.

Table 39: Script Record Fields
Field Name Value Type Meaning
[[Realm]] a Realm Record or undefined The realm within which this script was created. undefined if not yet assigned.
[[ECMAScriptCode]] a Script Parse Node The result of parsing the source text of this script.
[[LoadedModules]] a List of Records with fields [[Specifier]] (a String) and [[Module]] (a Module Record) A map from the specifier strings imported by this script to the resolved Module Record. The list does not contain two different Records with the same [[Specifier]].
[[HostDefined]] anything (default value is empty) Field reserved for use by host environments that need to associate additional information with a script.

16.1.5 ParseScript ( sourceText, realm, hostDefined )

The abstract operation ParseScript takes arguments sourceText (ECMAScript source text), realm (a Realm Record or undefined), and hostDefined (anything) and returns a Script Record or a non-empty List of SyntaxError objects. It creates a Script Record based upon the result of parsing sourceText as a Script. It performs the following steps when called:

  1. Let script be ParseText(sourceText, Script).
  2. If script is a List of errors, return script.
  3. Return Script Record { [[Realm]]: realm, [[ECMAScriptCode]]: script, [[LoadedModules]]: « », [[HostDefined]]: hostDefined }.
 Note

An implementation may parse script source text and analyse it for Early Error conditions prior to evaluation of ParseScript for that script source text. However, the reporting of any errors must be deferred until the point where this specification actually performs ParseScript upon that source text.

16.1.6 ScriptEvaluation ( scriptRecord )

The abstract operation ScriptEvaluation takes argument scriptRecord (a Script Record) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It performs the following steps when called:

  1. Let globalEnv be scriptRecord.[[Realm]].[[GlobalEnv]].
  2. Let scriptContext be a new ECMAScript code execution context.
  3. Set the Function of scriptContext to null.
  4. Set the Realm of scriptContext to scriptRecord.[[Realm]].
  5. Set the ScriptOrModule of scriptContext to scriptRecord.
  6. Set the VariableEnvironment of scriptContext to globalEnv.
  7. Set the LexicalEnvironment of scriptContext to globalEnv.
  8. Set the PrivateEnvironment of scriptContext to null.
  9. Suspend the running execution context.
  10. Push scriptContext onto the execution context stack; scriptContext is now the running execution context.
  11. Let script be scriptRecord.[[ECMAScriptCode]].
  12. Let result be Completion(GlobalDeclarationInstantiation(script, globalEnv)).
  13. If result.[[Type]] is normal, then
    1. Set result to Completion(Evaluation of script).
    2. If result.[[Type]] is normal and result.[[Value]] is empty, then
      1. Set result to NormalCompletion(undefined).
  14. Suspend scriptContext and remove it from the execution context stack.
  15. Assert: The execution context stack is not empty.
  16. Resume the context that is now on the top of the execution context stack as the running execution context.
  17. Return ? result.

16.1.7 GlobalDeclarationInstantiation ( script, env )

The abstract operation GlobalDeclarationInstantiation takes arguments script (a Script Parse Node) and env (a Global Environment Record) and returns either a normal completion containing unused or a throw completion. script is the Script for which the execution context is being established. env is the global environment in which bindings are to be created.

Note 1

When an execution context is established for evaluating scripts, declarations are instantiated in the current global environment. Each global binding declared in the code is instantiated.

It performs the following steps when called:

  1. Let lexNames be the LexicallyDeclaredNames of script.
  2. Let varNames be the VarDeclaredNames of script.
  3. For each element name of lexNames, do
    1. If env.HasVarDeclaration(name) is true, throw a SyntaxError exception.
    2. If env.HasLexicalDeclaration(name) is true, throw a SyntaxError exception.
    3. Let hasRestrictedGlobal be ? env.HasRestrictedGlobalProperty(name).
    4. If hasRestrictedGlobal is true, throw a SyntaxError exception.
  4. For each element name of varNames, do
    1. If env.HasLexicalDeclaration(name) is true, throw a SyntaxError exception.
  5. Let varDeclarations be the VarScopedDeclarations of script.
  6. Let functionsToInitialize be a new empty List.
  7. Let declaredFunctionNames be a new empty List.
  8. For each element d of varDeclarations, in reverse List order, do
    1. If d is not either a VariableDeclaration, a ForBinding, or a BindingIdentifier, then
      1. Assert: d is either a FunctionDeclaration, a GeneratorDeclaration, an AsyncFunctionDeclaration, or an AsyncGeneratorDeclaration.
      2. NOTE: If there are multiple function declarations for the same name, the last declaration is used.
      3. Let fn be the sole element of the BoundNames of d.
      4. If declaredFunctionNames does not contain fn, then
        1. Let fnDefinable be ? env.CanDeclareGlobalFunction(fn).
        2. If fnDefinable is false, throw a TypeError exception.
        3. Append fn to declaredFunctionNames.
        4. Insert d as the first element of functionsToInitialize.
  9. Let declaredVarNames be a new empty List.
  10. For each element d of varDeclarations, do
    1. If d is either a VariableDeclaration, a ForBinding, or a BindingIdentifier, then
      1. For each String vn of the BoundNames of d, do
        1. If declaredFunctionNames does not contain vn, then
          1. Let vnDefinable be ? env.CanDeclareGlobalVar(vn).
          2. If vnDefinable is false, throw a TypeError exception.
          3. If declaredVarNames does not contain vn, then
            1. Append vn to declaredVarNames.
  11. NOTE: No abnormal terminations occur after this algorithm step if the global object is an ordinary object. However, if the global object is a Proxy exotic object it may exhibit behaviours that cause abnormal terminations in some of the following steps.
  12. NOTE: Annex B.3.2.2 adds additional steps at this point.
  13. Let lexDeclarations be the LexicallyScopedDeclarations of script.
  14. Let privateEnv be null.
  15. For each element d of lexDeclarations, do
    1. NOTE: Lexically declared names are only instantiated here but not initialized.
    2. For each element dn of the BoundNames of d, do
      1. If IsConstantDeclaration of d is true, then
        1. Perform ? env.CreateImmutableBinding(dn, true).
      2. Else,
        1. Perform ? env.CreateMutableBinding(dn, false).
  16. For each Parse Node f of functionsToInitialize, do
    1. Let fn be the sole element of the BoundNames of f.
    2. Let fo be InstantiateFunctionObject of f with arguments env and privateEnv.
    3. Perform ? env.CreateGlobalFunctionBinding(fn, fo, false).
  17. For each String vn of declaredVarNames, do
    1. Perform ? env.CreateGlobalVarBinding(vn, false).
  18. Return unused.
 Note 2

Early errors specified in 16.1.1 prevent name conflicts between function/var declarations and let/const/class declarations as well as redeclaration of let/const/class bindings for declaration contained within a single Script. However, such conflicts and redeclarations that span more than one Script are detected as runtime errors during GlobalDeclarationInstantiation. If any such errors are detected, no bindings are instantiated for the script. However, if the global object is defined using Proxy exotic objects then the runtime tests for conflicting declarations may be unreliable resulting in an abrupt completion and some global declarations not being instantiated. If this occurs, the code for the Script is not evaluated.

Unlike explicit var or function declarations, properties that are directly created on the global object result in global bindings that may be shadowed by let/const/class declarations.

16.2 Modules

Syntax

Module : ModuleBodyopt ModuleBody : ModuleItemList ModuleItemList : ModuleItem ModuleItemList ModuleItem ModuleItem : ImportDeclaration ExportDeclaration StatementListItem[~Yield, Await, ~Return] ModuleExportName : IdentifierName StringLiteral

16.2.1 Module Semantics

16.2.1.1 Static Semantics: Early Errors

ModuleBody : ModuleItemList Note

The duplicate ExportedNames rule implies that multiple export default ExportDeclaration items within a ModuleBody is a Syntax Error. Additional error conditions relating to conflicting or duplicate declarations are checked during module linking prior to evaluation of a Module. If any such errors are detected the Module is not evaluated.

 ModuleExportName : StringLiteral

16.2.1.2 Static Semantics: ImportedLocalNames ( importEntries )

The abstract operation ImportedLocalNames takes argument importEntries (a List of ImportEntry Records) and returns a List of Strings. It creates a List of all of the local name bindings defined by importEntries. It performs the following steps when called:

  1. Let localNames be a new empty List.
  2. For each ImportEntry Record i of importEntries, do
    1. Append i.[[LocalName]] to localNames.
  3. Return localNames.

16.2.1.3 Static Semantics: ModuleRequests

The syntax-directed operation ModuleRequests takes no arguments and returns a List of Strings. It is defined piecewise over the following productions:

Module : [empty]
  1. Return a new empty List.
 ModuleItemList : ModuleItem
  1. Return ModuleRequests of ModuleItem.
 ModuleItemList : ModuleItemList ModuleItem
  1. Let moduleNames be ModuleRequests of ModuleItemList.
  2. Let additionalNames be ModuleRequests of ModuleItem.
  3. For each String name of additionalNames, do
    1. If moduleNames does not contain name, then
      1. Append name to moduleNames.
  4. Return moduleNames.
 ModuleItem : StatementListItem
  1. Return a new empty List.
 ImportDeclaration : import ImportClause FromClause ;
  1. Return ModuleRequests of FromClause.
 ModuleSpecifier : StringLiteral
  1. Return a List whose sole element is the SV of StringLiteral.
 ExportDeclaration : export ExportFromClause FromClause ;
  1. Return the ModuleRequests of FromClause.
 ExportDeclaration : export NamedExports ; export VariableStatement export Declaration export default HoistableDeclaration export default ClassDeclaration export default AssignmentExpression ;
  1. Return a new empty List.

16.2.1.4 Abstract Module Records

A Module Record encapsulates structural information about the imports and exports of a single module. This information is used to link the imports and exports of sets of connected modules. A Module Record includes four fields that are only used when evaluating a module.

For specification purposes Module Record values are values of the Record specification type and can be thought of as existing in a simple object-oriented hierarchy where Module Record is an abstract class with both abstract and concrete subclasses. This specification defines the abstract subclass named Cyclic Module Record and its concrete subclass named Source Text Module Record. Other specifications and implementations may define additional Module Record subclasses corresponding to alternative module definition facilities that they defined.

Module Record defines the fields listed in Table 40. All Module Definition subclasses include at least those fields. Module Record also defines the abstract method list in Table 41. All Module definition subclasses must provide concrete implementations of these abstract methods.

Table 40: Module Record Fields
Field Name Value Type Meaning
[[Realm]] a Realm Record The Realm within which this module was created.
[[Environment]] a Module Environment Record or empty The Environment Record containing the top level bindings for this module. This field is set when the module is linked.
[[Namespace]] an Object or empty The Module Namespace Object (28.3) if one has been created for this module.
[[HostDefined]] anything (default value is undefined) Field reserved for use by host environments that need to associate additional information with a module.
Table 41: Abstract Methods of Module Records
Method Purpose
LoadRequestedModules( [ hostDefined ] )

Prepares the module for linking by recursively loading all its dependencies, and returns a promise.
GetExportedNames([exportStarSet])

Return a list of all names that are either directly or indirectly exported from this module.

LoadRequestedModules must have completed successfully prior to invoking this method.
ResolveExport(exportName [, resolveSet])

Return the binding of a name exported by this module. Bindings are represented by a ResolvedBinding Record, of the form { [[Module]]: Module Record, [[BindingName]]: String | namespace }. If the export is a Module Namespace Object without a direct binding in any module, [[BindingName]] will be set to namespace. Return null if the name cannot be resolved, or ambiguous if multiple bindings were found.

Each time this operation is called with a specific exportName, resolveSet pair as arguments it must return the same result.

LoadRequestedModules must have completed successfully prior to invoking this method.
Link()

Prepare the module for evaluation by transitively resolving all module dependencies and creating a Module Environment Record.

LoadRequestedModules must have completed successfully prior to invoking this method.
Evaluate()

Returns a promise for the evaluation of this module and its dependencies, resolving on successful evaluation or if it has already been evaluated successfully, and rejecting for an evaluation error or if it has already been evaluated unsuccessfully. If the promise is rejected, hosts are expected to handle the promise rejection and rethrow the evaluation error.

Link must have completed successfully prior to invoking this method.

16.2.1.5 Cyclic Module Records

A Cyclic Module Record is used to represent information about a module that can participate in dependency cycles with other modules that are subclasses of the Cyclic Module Record type. Module Records that are not subclasses of the Cyclic Module Record type must not participate in dependency cycles with Source Text Module Records.

In addition to the fields defined in Table 40 Cyclic Module Records have the additional fields listed in Table 42

Table 42: Additional Fields of Cyclic Module Records
Field Name Value Type Meaning
[[Status]] new, unlinked, linking, linked, evaluating, evaluating-async, or evaluated Initially new. Transitions to unlinked, linking, linked, evaluating, possibly evaluating-async, evaluated (in that order) as the module progresses throughout its lifecycle. evaluating-async indicates this module is queued to execute on completion of its asynchronous dependencies or it is a module whose [[HasTLA]] field is true that has been executed and is pending top-level completion.
[[EvaluationError]] a throw completion or empty A throw completion representing the exception that occurred during evaluation. undefined if no exception occurred or if [[Status]] is not evaluated.
[[DFSIndex]] an integer or empty Auxiliary field used during Link and Evaluate only. If [[Status]] is either linking or evaluating, this non-negative number records the point at which the module was first visited during the depth-first traversal of the dependency graph.
[[DFSAncestorIndex]] an integer or empty Auxiliary field used during Link and Evaluate only. If [[Status]] is either linking or evaluating, this is either the module's own [[DFSIndex]] or that of an "earlier" module in the same strongly connected component.
[[RequestedModules]] a List of Strings A List of all the ModuleSpecifier strings used by the module represented by this record to request the importation of a module. The List is in source text occurrence order.
[[LoadedModules]] a List of Records with fields [[Specifier]] (a String) and [[Module]] (a Module Record) A map from the specifier strings used by the module represented by this record to request the importation of a module to the resolved Module Record. The list does not contain two different Records with the same [[Specifier]].
[[CycleRoot]] a Cyclic Module Record or empty The first visited module of the cycle, the root DFS ancestor of the strongly connected component. For a module not in a cycle, this would be the module itself. Once Evaluate has completed, a module's [[DFSAncestorIndex]] is the [[DFSIndex]] of its [[CycleRoot]].
[[HasTLA]] a Boolean Whether this module is individually asynchronous (for example, if it's a Source Text Module Record containing a top-level await). Having an asynchronous dependency does not mean this field is true. This field must not change after the module is parsed.
[[AsyncEvaluation]] a Boolean Whether this module is either itself asynchronous or has an asynchronous dependency. Note: The order in which this field is set is used to order queued executions, see 16.2.1.5.3.4.
[[TopLevelCapability]] a PromiseCapability Record or empty If this module is the [[CycleRoot]] of some cycle, and Evaluate() was called on some module in that cycle, this field contains the PromiseCapability Record for that entire evaluation. It is used to settle the Promise object that is returned from the Evaluate() abstract method. This field will be empty for any dependencies of that module, unless a top-level Evaluate() has been initiated for some of those dependencies.
[[AsyncParentModules]] a List of Cyclic Module Records If this module or a dependency has [[HasTLA]] true, and execution is in progress, this tracks the parent importers of this module for the top-level execution job. These parent modules will not start executing before this module has successfully completed execution.
[[PendingAsyncDependencies]] an integer or empty If this module has any asynchronous dependencies, this tracks the number of asynchronous dependency modules remaining to execute for this module. A module with asynchronous dependencies will be executed when this field reaches 0 and there are no execution errors.

In addition to the methods defined in Table 41 Cyclic Module Records have the additional methods listed in Table 43

Table 43: Additional Abstract Methods of Cyclic Module Records
Method Purpose
InitializeEnvironment() Initialize the Environment Record of the module, including resolving all imported bindings, and create the module's execution context.
ExecuteModule( [ promiseCapability ] ) Evaluate the module's code within its execution context. If this module has true in [[HasTLA]], then a PromiseCapability Record is passed as an argument, and the method is expected to resolve or reject the given capability. In this case, the method must not throw an exception, but instead reject the PromiseCapability Record if necessary.

A GraphLoadingState Record is a Record that contains information about the loading process of a module graph. It's used to continue loading after a call to HostLoadImportedModule. Each GraphLoadingState Record has the fields defined in Table 44:

Table 44: GraphLoadingState Record Fields
Field Name Value Type Meaning
[[PromiseCapability]] a PromiseCapability Record The promise to resolve when the loading process finishes.
[[IsLoading]] a Boolean It is true if the loading process has not finished yet, neither successfully nor with an error.
[[PendingModulesCount]] a non-negative integer It tracks the number of pending HostLoadImportedModule calls.
[[Visited]] a List of Cyclic Module Records It is a list of the Cyclic Module Records that have been already loaded by the current loading process, to avoid infinite loops with circular dependencies.
[[HostDefined]] anything (default value is empty) It contains host-defined data to pass from the LoadRequestedModules caller to HostLoadImportedModule.

16.2.1.5.1 LoadRequestedModules ( [ hostDefined ] )

The LoadRequestedModules concrete method of a Cyclic Module Record module takes optional argument hostDefined (anything) and returns a Promise. It populates the [[LoadedModules]] of all the Module Records in the dependency graph of module (most of the work is done by the auxiliary function InnerModuleLoading). It takes an optional hostDefined parameter that is passed to the HostLoadImportedModule hook. It performs the following steps when called:

  1. If hostDefined is not present, let hostDefined be empty.
  2. Let pc be ! NewPromiseCapability(%Promise%).
  3. Let state be the GraphLoadingState Record { [[IsLoading]]: true, [[PendingModulesCount]]: 1, [[Visited]]: « », [[PromiseCapability]]: pc, [[HostDefined]]: hostDefined }.
  4. Perform InnerModuleLoading(state, module).
  5. Return pc.[[Promise]].
 Note
The hostDefined parameter can be used to pass additional information necessary to fetch the imported modules. It is used, for example, by HTML to set the correct fetch destination for <link rel="preload" as="..."> tags. import() expressions never set the hostDefined parameter.

16.2.1.5.1.1 InnerModuleLoading ( state, module )

The abstract operation InnerModuleLoading takes arguments state (a GraphLoadingState Record) and module (a Module Record) and returns unused. It is used by LoadRequestedModules to recursively perform the actual loading process for module's dependency graph. It performs the following steps when called:

  1. Assert: state.[[IsLoading]] is true.
  2. If module is a Cyclic Module Record, module.[[Status]] is new, and state.[[Visited]] does not contain module, then
    1. Append module to state.[[Visited]].
    2. Let requestedModulesCount be the number of elements in module.[[RequestedModules]].
    3. Set state.[[PendingModulesCount]] to state.[[PendingModulesCount]] requestedModulesCount.
    4. For each String required of module.[[RequestedModules]], do
      1. If module.[[LoadedModules]] contains a Record whose [[Specifier]] is required, then
        1. Let record be that Record.
        2. Perform InnerModuleLoading(state, record.[[Module]]).
      2. Else,
        1. Perform HostLoadImportedModule(module, required, state.[[HostDefined]], state).
        2. NOTE: HostLoadImportedModule will call FinishLoadingImportedModule, which re-enters the graph loading process through ContinueModuleLoading.
      3. If state.[[IsLoading]] is false, return unused.
  3. Assert: state.[[PendingModulesCount]] ≥ 1.
  4. Set state.[[PendingModulesCount]] to state.[[PendingModulesCount]] - 1.
  5. If state.[[PendingModulesCount]] = 0, then
    1. Set state.[[IsLoading]] to false.
    2. For each Cyclic Module Record loaded of state.[[Visited]], do
      1. If loaded.[[Status]] is new, set loaded.[[Status]] to unlinked.
    3. Perform ! Call(state.[[PromiseCapability]].[[Resolve]], undefined, « undefined »).
  6. Return unused.

16.2.1.5.1.2 ContinueModuleLoading ( state, moduleCompletion )

The abstract operation ContinueModuleLoading takes arguments state (a GraphLoadingState Record) and moduleCompletion (either a normal completion containing a Module Record or a throw completion) and returns unused. It is used to re-enter the loading process after a call to HostLoadImportedModule. It performs the following steps when called:

  1. If state.[[IsLoading]] is false, return unused.
  2. If moduleCompletion is a normal completion, then
    1. Perform InnerModuleLoading(state, moduleCompletion.[[Value]]).
  3. Else,
    1. Set state.[[IsLoading]] to false.
    2. Perform ! Call(state.[[PromiseCapability]].[[Reject]], undefined, « moduleCompletion.[[Value]] »).
  4. Return unused.

16.2.1.5.2 Link ( )

The Link concrete method of a Cyclic Module Record module takes no arguments and returns either a normal completion containing unused or a throw completion. On success, Link transitions this module's [[Status]] from unlinked to linked. On failure, an exception is thrown and this module's [[Status]] remains unlinked. (Most of the work is done by the auxiliary function InnerModuleLinking.) It performs the following steps when called:

  1. Assert: module.[[Status]] is one of unlinked, linked, evaluating-async, or evaluated.
  2. Let stack be a new empty List.
  3. Let result be Completion(InnerModuleLinking(module, stack, 0)).
  4. If result is an abrupt completion, then
    1. For each Cyclic Module Record m of stack, do
      1. Assert: m.[[Status]] is linking.
      2. Set m.[[Status]] to unlinked.
    2. Assert: module.[[Status]] is unlinked.
    3. Return ? result.
  5. Assert: module.[[Status]] is one of linked, evaluating-async, or evaluated.
  6. Assert: stack is empty.
  7. Return unused.

16.2.1.5.2.1 InnerModuleLinking ( module, stack, index )

The abstract operation InnerModuleLinking takes arguments module (a Module Record), stack (a List of Cyclic Module Records), and index (a non-negative integer) and returns either a normal completion containing a non-negative integer or a throw completion. It is used by Link to perform the actual linking process for module, as well as recursively on all other modules in the dependency graph. The stack and index parameters, as well as a module's [[DFSIndex]] and [[DFSAncestorIndex]] fields, keep track of the depth-first search (DFS) traversal. In particular, [[DFSAncestorIndex]] is used to discover strongly connected components (SCCs), such that all modules in an SCC transition to linked together. It performs the following steps when called:

  1. If module is not a Cyclic Module Record, then
    1. Perform ? module.Link().
    2. Return index.
  2. If module.[[Status]] is one of linking, linked, evaluating-async, or evaluated, then
    1. Return index.
  3. Assert: module.[[Status]] is unlinked.
  4. Set module.[[Status]] to linking.
  5. Set module.[[DFSIndex]] to index.
  6. Set module.[[DFSAncestorIndex]] to index.
  7. Set index to index 1.
  8. Append module to stack.
  9. For each String required of module.[[RequestedModules]], do
    1. Let requiredModule be GetImportedModule(module, required).
    2. Set index to ? InnerModuleLinking(requiredModule, stack, index).
    3. If requiredModule is a Cyclic Module Record, then
      1. Assert: requiredModule.[[Status]] is one of linking, linked, evaluating-async, or evaluated.
      2. Assert: requiredModule.[[Status]] is linking if and only if stack contains requiredModule.
      3. If requiredModule.[[Status]] is linking, then
        1. Set module.[[DFSAncestorIndex]] to min(module.[[DFSAncestorIndex]], requiredModule.[[DFSAncestorIndex]]).
  10. Perform ? module.InitializeEnvironment().
  11. Assert: module occurs exactly once in stack.
  12. Assert: module.[[DFSAncestorIndex]] ≤ module.[[DFSIndex]].
  13. If module.[[DFSAncestorIndex]] = module.[[DFSIndex]], then
    1. Let done be false.
    2. Repeat, while done is false,
      1. Let requiredModule be the last element of stack.
      2. Remove the last element of stack.
      3. Assert: requiredModule is a Cyclic Module Record.
      4. Set requiredModule.[[Status]] to linked.
      5. If requiredModule and module are the same Module Record, set done to true.
  14. Return index.

16.2.1.5.3 Evaluate ( )

The Evaluate concrete method of a Cyclic Module Record module takes no arguments and returns a Promise. Evaluate transitions this module's [[Status]] from linked to either evaluating-async or evaluated. The first time it is called on a module in a given strongly connected component, Evaluate creates and returns a Promise which resolves when the module has finished evaluating. This Promise is stored in the [[TopLevelCapability]] field of the [[CycleRoot]] for the component. Future invocations of Evaluate on any module in the component return the same Promise. (Most of the work is done by the auxiliary function InnerModuleEvaluation.) It performs the following steps when called:

  1. Assert: This call to Evaluate is not happening at the same time as another call to Evaluate within the surrounding agent.
  2. Assert: module.[[Status]] is one of linked, evaluating-async, or evaluated.
  3. If module.[[Status]] is either evaluating-async or evaluated, set module to module.[[CycleRoot]].
  4. If module.[[TopLevelCapability]] is not empty, then
    1. Return module.[[TopLevelCapability]].[[Promise]].
  5. Let stack be a new empty List.
  6. Let capability be ! NewPromiseCapability(%Promise%).
  7. Set module.[[TopLevelCapability]] to capability.
  8. Let result be Completion(InnerModuleEvaluation(module, stack, 0)).
  9. If result is an abrupt completion, then
    1. For each Cyclic Module Record m of stack, do
      1. Assert: m.[[Status]] is evaluating.
      2. Set m.[[Status]] to evaluated.
      3. Set m.[[EvaluationError]] to result.
    2. Assert: module.[[Status]] is evaluated.
    3. Assert: module.[[EvaluationError]] is result.
    4. Perform ! Call(capability.[[Reject]], undefined, « result.[[Value]] »).
  10. Else,
    1. Assert: module.[[Status]] is either evaluating-async or evaluated.
    2. Assert: module.[[EvaluationError]] is empty.
    3. If module.[[AsyncEvaluation]] is false, then
      1. Assert: module.[[Status]] is evaluated.
      2. Perform ! Call(capability.[[Resolve]], undefined, « undefined »).
    4. Assert: stack is empty.
  11. Return capability.[[Promise]].

16.2.1.5.3.1 InnerModuleEvaluation ( module, stack, index )

The abstract operation InnerModuleEvaluation takes arguments module (a Module Record), stack (a List of Cyclic Module Records), and index (a non-negative integer) and returns either a normal completion containing a non-negative integer or a throw completion. It is used by Evaluate to perform the actual evaluation process for module, as well as recursively on all other modules in the dependency graph. The stack and index parameters, as well as module's [[DFSIndex]] and [[DFSAncestorIndex]] fields, are used the same way as in InnerModuleLinking. It performs the following steps when called:

  1. If module is not a Cyclic Module Record, then
    1. Let promise be ! module.Evaluate().
    2. Assert: promise.[[PromiseState]] is not pending.
    3. If promise.[[PromiseState]] is rejected, then
      1. Return ThrowCompletion(promise.[[PromiseResult]]).
    4. Return index.
  2. If module.[[Status]] is either evaluating-async or evaluated, then
    1. If module.[[EvaluationError]] is empty, return index.
    2. Otherwise, return ? module.[[EvaluationError]].
  3. If module.[[Status]] is evaluating, return index.
  4. Assert: module.[[Status]] is linked.
  5. Set module.[[Status]] to evaluating.
  6. Set module.[[DFSIndex]] to index.
  7. Set module.[[DFSAncestorIndex]] to index.
  8. Set module.[[PendingAsyncDependencies]] to 0.
  9. Set index to index 1.
  10. Append module to stack.
  11. For each String required of module.[[RequestedModules]], do
    1. Let requiredModule be GetImportedModule(module, required).
    2. Set index to ? InnerModuleEvaluation(requiredModule, stack, index).
    3. If requiredModule is a Cyclic Module Record, then
      1. Assert: requiredModule.[[Status]] is one of evaluating, evaluating-async, or evaluated.
      2. Assert: requiredModule.[[Status]] is evaluating if and only if stack contains requiredModule.
      3. If requiredModule.[[Status]] is evaluating, then
        1. Set module.[[DFSAncestorIndex]] to min(module.[[DFSAncestorIndex]], requiredModule.[[DFSAncestorIndex]]).
      4. Else,
        1. Set requiredModule to requiredModule.[[CycleRoot]].
        2. Assert: requiredModule.[[Status]] is either evaluating-async or evaluated.
        3. If requiredModule.[[EvaluationError]] is not empty, return ? requiredModule.[[EvaluationError]].
      5. If requiredModule.[[AsyncEvaluation]] is true, then
        1. Set module.[[PendingAsyncDependencies]] to module.[[PendingAsyncDependencies]] 1.
        2. Append module to requiredModule.[[AsyncParentModules]].
  12. If module.[[PendingAsyncDependencies]] > 0 or module.[[HasTLA]] is true, then
    1. Assert: module.[[AsyncEvaluation]] is false and was never previously set to true.
    2. Set module.[[AsyncEvaluation]] to true.
    3. NOTE: The order in which module records have their [[AsyncEvaluation]] fields transition to true is significant. (See 16.2.1.5.3.4.)
    4. If module.[[PendingAsyncDependencies]] = 0, perform ExecuteAsyncModule(module).
  13. Otherwise, perform ? module.ExecuteModule().
  14. Assert: module occurs exactly once in stack.
  15. Assert: module.[[DFSAncestorIndex]] ≤ module.[[DFSIndex]].
  16. If module.[[DFSAncestorIndex]] = module.[[DFSIndex]], then
    1. Let done be false.
    2. Repeat, while done is false,
      1. Let requiredModule be the last element of stack.
      2. Remove the last element of stack.
      3. Assert: requiredModule is a Cyclic Module Record.
      4. If requiredModule.[[AsyncEvaluation]] is false, set requiredModule.[[Status]] to evaluated.
      5. Otherwise, set requiredModule.[[Status]] to evaluating-async.
      6. If requiredModule and module are the same Module Record, set done to true.
      7. Set requiredModule.[[CycleRoot]] to module.
  17. Return index.
 Note 1

A module is evaluating while it is being traversed by InnerModuleEvaluation. A module is evaluated on execution completion or evaluating-async during execution if its [[HasTLA]] field is true or if it has asynchronous dependencies.

 Note 2

Any modules depending on a module of an asynchronous cycle when that cycle is not evaluating will instead depend on the execution of the root of the cycle via [[CycleRoot]]. This ensures that the cycle state can be treated as a single strongly connected component through its root module state.

16.2.1.5.3.2 ExecuteAsyncModule ( module )

The abstract operation ExecuteAsyncModule takes argument module (a Cyclic Module Record) and returns unused. It performs the following steps when called:

  1. Assert: module.[[Status]] is either evaluating or evaluating-async.
  2. Assert: module.[[HasTLA]] is true.
  3. Let capability be ! NewPromiseCapability(%Promise%).
  4. Let fulfilledClosure be a new Abstract Closure with no parameters that captures module and performs the following steps when called:
    1. Perform AsyncModuleExecutionFulfilled(module).
    2. Return undefined.
  5. Let onFulfilled be CreateBuiltinFunction(fulfilledClosure, 0, "", « »).
  6. Let rejectedClosure be a new Abstract Closure with parameters (error) that captures module and performs the following steps when called:
    1. Perform AsyncModuleExecutionRejected(module, error).
    2. Return undefined.
  7. Let onRejected be CreateBuiltinFunction(rejectedClosure, 0, "", « »).
  8. Perform PerformPromiseThen(capability.[[Promise]], onFulfilled, onRejected).
  9. Perform ! module.ExecuteModule(capability).
  10. Return unused.

16.2.1.5.3.3 GatherAvailableAncestors ( module, execList )

The abstract operation GatherAvailableAncestors takes arguments module (a Cyclic Module Record) and execList (a List of Cyclic Module Records) and returns unused. It performs the following steps when called:

  1. For each Cyclic Module Record m of module.[[AsyncParentModules]], do
    1. If execList does not contain m and m.[[CycleRoot]].[[EvaluationError]] is empty, then
      1. Assert: m.[[Status]] is evaluating-async.
      2. Assert: m.[[EvaluationError]] is empty.
      3. Assert: m.[[AsyncEvaluation]] is true.
      4. Assert: m.[[PendingAsyncDependencies]] > 0.
      5. Set m.[[PendingAsyncDependencies]] to m.[[PendingAsyncDependencies]] - 1.
      6. If m.[[PendingAsyncDependencies]] = 0, then
        1. Append m to execList.
        2. If m.[[HasTLA]] is false, perform GatherAvailableAncestors(m, execList).
  2. Return unused.
 Note

When an asynchronous execution for a root module is fulfilled, this function determines the list of modules which are able to synchronously execute together on this completion, populating them in execList.

16.2.1.5.3.4 AsyncModuleExecutionFulfilled ( module )

The abstract operation AsyncModuleExecutionFulfilled takes argument module (a Cyclic Module Record) and returns unused. It performs the following steps when called:

  1. If module.[[Status]] is evaluated, then
    1. Assert: module.[[EvaluationError]] is not empty.
    2. Return unused.
  2. Assert: module.[[Status]] is evaluating-async.
  3. Assert: module.[[AsyncEvaluation]] is true.
  4. Assert: module.[[EvaluationError]] is empty.
  5. Set module.[[AsyncEvaluation]] to false.
  6. Set module.[[Status]] to evaluated.
  7. If module.[[TopLevelCapability]] is not empty, then
    1. Assert: module.[[CycleRoot]] is module.
    2. Perform ! Call(module.[[TopLevelCapability]].[[Resolve]], undefined, « undefined »).
  8. Let execList be a new empty List.
  9. Perform GatherAvailableAncestors(module, execList).
  10. Let sortedExecList be a List whose elements are the elements of execList, in the order in which they had their [[AsyncEvaluation]] fields set to true in InnerModuleEvaluation.
  11. Assert: All elements of sortedExecList have their [[AsyncEvaluation]] field set to true, [[PendingAsyncDependencies]] field set to 0, and [[EvaluationError]] field set to empty.
  12. For each Cyclic Module Record m of sortedExecList, do
    1. If m.[[Status]] is evaluated, then
      1. Assert: m.[[EvaluationError]] is not empty.
    2. Else if m.[[HasTLA]] is true, then
      1. Perform ExecuteAsyncModule(m).
    3. Else,
      1. Let result be m.ExecuteModule().
      2. If result is an abrupt completion, then
        1. Perform AsyncModuleExecutionRejected(m, result.[[Value]]).
      3. Else,
        1. Set m.[[Status]] to evaluated.
        2. If m.[[TopLevelCapability]] is not empty, then
          1. Assert: m.[[CycleRoot]] is m.
          2. Perform ! Call(m.[[TopLevelCapability]].[[Resolve]], undefined, « undefined »).
  13. Return unused.

16.2.1.5.3.5 AsyncModuleExecutionRejected ( module, error )

The abstract operation AsyncModuleExecutionRejected takes arguments module (a Cyclic Module Record) and error (an ECMAScript language value) and returns unused. It performs the following steps when called:

  1. If module.[[Status]] is evaluated, then
    1. Assert: module.[[EvaluationError]] is not empty.
    2. Return unused.
  2. Assert: module.[[Status]] is evaluating-async.
  3. Assert: module.[[AsyncEvaluation]] is true.
  4. Assert: module.[[EvaluationError]] is empty.
  5. Set module.[[EvaluationError]] to ThrowCompletion(error).
  6. Set module.[[Status]] to evaluated.
  7. For each Cyclic Module Record m of module.[[AsyncParentModules]], do
    1. Perform AsyncModuleExecutionRejected(m, error).
  8. If module.[[TopLevelCapability]] is not empty, then
    1. Assert: module.[[CycleRoot]] is module.
    2. Perform ! Call(module.[[TopLevelCapability]].[[Reject]], undefined, « error »).
  9. Return unused.

16.2.1.5.4 Example Cyclic Module Record Graphs

This non-normative section gives a series of examples of the linking and evaluation of a few common module graphs, with a specific focus on how errors can occur.

First consider the following simple module graph:

Figure 2: A simple module graph
A module graph in which module A depends on module B, and module B depends on module C

Let's first assume that there are no error conditions. When a host first calls A.LoadRequestedModules(), this will complete successfully by assumption, and recursively load the dependencies of B and C as well (respectively, C and none), and then set A.[[Status]] = B.[[Status]] = C.[[Status]] = unlinked. Then, when the host calls A.Link(), it will complete successfully (again by assumption) such that A.[[Status]] = B.[[Status]] = C.[[Status]] = linked. These preparatory steps can be performed at any time. Later, when the host is ready to incur any possible side effects of the modules, it can call A.Evaluate(), which will complete successfully, returning a Promise resolving to undefined (again by assumption), recursively having evaluated first C and then B. Each module's [[Status]] at this point will be evaluated.

Consider then cases involving linking errors, after a successful call to A.LoadRequestedModules(). If InnerModuleLinking of C succeeds but, thereafter, fails for B, for example because it imports something that C does not provide, then the original A.Link() will fail, and both A and B's [[Status]] remain unlinked. C's [[Status]] has become linked, though.

Finally, consider a case involving evaluation errors after a successful call to Link(). If InnerModuleEvaluation of C succeeds but, thereafter, fails for B, for example because B contains code that throws an exception, then the original A.Evaluate() will fail, returning a rejected Promise. The resulting exception will be recorded in both A and B's [[EvaluationError]] fields, and their [[Status]] will become evaluated. C will also become evaluated but, in contrast to A and B, will remain without an [[EvaluationError]], as it successfully completed evaluation. Storing the exception ensures that any time a host tries to reuse A or B by calling their Evaluate() method, it will encounter the same exception. (Hosts are not required to reuse Cyclic Module Records; similarly, hosts are not required to expose the exception objects thrown by these methods. However, the specification enables such uses.)

Now consider a different type of error condition:

Figure 3: A module graph with an unresolvable module
A module graph in which module A depends on a missing (unresolvable) module, represented by ???

In this scenario, module A declares a dependency on some other module, but no Module Record exists for that module, i.e. HostLoadImportedModule calls FinishLoadingImportedModule with an exception when asked for it. This could occur for a variety of reasons, such as the corresponding resource not existing, or the resource existing but ParseModule returning some errors when trying to parse the resulting source text. Hosts can choose to expose the cause of failure via the completion they pass to FinishLoadingImportedModule. In any case, this exception causes a loading failure, which results in A's [[Status]] remaining new.

The difference here between loading, linking and evaluation errors is due to the following characteristic:

  • Evaluation must be only performed once, as it can cause side effects; it is thus important to remember whether evaluation has already been performed, even if unsuccessfully. (In the error case, it makes sense to also remember the exception because otherwise subsequent Evaluate() calls would have to synthesize a new one.)
  • Linking, on the other hand, is side-effect-free, and thus even if it fails, it can be retried at a later time with no issues.
  • Loading closely interacts with the host, and it may be desiderable for some of them to allow users to retry failed loads (for example, if the failure is caused by temporarily bad network conditions).

Now, consider a module graph with a cycle:

Figure 4: A cyclic module graph
A module graph in which module A depends on module B and C, but module B also depends on module A

Here we assume that the entry point is module A, so that the host proceeds by calling A.LoadRequestedModules(), which performs InnerModuleLoading on A. This in turn calls InnerModuleLoading on B and C. Because of the cycle, this again triggers InnerModuleLoading on A, but at this point it is a no-op since A's dependencies loading has already been triggered during this LoadRequestedModules process. When all the modules in the graph have been successfully loaded, their [[Status]] transitions from new to unlinked at the same time.

Then the host proceeds by calling A.Link(), which performs InnerModuleLinking on A. This in turn calls InnerModuleLinking on B. Because of the cycle, this again triggers InnerModuleLinking on A, but at this point it is a no-op since A.[[Status]] is already linking. B.[[Status]] itself remains linking when control gets back to A and InnerModuleLinking is triggered on C. After this returns with C.[[Status]] being linked, both A and B transition from linking to linked together; this is by design, since they form a strongly connected component. It's possible to transition the status of modules in the same SCC at the same time because during this phase the module graph is traversed with a depth-first search.

An analogous story occurs for the evaluation phase of a cyclic module graph, in the success case.

Now consider a case where A has a linking error; for example, it tries to import a binding from C that does not exist. In that case, the above steps still occur, including the early return from the second call to InnerModuleLinking on A. However, once we unwind back to the original InnerModuleLinking on A, it fails during InitializeEnvironment, namely right after C.ResolveExport(). The thrown SyntaxError exception propagates up to A.Link, which resets all modules that are currently on its stack (these are always exactly the modules that are still linking). Hence both A and B become unlinked. Note that C is left as linked.

Alternatively, consider a case where A has an evaluation error; for example, its source code throws an exception. In that case, the evaluation-time analog of the above steps still occurs, including the early return from the second call to InnerModuleEvaluation on A. However, once we unwind back to the original InnerModuleEvaluation on A, it fails by assumption. The exception thrown propagates up to A.Evaluate(), which records the error in all modules that are currently on its stack (i.e., the modules that are still evaluating) as well as via [[AsyncParentModules]], which form a chain for modules which contain or depend on top-level await through the whole dependency graph through the AsyncModuleExecutionRejected algorithm. Hence both A and B become evaluated and the exception is recorded in both A and B's [[EvaluationError]] fields, while C is left as evaluated with no [[EvaluationError]].

Lastly, consider a module graph with a cycle, where all modules complete asynchronously:

Figure 5: An asynchronous cyclic module graph
A module graph in which module A depends on module B and C, module B depends on module D, module C depends on module D and E, and module D depends on module A

Loading and linking happen as before, and all modules end up with [[Status]] set to linked.

Calling A.Evaluate() calls InnerModuleEvaluation on A, B, and D, which all transition to evaluating. Then InnerModuleEvaluation is called on A again, which is a no-op because it is already evaluating. At this point, D.[[PendingAsyncDependencies]] is 0, so ExecuteAsyncModule(D) is called and we call D.ExecuteModule with a new PromiseCapability tracking the asynchronous execution of D. We unwind back to the InnerModuleEvaluation on B, setting B.[[PendingAsyncDependencies]] to 1 and B.[[AsyncEvaluation]] to true. We unwind back to the original InnerModuleEvaluation on A, setting A.[[PendingAsyncDependencies]] to 1. In the next iteration of the loop over A's dependencies, we call InnerModuleEvaluation on C and thus on D (again a no-op) and E. As E has no dependencies and is not part of a cycle, we call ExecuteAsyncModule(E) in the same manner as D and E is immediately removed from the stack. We unwind once more to the original InnerModuleEvaluation on A, setting C.[[AsyncEvaluation]] to true. Now we finish the loop over A's dependencies, set A.[[AsyncEvaluation]] to true, and remove the entire strongly connected component from the stack, transitioning all of the modules to evaluating-async at once. At this point, the fields of the modules are as given in Table 45.

Table 45: Module fields after the initial Evaluate() call
Module [[DFSIndex]] [[DFSAncestorIndex]] [[Status]] [[AsyncEvaluation]] [[AsyncParentModules]] [[PendingAsyncDependencies]]
A 0 0 evaluating-async true « » 2 (B and C)
B 1 0 evaluating-async true « A » 1 (D)
C 2 0 evaluating-async true « A » 2 (D and E)
D 3 0 evaluating-async true « B, C » 0
E 4 4 evaluating-async true « C » 0

Let us assume that E finishes executing first. When that happens, AsyncModuleExecutionFulfilled is called, E.[[Status]] is set to evaluated and C.[[PendingAsyncDependencies]] is decremented to become 1. The fields of the updated modules are as given in Table 46.

Table 46: Module fields after module E finishes executing
Module [[DFSIndex]] [[DFSAncestorIndex]] [[Status]] [[AsyncEvaluation]] [[AsyncParentModules]] [[PendingAsyncDependencies]]
C 2 0 evaluating-async true « A » 1 (D)
E 4 4 evaluated true « C » 0

D is next to finish (as it was the only module that was still executing). When that happens, AsyncModuleExecutionFulfilled is called again and D.[[Status]] is set to evaluated. Then B.[[PendingAsyncDependencies]] is decremented to become 0, ExecuteAsyncModule is called on B, and it starts executing. C.[[PendingAsyncDependencies]] is also decremented to become 0, and C starts executing (potentially in parallel to B if B contains an await). The fields of the updated modules are as given in Table 47.

Table 47: Module fields after module D finishes executing
Module [[DFSIndex]] [[DFSAncestorIndex]] [[Status]] [[AsyncEvaluation]] [[AsyncParentModules]] [[PendingAsyncDependencies]]
B 1 0 evaluating-async true « A » 0
C 2 0 evaluating-async true « A » 0
D 3 0 evaluated true « B, C » 0

Let us assume that C finishes executing next. When that happens, AsyncModuleExecutionFulfilled is called again, C.[[Status]] is set to evaluated and A.[[PendingAsyncDependencies]] is decremented to become 1. The fields of the updated modules are as given in Table 48.

Table 48: Module fields after module C finishes executing
Module [[DFSIndex]] [[DFSAncestorIndex]] [[Status]] [[AsyncEvaluation]] [[AsyncParentModules]] [[PendingAsyncDependencies]]
A 0 0 evaluating-async true « » 1 (B)
C 2 0 evaluated true « A » 0

Then, B finishes executing. When that happens, AsyncModuleExecutionFulfilled is called again and B.[[Status]] is set to evaluated. A.[[PendingAsyncDependencies]] is decremented to become 0, so ExecuteAsyncModule is called and it starts executing. The fields of the updated modules are as given in Table 49.

Table 49: Module fields after module B finishes executing
Module [[DFSIndex]] [[DFSAncestorIndex]] [[Status]] [[AsyncEvaluation]] [[AsyncParentModules]] [[PendingAsyncDependencies]]
A 0 0 evaluating-async true « » 0
B 1 0 evaluated true « A » 0

Finally, A finishes executing. When that happens, AsyncModuleExecutionFulfilled is called again and A.[[Status]] is set to evaluated. At this point, the Promise in A.[[TopLevelCapability]] (which was returned from A.Evaluate()) is resolved, and this concludes the handling of this module graph. The fields of the updated module are as given in Table 50.

Table 50: Module fields after module A finishes executing
Module [[DFSIndex]] [[DFSAncestorIndex]] [[Status]] [[AsyncEvaluation]] [[AsyncParentModules]] [[PendingAsyncDependencies]]
A 0 0 evaluated true « » 0

Alternatively, consider a failure case where C fails execution and returns an error before B has finished executing. When that happens, AsyncModuleExecutionRejected is called, which sets C.[[Status]] to evaluated and C.[[EvaluationError]] to the error. It then propagates this error to all of the AsyncParentModules by performing AsyncModuleExecutionRejected on each of them. The fields of the updated modules are as given in Table 51.

Table 51: Module fields after module C finishes with an error
Module [[DFSIndex]] [[DFSAncestorIndex]] [[Status]] [[AsyncEvaluation]] [[AsyncParentModules]] [[PendingAsyncDependencies]] [[EvaluationError]]
A 0 0 evaluated true « » 1 (B) empty
C 2 1 evaluated true « A » 0 C's evaluation error

A will be rejected with the same error as C since C will call AsyncModuleExecutionRejected on A with C's error. A.[[Status]] is set to evaluated. At this point the Promise in A.[[TopLevelCapability]] (which was returned from A.Evaluate()) is rejected. The fields of the updated module are as given in Table 52.

Table 52: Module fields after module A is rejected
Module [[DFSIndex]] [[DFSAncestorIndex]] [[Status]] [[AsyncEvaluation]] [[AsyncParentModules]] [[PendingAsyncDependencies]] [[EvaluationError]]
A 0 0 evaluated true « » 0 C's Evaluation Error

Then, B finishes executing without an error. When that happens, AsyncModuleExecutionFulfilled is called again and B.[[Status]] is set to evaluated. GatherAvailableAncestors is called on B. However, A.[[CycleRoot]] is A which has an evaluation error, so it will not be added to the returned sortedExecList and AsyncModuleExecutionFulfilled will return without further processing. Any future importer of B will resolve the rejection of B.[[CycleRoot]].[[EvaluationError]] from the evaluation error from C that was set on the cycle root A. The fields of the updated modules are as given in Table 53.

Table 53: Module fields after module B finishes executing in an erroring graph
Module [[DFSIndex]] [[DFSAncestorIndex]] [[Status]] [[AsyncEvaluation]] [[AsyncParentModules]] [[PendingAsyncDependencies]] [[EvaluationError]]
A 0 0 evaluated true « » 0 C's Evaluation Error
B 1 0 evaluated true « A » 0 empty

16.2.1.6 Source Text Module Records

A Source Text Module Record is used to represent information about a module that was defined from ECMAScript source text (11) that was parsed using the goal symbol Module. Its fields contain digested information about the names that are imported by the module and its concrete methods use this digest to link, link, and evaluate the module.

A Source Text Module Record can exist in a module graph with other subclasses of the abstract Module Record type, and can participate in cycles with other subclasses of the Cyclic Module Record type.

In addition to the fields defined in Table 42, Source Text Module Records have the additional fields listed in Table 54. Each of these fields is initially set in ParseModule.

Table 54: Additional Fields of Source Text Module Records
Field Name Value Type Meaning
[[ECMAScriptCode]] a Parse Node The result of parsing the source text of this module using Module as the goal symbol.
[[Context]] an ECMAScript code execution context or empty The execution context associated with this module. It is empty until the module's environment has been initialized.
[[ImportMeta]] an Object or empty An object exposed through the import.meta meta property. It is empty until it is accessed by ECMAScript code.
[[ImportEntries]] a List of ImportEntry Records A List of ImportEntry records derived from the code of this module.
[[LocalExportEntries]] a List of ExportEntry Records A List of ExportEntry records derived from the code of this module that correspond to declarations that occur within the module.
[[IndirectExportEntries]] a List of ExportEntry Records A List of ExportEntry records derived from the code of this module that correspond to reexported imports that occur within the module or exports from export * as namespace declarations.
[[StarExportEntries]] a List of ExportEntry Records A List of ExportEntry records derived from the code of this module that correspond to export * declarations that occur within the module, not including export * as namespace declarations.

An ImportEntry Record is a Record that digests information about a single declarative import. Each ImportEntry Record has the fields defined in Table 55:

Table 55: ImportEntry Record Fields
Field Name Value Type Meaning
[[ModuleRequest]] a String String value of the ModuleSpecifier of the ImportDeclaration.
[[ImportName]] a String or namespace-object The name under which the desired binding is exported by the module identified by [[ModuleRequest]]. The value namespace-object indicates that the import request is for the target module's namespace object.
[[LocalName]] a String The name that is used to locally access the imported value from within the importing module.
Note 1

Table 56 gives examples of ImportEntry records fields used to represent the syntactic import forms:

Table 56 (Informative): Import Forms Mappings to ImportEntry Records
Import Statement Form [[ModuleRequest]] [[ImportName]] [[LocalName]]
import v from "mod"; "mod" "default" "v"
import * as ns from "mod"; "mod" namespace-object "ns"
import {x} from "mod"; "mod" "x" "x"
import {x as v} from "mod"; "mod" "x" "v"
import "mod"; An ImportEntry Record is not created.

An ExportEntry Record is a Record that digests information about a single declarative export. Each ExportEntry Record has the fields defined in Table 57:

Table 57: ExportEntry Record Fields
Field Name Value Type Meaning
[[ExportName]] a String or null The name used to export this binding by this module.
[[ModuleRequest]] a String or null The String value of the ModuleSpecifier of the ExportDeclaration. null if the ExportDeclaration does not have a ModuleSpecifier.
[[ImportName]] a String, null, all, or all-but-default The name under which the desired binding is exported by the module identified by [[ModuleRequest]]. null if the ExportDeclaration does not have a ModuleSpecifier. all is used for export * as ns from "mod" declarations. all-but-default is used for export * from "mod" declarations.
[[LocalName]] a String or null The name that is used to locally access the exported value from within the importing module. null if the exported value is not locally accessible from within the module.
Note 2

Table 58 gives examples of the ExportEntry record fields used to represent the syntactic export forms:

Table 58 (Informative): Export Forms Mappings to ExportEntry Records
Export Statement Form [[ExportName]] [[ModuleRequest]] [[ImportName]] [[LocalName]]
export var v; "v" null null "v"
export default function f() {} "default" null null "f"
export default function () {} "default" null null "*default*"
export default 42; "default" null null "*default*"
export {x}; "x" null null "x"
export {v as x}; "x" null null "v"
export {x} from "mod"; "x" "mod" "x" null
export {v as x} from "mod"; "x" "mod" "v" null
export * from "mod"; null "mod" all-but-default null
export * as ns from "mod"; "ns" "mod" all null

The following definitions specify the required concrete methods and other abstract operations for Source Text Module Records

16.2.1.6.1 ParseModule ( sourceText, realm, hostDefined )

The abstract operation ParseModule takes arguments sourceText (ECMAScript source text), realm (a Realm Record), and hostDefined (anything) and returns a Source Text Module Record or a non-empty List of SyntaxError objects. It creates a Source Text Module Record based upon the result of parsing sourceText as a Module. It performs the following steps when called:

  1. Let body be ParseText(sourceText, Module).
  2. If body is a List of errors, return body.
  3. Let requestedModules be the ModuleRequests of body.
  4. Let importEntries be ImportEntries of body.
  5. Let importedBoundNames be ImportedLocalNames(importEntries).
  6. Let indirectExportEntries be a new empty List.
  7. Let localExportEntries be a new empty List.
  8. Let starExportEntries be a new empty List.
  9. Let exportEntries be ExportEntries of body.
  10. For each ExportEntry Record ee of exportEntries, do
    1. If ee.[[ModuleRequest]] is null, then
      1. If importedBoundNames does not contain ee.[[LocalName]], then
        1. Append ee to localExportEntries.
      2. Else,
        1. Let ie be the element of importEntries whose [[LocalName]] is ee.[[LocalName]].
        2. If ie.[[ImportName]] is namespace-object, then
          1. NOTE: This is a re-export of an imported module namespace object.
          2. Append ee to localExportEntries.
        3. Else,
          1. NOTE: This is a re-export of a single name.
          2. Append the ExportEntry Record { [[ModuleRequest]]: ie.[[ModuleRequest]], [[ImportName]]: ie.[[ImportName]], [[LocalName]]: null, [[ExportName]]: ee.[[ExportName]] } to indirectExportEntries.
    2. Else if ee.[[ImportName]] is all-but-default, then
      1. Assert: ee.[[ExportName]] is null.
      2. Append ee to starExportEntries.
    3. Else,
      1. Append ee to indirectExportEntries.
  11. Let async be body Contains await.
  12. Return Source Text Module Record { [[Realm]]: realm, [[Environment]]: empty, [[Namespace]]: empty, [[CycleRoot]]: empty, [[HasTLA]]: async, [[AsyncEvaluation]]: false, [[TopLevelCapability]]: empty, [[AsyncParentModules]]: « », [[PendingAsyncDependencies]]: empty, [[Status]]: new, [[EvaluationError]]: empty, [[HostDefined]]: hostDefined, [[ECMAScriptCode]]: body, [[Context]]: empty, [[ImportMeta]]: empty, [[RequestedModules]]: requestedModules, [[LoadedModules]]: « », [[ImportEntries]]: importEntries, [[LocalExportEntries]]: localExportEntries, [[IndirectExportEntries]]: indirectExportEntries, [[StarExportEntries]]: starExportEntries, [[DFSIndex]]: empty, [[DFSAncestorIndex]]: empty }.
 Note

An implementation may parse module source text and analyse it for Early Error conditions prior to the evaluation of ParseModule for that module source text. However, the reporting of any errors must be deferred until the point where this specification actually performs ParseModule upon that source text.

16.2.1.6.2 GetExportedNames ( [ exportStarSet ] )

The GetExportedNames concrete method of a Source Text Module Record module takes optional argument exportStarSet (a List of Source Text Module Records) and returns a List of either Strings or null. It performs the following steps when called:

  1. Assert: module.[[Status]] is not new.
  2. If exportStarSet is not present, set exportStarSet to a new empty List.
  3. If exportStarSet contains module, then
    1. Assert: We've reached the starting point of an export * circularity.
    2. Return a new empty List.
  4. Append module to exportStarSet.
  5. Let exportedNames be a new empty List.
  6. For each ExportEntry Record e of module.[[LocalExportEntries]], do
    1. Assert: module provides the direct binding for this export.
    2. Append e.[[ExportName]] to exportedNames.
  7. For each ExportEntry Record e of module.[[IndirectExportEntries]], do
    1. Assert: module imports a specific binding for this export.
    2. Append e.[[ExportName]] to exportedNames.
  8. For each ExportEntry Record e of module.[[StarExportEntries]], do
    1. Let requestedModule be GetImportedModule(module, e.[[ModuleRequest]]).
    2. Let starNames be requestedModule.GetExportedNames(exportStarSet).
    3. For each element n of starNames, do
      1. If SameValue(n, "default") is false, then
        1. If exportedNames does not contain n, then
          1. Append n to exportedNames.
  9. Return exportedNames.
 Note

GetExportedNames does not filter out or throw an exception for names that have ambiguous star export bindings.

16.2.1.6.3 ResolveExport ( exportName [ , resolveSet ] )

The ResolveExport concrete method of a Source Text Module Record module takes argument exportName (a String) and optional argument resolveSet (a List of Records with fields [[Module]] (a Module Record) and [[ExportName]] (a String)) and returns a ResolvedBinding Record, null, or ambiguous.

ResolveExport attempts to resolve an imported binding to the actual defining module and local binding name. The defining module may be the module represented by the Module Record this method was invoked on or some other module that is imported by that module. The parameter resolveSet is used to detect unresolved circular import/export paths. If a pair consisting of specific Module Record and exportName is reached that is already in resolveSet, an import circularity has been encountered. Before recursively calling ResolveExport, a pair consisting of module and exportName is added to resolveSet.

If a defining module is found, a ResolvedBinding Record { [[Module]], [[BindingName]] } is returned. This record identifies the resolved binding of the originally requested export, unless this is the export of a namespace with no local binding. In this case, [[BindingName]] will be set to namespace. If no definition was found or the request is found to be circular, null is returned. If the request is found to be ambiguous, ambiguous is returned.

It performs the following steps when called:

  1. Assert: module.[[Status]] is not new.
  2. If resolveSet is not present, set resolveSet to a new empty List.
  3. For each Record { [[Module]], [[ExportName]] } r of resolveSet, do
    1. If module and r.[[Module]] are the same Module Record and SameValue(exportName, r.[[ExportName]]) is true, then
      1. Assert: This is a circular import request.
      2. Return null.
  4. Append the Record { [[Module]]: module, [[ExportName]]: exportName } to resolveSet.
  5. For each ExportEntry Record e of module.[[LocalExportEntries]], do
    1. If SameValue(exportName, e.[[ExportName]]) is true, then
      1. Assert: module provides the direct binding for this export.
      2. Return ResolvedBinding Record { [[Module]]: module, [[BindingName]]: e.[[LocalName]] }.
  6. For each ExportEntry Record e of module.[[IndirectExportEntries]], do
    1. If SameValue(exportName, e.[[ExportName]]) is true, then
      1. Let importedModule be GetImportedModule(module, e.[[ModuleRequest]]).
      2. If e.[[ImportName]] is all, then
        1. Assert: module does not provide the direct binding for this export.
        2. Return ResolvedBinding Record { [[Module]]: importedModule, [[BindingName]]: namespace }.
      3. Else,
        1. Assert: module imports a specific binding for this export.
        2. Return importedModule.ResolveExport(e.[[ImportName]], resolveSet).
  7. If SameValue(exportName, "default") is true, then
    1. Assert: A default export was not explicitly defined by this module.
    2. Return null.
    3. NOTE: A default export cannot be provided by an export * from "mod" declaration.
  8. Let starResolution be null.
  9. For each ExportEntry Record e of module.[[StarExportEntries]], do
    1. Let importedModule be GetImportedModule(module, e.[[ModuleRequest]]).
    2. Let resolution be importedModule.ResolveExport(exportName, resolveSet).
    3. If resolution is ambiguous, return ambiguous.
    4. If resolution is not null, then
      1. Assert: resolution is a ResolvedBinding Record.
      2. If starResolution is null, set starResolution to resolution.
      3. Else,
        1. Assert: There is more than one * import that includes the requested name.
        2. If resolution.[[Module]] and starResolution.[[Module]] are not the same Module Record, return ambiguous.
        3. If resolution.[[BindingName]] is not starResolution.[[BindingName]] and either resolution.[[BindingName]] or starResolution.[[BindingName]] is namespace, return ambiguous.
        4. If resolution.[[BindingName]] is a String, starResolution.[[BindingName]] is a String, and SameValue(resolution.[[BindingName]], starResolution.[[BindingName]]) is false, return ambiguous.
  10. Return starResolution.

16.2.1.6.4 InitializeEnvironment ( )

The InitializeEnvironment concrete method of a Source Text Module Record module takes no arguments and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

  1. For each ExportEntry Record e of module.[[IndirectExportEntries]], do
    1. Let resolution be module.ResolveExport(e.[[ExportName]]).
    2. If resolution is either null or ambiguous, throw a SyntaxError exception.
    3. Assert: resolution is a ResolvedBinding Record.
  2. Assert: All named exports from module are resolvable.
  3. Let realm be module.[[Realm]].
  4. Assert: realm is not undefined.
  5. Let env be NewModuleEnvironment(realm.[[GlobalEnv]]).
  6. Set module.[[Environment]] to env.
  7. For each ImportEntry Record in of module.[[ImportEntries]], do
    1. Let importedModule be GetImportedModule(module, in.[[ModuleRequest]]).
    2. If in.[[ImportName]] is namespace-object, then
      1. Let namespace be GetModuleNamespace(importedModule).
      2. Perform ! env.CreateImmutableBinding(in.[[LocalName]], true).
      3. Perform ! env.InitializeBinding(in.[[LocalName]], namespace).
    3. Else,
      1. Let resolution be importedModule.ResolveExport(in.[[ImportName]]).
      2. If resolution is either null or ambiguous, throw a SyntaxError exception.
      3. If resolution.[[BindingName]] is namespace, then
        1. Let namespace be GetModuleNamespace(resolution.[[Module]]).
        2. Perform ! env.CreateImmutableBinding(in.[[LocalName]], true).
        3. Perform ! env.InitializeBinding(in.[[LocalName]], namespace).
      4. Else,
        1. Perform env.CreateImportBinding(in.[[LocalName]], resolution.[[Module]], resolution.[[BindingName]]).
  8. Let moduleContext be a new ECMAScript code execution context.
  9. Set the Function of moduleContext to null.
  10. Assert: module.[[Realm]] is not undefined.
  11. Set the Realm of moduleContext to module.[[Realm]].
  12. Set the ScriptOrModule of moduleContext to module.
  13. Set the VariableEnvironment of moduleContext to module.[[Environment]].
  14. Set the LexicalEnvironment of moduleContext to module.[[Environment]].
  15. Set the PrivateEnvironment of moduleContext to null.
  16. Set module.[[Context]] to moduleContext.
  17. Push moduleContext onto the execution context stack; moduleContext is now the running execution context.
  18. Let code be module.[[ECMAScriptCode]].
  19. Let varDeclarations be the VarScopedDeclarations of code.
  20. Let declaredVarNames be a new empty List.
  21. For each element d of varDeclarations, do
    1. For each element dn of the BoundNames of d, do
      1. If declaredVarNames does not contain dn, then
        1. Perform ! env.CreateMutableBinding(dn, false).
        2. Perform ! env.InitializeBinding(dn, undefined).
        3. Append dn to declaredVarNames.
  22. Let lexDeclarations be the LexicallyScopedDeclarations of code.
  23. Let privateEnv be null.
  24. For each element d of lexDeclarations, do
    1. For each element dn of the BoundNames of d, do
      1. If IsConstantDeclaration of d is true, then
        1. Perform ! env.CreateImmutableBinding(dn, true).
      2. Else,
        1. Perform ! env.CreateMutableBinding(dn, false).
      3. If d is either a FunctionDeclaration, a GeneratorDeclaration, an AsyncFunctionDeclaration, or an AsyncGeneratorDeclaration, then
        1. Let fo be InstantiateFunctionObject of d with arguments env and privateEnv.
        2. Perform ! env.InitializeBinding(dn, fo).
  25. Remove moduleContext from the execution context stack.
  26. Return unused.

16.2.1.6.5 ExecuteModule ( [ capability ] )

The ExecuteModule concrete method of a Source Text Module Record module takes optional argument capability (a PromiseCapability Record) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

  1. Let moduleContext be a new ECMAScript code execution context.
  2. Set the Function of moduleContext to null.
  3. Set the Realm of moduleContext to module.[[Realm]].
  4. Set the ScriptOrModule of moduleContext to module.
  5. Assert: module has been linked and declarations in its module environment have been instantiated.
  6. Set the VariableEnvironment of moduleContext to module.[[Environment]].
  7. Set the LexicalEnvironment of moduleContext to module.[[Environment]].
  8. Suspend the running execution context.
  9. If module.[[HasTLA]] is false, then
    1. Assert: capability is not present.
    2. Push moduleContext onto the execution context stack; moduleContext is now the running execution context.
    3. Let result be Completion(Evaluation of module.[[ECMAScriptCode]]).
    4. Suspend moduleContext and remove it from the execution context stack.
    5. Resume the context that is now on the top of the execution context stack as the running execution context.
    6. If result is an abrupt completion, then
      1. Return ? result.
  10. Else,
    1. Assert: capability is a PromiseCapability Record.
    2. Perform AsyncBlockStart(capability, module.[[ECMAScriptCode]], moduleContext).
  11. Return unused.

16.2.1.7 GetImportedModule ( referrer, specifier )

The abstract operation GetImportedModule takes arguments referrer (a Cyclic Module Record) and specifier (a String) and returns a Module Record. It performs the following steps when called:

  1. Assert: Exactly one element of referrer.[[LoadedModules]] is a Record whose [[Specifier]] is specifier, since LoadRequestedModules has completed successfully on referrer prior to invoking this abstract operation.
  2. Let record be the Record in referrer.[[LoadedModules]] whose [[Specifier]] is specifier.
  3. Return record.[[Module]].

16.2.1.8 HostLoadImportedModule ( referrer, specifier, hostDefined, payload )

The host-defined abstract operation HostLoadImportedModule takes arguments referrer (a Script Record, a Cyclic Module Record, or a Realm Record), specifier (a String), hostDefined (anything), and payload (a GraphLoadingState Record or a PromiseCapability Record) and returns unused.

Note

An example of when referrer can be a Realm Record is in a web browser host. There, if a user clicks on a control given by

<button type="button" onclick="import('./foo.mjs')">Click me</button>

there will be no active script or module at the time the import() expression runs. More generally, this can happen in any situation where the host pushes execution contexts with null ScriptOrModule components onto the execution context stack.

An implementation of HostLoadImportedModule must conform to the following requirements:

The actual process performed is host-defined, but typically consists of performing whatever I/O operations are necessary to load the appropriate Module Record. Multiple different (referrer, specifier) pairs may map to the same Module Record instance. The actual mapping semantics is host-defined but typically a normalization process is applied to specifier as part of the mapping process. A typical normalization process would include actions such as expansion of relative and abbreviated path specifiers.

16.2.1.9 FinishLoadingImportedModule ( referrer, specifier, payload, result )

The abstract operation FinishLoadingImportedModule takes arguments referrer (a Script Record, a Cyclic Module Record, or a Realm Record), specifier (a String), payload (a GraphLoadingState Record or a PromiseCapability Record), and result (either a normal completion containing a Module Record or a throw completion) and returns unused. It performs the following steps when called:

  1. If result is a normal completion, then
    1. If referrer.[[LoadedModules]] contains a Record whose [[Specifier]] is specifier, then
      1. Assert: That Record's [[Module]] is result.[[Value]].
    2. Else, append the Record { [[Specifier]]: specifier, [[Module]]: result.[[Value]] } to referrer.[[LoadedModules]].
  2. If payload is a GraphLoadingState Record, then
    1. Perform ContinueModuleLoading(payload, result).
  3. Else,
    1. Perform ContinueDynamicImport(payload, result).
  4. Return unused.

16.2.1.10 GetModuleNamespace ( module )

The abstract operation GetModuleNamespace takes argument module (an instance of a concrete subclass of Module Record) and returns a Module Namespace Object or empty. It retrieves the Module Namespace Object representing module's exports, lazily creating it the first time it was requested, and storing it in module.[[Namespace]] for future retrieval. It performs the following steps when called:

  1. Assert: If module is a Cyclic Module Record, then module.[[Status]] is not new or unlinked.
  2. Let namespace be module.[[Namespace]].
  3. If namespace is empty, then
    1. Let exportedNames be module.GetExportedNames().
    2. Let unambiguousNames be a new empty List.
    3. For each element name of exportedNames, do
      1. Let resolution be module.ResolveExport(name).
      2. If resolution is a ResolvedBinding Record, append name to unambiguousNames.
    4. Set namespace to ModuleNamespaceCreate(module, unambiguousNames).
  4. Return namespace.
 Note

GetModuleNamespace never throws. Instead, unresolvable names are simply excluded from the namespace at this point. They will lead to a real linking error later unless they are all ambiguous star exports that are not explicitly requested anywhere.

16.2.1.11 Runtime Semantics: Evaluation

Module : [empty]
  1. Return undefined.
 ModuleBody : ModuleItemList
  1. Let result be Completion(Evaluation of ModuleItemList).
  2. If result.[[Type]] is normal and result.[[Value]] is empty, then
    1. Return undefined.
  3. Return ? result.
 ModuleItemList : ModuleItemList ModuleItem
  1. Let sl be ? Evaluation of ModuleItemList.
  2. Let s be Completion(Evaluation of ModuleItem).
  3. Return ? UpdateEmpty(s, sl).
 Note

The value of a ModuleItemList is the value of the last value-producing item in the ModuleItemList.

 ModuleItem : ImportDeclaration
  1. Return empty.

16.2.2 Imports

Syntax

ImportDeclaration : import ImportClause FromClause ; import ModuleSpecifier ; ImportClause : ImportedDefaultBinding NameSpaceImport NamedImports ImportedDefaultBinding , NameSpaceImport ImportedDefaultBinding , NamedImports ImportedDefaultBinding : ImportedBinding NameSpaceImport : * as ImportedBinding NamedImports : { } { ImportsList } { ImportsList , } FromClause : from ModuleSpecifier ImportsList : ImportSpecifier ImportsList , ImportSpecifier ImportSpecifier : ImportedBinding ModuleExportName as ImportedBinding ModuleSpecifier : StringLiteral ImportedBinding : BindingIdentifier[~Yield, Await]

16.2.2.1 Static Semantics: Early Errors

ModuleItem : ImportDeclaration

16.2.2.2 Static Semantics: ImportEntries

The syntax-directed operation ImportEntries takes no arguments and returns a List of ImportEntry Records. It is defined piecewise over the following productions:

Module : [empty]
  1. Return a new empty List.
 ModuleItemList : ModuleItemList ModuleItem
  1. Let entries1 be ImportEntries of ModuleItemList.
  2. Let entries2 be ImportEntries of ModuleItem.
  3. Return the list-concatenation of entries1 and entries2.
 ModuleItem : ExportDeclaration StatementListItem
  1. Return a new empty List.
 ImportDeclaration : import ImportClause FromClause ;
  1. Let module be the sole element of ModuleRequests of FromClause.
  2. Return ImportEntriesForModule of ImportClause with argument module.
 ImportDeclaration : import ModuleSpecifier ;
  1. Return a new empty List.

16.2.2.3 Static Semantics: ImportEntriesForModule

The syntax-directed operation ImportEntriesForModule takes argument module (a String) and returns a List of ImportEntry Records. It is defined piecewise over the following productions:

ImportClause : ImportedDefaultBinding , NameSpaceImport
  1. Let entries1 be ImportEntriesForModule of ImportedDefaultBinding with argument module.
  2. Let entries2 be ImportEntriesForModule of NameSpaceImport with argument module.
  3. Return the list-concatenation of entries1 and entries2.
 ImportClause : ImportedDefaultBinding , NamedImports
  1. Let entries1 be ImportEntriesForModule of ImportedDefaultBinding with argument module.
  2. Let entries2 be ImportEntriesForModule of NamedImports with argument module.
  3. Return the list-concatenation of entries1 and entries2.
 ImportedDefaultBinding : ImportedBinding
  1. Let localName be the sole element of BoundNames of ImportedBinding.
  2. Let defaultEntry be the ImportEntry Record { [[ModuleRequest]]: module, [[ImportName]]: "default", [[LocalName]]: localName }.
  3. Return « defaultEntry ».
 NameSpaceImport : * as ImportedBinding
  1. Let localName be the StringValue of ImportedBinding.
  2. Let entry be the ImportEntry Record { [[ModuleRequest]]: module, [[ImportName]]: namespace-object, [[LocalName]]: localName }.
  3. Return « entry ».
 NamedImports : { }
  1. Return a new empty List.
 ImportsList : ImportsList , ImportSpecifier
  1. Let specs1 be the ImportEntriesForModule of ImportsList with argument module.
  2. Let specs2 be the ImportEntriesForModule of ImportSpecifier with argument module.
  3. Return the list-concatenation of specs1 and specs2.
 ImportSpecifier : ImportedBinding
  1. Let localName be the sole element of BoundNames of ImportedBinding.
  2. Let entry be the ImportEntry Record { [[ModuleRequest]]: module, [[ImportName]]: localName, [[LocalName]]: localName }.
  3. Return « entry ».
 ImportSpecifier : ModuleExportName as ImportedBinding
  1. Let importName be the StringValue of ModuleExportName.
  2. Let localName be the StringValue of ImportedBinding.
  3. Let entry be the ImportEntry Record { [[ModuleRequest]]: module, [[ImportName]]: importName, [[LocalName]]: localName }.
  4. Return « entry ».

16.2.3 Exports

Syntax

ExportDeclaration : export ExportFromClause FromClause ; export NamedExports ; export VariableStatement[~Yield, Await] export Declaration[~Yield, Await] export default HoistableDeclaration[~Yield, Await, Default] export default ClassDeclaration[~Yield, Await, Default] export default [lookahead ∉ { function, async [no LineTerminator here] function, class }] AssignmentExpression[ In, ~Yield, Await] ; ExportFromClause : * * as ModuleExportName NamedExports NamedExports : { } { ExportsList } { ExportsList , } ExportsList : ExportSpecifier ExportsList , ExportSpecifier ExportSpecifier : ModuleExportName ModuleExportName as ModuleExportName

16.2.3.1 Static Semantics: Early Errors

ExportDeclaration : export NamedExports ; Note

The above rule means that each ReferencedBindings of NamedExports is treated as an IdentifierReference.

16.2.3.2 Static Semantics: ExportedBindings

The syntax-directed operation ExportedBindings takes no arguments and returns a List of Strings.

Note

ExportedBindings are the locally bound names that are explicitly associated with a Module's ExportedNames.

It is defined piecewise over the following productions:

ModuleItemList : ModuleItemList ModuleItem
  1. Let names1 be ExportedBindings of ModuleItemList.
  2. Let names2 be ExportedBindings of ModuleItem.
  3. Return the list-concatenation of names1 and names2.
 ModuleItem : ImportDeclaration StatementListItem
  1. Return a new empty List.
 ExportDeclaration : export ExportFromClause FromClause ;
  1. Return a new empty List.
 ExportDeclaration : export NamedExports ;
  1. Return the ExportedBindings of NamedExports.
 ExportDeclaration : export VariableStatement
  1. Return the BoundNames of VariableStatement.
 ExportDeclaration : export Declaration
  1. Return the BoundNames of Declaration.
 ExportDeclaration : export default HoistableDeclaration export default ClassDeclaration export default AssignmentExpression ;
  1. Return the BoundNames of this ExportDeclaration.
 NamedExports : { }
  1. Return a new empty List.
 ExportsList : ExportsList , ExportSpecifier
  1. Let names1 be the ExportedBindings of ExportsList.
  2. Let names2 be the ExportedBindings of ExportSpecifier.
  3. Return the list-concatenation of names1 and names2.
 ExportSpecifier : ModuleExportName
  1. Return a List whose sole element is the StringValue of ModuleExportName.
 ExportSpecifier : ModuleExportName as ModuleExportName
  1. Return a List whose sole element is the StringValue of the first ModuleExportName.

16.2.3.3 Static Semantics: ExportedNames

The syntax-directed operation ExportedNames takes no arguments and returns a List of Strings.

Note

ExportedNames are the externally visible names that a Module explicitly maps to one of its local name bindings.

It is defined piecewise over the following productions:

ModuleItemList : ModuleItemList ModuleItem
  1. Let names1 be ExportedNames of ModuleItemList.
  2. Let names2 be ExportedNames of ModuleItem.
  3. Return the list-concatenation of names1 and names2.
 ModuleItem : ExportDeclaration
  1. Return the ExportedNames of ExportDeclaration.
 ModuleItem : ImportDeclaration StatementListItem
  1. Return a new empty List.
 ExportDeclaration : export ExportFromClause FromClause ;
  1. Return the ExportedNames of ExportFromClause.
 ExportFromClause : *
  1. Return a new empty List.
 ExportFromClause : * as ModuleExportName
  1. Return a List whose sole element is the StringValue of ModuleExportName.
 ExportFromClause : NamedExports
  1. Return the ExportedNames of NamedExports.
 ExportDeclaration : export VariableStatement
  1. Return the BoundNames of VariableStatement.
 ExportDeclaration : export Declaration
  1. Return the BoundNames of Declaration.
 ExportDeclaration : export default HoistableDeclaration export default ClassDeclaration export default AssignmentExpression ;
  1. Return « "default" ».
 NamedExports : { }
  1. Return a new empty List.
 ExportsList : ExportsList , ExportSpecifier
  1. Let names1 be the ExportedNames of ExportsList.
  2. Let names2 be the ExportedNames of ExportSpecifier.
  3. Return the list-concatenation of names1 and names2.
 ExportSpecifier : ModuleExportName
  1. Return a List whose sole element is the StringValue of ModuleExportName.
 ExportSpecifier : ModuleExportName as ModuleExportName
  1. Return a List whose sole element is the StringValue of the second ModuleExportName.

16.2.3.4 Static Semantics: ExportEntries

The syntax-directed operation ExportEntries takes no arguments and returns a List of ExportEntry Records. It is defined piecewise over the following productions:

Module : [empty]
  1. Return a new empty List.
 ModuleItemList : ModuleItemList ModuleItem
  1. Let entries1 be ExportEntries of ModuleItemList.
  2. Let entries2 be ExportEntries of ModuleItem.
  3. Return the list-concatenation of entries1 and entries2.
 ModuleItem : ImportDeclaration StatementListItem
  1. Return a new empty List.
 ExportDeclaration : export ExportFromClause FromClause ;
  1. Let module be the sole element of ModuleRequests of FromClause.
  2. Return ExportEntriesForModule of ExportFromClause with argument module.
 ExportDeclaration : export NamedExports ;
  1. Return ExportEntriesForModule of NamedExports with argument null.
 ExportDeclaration : export VariableStatement
  1. Let entries be a new empty List.
  2. Let names be the BoundNames of VariableStatement.
  3. For each element name of names, do
    1. Append the ExportEntry Record { [[ModuleRequest]]: null, [[ImportName]]: null, [[LocalName]]: name, [[ExportName]]: name } to entries.
  4. Return entries.
 ExportDeclaration : export Declaration
  1. Let entries be a new empty List.
  2. Let names be the BoundNames of Declaration.
  3. For each element name of names, do
    1. Append the ExportEntry Record { [[ModuleRequest]]: null, [[ImportName]]: null, [[LocalName]]: name, [[ExportName]]: name } to entries.
  4. Return entries.
 ExportDeclaration : export default HoistableDeclaration
  1. Let names be BoundNames of HoistableDeclaration.
  2. Let localName be the sole element of names.
  3. Return a List whose sole element is a new ExportEntry Record { [[ModuleRequest]]: null, [[ImportName]]: null, [[LocalName]]: localName, [[ExportName]]: "default" }.
 ExportDeclaration : export default ClassDeclaration
  1. Let names be BoundNames of ClassDeclaration.
  2. Let localName be the sole element of names.
  3. Return a List whose sole element is a new ExportEntry Record { [[ModuleRequest]]: null, [[ImportName]]: null, [[LocalName]]: localName, [[ExportName]]: "default" }.
 ExportDeclaration : export default AssignmentExpression ;
  1. Let entry be the ExportEntry Record { [[ModuleRequest]]: null, [[ImportName]]: null, [[LocalName]]: "*default*", [[ExportName]]: "default" }.
  2. Return « entry ».
 Note

"*default*" is used within this specification as a synthetic name for anonymous default export values. See this note for more details.

16.2.3.5 Static Semantics: ExportEntriesForModule

The syntax-directed operation ExportEntriesForModule takes argument module (a String or null) and returns a List of ExportEntry Records. It is defined piecewise over the following productions:

ExportFromClause : *
  1. Let entry be the ExportEntry Record { [[ModuleRequest]]: module, [[ImportName]]: all-but-default, [[LocalName]]: null, [[ExportName]]: null }.
  2. Return « entry ».
 ExportFromClause : * as ModuleExportName
  1. Let exportName be the StringValue of ModuleExportName.
  2. Let entry be the ExportEntry Record { [[ModuleRequest]]: module, [[ImportName]]: all, [[LocalName]]: null, [[ExportName]]: exportName }.
  3. Return « entry ».
 NamedExports : { }
  1. Return a new empty List.
 ExportsList : ExportsList , ExportSpecifier
  1. Let specs1 be the ExportEntriesForModule of ExportsList with argument module.
  2. Let specs2 be the ExportEntriesForModule of ExportSpecifier with argument module.
  3. Return the list-concatenation of specs1 and specs2.
 ExportSpecifier : ModuleExportName
  1. Let sourceName be the StringValue of ModuleExportName.
  2. If module is null, then
    1. Let localName be sourceName.
    2. Let importName be null.
  3. Else,
    1. Let localName be null.
    2. Let importName be sourceName.
  4. Return a List whose sole element is a new ExportEntry Record { [[ModuleRequest]]: module, [[ImportName]]: importName, [[LocalName]]: localName, [[ExportName]]: sourceName }.
 ExportSpecifier : ModuleExportName as ModuleExportName
  1. Let sourceName be the StringValue of the first ModuleExportName.
  2. Let exportName be the StringValue of the second ModuleExportName.
  3. If module is null, then
    1. Let localName be sourceName.
    2. Let importName be null.
  4. Else,
    1. Let localName be null.
    2. Let importName be sourceName.
  5. Return a List whose sole element is a new ExportEntry Record { [[ModuleRequest]]: module, [[ImportName]]: importName, [[LocalName]]: localName, [[ExportName]]: exportName }.

16.2.3.6 Static Semantics: ReferencedBindings

The syntax-directed operation ReferencedBindings takes no arguments and returns a List of Parse Nodes. It is defined piecewise over the following productions:

NamedExports : { }
  1. Return a new empty List.
 ExportsList : ExportsList , ExportSpecifier
  1. Let names1 be the ReferencedBindings of ExportsList.
  2. Let names2 be the ReferencedBindings of ExportSpecifier.
  3. Return the list-concatenation of names1 and names2.
 ExportSpecifier : ModuleExportName as ModuleExportName
  1. Return the ReferencedBindings of the first ModuleExportName.
 ModuleExportName : IdentifierName
  1. Return a List whose sole element is the IdentifierName.
 ModuleExportName : StringLiteral
  1. Return a List whose sole element is the StringLiteral.

16.2.3.7 Runtime Semantics: Evaluation

ExportDeclaration : export ExportFromClause FromClause ; export NamedExports ;
  1. Return empty.
 ExportDeclaration : export VariableStatement
  1. Return ? Evaluation of VariableStatement.
 ExportDeclaration : export Declaration
  1. Return ? Evaluation of Declaration.
 ExportDeclaration : export default HoistableDeclaration
  1. Return ? Evaluation of HoistableDeclaration.
 ExportDeclaration : export default ClassDeclaration
  1. Let value be ? BindingClassDeclarationEvaluation of ClassDeclaration.
  2. Let className be the sole element of BoundNames of ClassDeclaration.
  3. If className is "*default*", then
    1. Let env be the running execution context's LexicalEnvironment.
    2. Perform ? InitializeBoundName("*default*", value, env).
  4. Return empty.
 ExportDeclaration : export default AssignmentExpression ;
  1. If IsAnonymousFunctionDefinition(AssignmentExpression) is true, then
    1. Let value be ? NamedEvaluation of AssignmentExpression with argument "default".
  2. Else,
    1. Let rhs be ? Evaluation of AssignmentExpression.
    2. Let value be ? GetValue(rhs).
  3. Let env be the running execution context's LexicalEnvironment.
  4. Perform ? InitializeBoundName("*default*", value, env).
  5. Return empty.

17 Error Handling and Language Extensions

An implementation must report most errors at the time the relevant ECMAScript language construct is evaluated. An early error is an error that can be detected and reported prior to the evaluation of any construct in the Script containing the error. The presence of an early error prevents the evaluation of the construct. An implementation must report early errors in a Script as part of parsing that Script in ParseScript. Early errors in a Module are reported at the point when the Module would be evaluated and the Module is never initialized. Early errors in eval code are reported at the time eval is called and prevent evaluation of the eval code. All errors that are not early errors are runtime errors.

An implementation must report as an early error any occurrence of a condition that is listed in a “Static Semantics: Early Errors” subclause of this specification.

An implementation shall not treat other kinds of errors as early errors even if the compiler can prove that a construct cannot execute without error under any circumstances. An implementation may issue an early warning in such a case, but it should not report the error until the relevant construct is actually executed.

An implementation shall report all errors as specified, except for the following:

17.1 Forbidden Extensions

An implementation must not extend this specification in the following ways:

18 ECMAScript Standard Built-in Objects

There are certain built-in objects available whenever an ECMAScript Script or Module begins execution. One, the global object, is part of the global environment of the executing program. Others are accessible as initial properties of the global object or indirectly as properties of accessible built-in objects.

Unless specified otherwise, a built-in object that is callable as a function is a built-in function object with the characteristics described in 10.3. Unless specified otherwise, the [[Extensible]] internal slot of a built-in object initially has the value true. Every built-in function object has a [[Realm]] internal slot whose value is the Realm Record of the realm for which the object was initially created.

Many built-in objects are functions: they can be invoked with arguments. Some of them furthermore are constructors: they are functions intended for use with the new operator. For each built-in function, this specification describes the arguments required by that function and the properties of that function object. For each built-in constructor, this specification furthermore describes properties of the prototype object of that constructor and properties of specific object instances returned by a new expression that invokes that constructor.

Unless otherwise specified in the description of a particular function, if a built-in function or constructor is given fewer arguments than the function is specified to require, the function or constructor shall behave exactly as if it had been given sufficient additional arguments, each such argument being the undefined value. Such missing arguments are considered to be “not present” and may be identified in that manner by specification algorithms. In the description of a particular function, the terms “this value” and “NewTarget” have the meanings given in 10.3.

Unless otherwise specified in the description of a particular function, if a built-in function or constructor described is given more arguments than the function is specified to allow, the extra arguments are evaluated by the call and then ignored by the function. However, an implementation may define implementation specific behaviour relating to such arguments as long as the behaviour is not the throwing of a TypeError exception that is predicated simply on the presence of an extra argument.

Note 1

Implementations that add additional capabilities to the set of built-in functions are encouraged to do so by adding new functions rather than adding new parameters to existing functions.

Unless otherwise specified every built-in function and every built-in constructor has the Function prototype object, which is the initial value of the expression Function.prototype (20.2.3), as the value of its [[Prototype]] internal slot.

Unless otherwise specified every built-in prototype object has the Object prototype object, which is the initial value of the expression Object.prototype (20.1.3), as the value of its [[Prototype]] internal slot, except the Object prototype object itself.

Built-in function objects that are not identified as constructors do not implement the [[Construct]] internal method unless otherwise specified in the description of a particular function.

Each built-in function defined in this specification is created by calling the CreateBuiltinFunction abstract operation (10.3.3). The values of the length and name parameters are the initial values of the "length" and "name" properties as discussed below. The values of the prefix parameter are similarly discussed below.

Every built-in function object, including constructors, has a "length" property whose value is a non-negative integral Number. Unless otherwise specified, this value is the number of required parameters shown in the subclause heading for the function description. Optional parameters and rest parameters are not included in the parameter count.

Note 2

For example, the function object that is the initial value of the "map" property of the Array prototype object is described under the subclause heading «Array.prototype.map (callbackFn [ , thisArg])» which shows the two named arguments callbackFn and thisArg, the latter being optional; therefore the value of the "length" property of that function object is 1𝔽.

Unless otherwise specified, the "length" property of a built-in function object has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

Every built-in function object, including constructors, has a "name" property whose value is a String. Unless otherwise specified, this value is the name that is given to the function in this specification. Functions that are identified as anonymous functions use the empty String as the value of the "name" property. For functions that are specified as properties of objects, the name value is the property name string used to access the function. Functions that are specified as get or set accessor functions of built-in properties have "get" or "set" (respectively) passed to the prefix parameter when calling CreateBuiltinFunction.

The value of the "name" property is explicitly specified for each built-in functions whose property key is a Symbol value. If such an explicitly specified value starts with the prefix "get " or "set " and the function for which it is specified is a get or set accessor function of a built-in property, the value without the prefix is passed to the name parameter, and the value "get" or "set" (respectively) is passed to the prefix parameter when calling CreateBuiltinFunction.

Unless otherwise specified, the "name" property of a built-in function object has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

Every other data property described in clauses 19 through 28 and in Annex B.2 has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true } unless otherwise specified.

Every accessor property described in clauses 19 through 28 and in Annex B.2 has the attributes { [[Enumerable]]: false, [[Configurable]]: true } unless otherwise specified. If only a get accessor function is described, the set accessor function is the default value, undefined. If only a set accessor is described the get accessor is the default value, undefined.

19 The Global Object

The global object:

19.1 Value Properties of the Global Object

19.1.1 globalThis

The initial value of the "globalThis" property of the global object in a Realm Record realm is realm.[[GlobalEnv]].[[GlobalThisValue]].

This property has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }.

19.1.2 Infinity

The value of Infinity is ∞𝔽 (see 6.1.6.1). This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.1.3 NaN

The value of NaN is NaN (see 6.1.6.1). This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.1.4 undefined

The value of undefined is undefined (see 6.1.1). This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

19.2 Function Properties of the Global Object

19.2.1 eval ( x )

This function is the %eval% intrinsic object.

It performs the following steps when called:

  1. Return ? PerformEval(x, false, false).

19.2.1.1 PerformEval ( x, strictCaller, direct )

The abstract operation PerformEval takes arguments x (an ECMAScript language value), strictCaller (a Boolean), and direct (a Boolean) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

  1. Assert: If direct is false, then strictCaller is also false.
  2. If x is not a String, return x.
  3. Let evalRealm be the current Realm Record.
  4. NOTE: In the case of a direct eval, evalRealm is the realm of both the caller of eval and of the eval function itself.
  5. Perform ? HostEnsureCanCompileStrings(evalRealm).
  6. Let inFunction be false.
  7. Let inMethod be false.
  8. Let inDerivedConstructor be false.
  9. Let inClassFieldInitializer be false.
  10. If direct is true, then
    1. Let thisEnvRec be GetThisEnvironment().
    2. If thisEnvRec is a Function Environment Record, then
      1. Let F be thisEnvRec.[[FunctionObject]].
      2. Set inFunction to true.
      3. Set inMethod to thisEnvRec.HasSuperBinding().
      4. If F.[[ConstructorKind]] is derived, set inDerivedConstructor to true.
      5. Let classFieldInitializerName be F.[[ClassFieldInitializerName]].
      6. If classFieldInitializerName is not empty, set inClassFieldInitializer to true.
  11. Perform the following substeps in an implementation-defined order, possibly interleaving parsing and error detection:
    1. Let script be ParseText(StringToCodePoints(x), Script).
    2. If script is a List of errors, throw a SyntaxError exception.
    3. If script Contains ScriptBody is false, return undefined.
    4. Let body be the ScriptBody of script.
    5. If inFunction is false and body Contains NewTarget, throw a SyntaxError exception.
    6. If inMethod is false and body Contains SuperProperty, throw a SyntaxError exception.
    7. If inDerivedConstructor is false and body Contains SuperCall, throw a SyntaxError exception.
    8. If inClassFieldInitializer is true and ContainsArguments of body is true, throw a SyntaxError exception.
  12. If strictCaller is true, let strictEval be true.
  13. Else, let strictEval be IsStrict of script.
  14. Let runningContext be the running execution context.
  15. NOTE: If direct is true, runningContext will be the execution context that performed the direct eval. If direct is false, runningContext will be the execution context for the invocation of the eval function.
  16. If direct is true, then
    1. Let lexEnv be NewDeclarativeEnvironment(runningContext's LexicalEnvironment).
    2. Let varEnv be runningContext's VariableEnvironment.
    3. Let privateEnv be runningContext's PrivateEnvironment.
  17. Else,
    1. Let lexEnv be NewDeclarativeEnvironment(evalRealm.[[GlobalEnv]]).
    2. Let varEnv be evalRealm.[[GlobalEnv]].
    3. Let privateEnv be null.
  18. If strictEval is true, set varEnv to lexEnv.
  19. If runningContext is not already suspended, suspend runningContext.
  20. Let evalContext be a new ECMAScript code execution context.
  21. Set evalContext's Function to null.
  22. Set evalContext's Realm to evalRealm.
  23. Set evalContext's ScriptOrModule to runningContext's ScriptOrModule.
  24. Set evalContext's VariableEnvironment to varEnv.
  25. Set evalContext's LexicalEnvironment to lexEnv.
  26. Set evalContext's PrivateEnvironment to privateEnv.
  27. Push evalContext onto the execution context stack; evalContext is now the running execution context.
  28. Let result be Completion(EvalDeclarationInstantiation(body, varEnv, lexEnv, privateEnv, strictEval)).
  29. If result.[[Type]] is normal, then
    1. Set result to Completion(Evaluation of body).
  30. If result.[[Type]] is normal and result.[[Value]] is empty, then
    1. Set result to NormalCompletion(undefined).
  31. Suspend evalContext and remove it from the execution context stack.
  32. Resume the context that is now on the top of the execution context stack as the running execution context.
  33. Return ? result.
 Note

The eval code cannot instantiate variable or function bindings in the variable environment of the calling context that invoked the eval if either the code of the calling context or the eval code is strict mode code. Instead such bindings are instantiated in a new VariableEnvironment that is only accessible to the eval code. Bindings introduced by let, const, or class declarations are always instantiated in a new LexicalEnvironment.

19.2.1.2 HostEnsureCanCompileStrings ( calleeRealm )

The host-defined abstract operation HostEnsureCanCompileStrings takes argument calleeRealm (a Realm Record) and returns either a normal completion containing unused or a throw completion. It allows host environments to block certain ECMAScript functions which allow developers to interpret and evaluate strings as ECMAScript code.

An implementation of HostEnsureCanCompileStrings must conform to the following requirements:

The default implementation of HostEnsureCanCompileStrings is to return NormalCompletion(unused).

19.2.1.3 EvalDeclarationInstantiation ( body, varEnv, lexEnv, privateEnv, strict )

The abstract operation EvalDeclarationInstantiation takes arguments body (a ScriptBody Parse Node), varEnv (an Environment Record), lexEnv (a Declarative Environment Record), privateEnv (a PrivateEnvironment Record or null), and strict (a Boolean) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

  1. Let varNames be the VarDeclaredNames of body.
  2. Let varDeclarations be the VarScopedDeclarations of body.
  3. If strict is false, then
    1. If varEnv is a Global Environment Record, then
      1. For each element name of varNames, do
        1. If varEnv.HasLexicalDeclaration(name) is true, throw a SyntaxError exception.
        2. NOTE: eval will not create a global var declaration that would be shadowed by a global lexical declaration.
    2. Let thisEnv be lexEnv.
    3. Assert: The following loop will terminate.
    4. Repeat, while thisEnv is not varEnv,
      1. If thisEnv is not an Object Environment Record, then
        1. NOTE: The environment of with statements cannot contain any lexical declaration so it doesn't need to be checked for var/let hoisting conflicts.
        2. For each element name of varNames, do
          1. If ! thisEnv.HasBinding(name) is true, then
            1. Throw a SyntaxError exception.
            2. NOTE: Annex B.3.4 defines alternate semantics for the above step.
          2. NOTE: A direct eval will not hoist var declaration over a like-named lexical declaration.
      2. Set thisEnv to thisEnv.[[OuterEnv]].
  4. Let privateIdentifiers be a new empty List.
  5. Let pointer be privateEnv.
  6. Repeat, while pointer is not null,
    1. For each Private Name binding of pointer.[[Names]], do
      1. If privateIdentifiers does not contain binding.[[Description]], append binding.[[Description]] to privateIdentifiers.
    2. Set pointer to pointer.[[OuterPrivateEnvironment]].
  7. If AllPrivateIdentifiersValid of body with argument privateIdentifiers is false, throw a SyntaxError exception.
  8. Let functionsToInitialize be a new empty List.
  9. Let declaredFunctionNames be a new empty List.
  10. For each element d of varDeclarations, in reverse List order, do
    1. If d is not either a VariableDeclaration, a ForBinding, or a BindingIdentifier, then
      1. Assert: d is either a FunctionDeclaration, a GeneratorDeclaration, an AsyncFunctionDeclaration, or an AsyncGeneratorDeclaration.
      2. NOTE: If there are multiple function declarations for the same name, the last declaration is used.
      3. Let fn be the sole element of the BoundNames of d.
      4. If declaredFunctionNames does not contain fn, then
        1. If varEnv is a Global Environment Record, then
          1. Let fnDefinable be ? varEnv.CanDeclareGlobalFunction(fn).
          2. If fnDefinable is false, throw a TypeError exception.
        2. Append fn to declaredFunctionNames.
        3. Insert d as the first element of functionsToInitialize.
  11. NOTE: Annex B.3.2.3 adds additional steps at this point.
  12. Let declaredVarNames be a new empty List.
  13. For each element d of varDeclarations, do
    1. If d is either a VariableDeclaration, a ForBinding, or a BindingIdentifier, then
      1. For each String vn of the BoundNames of d, do
        1. If declaredFunctionNames does not contain vn, then
          1. If varEnv is a Global Environment Record, then
            1. Let vnDefinable be ? varEnv.CanDeclareGlobalVar(vn).
            2. If vnDefinable is false, throw a TypeError exception.
          2. If declaredVarNames does not contain vn, then
            1. Append vn to declaredVarNames.
  14. NOTE: No abnormal terminations occur after this algorithm step unless varEnv is a Global Environment Record and the global object is a Proxy exotic object.
  15. Let lexDeclarations be the LexicallyScopedDeclarations of body.
  16. For each element d of lexDeclarations, do
    1. NOTE: Lexically declared names are only instantiated here but not initialized.
    2. For each element dn of the BoundNames of d, do
      1. If IsConstantDeclaration of d is true, then
        1. Perform ? lexEnv.CreateImmutableBinding(dn, true).
      2. Else,
        1. Perform ? lexEnv.CreateMutableBinding(dn, false).
  17. For each Parse Node f of functionsToInitialize, do
    1. Let fn be the sole element of the BoundNames of f.
    2. Let fo be InstantiateFunctionObject of f with arguments lexEnv and privateEnv.
    3. If varEnv is a Global Environment Record, then
      1. Perform ? varEnv.CreateGlobalFunctionBinding(fn, fo, true).
    4. Else,
      1. Let bindingExists be ! varEnv.HasBinding(fn).
      2. If bindingExists is false, then
        1. NOTE: The following invocation cannot return an abrupt completion because of the validation preceding step 14.
        2. Perform ! varEnv.CreateMutableBinding(fn, true).
        3. Perform ! varEnv.InitializeBinding(fn, fo).
      3. Else,
        1. Perform ! varEnv.SetMutableBinding(fn, fo, false).
  18. For each String vn of declaredVarNames, do
    1. If varEnv is a Global Environment Record, then
      1. Perform ? varEnv.CreateGlobalVarBinding(vn, true).
    2. Else,
      1. Let bindingExists be ! varEnv.HasBinding(vn).
      2. If bindingExists is false, then
        1. NOTE: The following invocation cannot return an abrupt completion because of the validation preceding step 14.
        2. Perform ! varEnv.CreateMutableBinding(vn, true).
        3. Perform ! varEnv.InitializeBinding(vn, undefined).
  19. Return unused.
 Note

An alternative version of this algorithm is described in B.3.4.

19.2.2 isFinite ( number )

This function is the %isFinite% intrinsic object.

It performs the following steps when called:

  1. Let num be ? ToNumber(number).
  2. If num is not finite, return false.
  3. Otherwise, return true.

19.2.3 isNaN ( number )

This function is the %isNaN% intrinsic object.

It performs the following steps when called:

  1. Let num be ? ToNumber(number).
  2. If num is NaN, return true.
  3. Otherwise, return false.
 Note

A reliable way for ECMAScript code to test if a value X is NaN is an expression of the form X !== X. The result will be true if and only if X is NaN.

19.2.4 parseFloat ( string )

This function produces a Number value dictated by interpretation of the contents of the string argument as a decimal literal.

It is the %parseFloat% intrinsic object.

It performs the following steps when called:

  1. Let inputString be ? ToString(string).
  2. Let trimmedString be ! TrimString(inputString, start).
  3. Let trimmed be StringToCodePoints(trimmedString).
  4. Let trimmedPrefix be the longest prefix of trimmed that satisfies the syntax of a StrDecimalLiteral, which might be trimmed itself. If there is no such prefix, return NaN.
  5. Let parsedNumber be ParseText(trimmedPrefix, StrDecimalLiteral).
  6. Assert: parsedNumber is a Parse Node.
  7. Return StringNumericValue of parsedNumber.
 Note

This function may interpret only a leading portion of string as a Number value; it ignores any code units that cannot be interpreted as part of the notation of a decimal literal, and no indication is given that any such code units were ignored.

19.2.5 parseInt ( string, radix )

This function produces an integral Number dictated by interpretation of the contents of string according to the specified radix. Leading white space in string is ignored. If radix coerces to 0 (such as when it is undefined), it is assumed to be 10 except when the number representation begins with "0x" or "0X", in which case it is assumed to be 16. If radix is 16, the number representation may optionally begin with "0x" or "0X".

It is the %parseInt% intrinsic object.

It performs the following steps when called:

  1. Let inputString be ? ToString(string).
  2. Let S be ! TrimString(inputString, start).
  3. Let sign be 1.
  4. If S is not empty and the first code unit of S is the code unit 0x002D (HYPHEN-MINUS), set sign to -1.
  5. If S is not empty and the first code unit of S is either the code unit 0x002B (PLUS SIGN) or the code unit 0x002D (HYPHEN-MINUS), set S to the substring of S from index 1.
  6. Let R be ℝ(? ToInt32(radix)).
  7. Let stripPrefix be true.
  8. If R ≠ 0, then
    1. If R < 2 or R > 36, return NaN.
    2. If R ≠ 16, set stripPrefix to false.
  9. Else,
    1. Set R to 10.
  10. If stripPrefix is true, then
    1. If the length of S is at least 2 and the first two code units of S are either "0x" or "0X", then
      1. Set S to the substring of S from index 2.
      2. Set R to 16.
  11. If S contains a code unit that is not a radix-R digit, let end be the index within S of the first such code unit; otherwise, let end be the length of S.
  12. Let Z be the substring of S from 0 to end.
  13. If Z is empty, return NaN.
  14. Let mathInt be the integer value that is represented by Z in radix-R notation, using the letters A-Z and a-z for digits with values 10 through 35. (However, if R = 10 and Z contains more than 20 significant digits, every significant digit after the 20th may be replaced by a 0 digit, at the option of the implementation; and if R is not one of 2, 4, 8, 10, 16, or 32, then mathInt may be an implementation-approximated integer representing the integer value denoted by Z in radix-R notation.)
  15. If mathInt = 0, then
    1. If sign = -1, return -0𝔽.
    2. Return 0𝔽.
  16. Return 𝔽(sign × mathInt).
 Note

This function may interpret only a leading portion of string as an integer value; it ignores any code units that cannot be interpreted as part of the notation of an integer, and no indication is given that any such code units were ignored.

19.2.6 URI Handling Functions

Uniform Resource Identifiers, or URIs, are Strings that identify resources (e.g. web pages or files) and transport protocols by which to access them (e.g. HTTP or FTP) on the Internet. The ECMAScript language itself does not provide any support for using URIs except for functions that encode and decode URIs as described in this section. encodeURI and decodeURI are intended to work with complete URIs; they assume that any reserved characters are intended to have special meaning (e.g., as delimiters) and so are not encoded. encodeURIComponent and decodeURIComponent are intended to work with the individual components of a URI; they assume that any reserved characters represent text and must be encoded to avoid special meaning when the component is part of a complete URI.

Note 1

The set of reserved characters is based upon RFC 2396 and does not reflect changes introduced by the more recent RFC 3986.

 Note 2

Many implementations of ECMAScript provide additional functions and methods that manipulate web pages; these functions are beyond the scope of this standard.

19.2.6.1 decodeURI ( encodedURI )

This function computes a new version of a URI in which each escape sequence and UTF-8 encoding of the sort that might be introduced by the encodeURI function is replaced with the UTF-16 encoding of the code point that it represents. Escape sequences that could not have been introduced by encodeURI are not replaced.

It is the �codeURI% intrinsic object.

It performs the following steps when called:

  1. Let uriString be ? ToString(encodedURI).
  2. Let preserveEscapeSet be ";/?:@&= $,#".
  3. Return ? Decode(uriString, preserveEscapeSet).

19.2.6.2 decodeURIComponent ( encodedURIComponent )

This function computes a new version of a URI in which each escape sequence and UTF-8 encoding of the sort that might be introduced by the encodeURIComponent function is replaced with the UTF-16 encoding of the code point that it represents.

It is the �codeURIComponent% intrinsic object.

It performs the following steps when called:

  1. Let componentString be ? ToString(encodedURIComponent).
  2. Let preserveEscapeSet be the empty String.
  3. Return ? Decode(componentString, preserveEscapeSet).

19.2.6.3 encodeURI ( uri )

This function computes a new version of a UTF-16 encoded (6.1.4) URI in which each instance of certain code points is replaced by one, two, three, or four escape sequences representing the UTF-8 encoding of the code point.

It is the %encodeURI% intrinsic object.

It performs the following steps when called:

  1. Let uriString be ? ToString(uri).
  2. Let extraUnescaped be ";/?:@&= $,#".
  3. Return ? Encode(uriString, extraUnescaped).

19.2.6.4 encodeURIComponent ( uriComponent )

This function computes a new version of a UTF-16 encoded (6.1.4) URI in which each instance of certain code points is replaced by one, two, three, or four escape sequences representing the UTF-8 encoding of the code point.

It is the %encodeURIComponent% intrinsic object.

It performs the following steps when called:

  1. Let componentString be ? ToString(uriComponent).
  2. Let extraUnescaped be the empty String.
  3. Return ? Encode(componentString, extraUnescaped).

19.2.6.5 Encode ( string, extraUnescaped )

The abstract operation Encode takes arguments string (a String) and extraUnescaped (a String) and returns either a normal completion containing a String or a throw completion. It performs URI encoding and escaping, interpreting string as a sequence of UTF-16 encoded code points as described in 6.1.4. If a character is identified as unreserved in RFC 2396 or appears in extraUnescaped, it is not escaped. It performs the following steps when called:

  1. Let len be the length of string.
  2. Let R be the empty String.
  3. Let alwaysUnescaped be the string-concatenation of the ASCII word characters and "-.!~*'()".
  4. Let unescapedSet be the string-concatenation of alwaysUnescaped and extraUnescaped.
  5. Let k be 0.
  6. Repeat, while k < len,
    1. Let C be the code unit at index k within string.
    2. If unescapedSet contains C, then
      1. Set k to k 1.
      2. Set R to the string-concatenation of R and C.
    3. Else,
      1. Let cp be CodePointAt(string, k).
      2. If cp.[[IsUnpairedSurrogate]] is true, throw a URIError exception.
      3. Set k to k cp.[[CodeUnitCount]].
      4. Let Octets be the List of octets resulting by applying the UTF-8 transformation to cp.[[CodePoint]].
      5. For each element octet of Octets, do
        1. Let hex be the String representation of octet, formatted as an uppercase hexadecimal number.
        2. Set R to the string-concatenation of R, "%", and ! StringPad(hex, 2𝔽, "0", start).
  7. Return R.
 Note

Because percent-encoding is used to represent individual octets, a single code point may be expressed as multiple consecutive escape sequences (one for each of its 8-bit UTF-8 code units).

19.2.6.6 Decode ( string, preserveEscapeSet )

The abstract operation Decode takes arguments string (a String) and preserveEscapeSet (a String) and returns either a normal completion containing a String or a throw completion. It performs URI unescaping and decoding, preserving any escape sequences that correspond to Basic Latin characters in preserveEscapeSet. It performs the following steps when called:

  1. Let len be the length of string.
  2. Let R be the empty String.
  3. Let k be 0.
  4. Repeat, while k < len,
    1. Let C be the code unit at index k within string.
    2. Let S be C.
    3. If C is the code unit 0x0025 (PERCENT SIGN), then
      1. If k 3 > len, throw a URIError exception.
      2. Let escape be the substring of string from k to k 3.
      3. Let B be ParseHexOctet(string, k 1).
      4. If B is not an integer, throw a URIError exception.
      5. Set k to k 2.
      6. Let n be the number of leading 1 bits in B.
      7. If n = 0, then
        1. Let asciiChar be the code unit whose numeric value is B.
        2. If preserveEscapeSet contains asciiChar, set S to escape. Otherwise, set S to asciiChar.
      8. Else,
        1. If n = 1 or n > 4, throw a URIError exception.
        2. Let Octets be « B ».
        3. Let j be 1.
        4. Repeat, while j < n,
          1. Set k to k 1.
          2. If k 3 > len, throw a URIError exception.
          3. If the code unit at index k within string is not the code unit 0x0025 (PERCENT SIGN), throw a URIError exception.
          4. Let continuationByte be ParseHexOctet(string, k 1).
          5. If continuationByte is not an integer, throw a URIError exception.
          6. Append continuationByte to Octets.
          7. Set k to k 2.
          8. Set j to j 1.
        5. Assert: The length of Octets is n.
        6. If Octets does not contain a valid UTF-8 encoding of a Unicode code point, throw a URIError exception.
        7. Let V be the code point obtained by applying the UTF-8 transformation to Octets, that is, from a List of octets into a 21-bit value.
        8. Set S to UTF16EncodeCodePoint(V).
    4. Set R to the string-concatenation of R and S.
    5. Set k to k 1.
  5. Return R.
 Note

RFC 3629 prohibits the decoding of invalid UTF-8 octet sequences. For example, the invalid sequence 0xC0 0x80 must not decode into the code unit 0x0000. Implementations of the Decode algorithm are required to throw a URIError when encountering such invalid sequences.

19.2.6.7 ParseHexOctet ( string, position )

The abstract operation ParseHexOctet takes arguments string (a String) and position (a non-negative integer) and returns either a non-negative integer or a non-empty List of SyntaxError objects. It parses a sequence of two hexadecimal characters at the specified position in string into an unsigned 8-bit integer. It performs the following steps when called:

  1. Let len be the length of string.
  2. Assert: position 2 ≤ len.
  3. Let hexDigits be the substring of string from position to position 2.
  4. Let parseResult be ParseText(StringToCodePoints(hexDigits), HexDigits[~Sep]).
  5. If parseResult is not a Parse Node, return parseResult.
  6. Let n be the MV of parseResult.
  7. Assert: n is in the inclusive interval from 0 to 255.
  8. Return n.

19.3 Constructor Properties of the Global Object

19.3.1 AggregateError ( . . . )

See 20.5.7.1.

19.3.2 Array ( . . . )

See 23.1.1.

19.3.3 ArrayBuffer ( . . . )

See 25.1.3.

19.3.4 BigInt ( . . . )

See 21.2.1.

19.3.5 BigInt64Array ( . . . )

See 23.2.5.

19.3.6 BigUint64Array ( . . . )

See 23.2.5.

19.3.7 Boolean ( . . . )

See 20.3.1.

19.3.8 DataView ( . . . )

See 25.3.2.

19.3.9 Date ( . . . )

See 21.4.2.

19.3.10 Error ( . . . )

See 20.5.1.

19.3.11 EvalError ( . . . )

See 20.5.5.1.

19.3.12 FinalizationRegistry ( . . . )

See 26.2.1.

19.3.13 Float32Array ( . . . )

See 23.2.5.

19.3.14 Float64Array ( . . . )

See 23.2.5.

19.3.15 Function ( . . . )

See 20.2.1.

19.3.16 Int8Array ( . . . )

See 23.2.5.

19.3.17 Int16Array ( . . . )

See 23.2.5.

19.3.18 Int32Array ( . . . )

See 23.2.5.

19.3.19 Map ( . . . )

See 24.1.1.

19.3.20 Number ( . . . )

See 21.1.1.

19.3.21 Object ( . . . )

See 20.1.1.

19.3.22 Promise ( . . . )

See 27.2.3.

19.3.23 Proxy ( . . . )

See 28.2.1.

19.3.24 RangeError ( . . . )

See 20.5.5.2.

19.3.25 ReferenceError ( . . . )

See 20.5.5.3.

19.3.26 RegExp ( . . . )

See 22.2.4.

19.3.27 Set ( . . . )

See 24.2.1.

19.3.28 SharedArrayBuffer ( . . . )

See 25.2.2.

19.3.29 String ( . . . )

See 22.1.1.

19.3.30 Symbol ( . . . )

See 20.4.1.

19.3.31 SyntaxError ( . . . )

See 20.5.5.4.

19.3.32 TypeError ( . . . )

See 20.5.5.5.

19.3.33 Uint8Array ( . . . )

See 23.2.5.

19.3.34 Uint8ClampedArray ( . . . )

See 23.2.5.

19.3.35 Uint16Array ( . . . )

See 23.2.5.

19.3.36 Uint32Array ( . . . )

See 23.2.5.

19.3.37 URIError ( . . . )

See 20.5.5.6.

19.3.38 WeakMap ( . . . )

See 24.3.1.

19.3.39 WeakRef ( . . . )

See 26.1.1.

19.3.40 WeakSet ( . . . )

See 24.4.

19.4 Other Properties of the Global Object

19.4.1 Atomics

See 25.4.

19.4.2 JSON

See 25.5.

19.4.3 Math

See 21.3.

19.4.4 Reflect

See 28.1.

20 Fundamental Objects

20.1 Object Objects

20.1.1 The Object Constructor

The Object constructor:

  • is %Object%.
  • is the initial value of the "Object" property of the global object.
  • creates a new ordinary object when called as a constructor.
  • performs a type conversion when called as a function rather than as a constructor.
  • may be used as the value of an extends clause of a class definition.

20.1.1.1 Object ( [ value ] )

This function performs the following steps when called:

  1. If NewTarget is neither undefined nor the active function object, then
    1. Return ? OrdinaryCreateFromConstructor(NewTarget, "%Object.prototype%").
  2. If value is either undefined or null, return OrdinaryObjectCreate(%Object.prototype%).
  3. Return ! ToObject(value).

The "length" property of this function is 1𝔽.

20.1.2 Properties of the Object Constructor

The Object constructor:

  • has a [[Prototype]] internal slot whose value is %Function.prototype%.
  • has a "length" property.
  • has the following additional properties:

20.1.2.1 Object.assign ( target, ...sources )

This function copies the values of all of the enumerable own properties from one or more source objects to a target object.

It performs the following steps when called:

  1. Let to be ? ToObject(target).
  2. If only one argument was passed, return to.
  3. For each element nextSource of sources, do
    1. If nextSource is neither undefined nor null, then
      1. Let from be ! ToObject(nextSource).
      2. Let keys be ? from.[[OwnPropertyKeys]]().
      3. For each element nextKey of keys, do
        1. Let desc be ? from.[[GetOwnProperty]](nextKey).
        2. If desc is not undefined and desc.[[Enumerable]] is true, then
          1. Let propValue be ? Get(from, nextKey).
          2. Perform ? Set(to, nextKey, propValue, true).
  4. Return to.

The "length" property of this function is 2𝔽.

20.1.2.2 Object.create ( O, Properties )

This function creates a new object with a specified prototype.

It performs the following steps when called:

  1. If O is not an Object and O is not null, throw a TypeError exception.
  2. Let obj be OrdinaryObjectCreate(O).
  3. If Properties is not undefined, then
    1. Return ? ObjectDefineProperties(obj, Properties).
  4. Return obj.

20.1.2.3 Object.defineProperties ( O, Properties )

This function adds own properties and/or updates the attributes of existing own properties of an object.

It performs the following steps when called:

  1. If O is not an Object, throw a TypeError exception.
  2. Return ? ObjectDefineProperties(O, Properties).

20.1.2.3.1 ObjectDefineProperties ( O, Properties )

The abstract operation ObjectDefineProperties takes arguments O (an Object) and Properties (an ECMAScript language value) and returns either a normal completion containing an Object or a throw completion. It performs the following steps when called:

  1. Let props be ? ToObject(Properties).
  2. Let keys be ? props.[[OwnPropertyKeys]]().
  3. Let descriptors be a new empty List.
  4. For each element nextKey of keys, do
    1. Let propDesc be ? props.[[GetOwnProperty]](nextKey).
    2. If propDesc is not undefined and propDesc.[[Enumerable]] is true, then
      1. Let descObj be ? Get(props, nextKey).
      2. Let desc be ? ToPropertyDescriptor(descObj).
      3. Append the pair (a two element List) consisting of nextKey and desc to the end of descriptors.
  5. For each element pair of descriptors, do
    1. Let P be the first element of pair.
    2. Let desc be the second element of pair.
    3. Perform ? DefinePropertyOrThrow(O, P, desc).
  6. Return O.

20.1.2.4 Object.defineProperty ( O, P, Attributes )

This function adds an own property and/or updates the attributes of an existing own property of an object.

It performs the following steps when called:

  1. If O is not an Object, throw a TypeError exception.
  2. Let key be ? ToPropertyKey(P).
  3. Let desc be ? ToPropertyDescriptor(Attributes).
  4. Perform ? DefinePropertyOrThrow(O, key, desc).
  5. Return O.

20.1.2.5 Object.entries ( O )

This function performs the following steps when called:

  1. Let obj be ? ToObject(O).
  2. Let entryList be ? EnumerableOwnProperties(obj, key value).
  3. Return CreateArrayFromList(entryList).

20.1.2.6 Object.freeze ( O )

This function performs the following steps when called:

  1. If O is not an Object, return O.
  2. Let status be ? SetIntegrityLevel(O, frozen).
  3. If status is false, throw a TypeError exception.
  4. Return O.

20.1.2.7 Object.fromEntries ( iterable )

This function performs the following steps when called:

  1. Perform ? RequireObjectCoercible(iterable).
  2. Let obj be OrdinaryObjectCreate(%Object.prototype%).
  3. Assert: obj is an extensible ordinary object with no own properties.
  4. Let closure be a new Abstract Closure with parameters (key, value) that captures obj and performs the following steps when called:
    1. Let propertyKey be ? ToPropertyKey(key).
    2. Perform ! CreateDataPropertyOrThrow(obj, propertyKey, value).
    3. Return undefined.
  5. Let adder be CreateBuiltinFunction(closure, 2, "", « »).
  6. Return ? AddEntriesFromIterable(obj, iterable, adder).
 Note
The function created for adder is never directly accessible to ECMAScript code.

20.1.2.8 Object.getOwnPropertyDescriptor ( O, P )

This function performs the following steps when called:

  1. Let obj be ? ToObject(O).
  2. Let key be ? ToPropertyKey(P).
  3. Let desc be ? obj.[[GetOwnProperty]](key).
  4. Return FromPropertyDescriptor(desc).

20.1.2.9 Object.getOwnPropertyDescriptors ( O )

This function performs the following steps when called:

  1. Let obj be ? ToObject(O).
  2. Let ownKeys be ? obj.[[OwnPropertyKeys]]().
  3. Let descriptors be OrdinaryObjectCreate(%Object.prototype%).
  4. For each element key of ownKeys, do
    1. Let desc be ? obj.[[GetOwnProperty]](key).
    2. Let descriptor be FromPropertyDescriptor(desc).
    3. If descriptor is not undefined, perform ! CreateDataPropertyOrThrow(descriptors, key, descriptor).
  5. Return descriptors.

20.1.2.10 Object.getOwnPropertyNames ( O )

This function performs the following steps when called:

  1. Return CreateArrayFromList(? GetOwnPropertyKeys(O, string)).

20.1.2.11 Object.getOwnPropertySymbols ( O )

This function performs the following steps when called:

  1. Return CreateArrayFromList(? GetOwnPropertyKeys(O, symbol)).

20.1.2.11.1 GetOwnPropertyKeys ( O, type )

The abstract operation GetOwnPropertyKeys takes arguments O (an ECMAScript language value) and type (string or symbol) and returns either a normal completion containing a List of property keys or a throw completion. It performs the following steps when called:

  1. Let obj be ? ToObject(O).
  2. Let keys be ? obj.[[OwnPropertyKeys]]().
  3. Let nameList be a new empty List.
  4. For each element nextKey of keys, do
    1. If nextKey is a Symbol and type is symbol, or if nextKey is a String and type is string, then
      1. Append nextKey to nameList.
  5. Return nameList.

20.1.2.12 Object.getPrototypeOf ( O )

This function performs the following steps when called:

  1. Let obj be ? ToObject(O).
  2. Return ? obj.[[GetPrototypeOf]]().

20.1.2.13 Object.hasOwn ( O, P )

This function performs the following steps when called:

  1. Let obj be ? ToObject(O).
  2. Let key be ? ToPropertyKey(P).
  3. Return ? HasOwnProperty(obj, key).

20.1.2.14 Object.is ( value1, value2 )

This function performs the following steps when called:

  1. Return SameValue(value1, value2).

20.1.2.15 Object.isExtensible ( O )

This function performs the following steps when called:

  1. If O is not an Object, return false.
  2. Return ? IsExtensible(O).

20.1.2.16 Object.isFrozen ( O )

This function performs the following steps when called:

  1. If O is not an Object, return true.
  2. Return ? TestIntegrityLevel(O, frozen).

20.1.2.17 Object.isSealed ( O )

This function performs the following steps when called:

  1. If O is not an Object, return true.
  2. Return ? TestIntegrityLevel(O, sealed).

20.1.2.18 Object.keys ( O )

This function performs the following steps when called:

  1. Let obj be ? ToObject(O).
  2. Let keyList be ? EnumerableOwnProperties(obj, key).
  3. Return CreateArrayFromList(keyList).

20.1.2.19 Object.preventExtensions ( O )

This function performs the following steps when called:

  1. If O is not an Object, return O.
  2. Let status be ? O.[[PreventExtensions]]().
  3. If status is false, throw a TypeError exception.
  4. Return O.

20.1.2.20 Object.prototype

The initial value of Object.prototype is the Object prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.1.2.21 Object.seal ( O )

This function performs the following steps when called:

  1. If O is not an Object, return O.
  2. Let status be ? SetIntegrityLevel(O, sealed).
  3. If status is false, throw a TypeError exception.
  4. Return O.

20.1.2.22 Object.setPrototypeOf ( O, proto )

This function performs the following steps when called:

  1. Set O to ? RequireObjectCoercible(O).
  2. If proto is not an Object and proto is not null, throw a TypeError exception.
  3. If O is not an Object, return O.
  4. Let status be ? O.[[SetPrototypeOf]](proto).
  5. If status is false, throw a TypeError exception.
  6. Return O.

20.1.2.23 Object.values ( O )

This function performs the following steps when called:

  1. Let obj be ? ToObject(O).
  2. Let valueList be ? EnumerableOwnProperties(obj, value).
  3. Return CreateArrayFromList(valueList).

20.1.3 Properties of the Object Prototype Object

The Object prototype object:

  • is %Object.prototype%.
  • has an [[Extensible]] internal slot whose value is true.
  • has the internal methods defined for ordinary objects, except for the [[SetPrototypeOf]] method, which is as defined in 10.4.7.1. (Thus, it is an immutable prototype exotic object.)
  • has a [[Prototype]] internal slot whose value is null.

20.1.3.1 Object.prototype.constructor

The initial value of Object.prototype.constructor is %Object%.

20.1.3.2 Object.prototype.hasOwnProperty ( V )

This method performs the following steps when called:

  1. Let P be ? ToPropertyKey(V).
  2. Let O be ? ToObject(this value).
  3. Return ? HasOwnProperty(O, P).
 Note

The ordering of steps 1 and 2 is chosen to ensure that any exception that would have been thrown by step 1 in previous editions of this specification will continue to be thrown even if the this value is undefined or null.

20.1.3.3 Object.prototype.isPrototypeOf ( V )

This method performs the following steps when called:

  1. If V is not an Object, return false.
  2. Let O be ? ToObject(this value).
  3. Repeat,
    1. Set V to ? V.[[GetPrototypeOf]]().
    2. If V is null, return false.
    3. If SameValue(O, V) is true, return true.
 Note

The ordering of steps 1 and 2 preserves the behaviour specified by previous editions of this specification for the case where V is not an object and the this value is undefined or null.

20.1.3.4 Object.prototype.propertyIsEnumerable ( V )

This method performs the following steps when called:

  1. Let P be ? ToPropertyKey(V).
  2. Let O be ? ToObject(this value).
  3. Let desc be ? O.[[GetOwnProperty]](P).
  4. If desc is undefined, return false.
  5. Return desc.[[Enumerable]].
 Note 1

This method does not consider objects in the prototype chain.

 Note 2

The ordering of steps 1 and 2 is chosen to ensure that any exception that would have been thrown by step 1 in previous editions of this specification will continue to be thrown even if the this value is undefined or null.

20.1.3.5 Object.prototype.toLocaleString ( [ reserved1 [ , reserved2 ] ] )

This method performs the following steps when called:

  1. Let O be the this value.
  2. Return ? Invoke(O, "toString").

The optional parameters to this method are not used but are intended to correspond to the parameter pattern used by ECMA-402 toLocaleString methods. Implementations that do not include ECMA-402 support must not use those parameter positions for other purposes.

Note 1

This method provides a generic toLocaleString implementation for objects that have no locale-sensitive toString behaviour. Array, Number, Date, and %TypedArray% provide their own locale-sensitive toLocaleString methods.

 Note 2

ECMA-402 intentionally does not provide an alternative to this default implementation.

20.1.3.6 Object.prototype.toString ( )

This method performs the following steps when called:

  1. If the this value is undefined, return "[object Undefined]".
  2. If the this value is null, return "[object Null]".
  3. Let O be ! ToObject(this value).
  4. Let isArray be ? IsArray(O).
  5. If isArray is true, let builtinTag be "Array".
  6. Else if O has a [[ParameterMap]] internal slot, let builtinTag be "Arguments".
  7. Else if O has a [[Call]] internal method, let builtinTag be "Function".
  8. Else if O has an [[ErrorData]] internal slot, let builtinTag be "Error".
  9. Else if O has a [[BooleanData]] internal slot, let builtinTag be "Boolean".
  10. Else if O has a [[NumberData]] internal slot, let builtinTag be "Number".
  11. Else if O has a [[StringData]] internal slot, let builtinTag be "String".
  12. Else if O has a [[DateValue]] internal slot, let builtinTag be "Date".
  13. Else if O has a [[RegExpMatcher]] internal slot, let builtinTag be "RegExp".
  14. Else, let builtinTag be "Object".
  15. Let tag be ? Get(O, @@toStringTag).
  16. If tag is not a String, set tag to builtinTag.
  17. Return the string-concatenation of "[object ", tag, and "]".
 Note

Historically, this method was occasionally used to access the String value of the [[Class]] internal slot that was used in previous editions of this specification as a nominal type tag for various built-in objects. The above definition of toString preserves compatibility for legacy code that uses toString as a test for those specific kinds of built-in objects. It does not provide a reliable type testing mechanism for other kinds of built-in or program defined objects. In addition, programs can use @@toStringTag in ways that will invalidate the reliability of such legacy type tests.

20.1.3.7 Object.prototype.valueOf ( )

This method performs the following steps when called:

  1. Return ? ToObject(this value).
Normative Optional, Legacy

20.1.3.8 Object.prototype.__proto__

Object.prototype.__proto__ is an accessor property with attributes { [[Enumerable]]: false, [[Configurable]]: true }. The [[Get]] and [[Set]] attributes are defined as follows:

20.1.3.8.1 get Object.prototype.__proto__

The value of the [[Get]] attribute is a built-in function that requires no arguments. It performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Return ? O.[[GetPrototypeOf]]().

20.1.3.8.2 set Object.prototype.__proto__

The value of the [[Set]] attribute is a built-in function that takes an argument proto. It performs the following steps when called:

  1. Let O be ? RequireObjectCoercible(this value).
  2. If proto is not an Object and proto is not null, return undefined.
  3. If O is not an Object, return undefined.
  4. Let status be ? O.[[SetPrototypeOf]](proto).
  5. If status is false, throw a TypeError exception.
  6. Return undefined.
Normative Optional, Legacy

20.1.3.9 Legacy Object.prototype Accessor Methods

20.1.3.9.1 Object.prototype.__defineGetter__ ( P, getter )

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. If IsCallable(getter) is false, throw a TypeError exception.
  3. Let desc be PropertyDescriptor { [[Get]]: getter, [[Enumerable]]: true, [[Configurable]]: true }.
  4. Let key be ? ToPropertyKey(P).
  5. Perform ? DefinePropertyOrThrow(O, key, desc).
  6. Return undefined.

20.1.3.9.2 Object.prototype.__defineSetter__ ( P, setter )

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. If IsCallable(setter) is false, throw a TypeError exception.
  3. Let desc be PropertyDescriptor { [[Set]]: setter, [[Enumerable]]: true, [[Configurable]]: true }.
  4. Let key be ? ToPropertyKey(P).
  5. Perform ? DefinePropertyOrThrow(O, key, desc).
  6. Return undefined.

20.1.3.9.3 Object.prototype.__lookupGetter__ ( P )

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let key be ? ToPropertyKey(P).
  3. Repeat,
    1. Let desc be ? O.[[GetOwnProperty]](key).
    2. If desc is not undefined, then
      1. If IsAccessorDescriptor(desc) is true, return desc.[[Get]].
      2. Return undefined.
    3. Set O to ? O.[[GetPrototypeOf]]().
    4. If O is null, return undefined.

20.1.3.9.4 Object.prototype.__lookupSetter__ ( P )

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let key be ? ToPropertyKey(P).
  3. Repeat,
    1. Let desc be ? O.[[GetOwnProperty]](key).
    2. If desc is not undefined, then
      1. If IsAccessorDescriptor(desc) is true, return desc.[[Set]].
      2. Return undefined.
    3. Set O to ? O.[[GetPrototypeOf]]().
    4. If O is null, return undefined.

20.1.4 Properties of Object Instances

Object instances have no special properties beyond those inherited from the Object prototype object.

20.2 Function Objects

20.2.1 The Function Constructor

The Function constructor:

  • is %Function%.
  • is the initial value of the "Function" property of the global object.
  • creates and initializes a new function object when called as a function rather than as a constructor. Thus the function call Function(…) is equivalent to the object creation expression new Function(…) with the same arguments.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Function behaviour must include a super call to the Function constructor to create and initialize a subclass instance with the internal slots necessary for built-in function behaviour. All ECMAScript syntactic forms for defining function objects create instances of Function. There is no syntactic means to create instances of Function subclasses except for the built-in GeneratorFunction, AsyncFunction, and AsyncGeneratorFunction subclasses.

20.2.1.1 Function ( ...parameterArgs, bodyArg )

The last argument (if any) specifies the body (executable code) of a function; any preceding arguments specify formal parameters.

This function performs the following steps when called:

  1. Let C be the active function object.
  2. If bodyArg is not present, set bodyArg to the empty String.
  3. Return ? CreateDynamicFunction(C, NewTarget, normal, parameterArgs, bodyArg).
 Note

It is permissible but not necessary to have one argument for each formal parameter to be specified. For example, all three of the following expressions produce the same result:

new Function("a", "b", "c", "return a b c")
new Function("a, b, c", "return a b c")
new Function("a,b", "c", "return a b c")

20.2.1.1.1 CreateDynamicFunction ( constructor, newTarget, kind, parameterArgs, bodyArg )

The abstract operation CreateDynamicFunction takes arguments constructor (a constructor), newTarget (a constructor), kind (normal, generator, async, or asyncGenerator), parameterArgs (a List of ECMAScript language values), and bodyArg (an ECMAScript language value) and returns either a normal completion containing a function object or a throw completion. constructor is the constructor function that is performing this action. newTarget is the constructor that new was initially applied to. parameterArgs and bodyArg reflect the argument values that were passed to constructor. It performs the following steps when called:

  1. Let currentRealm be the current Realm Record.
  2. Perform ? HostEnsureCanCompileStrings(currentRealm).
  3. If newTarget is undefined, set newTarget to constructor.
  4. If kind is normal, then
    1. Let prefix be "function".
    2. Let exprSym be the grammar symbol FunctionExpression.
    3. Let bodySym be the grammar symbol FunctionBody[~Yield, ~Await].
    4. Let parameterSym be the grammar symbol FormalParameters[~Yield, ~Await].
    5. Let fallbackProto be "%Function.prototype%".
  5. Else if kind is generator, then
    1. Let prefix be "function*".
    2. Let exprSym be the grammar symbol GeneratorExpression.
    3. Let bodySym be the grammar symbol GeneratorBody.
    4. Let parameterSym be the grammar symbol FormalParameters[ Yield, ~Await].
    5. Let fallbackProto be "%GeneratorFunction.prototype%".
  6. Else if kind is async, then
    1. Let prefix be "async function".
    2. Let exprSym be the grammar symbol AsyncFunctionExpression.
    3. Let bodySym be the grammar symbol AsyncFunctionBody.
    4. Let parameterSym be the grammar symbol FormalParameters[~Yield, Await].
    5. Let fallbackProto be "%AsyncFunction.prototype%".
  7. Else,
    1. Assert: kind is asyncGenerator.
    2. Let prefix be "async function*".
    3. Let exprSym be the grammar symbol AsyncGeneratorExpression.
    4. Let bodySym be the grammar symbol AsyncGeneratorBody.
    5. Let parameterSym be the grammar symbol FormalParameters[ Yield, Await].
    6. Let fallbackProto be "%AsyncGeneratorFunction.prototype%".
  8. Let argCount be the number of elements in parameterArgs.
  9. Let P be the empty String.
  10. If argCount > 0, then
    1. Let firstArg be parameterArgs[0].
    2. Set P to ? ToString(firstArg).
    3. Let k be 1.
    4. Repeat, while k < argCount,
      1. Let nextArg be parameterArgs[k].
      2. Let nextArgString be ? ToString(nextArg).
      3. Set P to the string-concatenation of P, "," (a comma), and nextArgString.
      4. Set k to k 1.
  11. Let bodyString be the string-concatenation of 0x000A (LINE FEED), ? ToString(bodyArg), and 0x000A (LINE FEED).
  12. Let sourceString be the string-concatenation of prefix, " anonymous(", P, 0x000A (LINE FEED), ") {", bodyString, and "}".
  13. Let sourceText be StringToCodePoints(sourceString).
  14. Let parameters be ParseText(StringToCodePoints(P), parameterSym).
  15. If parameters is a List of errors, throw a SyntaxError exception.
  16. Let body be ParseText(StringToCodePoints(bodyString), bodySym).
  17. If body is a List of errors, throw a SyntaxError exception.
  18. NOTE: The parameters and body are parsed separately to ensure that each is valid alone. For example, new Function("/*", "*/ ) {") does not evaluate to a function.
  19. NOTE: If this step is reached, sourceText must have the syntax of exprSym (although the reverse implication does not hold). The purpose of the next two steps is to enforce any Early Error rules which apply to exprSym directly.
  20. Let expr be ParseText(sourceText, exprSym).
  21. If expr is a List of errors, throw a SyntaxError exception.
  22. Let proto be ? GetPrototypeFromConstructor(newTarget, fallbackProto).
  23. Let realmF be the current Realm Record.
  24. Let env be realmF.[[GlobalEnv]].
  25. Let privateEnv be null.
  26. Let F be OrdinaryFunctionCreate(proto, sourceText, parameters, body, non-lexical-this, env, privateEnv).
  27. Perform SetFunctionName(F, "anonymous").
  28. If kind is generator, then
    1. Let prototype be OrdinaryObjectCreate(%GeneratorFunction.prototype.prototype%).
    2. Perform ! DefinePropertyOrThrow(F, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
  29. Else if kind is asyncGenerator, then
    1. Let prototype be OrdinaryObjectCreate(%AsyncGeneratorFunction.prototype.prototype%).
    2. Perform ! DefinePropertyOrThrow(F, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
  30. Else if kind is normal, perform MakeConstructor(F).
  31. NOTE: Functions whose kind is async are not constructible and do not have a [[Construct]] internal method or a "prototype" property.
  32. Return F.
 Note

CreateDynamicFunction defines a "prototype" property on any function it creates whose kind is not async to provide for the possibility that the function will be used as a constructor.

20.2.2 Properties of the Function Constructor

The Function constructor:

20.2.2.1 Function.length

This is a data property with a value of 1. This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

20.2.2.2 Function.prototype

The value of Function.prototype is the Function prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.2.3 Properties of the Function Prototype Object

The Function prototype object:

  • is %Function.prototype%.
  • is itself a built-in function object.
  • accepts any arguments and returns undefined when invoked.
  • does not have a [[Construct]] internal method; it cannot be used as a constructor with the new operator.
  • has a [[Prototype]] internal slot whose value is %Object.prototype%.
  • does not have a "prototype" property.
  • has a "length" property whose value is 0𝔽.
  • has a "name" property whose value is the empty String.
Note

The Function prototype object is specified to be a function object to ensure compatibility with ECMAScript code that was created prior to the ECMAScript 2015 specification.

20.2.3.1 Function.prototype.apply ( thisArg, argArray )

This method performs the following steps when called:

  1. Let func be the this value.
  2. If IsCallable(func) is false, throw a TypeError exception.
  3. If argArray is either undefined or null, then
    1. Perform PrepareForTailCall().
    2. Return ? Call(func, thisArg).
  4. Let argList be ? CreateListFromArrayLike(argArray).
  5. Perform PrepareForTailCall().
  6. Return ? Call(func, thisArg, argList).
 Note 1

The thisArg value is passed without modification as the this value. This is a change from Edition 3, where an undefined or null thisArg is replaced with the global object and ToObject is applied to all other values and that result is passed as the this value. Even though the thisArg is passed without modification, non-strict functions still perform these transformations upon entry to the function.

 Note 2

If func is either an arrow function or a bound function exotic object, then the thisArg will be ignored by the function [[Call]] in step 6.

20.2.3.2 Function.prototype.bind ( thisArg, ...args )

This method performs the following steps when called:

  1. Let Target be the this value.
  2. If IsCallable(Target) is false, throw a TypeError exception.
  3. Let F be ? BoundFunctionCreate(Target, thisArg, args).
  4. Let L be 0.
  5. Let targetHasLength be ? HasOwnProperty(Target, "length").
  6. If targetHasLength is true, then
    1. Let targetLen be ? Get(Target, "length").
    2. If targetLen is a Number, then
      1. If targetLen is ∞𝔽, set L to ∞.
      2. Else if targetLen is -∞𝔽, set L to 0.
      3. Else,
        1. Let targetLenAsInt be ! ToIntegerOrInfinity(targetLen).
        2. Assert: targetLenAsInt is finite.
        3. Let argCount be the number of elements in args.
        4. Set L to max(targetLenAsInt - argCount, 0).
  7. Perform SetFunctionLength(F, L).
  8. Let targetName be ? Get(Target, "name").
  9. If targetName is not a String, set targetName to the empty String.
  10. Perform SetFunctionName(F, targetName, "bound").
  11. Return F.
 Note 1

Function objects created using Function.prototype.bind are exotic objects. They also do not have a "prototype" property.

 Note 2

If Target is either an arrow function or a bound function exotic object, then the thisArg passed to this method will not be used by subsequent calls to F.

20.2.3.3 Function.prototype.call ( thisArg, ...args )

This method performs the following steps when called:

  1. Let func be the this value.
  2. If IsCallable(func) is false, throw a TypeError exception.
  3. Perform PrepareForTailCall().
  4. Return ? Call(func, thisArg, args).
 Note 1

The thisArg value is passed without modification as the this value. This is a change from Edition 3, where an undefined or null thisArg is replaced with the global object and ToObject is applied to all other values and that result is passed as the this value. Even though the thisArg is passed without modification, non-strict functions still perform these transformations upon entry to the function.

 Note 2

If func is either an arrow function or a bound function exotic object, then the thisArg will be ignored by the function [[Call]] in step 4.

20.2.3.4 Function.prototype.constructor

The initial value of Function.prototype.constructor is %Function%.

20.2.3.5 Function.prototype.toString ( )

This method performs the following steps when called:

  1. Let func be the this value.
  2. If func is an Object, func has a [[SourceText]] internal slot, func.[[SourceText]] is a sequence of Unicode code points, and HostHasSourceTextAvailable(func) is true, then
    1. Return CodePointsToString(func.[[SourceText]]).
  3. If func is a built-in function object, return an implementation-defined String source code representation of func. The representation must have the syntax of a NativeFunction. Additionally, if func has an [[InitialName]] internal slot and func.[[InitialName]] is a String, the portion of the returned String that would be matched by NativeFunctionAccessoropt PropertyName must be the value of func.[[InitialName]].
  4. If func is an Object and IsCallable(func) is true, return an implementation-defined String source code representation of func. The representation must have the syntax of a NativeFunction.
  5. Throw a TypeError exception.
 NativeFunction : function NativeFunctionAccessoropt PropertyName[~Yield, ~Await]opt ( FormalParameters[~Yield, ~Await] ) { [ native code ] } NativeFunctionAccessor : get set

20.2.3.6 Function.prototype [ @@hasInstance ] ( V )

This method performs the following steps when called:

  1. Let F be the this value.
  2. Return ? OrdinaryHasInstance(F, V).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

Note

This is the default implementation of @@hasInstance that most functions inherit. @@hasInstance is called by the instanceof operator to determine whether a value is an instance of a specific constructor. An expression such as

v instanceof F

evaluates as

F[@@hasInstance](v)

A constructor function can control which objects are recognized as its instances by instanceof by exposing a different @@hasInstance method on the function.

This property is non-writable and non-configurable to prevent tampering that could be used to globally expose the target function of a bound function.

The value of the "name" property of this method is "[Symbol.hasInstance]".

20.2.4 Function Instances

Every Function instance is an ECMAScript function object and has the internal slots listed in Table 30. Function objects created using the Function.prototype.bind method (20.2.3.2) have the internal slots listed in Table 31.

Function instances have the following properties:

20.2.4.1 length

The value of the "length" property is an integral Number that indicates the typical number of arguments expected by the function. However, the language permits the function to be invoked with some other number of arguments. The behaviour of a function when invoked on a number of arguments other than the number specified by its "length" property depends on the function. This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

20.2.4.2 name

The value of the "name" property is a String that is descriptive of the function. The name has no semantic significance but is typically a variable or property name that is used to refer to the function at its point of definition in ECMAScript source text. This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

Anonymous functions objects that do not have a contextual name associated with them by this specification use the empty String as the value of the "name" property.

20.2.4.3 prototype

Function instances that can be used as a constructor have a "prototype" property. Whenever such a Function instance is created another ordinary object is also created and is the initial value of the function's "prototype" property. Unless otherwise specified, the value of the "prototype" property is used to initialize the [[Prototype]] internal slot of the object created when that function is invoked as a constructor.

This property has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.

Note

Function objects created using Function.prototype.bind, or by evaluating a MethodDefinition (that is not a GeneratorMethod or AsyncGeneratorMethod) or an ArrowFunction do not have a "prototype" property.

20.2.5 HostHasSourceTextAvailable ( func )

The host-defined abstract operation HostHasSourceTextAvailable takes argument func (a function object) and returns a Boolean. It allows host environments to prevent the source text from being provided for func.

An implementation of HostHasSourceTextAvailable must conform to the following requirements:

  • It must be deterministic with respect to its parameters. Each time it is called with a specific func as its argument, it must return the same result.

The default implementation of HostHasSourceTextAvailable is to return true.

20.3 Boolean Objects

20.3.1 The Boolean Constructor

The Boolean constructor:

  • is %Boolean%.
  • is the initial value of the "Boolean" property of the global object.
  • creates and initializes a new Boolean object when called as a constructor.
  • performs a type conversion when called as a function rather than as a constructor.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Boolean behaviour must include a super call to the Boolean constructor to create and initialize the subclass instance with a [[BooleanData]] internal slot.

20.3.1.1 Boolean ( value )

This function performs the following steps when called:

  1. Let b be ToBoolean(value).
  2. If NewTarget is undefined, return b.
  3. Let O be ? OrdinaryCreateFromConstructor(NewTarget, "%Boolean.prototype%", « [[BooleanData]] »).
  4. Set O.[[BooleanData]] to b.
  5. Return O.

20.3.2 Properties of the Boolean Constructor

The Boolean constructor:

  • has a [[Prototype]] internal slot whose value is %Function.prototype%.
  • has the following properties:

20.3.2.1 Boolean.prototype

The initial value of Boolean.prototype is the Boolean prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.3.3 Properties of the Boolean Prototype Object

The Boolean prototype object:

  • is %Boolean.prototype%.
  • is an ordinary object.
  • is itself a Boolean object; it has a [[BooleanData]] internal slot with the value false.
  • has a [[Prototype]] internal slot whose value is %Object.prototype%.

The abstract operation thisBooleanValue takes argument value. It performs the following steps when called:

  1. If value is a Boolean, return value.
  2. If value is an Object and value has a [[BooleanData]] internal slot, then
    1. Let b be value.[[BooleanData]].
    2. Assert: b is a Boolean.
    3. Return b.
  3. Throw a TypeError exception.

20.3.3.1 Boolean.prototype.constructor

The initial value of Boolean.prototype.constructor is %Boolean%.

20.3.3.2 Boolean.prototype.toString ( )

This method performs the following steps when called:

  1. Let b be ? thisBooleanValue(this value).
  2. If b is true, return "true"; else return "false".

20.3.3.3 Boolean.prototype.valueOf ( )

This method performs the following steps when called:

  1. Return ? thisBooleanValue(this value).

20.3.4 Properties of Boolean Instances

Boolean instances are ordinary objects that inherit properties from the Boolean prototype object. Boolean instances have a [[BooleanData]] internal slot. The [[BooleanData]] internal slot is the Boolean value represented by this Boolean object.

20.4 Symbol Objects

20.4.1 The Symbol Constructor

The Symbol constructor:

  • is %Symbol%.
  • is the initial value of the "Symbol" property of the global object.
  • returns a new Symbol value when called as a function.
  • is not intended to be used with the new operator.
  • is not intended to be subclassed.
  • may be used as the value of an extends clause of a class definition but a super call to it will cause an exception.

20.4.1.1 Symbol ( [ description ] )

This function performs the following steps when called:

  1. If NewTarget is not undefined, throw a TypeError exception.
  2. If description is undefined, let descString be undefined.
  3. Else, let descString be ? ToString(description).
  4. Return a new Symbol whose [[Description]] is descString.

20.4.2 Properties of the Symbol Constructor

The Symbol constructor:

  • has a [[Prototype]] internal slot whose value is %Function.prototype%.
  • has the following properties:

20.4.2.1 Symbol.asyncIterator

The initial value of Symbol.asyncIterator is the well known symbol @@asyncIterator (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.4.2.2 Symbol.for ( key )

This function performs the following steps when called:

  1. Let stringKey be ? ToString(key).
  2. For each element e of the GlobalSymbolRegistry List, do
    1. If SameValue(e.[[Key]], stringKey) is true, return e.[[Symbol]].
  3. Assert: GlobalSymbolRegistry does not currently contain an entry for stringKey.
  4. Let newSymbol be a new Symbol whose [[Description]] is stringKey.
  5. Append the Record { [[Key]]: stringKey, [[Symbol]]: newSymbol } to the GlobalSymbolRegistry List.
  6. Return newSymbol.

The GlobalSymbolRegistry is an append-only List that is globally available. It is shared by all realms. Prior to the evaluation of any ECMAScript code, it is initialized as a new empty List. Elements of the GlobalSymbolRegistry are Records with the structure defined in Table 59.

Table 59: GlobalSymbolRegistry Record Fields
Field Name Value Usage
[[Key]] a String A string key used to globally identify a Symbol.
[[Symbol]] a Symbol A symbol that can be retrieved from any realm.

20.4.2.3 Symbol.hasInstance

The initial value of Symbol.hasInstance is the well-known symbol @@hasInstance (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.4.2.4 Symbol.isConcatSpreadable

The initial value of Symbol.isConcatSpreadable is the well-known symbol @@isConcatSpreadable (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.4.2.5 Symbol.iterator

The initial value of Symbol.iterator is the well-known symbol @@iterator (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.4.2.6 Symbol.keyFor ( sym )

This function performs the following steps when called:

  1. If sym is not a Symbol, throw a TypeError exception.
  2. Return KeyForSymbol(sym).

20.4.2.7 Symbol.match

The initial value of Symbol.match is the well-known symbol @@match (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.4.2.8 Symbol.matchAll

The initial value of Symbol.matchAll is the well-known symbol @@matchAll (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.4.2.9 Symbol.prototype

The initial value of Symbol.prototype is the Symbol prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.4.2.10 Symbol.replace

The initial value of Symbol.replace is the well-known symbol @@replace (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.4.2.11 Symbol.search

The initial value of Symbol.search is the well-known symbol @@search (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.4.2.12 Symbol.species

The initial value of Symbol.species is the well-known symbol @@species (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.4.2.13 Symbol.split

The initial value of Symbol.split is the well-known symbol @@split (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.4.2.14 Symbol.toPrimitive

The initial value of Symbol.toPrimitive is the well-known symbol @@toPrimitive (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.4.2.15 Symbol.toStringTag

The initial value of Symbol.toStringTag is the well-known symbol @@toStringTag (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.4.2.16 Symbol.unscopables

The initial value of Symbol.unscopables is the well-known symbol @@unscopables (Table 1).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.4.3 Properties of the Symbol Prototype Object

The Symbol prototype object:

The abstract operation thisSymbolValue takes argument value. It performs the following steps when called:

  1. If value is a Symbol, return value.
  2. If value is an Object and value has a [[SymbolData]] internal slot, then
    1. Let s be value.[[SymbolData]].
    2. Assert: s is a Symbol.
    3. Return s.
  3. Throw a TypeError exception.

20.4.3.1 Symbol.prototype.constructor

The initial value of Symbol.prototype.constructor is %Symbol%.

20.4.3.2 get Symbol.prototype.description

Symbol.prototype.description is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Let s be the this value.
  2. Let sym be ? thisSymbolValue(s).
  3. Return sym.[[Description]].

20.4.3.3 Symbol.prototype.toString ( )

This method performs the following steps when called:

  1. Let sym be ? thisSymbolValue(this value).
  2. Return SymbolDescriptiveString(sym).

20.4.3.3.1 SymbolDescriptiveString ( sym )

The abstract operation SymbolDescriptiveString takes argument sym (a Symbol) and returns a String. It performs the following steps when called:

  1. Let desc be sym's [[Description]] value.
  2. If desc is undefined, set desc to the empty String.
  3. Assert: desc is a String.
  4. Return the string-concatenation of "Symbol(", desc, and ")".

20.4.3.4 Symbol.prototype.valueOf ( )

This method performs the following steps when called:

  1. Return ? thisSymbolValue(this value).

20.4.3.5 Symbol.prototype [ @@toPrimitive ] ( hint )

This method is called by ECMAScript language operators to convert a Symbol object to a primitive value.

It performs the following steps when called:

  1. Return ? thisSymbolValue(this value).
 Note

The argument is ignored.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

The value of the "name" property of this method is "[Symbol.toPrimitive]".

20.4.3.6 Symbol.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "Symbol".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

20.4.4 Properties of Symbol Instances

Symbol instances are ordinary objects that inherit properties from the Symbol prototype object. Symbol instances have a [[SymbolData]] internal slot. The [[SymbolData]] internal slot is the Symbol value represented by this Symbol object.

20.4.5 Abstract Operations for Symbols

20.4.5.1 KeyForSymbol ( sym )

The abstract operation KeyForSymbol takes argument sym (a Symbol) and returns a String or undefined. If sym is in the GlobalSymbolRegistry (see 20.4.2.2) the String used to register sym will be returned. It performs the following steps when called:

  1. For each element e of the GlobalSymbolRegistry List, do
    1. If SameValue(e.[[Symbol]], sym) is true, return e.[[Key]].
  2. Assert: GlobalSymbolRegistry does not currently contain an entry for sym.
  3. Return undefined.

20.5 Error Objects

Instances of Error objects are thrown as exceptions when runtime errors occur. The Error objects may also serve as base objects for user-defined exception classes.

When an ECMAScript implementation detects a runtime error, it throws a new instance of one of the NativeError objects defined in 20.5.5 or a new instance of AggregateError object defined in 20.5.7. Each of these objects has the structure described below, differing only in the name used as the constructor name instead of NativeError, in the "name" property of the prototype object, in the implementation-defined "message" property of the prototype object, and in the presence of the %AggregateError%-specific "errors" property.

20.5.1 The Error Constructor

The Error constructor:

  • is %Error%.
  • is the initial value of the "Error" property of the global object.
  • creates and initializes a new Error object when called as a function rather than as a constructor. Thus the function call Error(…) is equivalent to the object creation expression new Error(…) with the same arguments.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Error behaviour must include a super call to the Error constructor to create and initialize subclass instances with an [[ErrorData]] internal slot.

20.5.1.1 Error ( message [ , options ] )

This function performs the following steps when called:

  1. If NewTarget is undefined, let newTarget be the active function object; else let newTarget be NewTarget.
  2. Let O be ? OrdinaryCreateFromConstructor(newTarget, "%Error.prototype%", « [[ErrorData]] »).
  3. If message is not undefined, then
    1. Let msg be ? ToString(message).
    2. Perform CreateNonEnumerableDataPropertyOrThrow(O, "message", msg).
  4. Perform ? InstallErrorCause(O, options).
  5. Return O.

20.5.2 Properties of the Error Constructor

The Error constructor:

  • has a [[Prototype]] internal slot whose value is %Function.prototype%.
  • has the following properties:

20.5.2.1 Error.prototype

The initial value of Error.prototype is the Error prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.5.3 Properties of the Error Prototype Object

The Error prototype object:

  • is %Error.prototype%.
  • is an ordinary object.
  • is not an Error instance and does not have an [[ErrorData]] internal slot.
  • has a [[Prototype]] internal slot whose value is %Object.prototype%.

20.5.3.1 Error.prototype.constructor

The initial value of Error.prototype.constructor is %Error%.

20.5.3.2 Error.prototype.message

The initial value of Error.prototype.message is the empty String.

20.5.3.3 Error.prototype.name

The initial value of Error.prototype.name is "Error".

20.5.3.4 Error.prototype.toString ( )

This method performs the following steps when called:

  1. Let O be the this value.
  2. If O is not an Object, throw a TypeError exception.
  3. Let name be ? Get(O, "name").
  4. If name is undefined, set name to "Error"; otherwise set name to ? ToString(name).
  5. Let msg be ? Get(O, "message").
  6. If msg is undefined, set msg to the empty String; otherwise set msg to ? ToString(msg).
  7. If name is the empty String, return msg.
  8. If msg is the empty String, return name.
  9. Return the string-concatenation of name, the code unit 0x003A (COLON), the code unit 0x0020 (SPACE), and msg.

20.5.4 Properties of Error Instances

Error instances are ordinary objects that inherit properties from the Error prototype object and have an [[ErrorData]] internal slot whose value is undefined. The only specified uses of [[ErrorData]] is to identify Error, AggregateError, and NativeError instances as Error objects within Object.prototype.toString.

20.5.5 Native Error Types Used in This Standard

A new instance of one of the NativeError objects below or of the AggregateError object is thrown when a runtime error is detected. All NativeError objects share the same structure, as described in 20.5.6.

20.5.5.1 EvalError

The EvalError constructor is %EvalError%.

This exception is not currently used within this specification. This object remains for compatibility with previous editions of this specification.

20.5.5.2 RangeError

The RangeError constructor is %RangeError%.

Indicates a value that is not in the set or range of allowable values.

20.5.5.3 ReferenceError

The ReferenceError constructor is %ReferenceError%.

Indicate that an invalid reference has been detected.

20.5.5.4 SyntaxError

The SyntaxError constructor is %SyntaxError%.

Indicates that a parsing error has occurred.

20.5.5.5 TypeError

The TypeError constructor is %TypeError%.

TypeError is used to indicate an unsuccessful operation when none of the other NativeError objects are an appropriate indication of the failure cause.

20.5.5.6 URIError

The URIError constructor is %URIError%.

Indicates that one of the global URI handling functions was used in a way that is incompatible with its definition.

20.5.6 NativeError Object Structure

When an ECMAScript implementation detects a runtime error, it throws a new instance of one of the NativeError objects defined in 20.5.5. Each of these objects has the structure described below, differing only in the name used as the constructor name instead of NativeError, in the "name" property of the prototype object, and in the implementation-defined "message" property of the prototype object.

For each error object, references to NativeError in the definition should be replaced with the appropriate error object name from 20.5.5.

20.5.6.1 The NativeError Constructors

Each NativeError constructor:

  • creates and initializes a new NativeError object when called as a function rather than as a constructor. A call of the object as a function is equivalent to calling it as a constructor with the same arguments. Thus the function call NativeError(…) is equivalent to the object creation expression new NativeError(…) with the same arguments.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified NativeError behaviour must include a super call to the NativeError constructor to create and initialize subclass instances with an [[ErrorData]] internal slot.

20.5.6.1.1 NativeError ( message [ , options ] )

Each NativeError function performs the following steps when called:

  1. If NewTarget is undefined, let newTarget be the active function object; else let newTarget be NewTarget.
  2. Let O be ? OrdinaryCreateFromConstructor(newTarget, "%NativeError.prototype%", « [[ErrorData]] »).
  3. If message is not undefined, then
    1. Let msg be ? ToString(message).
    2. Perform CreateNonEnumerableDataPropertyOrThrow(O, "message", msg).
  4. Perform ? InstallErrorCause(O, options).
  5. Return O.

The actual value of the string passed in step 2 is either "%EvalError.prototype%", "%RangeError.prototype%", "%ReferenceError.prototype%", "%SyntaxError.prototype%", "%TypeError.prototype%", or "%URIError.prototype%" corresponding to which NativeError constructor is being defined.

20.5.6.2 Properties of the NativeError Constructors

Each NativeError constructor:

  • has a [[Prototype]] internal slot whose value is %Error%.
  • has a "name" property whose value is the String value "NativeError".
  • has the following properties:

20.5.6.2.1 NativeError.prototype

The initial value of NativeError.prototype is a NativeError prototype object (20.5.6.3). Each NativeError constructor has a distinct prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.5.6.3 Properties of the NativeError Prototype Objects

Each NativeError prototype object:

  • is an ordinary object.
  • is not an Error instance and does not have an [[ErrorData]] internal slot.
  • has a [[Prototype]] internal slot whose value is %Error.prototype%.

20.5.6.3.1 NativeError.prototype.constructor

The initial value of the "constructor" property of the prototype for a given NativeError constructor is the corresponding intrinsic object %NativeError% (20.5.6.1).

20.5.6.3.2 NativeError.prototype.message

The initial value of the "message" property of the prototype for a given NativeError constructor is the empty String.

20.5.6.3.3 NativeError.prototype.name

The initial value of the "name" property of the prototype for a given NativeError constructor is the String value consisting of the name of the constructor (the name used instead of NativeError).

20.5.6.4 Properties of NativeError Instances

NativeError instances are ordinary objects that inherit properties from their NativeError prototype object and have an [[ErrorData]] internal slot whose value is undefined. The only specified use of [[ErrorData]] is by Object.prototype.toString (20.1.3.6) to identify Error, AggregateError, or NativeError instances.

20.5.7 AggregateError Objects

20.5.7.1 The AggregateError Constructor

The AggregateError constructor:

  • is %AggregateError%.
  • is the initial value of the "AggregateError" property of the global object.
  • creates and initializes a new AggregateError object when called as a function rather than as a constructor. Thus the function call AggregateError(…) is equivalent to the object creation expression new AggregateError(…) with the same arguments.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified AggregateError behaviour must include a super call to the AggregateError constructor to create and initialize subclass instances with an [[ErrorData]] internal slot.

20.5.7.1.1 AggregateError ( errors, message [ , options ] )

This function performs the following steps when called:

  1. If NewTarget is undefined, let newTarget be the active function object; else let newTarget be NewTarget.
  2. Let O be ? OrdinaryCreateFromConstructor(newTarget, "%AggregateError.prototype%", « [[ErrorData]] »).
  3. If message is not undefined, then
    1. Let msg be ? ToString(message).
    2. Perform CreateNonEnumerableDataPropertyOrThrow(O, "message", msg).
  4. Perform ? InstallErrorCause(O, options).
  5. Let errorsList be ? IteratorToList(? GetIterator(errors, sync)).
  6. Perform ! DefinePropertyOrThrow(O, "errors", PropertyDescriptor { [[Configurable]]: true, [[Enumerable]]: false, [[Writable]]: true, [[Value]]: CreateArrayFromList(errorsList) }).
  7. Return O.

20.5.7.2 Properties of the AggregateError Constructor

The AggregateError constructor:

  • has a [[Prototype]] internal slot whose value is %Error%.
  • has the following properties:

20.5.7.2.1 AggregateError.prototype

The initial value of AggregateError.prototype is %AggregateError.prototype%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

20.5.7.3 Properties of the AggregateError Prototype Object

The AggregateError prototype object:

  • is %AggregateError.prototype%.
  • is an ordinary object.
  • is not an Error instance or an AggregateError instance and does not have an [[ErrorData]] internal slot.
  • has a [[Prototype]] internal slot whose value is %Error.prototype%.

20.5.7.3.1 AggregateError.prototype.constructor

The initial value of AggregateError.prototype.constructor is %AggregateError%.

20.5.7.3.2 AggregateError.prototype.message

The initial value of AggregateError.prototype.message is the empty String.

20.5.7.3.3 AggregateError.prototype.name

The initial value of AggregateError.prototype.name is "AggregateError".

20.5.7.4 Properties of AggregateError Instances

AggregateError instances are ordinary objects that inherit properties from their AggregateError prototype object and have an [[ErrorData]] internal slot whose value is undefined. The only specified use of [[ErrorData]] is by Object.prototype.toString (20.1.3.6) to identify Error, AggregateError, or NativeError instances.

20.5.8 Abstract Operations for Error Objects

20.5.8.1 InstallErrorCause ( O, options )

The abstract operation InstallErrorCause takes arguments O (an Object) and options (an ECMAScript language value) and returns either a normal completion containing unused or a throw completion. It is used to create a "cause" property on O when a "cause" property is present on options. It performs the following steps when called:

  1. If options is an Object and ? HasProperty(options, "cause") is true, then
    1. Let cause be ? Get(options, "cause").
    2. Perform CreateNonEnumerableDataPropertyOrThrow(O, "cause", cause).
  2. Return unused.

21 Numbers and Dates

21.1 Number Objects

21.1.1 The Number Constructor

The Number constructor:

  • is %Number%.
  • is the initial value of the "Number" property of the global object.
  • creates and initializes a new Number object when called as a constructor.
  • performs a type conversion when called as a function rather than as a constructor.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Number behaviour must include a super call to the Number constructor to create and initialize the subclass instance with a [[NumberData]] internal slot.

21.1.1.1 Number ( value )

This function performs the following steps when called:

  1. If value is present, then
    1. Let prim be ? ToNumeric(value).
    2. If prim is a BigInt, let n be 𝔽(ℝ(prim)).
    3. Otherwise, let n be prim.
  2. Else,
    1. Let n be 0𝔽.
  3. If NewTarget is undefined, return n.
  4. Let O be ? OrdinaryCreateFromConstructor(NewTarget, "%Number.prototype%", « [[NumberData]] »).
  5. Set O.[[NumberData]] to n.
  6. Return O.

21.1.2 Properties of the Number Constructor

The Number constructor:

  • has a [[Prototype]] internal slot whose value is %Function.prototype%.
  • has the following properties:

21.1.2.1 Number.EPSILON

The value of Number.EPSILON is the Number value for the magnitude of the difference between 1 and the smallest value greater than 1 that is representable as a Number value, which is approximately 2.2204460492503130808472633361816 × 10-16.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

21.1.2.2 Number.isFinite ( number )

This function performs the following steps when called:

  1. If number is not a Number, return false.
  2. If number is not finite, return false.
  3. Otherwise, return true.

21.1.2.3 Number.isInteger ( number )

This function performs the following steps when called:

  1. Return IsIntegralNumber(number).

21.1.2.4 Number.isNaN ( number )

This function performs the following steps when called:

  1. If number is not a Number, return false.
  2. If number is NaN, return true.
  3. Otherwise, return false.
 Note

This function differs from the global isNaN function (19.2.3) in that it does not convert its argument to a Number before determining whether it is NaN.

21.1.2.5 Number.isSafeInteger ( number )

Note

An integer n is considered "safe" if and only if the Number value for n is not the Number value for any other integer.

This function performs the following steps when called:

  1. If IsIntegralNumber(number) is true, then
    1. If abs(ℝ(number)) ≤ 253 - 1, return true.
  2. Return false.

21.1.2.6 Number.MAX_SAFE_INTEGER

Note

Due to rounding behaviour necessitated by precision limitations of IEEE 754-2019, the Number value for every integer greater than Number.MAX_SAFE_INTEGER is shared with at least one other integer. Such large-magnitude integers are therefore not safe, and are not guaranteed to be exactly representable as Number values or even to be distinguishable from each other. For example, both 9007199254740992 and 9007199254740993 evaluate to the Number value 9007199254740992𝔽.

The value of Number.MAX_SAFE_INTEGER is 9007199254740991𝔽 (𝔽(253 - 1)).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

21.1.2.7 Number.MAX_VALUE

The value of Number.MAX_VALUE is the largest positive finite value of the Number type, which is approximately 1.7976931348623157 × 10308.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

21.1.2.8 Number.MIN_SAFE_INTEGER

Note

Due to rounding behaviour necessitated by precision limitations of IEEE 754-2019, the Number value for every integer less than Number.MIN_SAFE_INTEGER is shared with at least one other integer. Such large-magnitude integers are therefore not safe, and are not guaranteed to be exactly representable as Number values or even to be distinguishable from each other. For example, both -9007199254740992 and -9007199254740993 evaluate to the Number value -9007199254740992𝔽.

The value of Number.MIN_SAFE_INTEGER is -9007199254740991𝔽 (𝔽(-(253 - 1))).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

21.1.2.9 Number.MIN_VALUE

The value of Number.MIN_VALUE is the smallest positive value of the Number type, which is approximately 5 × 10-324.

In the IEEE 754-2019 double precision binary representation, the smallest possible value is a denormalized number. If an implementation does not support denormalized values, the value of Number.MIN_VALUE must be the smallest non-zero positive value that can actually be represented by the implementation.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

21.1.2.10 Number.NaN

The value of Number.NaN is NaN.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

21.1.2.11 Number.NEGATIVE_INFINITY

The value of Number.NEGATIVE_INFINITY is -∞𝔽.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

21.1.2.12 Number.parseFloat ( string )

The initial value of the "parseFloat" property is %parseFloat%.

21.1.2.13 Number.parseInt ( string, radix )

The initial value of the "parseInt" property is %parseInt%.

21.1.2.14 Number.POSITIVE_INFINITY

The value of Number.POSITIVE_INFINITY is ∞𝔽.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

21.1.2.15 Number.prototype

The initial value of Number.prototype is the Number prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

21.1.3 Properties of the Number Prototype Object

The Number prototype object:

  • is %Number.prototype%.
  • is an ordinary object.
  • is itself a Number object; it has a [[NumberData]] internal slot with the value 0𝔽.
  • has a [[Prototype]] internal slot whose value is %Object.prototype%.

Unless explicitly stated otherwise, the methods of the Number prototype object defined below are not generic and the this value passed to them must be either a Number value or an object that has a [[NumberData]] internal slot that has been initialized to a Number value.

The abstract operation thisNumberValue takes argument value. It performs the following steps when called:

  1. If value is a Number, return value.
  2. If value is an Object and value has a [[NumberData]] internal slot, then
    1. Let n be value.[[NumberData]].
    2. Assert: n is a Number.
    3. Return n.
  3. Throw a TypeError exception.

The phrase “this Number value” within the specification of a method refers to the result returned by calling the abstract operation thisNumberValue with the this value of the method invocation passed as the argument.

21.1.3.1 Number.prototype.constructor

The initial value of Number.prototype.constructor is %Number%.

21.1.3.2 Number.prototype.toExponential ( fractionDigits )

This method returns a String containing this Number value represented in decimal exponential notation with one digit before the significand's decimal point and fractionDigits digits after the significand's decimal point. If fractionDigits is undefined, it includes as many significand digits as necessary to uniquely specify the Number (just like in ToString except that in this case the Number is always output in exponential notation).

It performs the following steps when called:

  1. Let x be ? thisNumberValue(this value).
  2. Let f be ? ToIntegerOrInfinity(fractionDigits).
  3. Assert: If fractionDigits is undefined, then f is 0.
  4. If x is not finite, return Number::toString(x, 10).
  5. If f < 0 or f > 100, throw a RangeError exception.
  6. Set x to ℝ(x).
  7. Let s be the empty String.
  8. If x < 0, then
    1. Set s to "-".
    2. Set x to -x.
  9. If x = 0, then
    1. Let m be the String value consisting of f 1 occurrences of the code unit 0x0030 (DIGIT ZERO).
    2. Let e be 0.
  10. Else,
    1. If fractionDigits is not undefined, then
      1. Let e and n be integers such that 10f ≤ n < 10f 1 and for which n × 10e - f - x is as close to zero as possible. If there are two such sets of e and n, pick the e and n for which n × 10e - f is larger.
    2. Else,
      1. Let e, n, and f be integers such that f ≥ 0, 10f ≤ n < 10f 1, 𝔽(n × 10e - f) is 𝔽(x), and f is as small as possible. Note that the decimal representation of n has f 1 digits, n is not divisible by 10, and the least significant digit of n is not necessarily uniquely determined by these criteria.
    3. Let m be the String value consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
  11. If f ≠ 0, then
    1. Let a be the first code unit of m.
    2. Let b be the other f code units of m.
    3. Set m to the string-concatenation of a, ".", and b.
  12. If e = 0, then
    1. Let c be " ".
    2. Let d be "0".
  13. Else,
    1. If e > 0, let c be " ".
    2. Else,
      1. Assert: e < 0.
      2. Let c be "-".
      3. Set e to -e.
    3. Let d be the String value consisting of the digits of the decimal representation of e (in order, with no leading zeroes).
  14. Set m to the string-concatenation of m, "e", c, and d.
  15. Return the string-concatenation of s and m.
 Note

For implementations that provide more accurate conversions than required by the rules above, it is recommended that the following alternative version of step 10.b.i be used as a guideline:

  1. Let e, n, and f be integers such that f ≥ 0, 10f ≤ n < 10f 1, 𝔽(n × 10e - f) is 𝔽(x), and f is as small as possible. If there are multiple possibilities for n, choose the value of n for which 𝔽(n × 10e - f) is closest in value to 𝔽(x). If there are two such possible values of n, choose the one that is even.

21.1.3.3 Number.prototype.toFixed ( fractionDigits )

Note 1

This method returns a String containing this Number value represented in decimal fixed-point notation with fractionDigits digits after the decimal point. If fractionDigits is undefined, 0 is assumed.

It performs the following steps when called:

  1. Let x be ? thisNumberValue(this value).
  2. Let f be ? ToIntegerOrInfinity(fractionDigits).
  3. Assert: If fractionDigits is undefined, then f is 0.
  4. If f is not finite, throw a RangeError exception.
  5. If f < 0 or f > 100, throw a RangeError exception.
  6. If x is not finite, return Number::toString(x, 10).
  7. Set x to ℝ(x).
  8. Let s be the empty String.
  9. If x < 0, then
    1. Set s to "-".
    2. Set x to -x.
  10. If x ≥ 1021, then
    1. Let m be ! ToString(𝔽(x)).
  11. Else,
    1. Let n be an integer for which n / 10f - x is as close to zero as possible. If there are two such n, pick the larger n.
    2. If n = 0, let m be "0". Otherwise, let m be the String value consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
    3. If f ≠ 0, then
      1. Let k be the length of m.
      2. If k ≤ f, then
        1. Let z be the String value consisting of f 1 - k occurrences of the code unit 0x0030 (DIGIT ZERO).
        2. Set m to the string-concatenation of z and m.
        3. Set k to f 1.
      3. Let a be the first k - f code units of m.
      4. Let b be the other f code units of m.
      5. Set m to the string-concatenation of a, ".", and b.
  12. Return the string-concatenation of s and m.
 Note 2

The output of toFixed may be more precise than toString for some values because toString only prints enough significant digits to distinguish the number from adjacent Number values. For example,

(1000000000000000128).toString() returns "1000000000000000100", while
(1000000000000000128).toFixed(0) returns "1000000000000000128".

21.1.3.4 Number.prototype.toLocaleString ( [ reserved1 [ , reserved2 ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of this method is used:

This method produces a String value that represents this Number value formatted according to the conventions of the host environment's current locale. This method is implementation-defined, and it is permissible, but not encouraged, for it to return the same thing as toString.

The meanings of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.

21.1.3.5 Number.prototype.toPrecision ( precision )

This method returns a String containing this Number value represented either in decimal exponential notation with one digit before the significand's decimal point and precision - 1 digits after the significand's decimal point or in decimal fixed notation with precision significant digits. If precision is undefined, it calls ToString instead.

It performs the following steps when called:

  1. Let x be ? thisNumberValue(this value).
  2. If precision is undefined, return ! ToString(x).
  3. Let p be ? ToIntegerOrInfinity(precision).
  4. If x is not finite, return Number::toString(x, 10).
  5. If p < 1 or p > 100, throw a RangeError exception.
  6. Set x to ℝ(x).
  7. Let s be the empty String.
  8. If x < 0, then
    1. Set s to the code unit 0x002D (HYPHEN-MINUS).
    2. Set x to -x.
  9. If x = 0, then
    1. Let m be the String value consisting of p occurrences of the code unit 0x0030 (DIGIT ZERO).
    2. Let e be 0.
  10. Else,
    1. Let e and n be integers such that 10p - 1 ≤ n < 10p and for which n × 10e - p 1 - x is as close to zero as possible. If there are two such sets of e and n, pick the e and n for which n × 10e - p 1 is larger.
    2. Let m be the String value consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
    3. If e < -6 or e ≥ p, then
      1. Assert: e ≠ 0.
      2. If p ≠ 1, then
        1. Let a be the first code unit of m.
        2. Let b be the other p - 1 code units of m.
        3. Set m to the string-concatenation of a, ".", and b.
      3. If e > 0, then
        1. Let c be the code unit 0x002B (PLUS SIGN).
      4. Else,
        1. Assert: e < 0.
        2. Let c be the code unit 0x002D (HYPHEN-MINUS).
        3. Set e to -e.
      5. Let d be the String value consisting of the digits of the decimal representation of e (in order, with no leading zeroes).
      6. Return the string-concatenation of s, m, the code unit 0x0065 (LATIN SMALL LETTER E), c, and d.
  11. If e = p - 1, return the string-concatenation of s and m.
  12. If e ≥ 0, then
    1. Set m to the string-concatenation of the first e 1 code units of m, the code unit 0x002E (FULL STOP), and the remaining p - (e 1) code units of m.
  13. Else,
    1. Set m to the string-concatenation of the code unit 0x0030 (DIGIT ZERO), the code unit 0x002E (FULL STOP), -(e 1) occurrences of the code unit 0x0030 (DIGIT ZERO), and the String m.
  14. Return the string-concatenation of s and m.

21.1.3.6 Number.prototype.toString ( [ radix ] )

Note

The optional radix should be an integral Number value in the inclusive interval from 2𝔽 to 36𝔽. If radix is undefined then 10𝔽 is used as the value of radix.

This method performs the following steps when called:

  1. Let x be ? thisNumberValue(this value).
  2. If radix is undefined, let radixMV be 10.
  3. Else, let radixMV be ? ToIntegerOrInfinity(radix).
  4. If radixMV is not in the inclusive interval from 2 to 36, throw a RangeError exception.
  5. Return Number::toString(x, radixMV).

This method is not generic; it throws a TypeError exception if its this value is not a Number or a Number object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

The "length" property of this method is 1𝔽.

21.1.3.7 Number.prototype.valueOf ( )

  1. Return ? thisNumberValue(this value).

21.1.4 Properties of Number Instances

Number instances are ordinary objects that inherit properties from the Number prototype object. Number instances also have a [[NumberData]] internal slot. The [[NumberData]] internal slot is the Number value represented by this Number object.

21.2 BigInt Objects

21.2.1 The BigInt Constructor

The BigInt constructor:

  • is %BigInt%.
  • is the initial value of the "BigInt" property of the global object.
  • performs a type conversion when called as a function rather than as a constructor.
  • is not intended to be used with the new operator or to be subclassed. It may be used as the value of an extends clause of a class definition but a super call to the BigInt constructor will cause an exception.

21.2.1.1 BigInt ( value )

This function performs the following steps when called:

  1. If NewTarget is not undefined, throw a TypeError exception.
  2. Let prim be ? ToPrimitive(value, number).
  3. If prim is a Number, return ? NumberToBigInt(prim).
  4. Otherwise, return ? ToBigInt(prim).

21.2.1.1.1 NumberToBigInt ( number )

The abstract operation NumberToBigInt takes argument number (a Number) and returns either a normal completion containing a BigInt or a throw completion. It performs the following steps when called:

  1. If IsIntegralNumber(number) is false, throw a RangeError exception.
  2. Return the BigInt value that represents ℝ(number).

21.2.2 Properties of the BigInt Constructor

The BigInt constructor:

  • has a [[Prototype]] internal slot whose value is %Function.prototype%.
  • has the following properties:

21.2.2.1 BigInt.asIntN ( bits, bigint )

This function performs the following steps when called:

  1. Set bits to ? ToIndex(bits).
  2. Set bigint to ? ToBigInt(bigint).
  3. Let mod be ℝ(bigint) modulo 2bits.
  4. If mod ≥ 2bits - 1, return ℤ(mod - 2bits); otherwise, return ℤ(mod).

21.2.2.2 BigInt.asUintN ( bits, bigint )

This function performs the following steps when called:

  1. Set bits to ? ToIndex(bits).
  2. Set bigint to ? ToBigInt(bigint).
  3. Return the BigInt value that represents ℝ(bigint) modulo 2bits.

21.2.2.3 BigInt.prototype

The initial value of BigInt.prototype is the BigInt prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

21.2.3 Properties of the BigInt Prototype Object

The BigInt prototype object:

The abstract operation thisBigIntValue takes argument value. It performs the following steps when called:

  1. If value is a BigInt, return value.
  2. If value is an Object and value has a [[BigIntData]] internal slot, then
    1. Assert: value.[[BigIntData]] is a BigInt.
    2. Return value.[[BigIntData]].
  3. Throw a TypeError exception.

The phrase “this BigInt value” within the specification of a method refers to the result returned by calling the abstract operation thisBigIntValue with the this value of the method invocation passed as the argument.

21.2.3.1 BigInt.prototype.constructor

The initial value of BigInt.prototype.constructor is %BigInt%.

21.2.3.2 BigInt.prototype.toLocaleString ( [ reserved1 [ , reserved2 ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of this method is used:

This method produces a String value that represents this BigInt value formatted according to the conventions of the host environment's current locale. This method is implementation-defined, and it is permissible, but not encouraged, for it to return the same thing as toString.

The meanings of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.

21.2.3.3 BigInt.prototype.toString ( [ radix ] )

Note

The optional radix should be an integral Number value in the inclusive interval from 2𝔽 to 36𝔽. If radix is undefined then 10𝔽 is used as the value of radix.

This method performs the following steps when called:

  1. Let x be ? thisBigIntValue(this value).
  2. If radix is undefined, let radixMV be 10.
  3. Else, let radixMV be ? ToIntegerOrInfinity(radix).
  4. If radixMV is not in the inclusive interval from 2 to 36, throw a RangeError exception.
  5. Return BigInt::toString(x, radixMV).

This method is not generic; it throws a TypeError exception if its this value is not a BigInt or a BigInt object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

21.2.3.4 BigInt.prototype.valueOf ( )

  1. Return ? thisBigIntValue(this value).

21.2.3.5 BigInt.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "BigInt".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

21.3 The Math Object

The Math object:

  • is %Math%.
  • is the initial value of the "Math" property of the global object.
  • is an ordinary object.
  • has a [[Prototype]] internal slot whose value is %Object.prototype%.
  • is not a function object.
  • does not have a [[Construct]] internal method; it cannot be used as a constructor with the new operator.
  • does not have a [[Call]] internal method; it cannot be invoked as a function.
Note

In this specification, the phrase “the Number value for x” has a technical meaning defined in 6.1.6.1.

21.3.1 Value Properties of the Math Object

21.3.1.1 Math.E

The Number value for e, the base of the natural logarithms, which is approximately 2.7182818284590452354.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

21.3.1.2 Math.LN10

The Number value for the natural logarithm of 10, which is approximately 2.302585092994046.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

21.3.1.3 Math.LN2

The Number value for the natural logarithm of 2, which is approximately 0.6931471805599453.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

21.3.1.4 Math.LOG10E

The Number value for the base-10 logarithm of e, the base of the natural logarithms; this value is approximately 0.4342944819032518.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

Note

The value of Math.LOG10E is approximately the reciprocal of the value of Math.LN10.

21.3.1.5 Math.LOG2E

The Number value for the base-2 logarithm of e, the base of the natural logarithms; this value is approximately 1.4426950408889634.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

Note

The value of Math.LOG2E is approximately the reciprocal of the value of Math.LN2.

21.3.1.6 Math.PI

The Number value for π, the ratio of the circumference of a circle to its diameter, which is approximately 3.1415926535897932.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

21.3.1.7 Math.SQRT1_2

The Number value for the square root of ½, which is approximately 0.7071067811865476.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

Note

The value of Math.SQRT1_2 is approximately the reciprocal of the value of Math.SQRT2.

21.3.1.8 Math.SQRT2

The Number value for the square root of 2, which is approximately 1.4142135623730951.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

21.3.1.9 Math [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "Math".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

21.3.2 Function Properties of the Math Object

Note

The behaviour of the functions acos, acosh, asin, asinh, atan, atanh, atan2, cbrt, cos, cosh, exp, expm1, hypot, log, log1p, log2, log10, pow, random, sin, sinh, sqrt, tan, and tanh is not precisely specified here except to require specific results for certain argument values that represent boundary cases of interest. For other argument values, these functions are intended to compute approximations to the results of familiar mathematical functions, but some latitude is allowed in the choice of approximation algorithms. The general intent is that an implementer should be able to use the same mathematical library for ECMAScript on a given hardware platform that is available to C programmers on that platform.

Although the choice of algorithms is left to the implementation, it is recommended (but not specified by this standard) that implementations use the approximation algorithms for IEEE 754-2019 arithmetic contained in fdlibm, the freely distributable mathematical library from Sun Microsystems (http://www.netlib.org/fdlibm).

21.3.2.1 Math.abs ( x )

This function returns the absolute value of x; the result has the same magnitude as x but has positive sign.

It performs the following steps when called:

  1. Let n be ? ToNumber(x).
  2. If n is NaN, return NaN.
  3. If n is -0𝔽, return 0𝔽.
  4. If n is -∞𝔽, return ∞𝔽.
  5. If n < -0𝔽, return -n.
  6. Return n.

21.3.2.2 Math.acos ( x )

This function returns the inverse cosine of x. The result is expressed in radians and is in the inclusive interval from 0𝔽 to 𝔽(π).

It performs the following steps when called:

  1. Let n be ? ToNumber(x).
  2. If n is NaN, n > 1𝔽, or n < -1𝔽, return NaN.
  3. If n is 1𝔽, return 0𝔽.
  4. Return an implementation-approximated Number value representing the result of the inverse cosine of ℝ(n).

21.3.2.3 Math.acosh ( x )

This function returns the inverse hyperbolic cosine of x.

It performs the following steps when called:

  1. Let n be ? ToNumber(x).
  2. If n is either NaN or ∞𝔽, return n.
  3. If n is 1𝔽, return 0𝔽.
  4. If n < 1𝔽, return NaN.
  5. Return an implementation-approximated Number value representing the result of the inverse hyperbolic cosine of ℝ(n).

21.3.2.4 Math.asin ( x )

This function returns the inverse sine of x. The result is expressed in radians and is in the inclusive interval from 𝔽(-π / 2) to 𝔽(π / 2).

It performs the following steps when called:

  1. Let n be ? ToNumber(x).
  2. If n is one of NaN, 0𝔽, or -0𝔽, return n.
  3. If n > 1𝔽 or n < -1𝔽, return NaN.
  4. Return an implementation-approximated Number value representing the result of the inverse sine of ℝ(n).

21.3.2.5 Math.asinh ( x )

This function returns the inverse hyperbolic sine of x.

It performs the following steps when called:

  1. Let n be ? ToNumber(x).
  2. If n is not finite or n is either 0𝔽 or -0𝔽, return n.
  3. Return an implementation-approximated Number value representing the result of the inverse hyperbolic sine of ℝ(n).

21.3.2.6 Math.atan ( x )

This function returns the inverse tangent of x. The result is expressed in radians and is in the inclusive interval from 𝔽(-π / 2) to 𝔽(π / 2).

It performs the following steps when called:

  1. Let n be ? ToNumber(x).
  2. If n is one of NaN, 0𝔽, or -0𝔽, return n.
  3. If n is ∞𝔽, return an implementation-approximated Number value representing π / 2.
  4. If n is -∞𝔽, return an implementation-approximated Number value representing -π / 2.
  5. Return an implementation-approximated Number value representing the result of the inverse tangent of ℝ(n).

21.3.2.7 Math.atanh ( x )

This function returns the inverse hyperbolic tangent of x.

It performs the following steps when called:

  1. Let n be ? ToNumber(x).
  2. If n is one of NaN, 0𝔽, or -0𝔽, return n.
  3. If n > 1𝔽 or n < -1𝔽, return NaN.
  4. If n is 1𝔽, return ∞𝔽.
  5. If n is -1𝔽, return -∞𝔽.
  6. Return an implementation-approximated Number value representing the result of the inverse hyperbolic tangent of ℝ(n).

21.3.2.8 Math.atan2 ( y, x )

This function returns the inverse tangent of the quotient y / x of the arguments y and x, where the signs of y and x are used to determine the quadrant of the result. Note that it is intentional and traditional for the two-argument inverse tangent function that the argument named y be first and the argument named x be second. The result is expressed in radians and is in the inclusive interval from -π to π.

It performs the following steps when called:

  1. Let ny be ? ToNumber(y).
  2. Let nx be ? ToNumber(x).
  3. If ny is NaN or nx is NaN, return NaN.
  4. If ny is ∞𝔽, then
    1. If nx is ∞𝔽, return an implementation-approximated Number value representing π / 4.
    2. If nx is -∞𝔽, return an implementation-approximated Number value representing 3π / 4.
    3. Return an implementation-approximated Number value representing π / 2.
  5. If ny is -∞𝔽, then
    1. If nx is ∞𝔽, return an implementation-approximated Number value representing -π / 4.
    2. If nx is -∞𝔽, return an implementation-approximated Number value representing -3π / 4.
    3. Return an implementation-approximated Number value representing -π / 2.
  6. If ny is 0𝔽, then
    1. If nx > 0𝔽 or nx is 0𝔽, return 0𝔽.
    2. Return an implementation-approximated Number value representing π.
  7. If ny is -0𝔽, then
    1. If nx > 0𝔽 or nx is 0𝔽, return -0𝔽.
    2. Return an implementation-approximated Number value representing -π.
  8. Assert: ny is finite and is neither 0𝔽 nor -0𝔽.
  9. If ny > 0𝔽, then
    1. If nx is ∞𝔽, return 0𝔽.
    2. If nx is -∞𝔽, return an implementation-approximated Number value representing π.
    3. If nx is either 0𝔽 or -0𝔽, return an implementation-approximated Number value representing π / 2.
  10. If ny < -0𝔽, then
    1. If nx is ∞𝔽, return -0𝔽.
    2. If nx is -∞𝔽, return an implementation-approximated Number value representing -π.
    3. If nx is either 0𝔽 or -0𝔽, return an implementation-approximated Number value representing -π / 2.
  11. Assert: nx is finite and is neither 0𝔽 nor -0𝔽.
  12. Return an implementation-approximated Number value representing the result of the inverse tangent of the quotient ℝ(ny) / ℝ(nx).

21.3.2.9 Math.cbrt ( x )

This function returns the cube root of x.

It performs the following steps when called:

  1. Let n be ? ToNumber(x).
  2. If n is not finite or n is either 0𝔽 or -0𝔽, return n.
  3. Return an implementation-approximated Number value representing the result of the cube root of ℝ(n).

21.3.2.10 Math.ceil ( x )

This function returns the smallest (closest to -∞) integral Number value that is not less than x. If x is already an integral Number, the result is x.

It performs the following steps when called:

  1. Let n be ? ToNumber(x).
  2. If n is not finite or n is either 0𝔽 or -0𝔽, return n.
  3. If n < -0𝔽 and n > -1𝔽, return -0𝔽.
  4. If n is an integral Number, return n.
  5. Return the smallest (closest to -∞) integral Number value that is not less than n.
 Note

The value of Math.ceil(x) is the same as the value of -Math.floor(-x).

21.3.2.11 Math.clz32 ( x )

This function performs the following steps when called:

  1. Let n be ? ToUint32(x).
  2. Let p be the number of leading zero bits in the unsigned 32-bit binary representation of n.
  3. Return 𝔽(p).
 Note

If n is either 0𝔽 or -0𝔽, this method returns 32𝔽. If the most significant bit of the 32-bit binary encoding of n is 1, this method returns 0𝔽.

21.3.2.12 Math.cos ( x )

This function returns the cosine of x. The argument is expressed in radians.

It performs the following steps when called:

  1. Let n be ? ToNumber(x).
  2. If n is not finite, return NaN.
  3. If n is either 0𝔽 or -0𝔽, return 1𝔽.
  4. Return an implementation-approximated Number value representing the result of the cosine of ℝ(n).

21.3.2.13 Math.cosh ( x )

This function returns the hyperbolic cosine of x.

It performs the following steps when called:

  1. Let n be ? ToNumber(x).
  2. If n is NaN, return NaN.
  3. If n is either ∞𝔽 or -∞𝔽, return ∞𝔽.
  4. If n is either 0𝔽 or -0𝔽, return 1𝔽.
  5. Return an implementation-approximated Number value representing the result of the hyperbolic cosine of ℝ(n).
 Note

The value of Math.cosh(x) is the same as the value of (Math.exp(x) Math.exp(-x)) / 2.

21.3.2.14 Math.exp ( x )

This function returns the exponential function of x (e raised to the power of x, where e is the base of the natural logarithms).

It performs the following steps when called:

  1. Let n be ? ToNumber(x).
  2. If n is either NaN or ∞𝔽, return n.
  3. If n is either 0𝔽 or -0𝔽, return 1𝔽.
  4. If n is -∞𝔽, return 0𝔽.
  5. Return an implementation-approximated Number value representing the result of the exponential function of ℝ(n).

21.3.2.15 Math.expm1 ( x )

This function returns the result of subtracting 1 from the exponential function of x (e raised to the power of x, where e is the base of the natural logarithms). The result is computed in a way that is accurate even when the value of x is close to 0.

It performs the following steps when called:

  1. Let n be ? ToNumber(x).
  2. If n is one of NaN, 0𝔽, -0𝔽, or ∞𝔽, return n.
  3. If n is -∞𝔽, return -1𝔽.
  4. Return an implementation-approximated Number value representing the result of subtracting 1 from the exponential function of ℝ(n).

21.3.2.16 Math.floor ( x )

This function returns the greatest (closest to ∞) integral Number value that is not greater than x. If x is already an integral Number, the result is x.

It performs the following steps when called:

  1. Let n be ? ToNumber(x).
  2. If n is not finite or n is either 0𝔽 or -0𝔽, return n.
  3. If n < 1𝔽 and n > 0𝔽, return 0𝔽.
  4. If n is an integral Number, return n.
  5. Return the greatest (closest to ∞) integral Number value that is not greater than n.
 Note

The value of Math.floor(x) is the same as the value of -Math.ceil(-x).

21.3.2.17 Math.fround ( x )

This function performs the following steps when called:

  1. Let n be ? ToNumber(x).
  2. If n is NaN, return NaN.
  3. If n is one of 0𝔽, -0𝔽, ∞𝔽, or -∞𝔽, return n.
  4. Let n32 be the result of converting n to a value in IEEE 754-2019 binary32 format using roundTiesToEven mode.
  5. Let n64 be the result of converting n32 to a value in IEEE 754-2019 binary64 format.
  6. Return the ECMAScript Number value corresponding to n64.

21.3.2.18 Math.hypot ( ...args )

Given zero or more arguments, this function returns the square root of the sum of squares of its arguments.

It performs the following steps when called:

  1. Let coerced be a new empty List.
  2. For each element arg of args, do
    1. Let n be ? ToNumber(arg).
    2. Append n to coerced.
  3. For each element number of coerced, do
    1. If number is either ∞𝔽 or -∞𝔽, return ∞𝔽.
  4. Let onlyZero be true.
  5. For each element number of coerced, do
    1. If number is NaN, return NaN.
    2. If number is neither 0𝔽 nor -0𝔽, set onlyZero to false.
  6. If onlyZero is true, return 0𝔽.
  7. Return an implementation-approximated Number value representing the square root of the sum of squares of the mathematical values of the elements of coerced.

The "length" property of this function is 2𝔽.

Note

Implementations should take care to avoid the loss of precision from overflows and underflows that are prone to occur in naive implementations when this function is called with two or more arguments.

21.3.2.19 Math.imul ( x, y )

This function performs the following steps when called:

  1. Let a be ℝ(? ToUint32(x)).
  2. Let b be ℝ(? ToUint32(y)).
  3. Let product be (a × b) modulo 232.
  4. If product ≥ 231, return 𝔽(product - 232); otherwise return 𝔽(product).

21.3.2.20 Math.log ( x )

This function returns the natural logarithm of x.

It performs the following steps when called:

  1. Let n be ? ToNumber(x).
  2. If n is either NaN or ∞𝔽, return n.
  3. If n is 1𝔽, return 0𝔽.
  4. If n is either 0𝔽 or -0𝔽, return -∞𝔽.
  5. If n < -0𝔽, return NaN.
  6. Return an implementation-approximated Number value representing the result of the natural logarithm of ℝ(n).

21.3.2.21 Math.log1p ( x )

This function returns the natural logarithm of 1 x. The result is computed in a way that is accurate even when the value of x is close to zero.

It performs the following steps when called:

  1. Let n be ? ToNumber(x).
  2. If n is one of NaN, 0𝔽, -0𝔽, or ∞𝔽, return n.
  3. If n is -1𝔽, return -∞𝔽.
  4. If n < -1𝔽, return NaN.
  5. Return an implementation-approximated Number value representing the result of the natural logarithm of 1 ℝ(n).

21.3.2.22 Math.log10 ( x )

This function returns the base 10 logarithm of x.

It performs the following steps when called:

  1. Let n be ? ToNumber(x).
  2. If n is either NaN or ∞𝔽, return n.
  3. If n is 1𝔽, return 0𝔽.
  4. If n is either 0𝔽 or -0𝔽, return -∞𝔽.
  5. If n < -0𝔽, return NaN.
  6. Return an implementation-approximated Number value representing the result of the base 10 logarithm of ℝ(n).

21.3.2.23 Math.log2 ( x )

This function returns the base 2 logarithm of x.

It performs the following steps when called:

  1. Let n be ? ToNumber(x).
  2. If n is either NaN or ∞𝔽, return n.
  3. If n is 1𝔽, return 0𝔽.
  4. If n is either 0𝔽 or -0𝔽, return -∞𝔽.
  5. If n < -0𝔽, return NaN.
  6. Return an implementation-approximated Number value representing the result of the base 2 logarithm of ℝ(n).

21.3.2.24 Math.max ( ...args )

Given zero or more arguments, this function calls ToNumber on each of the arguments and returns the largest of the resulting values.

It performs the following steps when called:

  1. Let coerced be a new empty List.
  2. For each element arg of args, do
    1. Let n be ? ToNumber(arg).
    2. Append n to coerced.
  3. Let highest be -∞𝔽.
  4. For each element number of coerced, do
    1. If number is NaN, return NaN.
    2. If number is 0𝔽 and highest is -0𝔽, set highest to 0𝔽.
    3. If number > highest, set highest to number.
  5. Return highest.
 Note

The comparison of values to determine the largest value is done using the IsLessThan algorithm except that 0𝔽 is considered to be larger than -0𝔽.

The "length" property of this function is 2𝔽.

21.3.2.25 Math.min ( ...args )

Given zero or more arguments, this function calls ToNumber on each of the arguments and returns the smallest of the resulting values.

It performs the following steps when called:

  1. Let coerced be a new empty List.
  2. For each element arg of args, do
    1. Let n be ? ToNumber(arg).
    2. Append n to coerced.
  3. Let lowest be ∞𝔽.
  4. For each element number of coerced, do
    1. If number is NaN, return NaN.
    2. If number is -0𝔽 and lowest is 0𝔽, set lowest to -0𝔽.
    3. If number < lowest, set lowest to number.
  5. Return lowest.
 Note

The comparison of values to determine the largest value is done using the IsLessThan algorithm except that 0𝔽 is considered to be larger than -0𝔽.

The "length" property of this function is 2𝔽.

21.3.2.26 Math.pow ( base, exponent )

This function performs the following steps when called:

  1. Set base to ? ToNumber(base).
  2. Set exponent to ? ToNumber(exponent).
  3. Return Number::exponentiate(base, exponent).

21.3.2.27 Math.random ( )

This function returns a Number value with positive sign, greater than or equal to 0𝔽 but strictly less than 1𝔽, chosen randomly or pseudo randomly with approximately uniform distribution over that range, using an implementation-defined algorithm or strategy.

Each Math.random function created for distinct realms must produce a distinct sequence of values from successive calls.

21.3.2.28 Math.round ( x )

This function returns the Number value that is closest to x and is integral. If two integral Numbers are equally close to x, then the result is the Number value that is closer to ∞. If x is already integral, the result is x.

It performs the following steps when called:

  1. Let n be ? ToNumber(x).
  2. If n is not finite or n is an integral Number, return n.
  3. If n < 0.5𝔽 and n > 0𝔽, return 0𝔽.
  4. If n < -0𝔽 and n ≥ -0.5𝔽, return -0𝔽.
  5. Return the integral Number closest to n, preferring the Number closer to ∞ in the case of a tie.
 Note 1

Math.round(3.5) returns 4, but Math.round(-3.5) returns -3.

 Note 2

The value of Math.round(x) is not always the same as the value of Math.floor(x 0.5). When x is -0𝔽 or x is less than 0𝔽 but greater than or equal to -0.5𝔽, Math.round(x) returns -0𝔽, but Math.floor(x 0.5) returns 0𝔽. Math.round(x) may also differ from the value of Math.floor(x 0.5)because of internal rounding when computing x 0.5.

21.3.2.29 Math.sign ( x )

This function returns the sign of x, indicating whether x is positive, negative, or zero.

It performs the following steps when called:

  1. Let n be ? ToNumber(x).
  2. If n is one of NaN, 0𝔽, or -0𝔽, return n.
  3. If n < -0𝔽, return -1𝔽.
  4. Return 1𝔽.

21.3.2.30 Math.sin ( x )

This function returns the sine of x. The argument is expressed in radians.

It performs the following steps when called:

  1. Let n be ? ToNumber(x).
  2. If n is one of NaN, 0𝔽, or -0𝔽, return n.
  3. If n is either ∞𝔽 or -∞𝔽, return NaN.
  4. Return an implementation-approximated Number value representing the result of the sine of ℝ(n).

21.3.2.31 Math.sinh ( x )

This function returns the hyperbolic sine of x.

It performs the following steps when called:

  1. Let n be ? ToNumber(x).
  2. If n is not finite or n is either 0𝔽 or -0𝔽, return n.
  3. Return an implementation-approximated Number value representing the result of the hyperbolic sine of ℝ(n).
 Note

The value of Math.sinh(x) is the same as the value of (Math.exp(x) - Math.exp(-x)) / 2.

21.3.2.32 Math.sqrt ( x )

This function returns the square root of x.

It performs the following steps when called:

  1. Let n be ? ToNumber(x).
  2. If n is one of NaN, 0𝔽, -0𝔽, or ∞𝔽, return n.
  3. If n < -0𝔽, return NaN.
  4. Return an implementation-approximated Number value representing the result of the square root of ℝ(n).

21.3.2.33 Math.tan ( x )

This function returns the tangent of x. The argument is expressed in radians.

It performs the following steps when called:

  1. Let n be ? ToNumber(x).
  2. If n is one of NaN, 0𝔽, or -0𝔽, return n.
  3. If n is either ∞𝔽 or -∞𝔽, return NaN.
  4. Return an implementation-approximated Number value representing the result of the tangent of ℝ(n).

21.3.2.34 Math.tanh ( x )

This function returns the hyperbolic tangent of x.

It performs the following steps when called:

  1. Let n be ? ToNumber(x).
  2. If n is one of NaN, 0𝔽, or -0𝔽, return n.
  3. If n is ∞𝔽, return 1𝔽.
  4. If n is -∞𝔽, return -1𝔽.
  5. Return an implementation-approximated Number value representing the result of the hyperbolic tangent of ℝ(n).
 Note

The value of Math.tanh(x) is the same as the value of (Math.exp(x) - Math.exp(-x)) / (Math.exp(x) Math.exp(-x)).

21.3.2.35 Math.trunc ( x )

This function returns the integral part of the number x, removing any fractional digits. If x is already integral, the result is x.

It performs the following steps when called:

  1. Let n be ? ToNumber(x).
  2. If n is not finite or n is either 0𝔽 or -0𝔽, return n.
  3. If n < 1𝔽 and n > 0𝔽, return 0𝔽.
  4. If n < -0𝔽 and n > -1𝔽, return -0𝔽.
  5. Return the integral Number nearest n in the direction of 0𝔽.

21.4 Date Objects

21.4.1 Overview of Date Objects and Definitions of Abstract Operations

The following abstract operations operate on time values (defined in 21.4.1.1). Note that, in every case, if any argument to one of these functions is NaN, the result will be NaN.

21.4.1.1 Time Values and Time Range

Time measurement in ECMAScript is analogous to time measurement in POSIX, in particular sharing definition in terms of the proleptic Gregorian calendar, an epoch of midnight at the beginning of 1 January 1970 UTC, and an accounting of every day as comprising exactly 86,400 seconds (each of which is 1000 milliseconds long).

An ECMAScript time value is a Number, either a finite integral Number representing an instant in time to millisecond precision or NaN representing no specific instant. A time value that is a multiple of 24 × 60 × 60 × 1000 = 86,400,000 (i.e., is 86,400,000 × d for some integer d) represents the instant at the start of the UTC day that follows the epoch by d whole UTC days (preceding the epoch for negative d). Every other finite time value t is defined relative to the greatest preceding time value s that is such a multiple, and represents the instant that occurs within the same UTC day as s but follows it by (t - s) milliseconds.

Time values do not account for UTC leap seconds—there are no time values representing instants within positive leap seconds, and there are time values representing instants removed from the UTC timeline by negative leap seconds. However, the definition of time values nonetheless yields piecewise alignment with UTC, with discontinuities only at leap second boundaries and zero difference outside of leap seconds.

A Number can exactly represent all integers from -9,007,199,254,740,992 to 9,007,199,254,740,992 (21.1.2.8 and 21.1.2.6). A time value supports a slightly smaller range of -8,640,000,000,000,000 to 8,640,000,000,000,000 milliseconds. This yields a supported time value range of exactly -100,000,000 days to 100,000,000 days relative to midnight at the beginning of 1 January 1970 UTC.

The exact moment of midnight at the beginning of 1 January 1970 UTC is represented by the time value 0𝔽.

Note

The 400 year cycle of the proleptic Gregorian calendar contains 97 leap years. This yields an average of 365.2425 days per year, which is 31,556,952,000 milliseconds. Therefore, the maximum range a Number could represent exactly with millisecond precision is approximately -285,426 to 285,426 years relative to 1970. The smaller range supported by a time value as specified in this section is approximately -273,790 to 273,790 years relative to 1970.

21.4.1.2 Day Number and Time within Day

A given time value t belongs to day number

Day(t) = 𝔽(floor(ℝ(t / msPerDay)))

where the number of milliseconds per day is

msPerDay = 86400000𝔽

The remainder is called the time within the day:

TimeWithinDay(t) = 𝔽(ℝ(t) modulo ℝ(msPerDay))

21.4.1.3 Year Number

ECMAScript uses a proleptic Gregorian calendar to map a day number to a year number and to determine the month and date within that year. In this calendar, leap years are precisely those which are (divisible by 4) and ((not divisible by 100) or (divisible by 400)). The number of days in year number y is therefore defined by

DaysInYear(y)
= 365𝔽 if (ℝ(y) modulo 4) ≠ 0
= 366𝔽 if (ℝ(y) modulo 4) = 0 and (ℝ(y) modulo 100) ≠ 0
= 365𝔽 if (ℝ(y) modulo 100) = 0 and (ℝ(y) modulo 400) ≠ 0
= 366𝔽 if (ℝ(y) modulo 400) = 0

All non-leap years have 365 days with the usual number of days per month and leap years have an extra day in February. The day number of the first day of year y is given by:

DayFromYear(y) = 𝔽(365 × (ℝ(y) - 1970) floor((ℝ(y) - 1969) / 4) - floor((ℝ(y) - 1901) / 100) floor((ℝ(y) - 1601) / 400))

The time value of the start of a year is:

TimeFromYear(y) = msPerDay × DayFromYear(y)

A time value determines a year by:

YearFromTime(t) = the largest integral Number y (closest to ∞) such that TimeFromYear(y) ≤ t

The leap-year function is 1𝔽 for a time within a leap year and otherwise is 0𝔽:

InLeapYear(t)
= 0𝔽 if DaysInYear(YearFromTime(t)) is 365𝔽
= 1𝔽 if DaysInYear(YearFromTime(t)) is 366𝔽

21.4.1.4 Month Number

Months are identified by an integral Number in the inclusive interval from 0𝔽 to 11𝔽. The mapping MonthFromTime(t) from a time value t to a month number is defined by:

MonthFromTime(t)
= 0𝔽 if 0𝔽 ≤ DayWithinYear(t) < 31𝔽
= 1𝔽 if 31𝔽 ≤ DayWithinYear(t) < 59𝔽 InLeapYear(t)
= 2𝔽 if 59𝔽 InLeapYear(t) ≤ DayWithinYear(t) < 90𝔽 InLeapYear(t)
= 3𝔽 if 90𝔽 InLeapYear(t) ≤ DayWithinYear(t) < 120𝔽 InLeapYear(t)
= 4𝔽 if 120𝔽 InLeapYear(t) ≤ DayWithinYear(t) < 151𝔽 InLeapYear(t)
= 5𝔽 if 151𝔽 InLeapYear(t) ≤ DayWithinYear(t) < 181𝔽 InLeapYear(t)
= 6𝔽 if 181𝔽 InLeapYear(t) ≤ DayWithinYear(t) < 212𝔽 InLeapYear(t)
= 7𝔽 if 212𝔽 InLeapYear(t) ≤ DayWithinYear(t) < 243𝔽 InLeapYear(t)
= 8𝔽 if 243𝔽 InLeapYear(t) ≤ DayWithinYear(t) < 273𝔽 InLeapYear(t)
= 9𝔽 if 273𝔽 InLeapYear(t) ≤ DayWithinYear(t) < 304𝔽 InLeapYear(t)
= 10𝔽 if 304𝔽 InLeapYear(t) ≤ DayWithinYear(t) < 334𝔽 InLeapYear(t)
= 11𝔽 if 334𝔽 InLeapYear(t) ≤ DayWithinYear(t) < 365𝔽 InLeapYear(t)

where

DayWithinYear(t) = Day(t) - DayFromYear(YearFromTime(t))

A month value of 0𝔽 specifies January; 1𝔽 specifies February; 2𝔽 specifies March; 3𝔽 specifies April; 4𝔽 specifies May; 5𝔽 specifies June; 6𝔽 specifies July; 7𝔽 specifies August; 8𝔽 specifies September; 9𝔽 specifies October; 10𝔽 specifies November; and 11𝔽 specifies December. Note that MonthFromTime( 0𝔽) = 0𝔽, corresponding to Thursday, 1 January 1970.

21.4.1.5 Date Number

A date number is identified by an integral Number in the inclusive interval from 1𝔽 to 31𝔽. The mapping DateFromTime(t) from a time value t to a date number is defined by:

DateFromTime(t)
= DayWithinYear(t) 1𝔽 if MonthFromTime(t) is 0𝔽
= DayWithinYear(t) - 30𝔽 if MonthFromTime(t) is 1𝔽
= DayWithinYear(t) - 58𝔽 - InLeapYear(t) if MonthFromTime(t) is 2𝔽
= DayWithinYear(t) - 89𝔽 - InLeapYear(t) if MonthFromTime(t) is 3𝔽
= DayWithinYear(t) - 119𝔽 - InLeapYear(t) if MonthFromTime(t) is 4𝔽
= DayWithinYear(t) - 150𝔽 - InLeapYear(t) if MonthFromTime(t) is 5𝔽
= DayWithinYear(t) - 180𝔽 - InLeapYear(t) if MonthFromTime(t) is 6𝔽
= DayWithinYear(t) - 211𝔽 - InLeapYear(t) if MonthFromTime(t) is 7𝔽
= DayWithinYear(t) - 242𝔽 - InLeapYear(t) if MonthFromTime(t) is 8𝔽
= DayWithinYear(t) - 272𝔽 - InLeapYear(t) if MonthFromTime(t) is 9𝔽
= DayWithinYear(t) - 303𝔽 - InLeapYear(t) if MonthFromTime(t) is 10𝔽
= DayWithinYear(t) - 333𝔽 - InLeapYear(t) if MonthFromTime(t) is 11𝔽

21.4.1.6 Week Day

The weekday for a particular time value t is defined as

WeekDay(t) = 𝔽(ℝ(Day(t) 4𝔽) modulo 7)

A weekday value of 0𝔽 specifies Sunday; 1𝔽 specifies Monday; 2𝔽 specifies Tuesday; 3𝔽 specifies Wednesday; 4𝔽 specifies Thursday; 5𝔽 specifies Friday; and 6𝔽 specifies Saturday. Note that WeekDay( 0𝔽) = 4𝔽, corresponding to Thursday, 1 January 1970.

21.4.1.7 GetUTCEpochNanoseconds ( year, month, day, hour, minute, second, millisecond, microsecond, nanosecond )

The abstract operation GetUTCEpochNanoseconds takes arguments year (an integer), month (an integer in the inclusive interval from 1 to 12), day (an integer in the inclusive interval from 1 to 31), hour (an integer in the inclusive interval from 0 to 23), minute (an integer in the inclusive interval from 0 to 59), second (an integer in the inclusive interval from 0 to 59), millisecond (an integer in the inclusive interval from 0 to 999), microsecond (an integer in the inclusive interval from 0 to 999), and nanosecond (an integer in the inclusive interval from 0 to 999) and returns a BigInt. The returned value represents a number of nanoseconds since the epoch that corresponds to the given ISO 8601 calendar date and wall-clock time in UTC. It performs the following steps when called:

  1. Let date be MakeDay(𝔽(year), 𝔽(month - 1), 𝔽(day)).
  2. Let time be MakeTime(𝔽(hour), 𝔽(minute), 𝔽(second), 𝔽(millisecond)).
  3. Let ms be MakeDate(date, time).
  4. Assert: ms is an integral Number.
  5. Return ℤ(ℝ(ms) × 106 microsecond × 103 nanosecond).

21.4.1.8 GetNamedTimeZoneEpochNanoseconds ( timeZoneIdentifier, year, month, day, hour, minute, second, millisecond, microsecond, nanosecond )

The implementation-defined abstract operation GetNamedTimeZoneEpochNanoseconds takes arguments timeZoneIdentifier (a String), year (an integer), month (an integer in the inclusive interval from 1 to 12), day (an integer in the inclusive interval from 1 to 31), hour (an integer in the inclusive interval from 0 to 23), minute (an integer in the inclusive interval from 0 to 59), second (an integer in the inclusive interval from 0 to 59), millisecond (an integer in the inclusive interval from 0 to 999), microsecond (an integer in the inclusive interval from 0 to 999), and nanosecond (an integer in the inclusive interval from 0 to 999) and returns a List of BigInts. Each value in the returned List represents a number of nanoseconds since the epoch that corresponds to the given ISO 8601 calendar date and wall-clock time in the named time zone identified by timeZoneIdentifier.

When the input represents a local time occurring more than once because of a negative time zone transition (e.g. when daylight saving time ends or the time zone offset is decreased due to a time zone rule change), the returned List will have more than one element and will be sorted by ascending numerical value. When the input represents a local time skipped because of a positive time zone transition (e.g. when daylight saving time begins or the time zone offset is increased due to a time zone rule change), the returned List will be empty. Otherwise, the returned List will have one element.

The default implementation of GetNamedTimeZoneEpochNanoseconds, to be used for ECMAScript implementations that do not include local political rules for any time zones, performs the following steps when called:

  1. Assert: timeZoneIdentifier is "UTC".
  2. Let epochNanoseconds be GetUTCEpochNanoseconds(year, month, day, hour, minute, second, millisecond, microsecond, nanosecond).
  3. Return « epochNanoseconds ».
 Note

It is recommended that implementations use the time zone information of the IANA Time Zone Database https://www.iana.org/time-zones/.

1:30 AM on 5 November 2017 in America/New_York is repeated twice, so GetNamedTimeZoneEpochNanoseconds("America/New_York", 2017, 11, 5, 1, 30, 0, 0, 0, 0) would return a List of length 2 in which the first element represents 05:30 UTC (corresponding with 01:30 US Eastern Daylight Time at UTC offset -04:00) and the second element represents 06:30 UTC (corresponding with 01:30 US Eastern Standard Time at UTC offset -05:00).

2:30 AM on 12 March 2017 in America/New_York does not exist, so GetNamedTimeZoneEpochNanoseconds("America/New_York", 2017, 3, 12, 2, 30, 0, 0, 0, 0) would return an empty List.

21.4.1.9 GetNamedTimeZoneOffsetNanoseconds ( timeZoneIdentifier, epochNanoseconds )

The implementation-defined abstract operation GetNamedTimeZoneOffsetNanoseconds takes arguments timeZoneIdentifier (a String) and epochNanoseconds (a BigInt) and returns an integer.

The returned integer represents the offset from UTC of the named time zone identified by timeZoneIdentifier, at the instant corresponding with epochNanoseconds relative to the epoch, both in nanoseconds.

The default implementation of GetNamedTimeZoneOffsetNanoseconds, to be used for ECMAScript implementations that do not include local political rules for any time zones, performs the following steps when called:

  1. Assert: timeZoneIdentifier is "UTC".
  2. Return 0.
 Note

Time zone offset values may be positive or negative.

21.4.1.10 DefaultTimeZone ( )

The implementation-defined abstract operation DefaultTimeZone takes no arguments and returns a String. It returns a String value representing the host environment's current time zone, which is either a String representing a UTC offset for which IsTimeZoneOffsetString returns true, or a String identifier accepted by GetNamedTimeZoneEpochNanoseconds and GetNamedTimeZoneOffsetNanoseconds.

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the DefaultTimeZone abstract operation as specified in the ECMA-402 specification.

The default implementation of DefaultTimeZone, to be used for ECMAScript implementations that do not include local political rules for any time zones, performs the following steps when called:

  1. Return "UTC".
 Note

To ensure the level of functionality that implementations commonly provide in the methods of the Date object, it is recommended that DefaultTimeZone return an IANA time zone name corresponding to the host environment's time zone setting, if such a thing exists. GetNamedTimeZoneEpochNanoseconds and GetNamedTimeZoneOffsetNanoseconds must reflect the local political rules for standard time and daylight saving time in that time zone, if such rules exist.

For example, if the host environment is a browser on a system where the user has chosen US Eastern Time as their time zone, DefaultTimeZone returns "America/New_York".

21.4.1.11 LocalTime ( t )

The abstract operation LocalTime takes argument t (a finite time value) and returns an integral Number. It converts t from UTC to local time. The local political rules for standard time and daylight saving time in effect at t should be used to determine the result in the way specified in this section. It performs the following steps when called:

  1. Let localTimeZone be DefaultTimeZone().
  2. If IsTimeZoneOffsetString(localTimeZone) is true, then
    1. Let offsetNs be ParseTimeZoneOffsetString(localTimeZone).
  3. Else,
    1. Let offsetNs be GetNamedTimeZoneOffsetNanoseconds(localTimeZone, ℤ(ℝ(t) × 106)).
  4. Let offsetMs be truncate(offsetNs / 106).
  5. Return t 𝔽(offsetMs).

If political rules for the local time t are not available within the implementation, the result is t because DefaultTimeZone returns "UTC" and GetNamedTimeZoneOffsetNanoseconds returns 0.

Note 1

It is recommended that implementations use the time zone information of the IANA Time Zone Database https://www.iana.org/time-zones/.

 Note 2

Two different input time values tUTC are converted to the same local time tlocal at a negative time zone transition when there are repeated times (e.g. the daylight saving time ends or the time zone adjustment is decreased.).

LocalTime(UTC(tlocal)) is not necessarily always equal to tlocal. Correspondingly, UTC(LocalTime(tUTC)) is not necessarily always equal to tUTC.

21.4.1.12 UTC ( t )

The abstract operation UTC takes argument t (a Number) and returns a time value. It converts t from local time to a UTC time value. The local political rules for standard time and daylight saving time in effect at t should be used to determine the result in the way specified in this section. It performs the following steps when called:

  1. Let localTimeZone be DefaultTimeZone().
  2. If IsTimeZoneOffsetString(localTimeZone) is true, then
    1. Let offsetNs be ParseTimeZoneOffsetString(localTimeZone).
  3. Else,
    1. Let possibleInstants be GetNamedTimeZoneEpochNanoseconds(localTimeZone, ℝ(YearFromTime(t)), ℝ(MonthFromTime(t)) 1, ℝ(DateFromTime(t)), ℝ(HourFromTime(t)), ℝ(MinFromTime(t)), ℝ(SecFromTime(t)), ℝ(msFromTime(t)), 0, 0).
    2. NOTE: The following steps ensure that when t represents local time repeating multiple times at a negative time zone transition (e.g. when the daylight saving time ends or the time zone offset is decreased due to a time zone rule change) or skipped local time at a positive time zone transition (e.g. when the daylight saving time starts or the time zone offset is increased due to a time zone rule change), t is interpreted using the time zone offset before the transition.
    3. If possibleInstants is not empty, then
      1. Let disambiguatedInstant be possibleInstants[0].
    4. Else,
      1. NOTE: t represents a local time skipped at a positive time zone transition (e.g. due to daylight saving time starting or a time zone rule change increasing the UTC offset).
      2. Let possibleInstantsBefore be GetNamedTimeZoneEpochNanoseconds(localTimeZone, ℝ(YearFromTime(tBefore)), ℝ(MonthFromTime(tBefore)) 1, ℝ(DateFromTime(tBefore)), ℝ(HourFromTime(tBefore)), ℝ(MinFromTime(tBefore)), ℝ(SecFromTime(tBefore)), ℝ(msFromTime(tBefore)), 0, 0), where tBefore is the largest integral Number < t for which possibleInstantsBefore is not empty (i.e., tBefore represents the last local time before the transition).
      3. Let disambiguatedInstant be the last element of possibleInstantsBefore.
    5. Let offsetNs be GetNamedTimeZoneOffsetNanoseconds(localTimeZone, disambiguatedInstant).
  4. Let offsetMs be truncate(offsetNs / 106).
  5. Return t - 𝔽(offsetMs).

Input t is nominally a time value but may be any Number value. The algorithm must not limit t to the time value range, so that inputs corresponding with a boundary of the time value range can be supported regardless of local UTC offset. For example, the maximum time value is 8.64 × 1015, corresponding with " 275760-09-13T00:00:00Z". In an environment where the local time zone offset is ahead of UTC by 1 hour at that instant, it is represented by the larger input of 8.64 × 1015 3.6 × 106, corresponding with " 275760-09-13T01:00:00 01:00".

If political rules for the local time t are not available within the implementation, the result is t because DefaultTimeZone returns "UTC" and GetNamedTimeZoneOffsetNanoseconds returns 0.

Note 1

It is recommended that implementations use the time zone information of the IANA Time Zone Database https://www.iana.org/time-zones/.

1:30 AM on 5 November 2017 in America/New_York is repeated twice (fall backward), but it must be interpreted as 1:30 AM UTC-04 instead of 1:30 AM UTC-05. In UTC(TimeClip(MakeDate(MakeDay(2017, 10, 5), MakeTime(1, 30, 0, 0)))), the value of offsetMs is -4 × msPerHour.

2:30 AM on 12 March 2017 in America/New_York does not exist, but it must be interpreted as 2:30 AM UTC-05 (equivalent to 3:30 AM UTC-04). In UTC(TimeClip(MakeDate(MakeDay(2017, 2, 12), MakeTime(2, 30, 0, 0)))), the value of offsetMs is -5 × msPerHour.

Note 2

UTC(LocalTime(tUTC)) is not necessarily always equal to tUTC. Correspondingly, LocalTime(UTC(tlocal)) is not necessarily always equal to tlocal.

21.4.1.13 Hours, Minutes, Second, and Milliseconds

The following abstract operations are useful in decomposing time values:

HourFromTime(t) = 𝔽(floor(ℝ(t / msPerHour)) modulo HoursPerDay)
MinFromTime(t) = 𝔽(floor(ℝ(t / msPerMinute)) modulo MinutesPerHour)
msFromTime(t) = 𝔽(ℝ(t) modulo ℝ(msPerSecond))

where

HoursPerDay = 24
MinutesPerHour = 60
SecondsPerMinute = 60
msPerSecond = 1000𝔽
msPerMinute = 60000𝔽 = msPerSecond × 𝔽(SecondsPerMinute)
msPerHour = 3600000𝔽 = msPerMinute × 𝔽(MinutesPerHour)

21.4.1.14 MakeTime ( hour, min, sec, ms )

The abstract operation MakeTime takes arguments hour (a Number), min (a Number), sec (a Number), and ms (a Number) and returns a Number. It calculates a number of milliseconds. It performs the following steps when called:

  1. If hour is not finite, min is not finite, sec is not finite, or ms is not finite, return NaN.
  2. Let h be 𝔽(! ToIntegerOrInfinity(hour)).
  3. Let m be 𝔽(! ToIntegerOrInfinity(min)).
  4. Let s be 𝔽(! ToIntegerOrInfinity(sec)).
  5. Let milli be 𝔽(! ToIntegerOrInfinity(ms)).
  6. Let t be ((h * msPerHour m * msPerMinute) s * msPerSecond) milli, performing the arithmetic according to IEEE 754-2019 rules (that is, as if using the ECMAScript operators * and ).
  7. Return t.

21.4.1.15 MakeDay ( year, month, date )

The abstract operation MakeDay takes arguments year (a Number), month (a Number), and date (a Number) and returns a Number. It calculates a number of days. It performs the following steps when called:

  1. If year is not finite, month is not finite, or date is not finite, return NaN.
  2. Let y be 𝔽(! ToIntegerOrInfinity(year)).
  3. Let m be 𝔽(! ToIntegerOrInfinity(month)).
  4. Let dt be 𝔽(! ToIntegerOrInfinity(date)).
  5. Let ym be y 𝔽(floor(ℝ(m) / 12)).
  6. If ym is not finite, return NaN.
  7. Let mn be 𝔽(ℝ(m) modulo 12).
  8. Find a finite time value t such that YearFromTime(t) is ym, MonthFromTime(t) is mn, and DateFromTime(t) is 1𝔽; but if this is not possible (because some argument is out of range), return NaN.
  9. Return Day(t) dt - 1𝔽.

21.4.1.16 MakeDate ( day, time )

The abstract operation MakeDate takes arguments day (a Number) and time (a Number) and returns a Number. It calculates a number of milliseconds. It performs the following steps when called:

  1. If day is not finite or time is not finite, return NaN.
  2. Let tv be day × msPerDay time.
  3. If tv is not finite, return NaN.
  4. Return tv.

21.4.1.17 TimeClip ( time )

The abstract operation TimeClip takes argument time (a Number) and returns a Number. It calculates a number of milliseconds. It performs the following steps when called:

  1. If time is not finite, return NaN.
  2. If abs(ℝ(time)) > 8.64 × 1015, return NaN.
  3. Return 𝔽(! ToIntegerOrInfinity(time)).

21.4.1.18 Date Time String Format

ECMAScript defines a string interchange format for date-times based upon a simplification of the ISO 8601 calendar date extended format. The format is as follows: YYYY-MM-DDTHH:mm:ss.sssZ

Where the elements are as follows:

YYYY is the year in the proleptic Gregorian calendar as four decimal digits from 0000 to 9999, or as an expanded year of " " or "-" followed by six decimal digits.
- "-" (hyphen) appears literally twice in the string.
MM is the month of the year as two decimal digits from 01 (January) to 12 (December).
DD is the day of the month as two decimal digits from 01 to 31.
T "T" appears literally in the string, to indicate the beginning of the time element.
HH is the number of complete hours that have passed since midnight as two decimal digits from 00 to 24.
: ":" (colon) appears literally twice in the string.
mm is the number of complete minutes since the start of the hour as two decimal digits from 00 to 59.
ss is the number of complete seconds since the start of the minute as two decimal digits from 00 to 59.
. "." (dot) appears literally in the string.
sss is the number of complete milliseconds since the start of the second as three decimal digits.
Z is the UTC offset representation specified as "Z" (for UTC with no offset) or as either " " or "-" followed by a time expression HH:mm (a subset of the time zone offset string format for indicating local time ahead of or behind UTC, respectively)

This format includes date-only forms:

YYYY
YYYY-MM
YYYY-MM-DD

It also includes “date-time” forms that consist of one of the above date-only forms immediately followed by one of the following time forms with an optional UTC offset representation appended:

THH:mm
THH:mm:ss
THH:mm:ss.sss

A string containing out-of-bounds or nonconforming elements is not a valid instance of this format.

Note 1

As every day both starts and ends with midnight, the two notations 00:00 and 24:00 are available to distinguish the two midnights that can be associated with one date. This means that the following two notations refer to exactly the same point in time: 1995-02-04T24:00 and 1995-02-05T00:00. This interpretation of the latter form as "end of a calendar day" is consistent with ISO 8601, even though that specification reserves it for describing time intervals and does not permit it within representations of single points in time.

 Note 2

There exists no international standard that specifies abbreviations for civil time zones like CET, EST, etc. and sometimes the same abbreviation is even used for two very different time zones. For this reason, both ISO 8601 and this format specify numeric representations of time zone offsets.

21.4.1.18.1 Expanded Years

Covering the full time value range of approximately 273,790 years forward or backward from 1 January 1970 (21.4.1.1) requires representing years before 0 or after 9999. ISO 8601 permits expansion of the year representation, but only by mutual agreement of the partners in information interchange. In the simplified ECMAScript format, such an expanded year representation shall have 6 digits and is always prefixed with a or - sign. The year 0 is considered positive and must be prefixed with a sign. The representation of the year 0 as -000000 is invalid. Strings matching the Date Time String Format with expanded years representing instants in time outside the range of a time value are treated as unrecognizable by Date.parse and cause that function to return NaN without falling back to implementation-specific behaviour or heuristics.

Note

Examples of date-time values with expanded years:

-271821-04-20T00:00:00Z 271822 B.C.
-000001-01-01T00:00:00Z 2 B.C.
000000-01-01T00:00:00Z 1 B.C.
000001-01-01T00:00:00Z 1 A.D.
001970-01-01T00:00:00Z 1970 A.D.
002009-12-15T00:00:00Z 2009 A.D.
275760-09-13T00:00:00Z 275760 A.D.

21.4.1.19 Time Zone Offset String Format

ECMAScript defines a string interchange format for UTC offsets, derived from ISO 8601. The format is described by the following grammar. The usage of Unicode code points in this grammar is listed in Table 60.

Table 60: Time Zone Offset String Code Points
Code Point Unicode Name Abbreviation
U 2212 MINUS SIGN <MINUS>

Syntax

UTCOffset ::: TemporalSign Hour TemporalSign Hour HourSubcomponents[ Extended] TemporalSign Hour HourSubcomponents[~Extended] TemporalSign ::: ASCIISign <MINUS> ASCIISign ::: one of - Hour ::: 0 DecimalDigit 1 DecimalDigit 20 21 22 23 HourSubcomponents[Extended] ::: TimeSeparator[?Extended] MinuteSecond TimeSeparator[?Extended] MinuteSecond TimeSeparator[?Extended] MinuteSecond TemporalDecimalFractionopt TimeSeparator[Extended] ::: [ Extended] : [~Extended] [empty] MinuteSecond ::: 0 DecimalDigit 1 DecimalDigit 2 DecimalDigit 3 DecimalDigit 4 DecimalDigit 5 DecimalDigit TemporalDecimalFraction ::: TemporalDecimalSeparator DecimalDigit TemporalDecimalSeparator DecimalDigit DecimalDigit TemporalDecimalSeparator DecimalDigit DecimalDigit DecimalDigit TemporalDecimalSeparator DecimalDigit DecimalDigit DecimalDigit DecimalDigit TemporalDecimalSeparator DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit TemporalDecimalSeparator DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit TemporalDecimalSeparator DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit TemporalDecimalSeparator DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit TemporalDecimalSeparator DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit TemporalDecimalSeparator ::: one of . ,

21.4.1.19.1 IsTimeZoneOffsetString ( offsetString )

The abstract operation IsTimeZoneOffsetString takes argument offsetString (a String) and returns a Boolean. The return value indicates whether offsetString conforms to the grammar given by UTCOffset. It performs the following steps when called:

  1. Let parseResult be ParseText(StringToCodePoints(offsetString), UTCOffset).
  2. If parseResult is a List of errors, return false.
  3. Return true.

21.4.1.19.2 ParseTimeZoneOffsetString ( offsetString )

The abstract operation ParseTimeZoneOffsetString takes argument offsetString (a String) and returns an integer. The return value is the UTC offset, as a number of nanoseconds, that corresponds to the String offsetString. It performs the following steps when called:

  1. Let parseResult be ParseText(StringToCodePoints(offsetString), UTCOffset).
  2. Assert: parseResult is not a List of errors.
  3. Assert: parseResult contains a TemporalSign Parse Node.
  4. Let parsedSign be the source text matched by the TemporalSign Parse Node contained within parseResult.
  5. If parsedSign is the single code point U 002D (HYPHEN-MINUS) or U 2212 (MINUS SIGN), then
    1. Let sign be -1.
  6. Else,
    1. Let sign be 1.
  7. NOTE: Applications of StringToNumber below do not lose precision, since each of the parsed values is guaranteed to be a sufficiently short string of decimal digits.
  8. Assert: parseResult contains an Hour Parse Node.
  9. Let parsedHours be the source text matched by the Hour Parse Node contained within parseResult.
  10. Let hours be ℝ(StringToNumber(CodePointsToString(parsedHours))).
  11. If parseResult does not contain a MinuteSecond Parse Node, then
    1. Let minutes be 0.
  12. Else,
    1. Let parsedMinutes be the source text matched by the first MinuteSecond Parse Node contained within parseResult.
    2. Let minutes be ℝ(StringToNumber(CodePointsToString(parsedMinutes))).
  13. If parseResult does not contain two MinuteSecond Parse Nodes, then
    1. Let seconds be 0.
  14. Else,
    1. Let parsedSeconds be the source text matched by the second MinuteSecond Parse Node contained within parseResult.
    2. Let seconds be ℝ(StringToNumber(CodePointsToString(parsedSeconds))).
  15. If parseResult does not contain a TemporalDecimalFraction Parse Node, then
    1. Let nanoseconds be 0.
  16. Else,
    1. Let parsedFraction be the source text matched by the TemporalDecimalFraction Parse Node contained within parseResult.
    2. Let fraction be the string-concatenation of CodePointsToString(parsedFraction) and "000000000".
    3. Let nanosecondsString be the substring of fraction from 1 to 10.
    4. Let nanoseconds be ℝ(StringToNumber(nanosecondsString)).
  17. Return sign × (((hours × 60 minutes) × 60 seconds) × 109 nanoseconds).

21.4.2 The Date Constructor

The Date constructor:

  • is �te%.
  • is the initial value of the "Date" property of the global object.
  • creates and initializes a new Date when called as a constructor.
  • returns a String representing the current time (UTC) when called as a function rather than as a constructor.
  • is a function whose behaviour differs based upon the number and types of its arguments.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Date behaviour must include a super call to the Date constructor to create and initialize the subclass instance with a [[DateValue]] internal slot.
  • has a "length" property whose value is 7𝔽.

21.4.2.1 Date ( ...values )

This function performs the following steps when called:

  1. If NewTarget is undefined, then
    1. Let now be the time value (UTC) identifying the current time.
    2. Return ToDateString(now).
  2. Let numberOfArgs be the number of elements in values.
  3. If numberOfArgs = 0, then
    1. Let dv be the time value (UTC) identifying the current time.
  4. Else if numberOfArgs = 1, then
    1. Let value be values[0].
    2. If value is an Object and value has a [[DateValue]] internal slot, then
      1. Let tv be ! thisTimeValue(value).
    3. Else,
      1. Let v be ? ToPrimitive(value).
      2. If v is a String, then
        1. Assert: The next step never returns an abrupt completion because v is a String.
        2. Let tv be the result of parsing v as a date, in exactly the same manner as for the parse method (21.4.3.2).
      3. Else,
        1. Let tv be ? ToNumber(v).
    4. Let dv be TimeClip(tv).
  5. Else,
    1. Assert: numberOfArgs ≥ 2.
    2. Let y be ? ToNumber(values[0]).
    3. Let m be ? ToNumber(values[1]).
    4. If numberOfArgs > 2, let dt be ? ToNumber(values[2]); else let dt be 1𝔽.
    5. If numberOfArgs > 3, let h be ? ToNumber(values[3]); else let h be 0𝔽.
    6. If numberOfArgs > 4, let min be ? ToNumber(values[4]); else let min be 0𝔽.
    7. If numberOfArgs > 5, let s be ? ToNumber(values[5]); else let s be 0𝔽.
    8. If numberOfArgs > 6, let milli be ? ToNumber(values[6]); else let milli be 0𝔽.
    9. If y is NaN, let yr be NaN.
    10. Else,
      1. Let yi be ! ToIntegerOrInfinity(y).
      2. If 0 ≤ yi ≤ 99, let yr be 1900𝔽 𝔽(yi); otherwise, let yr be y.
    11. Let finalDate be MakeDate(MakeDay(yr, m, dt), MakeTime(h, min, s, milli)).
    12. Let dv be TimeClip(UTC(finalDate)).
  6. Let O be ? OrdinaryCreateFromConstructor(NewTarget, "�te.prototype%", « [[DateValue]] »).
  7. Set O.[[DateValue]] to dv.
  8. Return O.

21.4.3 Properties of the Date Constructor

The Date constructor:

  • has a [[Prototype]] internal slot whose value is %Function.prototype%.
  • has the following properties:

21.4.3.1 Date.now ( )

This function returns the time value designating the UTC date and time of the occurrence of the call to it.

21.4.3.2 Date.parse ( string )

This function applies the ToString operator to its argument. If ToString results in an abrupt completion the Completion Record is immediately returned. Otherwise, this function interprets the resulting String as a date and time; it returns a Number, the UTC time value corresponding to the date and time. The String may be interpreted as a local time, a UTC time, or a time in some other time zone, depending on the contents of the String. The function first attempts to parse the String according to the format described in Date Time String Format (21.4.1.18), including expanded years. If the String does not conform to that format the function may fall back to any implementation-specific heuristics or implementation-specific date formats. Strings that are unrecognizable or contain out-of-bounds format element values shall cause this function to return NaN.

If the String conforms to the Date Time String Format, substitute values take the place of absent format elements. When the MM or DD elements are absent, "01" is used. When the HH, mm, or ss elements are absent, "00" is used. When the sss element is absent, "000" is used. When the UTC offset representation is absent, date-only forms are interpreted as a UTC time and date-time forms are interpreted as a local time.

If x is any Date whose milliseconds amount is zero within a particular implementation of ECMAScript, then all of the following expressions should produce the same numeric value in that implementation, if all the properties referenced have their initial values:

x.valueOf()
Date.parse(x.toString())
Date.parse(x.toUTCString())
Date.parse(x.toISOString())

However, the expression

Date.parse(x.toLocaleString())

is not required to produce the same Number value as the preceding three expressions and, in general, the value produced by this function is implementation-defined when given any String value that does not conform to the Date Time String Format (21.4.1.18) and that could not be produced in that implementation by the toString or toUTCString method.

21.4.3.3 Date.prototype

The initial value of Date.prototype is the Date prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

21.4.3.4 Date.UTC ( year [ , month [ , date [ , hours [ , minutes [ , seconds [ , ms ] ] ] ] ] ] )

This function performs the following steps when called:

  1. Let y be ? ToNumber(year).
  2. If month is present, let m be ? ToNumber(month); else let m be 0𝔽.
  3. If date is present, let dt be ? ToNumber(date); else let dt be 1𝔽.
  4. If hours is present, let h be ? ToNumber(hours); else let h be 0𝔽.
  5. If minutes is present, let min be ? ToNumber(minutes); else let min be 0𝔽.
  6. If seconds is present, let s be ? ToNumber(seconds); else let s be 0𝔽.
  7. If ms is present, let milli be ? ToNumber(ms); else let milli be 0𝔽.
  8. If y is NaN, let yr be NaN.
  9. Else,
    1. Let yi be ! ToIntegerOrInfinity(y).
    2. If 0 ≤ yi ≤ 99, let yr be 1900𝔽 𝔽(yi); otherwise, let yr be y.
  10. Return TimeClip(MakeDate(MakeDay(yr, m, dt), MakeTime(h, min, s, milli))).

The "length" property of this function is 7𝔽.

Note

This function differs from the Date constructor in two ways: it returns a time value as a Number, rather than creating a Date, and it interprets the arguments in UTC rather than as local time.

21.4.4 Properties of the Date Prototype Object

The Date prototype object:

  • is �te.prototype%.
  • is itself an ordinary object.
  • is not a Date instance and does not have a [[DateValue]] internal slot.
  • has a [[Prototype]] internal slot whose value is %Object.prototype%.

Unless explicitly defined otherwise, the methods of the Date prototype object defined below are not generic and the this value passed to them must be an object that has a [[DateValue]] internal slot that has been initialized to a time value.

The abstract operation thisTimeValue takes argument value. It performs the following steps when called:

  1. If value is an Object and value has a [[DateValue]] internal slot, then
    1. Return value.[[DateValue]].
  2. Throw a TypeError exception.

In following descriptions of functions that are properties of the Date prototype object, the phrase “this Date object” refers to the object that is the this value for the invocation of the function. If the Type of the this value is not Object, a TypeError exception is thrown. The phrase “this time value” within the specification of a method refers to the result returned by calling the abstract operation thisTimeValue with the this value of the method invocation passed as the argument.

21.4.4.1 Date.prototype.constructor

The initial value of Date.prototype.constructor is �te%.

21.4.4.2 Date.prototype.getDate ( )

This method performs the following steps when called:

  1. Let t be ? thisTimeValue(this value).
  2. If t is NaN, return NaN.
  3. Return DateFromTime(LocalTime(t)).

21.4.4.3 Date.prototype.getDay ( )

This method performs the following steps when called:

  1. Let t be ? thisTimeValue(this value).
  2. If t is NaN, return NaN.
  3. Return WeekDay(LocalTime(t)).

21.4.4.4 Date.prototype.getFullYear ( )

This method performs the following steps when called:

  1. Let t be ? thisTimeValue(this value).
  2. If t is NaN, return NaN.
  3. Return YearFromTime(LocalTime(t)).

21.4.4.5 Date.prototype.getHours ( )

This method performs the following steps when called:

  1. Let t be ? thisTimeValue(this value).
  2. If t is NaN, return NaN.
  3. Return HourFromTime(LocalTime(t)).

21.4.4.6 Date.prototype.getMilliseconds ( )

This method performs the following steps when called:

  1. Let t be ? thisTimeValue(this value).
  2. If t is NaN, return NaN.
  3. Return msFromTime(LocalTime(t)).

21.4.4.7 Date.prototype.getMinutes ( )

This method performs the following steps when called:

  1. Let t be ? thisTimeValue(this value).
  2. If t is NaN, return NaN.
  3. Return MinFromTime(LocalTime(t)).

21.4.4.8 Date.prototype.getMonth ( )

This method performs the following steps when called:

  1. Let t be ? thisTimeValue(this value).
  2. If t is NaN, return NaN.
  3. Return MonthFromTime(LocalTime(t)).

21.4.4.9 Date.prototype.getSeconds ( )

This method performs the following steps when called:

  1. Let t be ? thisTimeValue(this value).
  2. If t is NaN, return NaN.
  3. Return SecFromTime(LocalTime(t)).

21.4.4.10 Date.prototype.getTime ( )

This method performs the following steps when called:

  1. Return ? thisTimeValue(this value).

21.4.4.11 Date.prototype.getTimezoneOffset ( )

This method performs the following steps when called:

  1. Let t be ? thisTimeValue(this value).
  2. If t is NaN, return NaN.
  3. Return (t - LocalTime(t)) / msPerMinute.

21.4.4.12 Date.prototype.getUTCDate ( )

This method performs the following steps when called:

  1. Let t be ? thisTimeValue(this value).
  2. If t is NaN, return NaN.
  3. Return DateFromTime(t).

21.4.4.13 Date.prototype.getUTCDay ( )

This method performs the following steps when called:

  1. Let t be ? thisTimeValue(this value).
  2. If t is NaN, return NaN.
  3. Return WeekDay(t).

21.4.4.14 Date.prototype.getUTCFullYear ( )

This method performs the following steps when called:

  1. Let t be ? thisTimeValue(this value).
  2. If t is NaN, return NaN.
  3. Return YearFromTime(t).

21.4.4.15 Date.prototype.getUTCHours ( )

This method performs the following steps when called:

  1. Let t be ? thisTimeValue(this value).
  2. If t is NaN, return NaN.
  3. Return HourFromTime(t).

21.4.4.16 Date.prototype.getUTCMilliseconds ( )

This method performs the following steps when called:

  1. Let t be ? thisTimeValue(this value).
  2. If t is NaN, return NaN.
  3. Return msFromTime(t).

21.4.4.17 Date.prototype.getUTCMinutes ( )

This method performs the following steps when called:

  1. Let t be ? thisTimeValue(this value).
  2. If t is NaN, return NaN.
  3. Return MinFromTime(t).

21.4.4.18 Date.prototype.getUTCMonth ( )

This method performs the following steps when called:

  1. Let t be ? thisTimeValue(this value).
  2. If t is NaN, return NaN.
  3. Return MonthFromTime(t).

21.4.4.19 Date.prototype.getUTCSeconds ( )

This method performs the following steps when called:

  1. Let t be ? thisTimeValue(this value).
  2. If t is NaN, return NaN.
  3. Return SecFromTime(t).

21.4.4.20 Date.prototype.setDate ( date )

This method performs the following steps when called:

  1. Let t be ? thisTimeValue(this value).
  2. Let dt be ? ToNumber(date).
  3. If t is NaN, return NaN.
  4. Set t to LocalTime(t).
  5. Let newDate be MakeDate(MakeDay(YearFromTime(t), MonthFromTime(t), dt), TimeWithinDay(t)).
  6. Let u be TimeClip(UTC(newDate)).
  7. Set the [[DateValue]] internal slot of this Date object to u.
  8. Return u.

21.4.4.21 Date.prototype.setFullYear ( year [ , month [ , date ] ] )

This method performs the following steps when called:

  1. Let t be ? thisTimeValue(this value).
  2. Let y be ? ToNumber(year).
  3. If t is NaN, set t to 0𝔽; otherwise, set t to LocalTime(t).
  4. If month is not present, let m be MonthFromTime(t); otherwise, let m be ? ToNumber(month).
  5. If date is not present, let dt be DateFromTime(t); otherwise, let dt be ? ToNumber(date).
  6. Let newDate be MakeDate(MakeDay(y, m, dt), TimeWithinDay(t)).
  7. Let u be TimeClip(UTC(newDate)).
  8. Set the [[DateValue]] internal slot of this Date object to u.
  9. Return u.

The "length" property of this method is 3𝔽.

Note

If month is not present, this method behaves as if month was present with the value getMonth(). If date is not present, it behaves as if date was present with the value getDate().

21.4.4.22 Date.prototype.setHours ( hour [ , min [ , sec [ , ms ] ] ] )

This method performs the following steps when called:

  1. Let t be ? thisTimeValue(this value).
  2. Let h be ? ToNumber(hour).
  3. If min is present, let m be ? ToNumber(min).
  4. If sec is present, let s be ? ToNumber(sec).
  5. If ms is present, let milli be ? ToNumber(ms).
  6. If t is NaN, return NaN.
  7. Set t to LocalTime(t).
  8. If min is not present, let m be MinFromTime(t).
  9. If sec is not present, let s be SecFromTime(t).
  10. If ms is not present, let milli be msFromTime(t).
  11. Let date be MakeDate(Day(t), MakeTime(h, m, s, milli)).
  12. Let u be TimeClip(UTC(date)).
  13. Set the [[DateValue]] internal slot of this Date object to u.
  14. Return u.

The "length" property of this method is 4𝔽.

Note

If min is not present, this method behaves as if min was present with the value getMinutes(). If sec is not present, it behaves as if sec was present with the value getSeconds(). If ms is not present, it behaves as if ms was present with the value getMilliseconds().

21.4.4.23 Date.prototype.setMilliseconds ( ms )

This method performs the following steps when called:

  1. Let t be ? thisTimeValue(this value).
  2. Set ms to ? ToNumber(ms).
  3. If t is NaN, return NaN.
  4. Set t to LocalTime(t).
  5. Let time be MakeTime(HourFromTime(t), MinFromTime(t), SecFromTime(t), ms).
  6. Let u be TimeClip(UTC(MakeDate(Day(t), time))).
  7. Set the [[DateValue]] internal slot of this Date object to u.
  8. Return u.

21.4.4.24 Date.prototype.setMinutes ( min [ , sec [ , ms ] ] )

This method performs the following steps when called:

  1. Let t be ? thisTimeValue(this value).
  2. Let m be ? ToNumber(min).
  3. If sec is present, let s be ? ToNumber(sec).
  4. If ms is present, let milli be ? ToNumber(ms).
  5. If t is NaN, return NaN.
  6. Set t to LocalTime(t).
  7. If sec is not present, let s be SecFromTime(t).
  8. If ms is not present, let milli be msFromTime(t).
  9. Let date be MakeDate(Day(t), MakeTime(HourFromTime(t), m, s, milli)).
  10. Let u be TimeClip(UTC(date)).
  11. Set the [[DateValue]] internal slot of this Date object to u.
  12. Return u.

The "length" property of this method is 3𝔽.

Note

If sec is not present, this method behaves as if sec was present with the value getSeconds(). If ms is not present, this behaves as if ms was present with the value getMilliseconds().

21.4.4.25 Date.prototype.setMonth ( month [ , date ] )

This method performs the following steps when called:

  1. Let t be ? thisTimeValue(this value).
  2. Let m be ? ToNumber(month).
  3. If date is present, let dt be ? ToNumber(date).
  4. If t is NaN, return NaN.
  5. Set t to LocalTime(t).
  6. If date is not present, let dt be DateFromTime(t).
  7. Let newDate be MakeDate(MakeDay(YearFromTime(t), m, dt), TimeWithinDay(t)).
  8. Let u be TimeClip(UTC(newDate)).
  9. Set the [[DateValue]] internal slot of this Date object to u.
  10. Return u.

The "length" property of this method is 2𝔽.

Note

If date is not present, this method behaves as if date was present with the value getDate().

21.4.4.26 Date.prototype.setSeconds ( sec [ , ms ] )

This method performs the following steps when called:

  1. Let t be ? thisTimeValue(this value).
  2. Let s be ? ToNumber(sec).
  3. If ms is present, let milli be ? ToNumber(ms).
  4. If t is NaN, return NaN.
  5. Set t to LocalTime(t).
  6. If ms is not present, let milli be msFromTime(t).
  7. Let date be MakeDate(Day(t), MakeTime(HourFromTime(t), MinFromTime(t), s, milli)).
  8. Let u be TimeClip(UTC(date)).
  9. Set the [[DateValue]] internal slot of this Date object to u.
  10. Return u.

The "length" property of this method is 2𝔽.

Note

If ms is not present, this method behaves as if ms was present with the value getMilliseconds().

21.4.4.27 Date.prototype.setTime ( time )

This method performs the following steps when called:

  1. Perform ? thisTimeValue(this value).
  2. Let t be ? ToNumber(time).
  3. Let v be TimeClip(t).
  4. Set the [[DateValue]] internal slot of this Date object to v.
  5. Return v.

21.4.4.28 Date.prototype.setUTCDate ( date )

This method performs the following steps when called:

  1. Let t be ? thisTimeValue(this value).
  2. Let dt be ? ToNumber(date).
  3. If t is NaN, return NaN.
  4. Let newDate be MakeDate(MakeDay(YearFromTime(t), MonthFromTime(t), dt), TimeWithinDay(t)).
  5. Let v be TimeClip(newDate).
  6. Set the [[DateValue]] internal slot of this Date object to v.
  7. Return v.

21.4.4.29 Date.prototype.setUTCFullYear ( year [ , month [ , date ] ] )

This method performs the following steps when called:

  1. Let t be ? thisTimeValue(this value).
  2. If t is NaN, set t to 0𝔽.
  3. Let y be ? ToNumber(year).
  4. If month is not present, let m be MonthFromTime(t); otherwise, let m be ? ToNumber(month).
  5. If date is not present, let dt be DateFromTime(t); otherwise, let dt be ? ToNumber(date).
  6. Let newDate be MakeDate(MakeDay(y, m, dt), TimeWithinDay(t)).
  7. Let v be TimeClip(newDate).
  8. Set the [[DateValue]] internal slot of this Date object to v.
  9. Return v.

The "length" property of this method is 3𝔽.

Note

If month is not present, this method behaves as if month was present with the value getUTCMonth(). If date is not present, it behaves as if date was present with the value getUTCDate().

21.4.4.30 Date.prototype.setUTCHours ( hour [ , min [ , sec [ , ms ] ] ] )

This method performs the following steps when called:

  1. Let t be ? thisTimeValue(this value).
  2. Let h be ? ToNumber(hour).
  3. If min is present, let m be ? ToNumber(min).
  4. If sec is present, let s be ? ToNumber(sec).
  5. If ms is present, let milli be ? ToNumber(ms).
  6. If t is NaN, return NaN.
  7. If min is not present, let m be MinFromTime(t).
  8. If sec is not present, let s be SecFromTime(t).
  9. If ms is not present, let milli be msFromTime(t).
  10. Let date be MakeDate(Day(t), MakeTime(h, m, s, milli)).
  11. Let v be TimeClip(date).
  12. Set the [[DateValue]] internal slot of this Date object to v.
  13. Return v.

The "length" property of this method is 4𝔽.

Note

If min is not present, this method behaves as if min was present with the value getUTCMinutes(). If sec is not present, it behaves as if sec was present with the value getUTCSeconds(). If ms is not present, it behaves as if ms was present with the value getUTCMilliseconds().

21.4.4.31 Date.prototype.setUTCMilliseconds ( ms )

This method performs the following steps when called:

  1. Let t be ? thisTimeValue(this value).
  2. Set ms to ? ToNumber(ms).
  3. If t is NaN, return NaN.
  4. Let time be MakeTime(HourFromTime(t), MinFromTime(t), SecFromTime(t), ms).
  5. Let v be TimeClip(MakeDate(Day(t), time)).
  6. Set the [[DateValue]] internal slot of this Date object to v.
  7. Return v.

21.4.4.32 Date.prototype.setUTCMinutes ( min [ , sec [ , ms ] ] )

This method performs the following steps when called:

  1. Let t be ? thisTimeValue(this value).
  2. Let m be ? ToNumber(min).
  3. If sec is present, let s be ? ToNumber(sec).
  4. If ms is present, let milli be ? ToNumber(ms).
  5. If t is NaN, return NaN.
  6. If sec is not present, let s be SecFromTime(t).
  7. If ms is not present, let milli be msFromTime(t).
  8. Let date be MakeDate(Day(t), MakeTime(HourFromTime(t), m, s, milli)).
  9. Let v be TimeClip(date).
  10. Set the [[DateValue]] internal slot of this Date object to v.
  11. Return v.

The "length" property of this method is 3𝔽.

Note

If sec is not present, this method behaves as if sec was present with the value getUTCSeconds(). If ms is not present, it behaves as if ms was present with the value return by getUTCMilliseconds().

21.4.4.33 Date.prototype.setUTCMonth ( month [ , date ] )

This method performs the following steps when called:

  1. Let t be ? thisTimeValue(this value).
  2. Let m be ? ToNumber(month).
  3. If date is present, let dt be ? ToNumber(date).
  4. If t is NaN, return NaN.
  5. If date is not present, let dt be DateFromTime(t).
  6. Let newDate be MakeDate(MakeDay(YearFromTime(t), m, dt), TimeWithinDay(t)).
  7. Let v be TimeClip(newDate).
  8. Set the [[DateValue]] internal slot of this Date object to v.
  9. Return v.

The "length" property of this method is 2𝔽.

Note

If date is not present, this method behaves as if date was present with the value getUTCDate().

21.4.4.34 Date.prototype.setUTCSeconds ( sec [ , ms ] )

This method performs the following steps when called:

  1. Let t be ? thisTimeValue(this value).
  2. Let s be ? ToNumber(sec).
  3. If ms is present, let milli be ? ToNumber(ms).
  4. If t is NaN, return NaN.
  5. If ms is not present, let milli be msFromTime(t).
  6. Let date be MakeDate(Day(t), MakeTime(HourFromTime(t), MinFromTime(t), s, milli)).
  7. Let v be TimeClip(date).
  8. Set the [[DateValue]] internal slot of this Date object to v.
  9. Return v.

The "length" property of this method is 2𝔽.

Note

If ms is not present, this method behaves as if ms was present with the value getUTCMilliseconds().

21.4.4.35 Date.prototype.toDateString ( )

This method performs the following steps when called:

  1. Let O be this Date object.
  2. Let tv be ? thisTimeValue(O).
  3. If tv is NaN, return "Invalid Date".
  4. Let t be LocalTime(tv).
  5. Return DateString(t).

21.4.4.36 Date.prototype.toISOString ( )

If this time value is not a finite Number or if it corresponds with a year that cannot be represented in the Date Time String Format, this method throws a RangeError exception. Otherwise, it returns a String representation of this time value in that format on the UTC time scale, including all format elements and the UTC offset representation "Z".

21.4.4.37 Date.prototype.toJSON ( key )

This method provides a String representation of a Date for use by JSON.stringify (25.5.2).

It performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let tv be ? ToPrimitive(O, number).
  3. If tv is a Number and tv is not finite, return null.
  4. Return ? Invoke(O, "toISOString").
 Note 1

The argument is ignored.

 Note 2

This method is intentionally generic; it does not require that its this value be a Date. Therefore, it can be transferred to other kinds of objects for use as a method. However, it does require that any such object have a toISOString method.

21.4.4.38 Date.prototype.toLocaleDateString ( [ reserved1 [ , reserved2 ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of this method is used:

This method returns a String value. The contents of the String are implementation-defined, but are intended to represent the “date” portion of the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of the host environment's current locale.

The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.

21.4.4.39 Date.prototype.toLocaleString ( [ reserved1 [ , reserved2 ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of this method is used:

This method returns a String value. The contents of the String are implementation-defined, but are intended to represent the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of the host environment's current locale.

The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.

21.4.4.40 Date.prototype.toLocaleTimeString ( [ reserved1 [ , reserved2 ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of this method is used:

This method returns a String value. The contents of the String are implementation-defined, but are intended to represent the “time” portion of the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of the host environment's current locale.

The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.

21.4.4.41 Date.prototype.toString ( )

This method performs the following steps when called:

  1. Let tv be ? thisTimeValue(this value).
  2. Return ToDateString(tv).
 Note 1

For any Date d such that d.[[DateValue]] is evenly divisible by 1000, the result of Date.parse(d.toString()) = d.valueOf(). See 21.4.3.2.

 Note 2

This method is not generic; it throws a TypeError exception if its this value is not a Date. Therefore, it cannot be transferred to other kinds of objects for use as a method.

21.4.4.41.1 TimeString ( tv )

The abstract operation TimeString takes argument tv (a Number, but not NaN) and returns a String. It performs the following steps when called:

  1. Let hour be ToZeroPaddedDecimalString(ℝ(HourFromTime(tv)), 2).
  2. Let minute be ToZeroPaddedDecimalString(ℝ(MinFromTime(tv)), 2).
  3. Let second be ToZeroPaddedDecimalString(ℝ(SecFromTime(tv)), 2).
  4. Return the string-concatenation of hour, ":", minute, ":", second, the code unit 0x0020 (SPACE), and "GMT".

21.4.4.41.2 DateString ( tv )

The abstract operation DateString takes argument tv (a Number, but not NaN) and returns a String. It performs the following steps when called:

  1. Let weekday be the Name of the entry in Table 61 with the Number WeekDay(tv).
  2. Let month be the Name of the entry in Table 62 with the Number MonthFromTime(tv).
  3. Let day be ToZeroPaddedDecimalString(ℝ(DateFromTime(tv)), 2).
  4. Let yv be YearFromTime(tv).
  5. If yv is 0𝔽 or yv > 0𝔽, let yearSign be the empty String; otherwise, let yearSign be "-".
  6. Let paddedYear be ToZeroPaddedDecimalString(abs(ℝ(yv)), 4).
  7. Return the string-concatenation of weekday, the code unit 0x0020 (SPACE), month, the code unit 0x0020 (SPACE), day, the code unit 0x0020 (SPACE), yearSign, and paddedYear.
Table 61: Names of days of the week
Number Name
0𝔽 "Sun"
1𝔽 "Mon"
2𝔽 "Tue"
3𝔽 "Wed"
4𝔽 "Thu"
5𝔽 "Fri"
6𝔽 "Sat"
Table 62: Names of months of the year
Number Name
0𝔽 "Jan"
1𝔽 "Feb"
2𝔽 "Mar"
3𝔽 "Apr"
4𝔽 "May"
5𝔽 "Jun"
6𝔽 "Jul"
7𝔽 "Aug"
8𝔽 "Sep"
9𝔽 "Oct"
10𝔽 "Nov"
11𝔽 "Dec"

21.4.4.41.3 TimeZoneString ( tv )

The abstract operation TimeZoneString takes argument tv (an integral Number) and returns a String. It performs the following steps when called:

  1. Let localTimeZone be DefaultTimeZone().
  2. If IsTimeZoneOffsetString(localTimeZone) is true, then
    1. Let offsetNs be ParseTimeZoneOffsetString(localTimeZone).
  3. Else,
    1. Let offsetNs be GetNamedTimeZoneOffsetNanoseconds(localTimeZone, ℤ(ℝ(tv) × 106)).
  4. Let offset be 𝔽(truncate(offsetNs / 106)).
  5. If offset is 0𝔽 or offset > 0𝔽, then
    1. Let offsetSign be " ".
    2. Let absOffset be offset.
  6. Else,
    1. Let offsetSign be "-".
    2. Let absOffset be -offset.
  7. Let offsetMin be ToZeroPaddedDecimalString(ℝ(MinFromTime(absOffset)), 2).
  8. Let offsetHour be ToZeroPaddedDecimalString(ℝ(HourFromTime(absOffset)), 2).
  9. Let tzName be an implementation-defined string that is either the empty String or the string-concatenation of the code unit 0x0020 (SPACE), the code unit 0x0028 (LEFT PARENTHESIS), an implementation-defined timezone name, and the code unit 0x0029 (RIGHT PARENTHESIS).
  10. Return the string-concatenation of offsetSign, offsetHour, offsetMin, and tzName.

21.4.4.41.4 ToDateString ( tv )

The abstract operation ToDateString takes argument tv (an integral Number or NaN) and returns a String. It performs the following steps when called:

  1. If tv is NaN, return "Invalid Date".
  2. Let t be LocalTime(tv).
  3. Return the string-concatenation of DateString(t), the code unit 0x0020 (SPACE), TimeString(t), and TimeZoneString(tv).

21.4.4.42 Date.prototype.toTimeString ( )

This method performs the following steps when called:

  1. Let O be this Date object.
  2. Let tv be ? thisTimeValue(O).
  3. If tv is NaN, return "Invalid Date".
  4. Let t be LocalTime(tv).
  5. Return the string-concatenation of TimeString(t) and TimeZoneString(tv).

21.4.4.43 Date.prototype.toUTCString ( )

This method returns a String value representing the instance in time corresponding to this time value. The format of the String is based upon "HTTP-date" from RFC 7231, generalized to support the full range of times supported by ECMAScript Dates.

It performs the following steps when called:

  1. Let O be this Date object.
  2. Let tv be ? thisTimeValue(O).
  3. If tv is NaN, return "Invalid Date".
  4. Let weekday be the Name of the entry in Table 61 with the Number WeekDay(tv).
  5. Let month be the Name of the entry in Table 62 with the Number MonthFromTime(tv).
  6. Let day be ToZeroPaddedDecimalString(ℝ(DateFromTime(tv)), 2).
  7. Let yv be YearFromTime(tv).
  8. If yv is 0𝔽 or yv > 0𝔽, let yearSign be the empty String; otherwise, let yearSign be "-".
  9. Let paddedYear be ToZeroPaddedDecimalString(abs(ℝ(yv)), 4).
  10. Return the string-concatenation of weekday, ",", the code unit 0x0020 (SPACE), day, the code unit 0x0020 (SPACE), month, the code unit 0x0020 (SPACE), yearSign, paddedYear, the code unit 0x0020 (SPACE), and TimeString(tv).

21.4.4.44 Date.prototype.valueOf ( )

This method performs the following steps when called:

  1. Return ? thisTimeValue(this value).

21.4.4.45 Date.prototype [ @@toPrimitive ] ( hint )

This method is called by ECMAScript language operators to convert a Date to a primitive value. The allowed values for hint are "default", "number", and "string". Dates are unique among built-in ECMAScript object in that they treat "default" as being equivalent to "string", All other built-in ECMAScript objects treat "default" as being equivalent to "number".

It performs the following steps when called:

  1. Let O be the this value.
  2. If O is not an Object, throw a TypeError exception.
  3. If hint is either "string" or "default", then
    1. Let tryFirst be string.
  4. Else if hint is "number", then
    1. Let tryFirst be number.
  5. Else, throw a TypeError exception.
  6. Return ? OrdinaryToPrimitive(O, tryFirst).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

The value of the "name" property of this method is "[Symbol.toPrimitive]".

21.4.5 Properties of Date Instances

Date instances are ordinary objects that inherit properties from the Date prototype object. Date instances also have a [[DateValue]] internal slot. The [[DateValue]] internal slot is the time value represented by this Date.

22 Text Processing

22.1 String Objects

22.1.1 The String Constructor

The String constructor:

  • is %String%.
  • is the initial value of the "String" property of the global object.
  • creates and initializes a new String object when called as a constructor.
  • performs a type conversion when called as a function rather than as a constructor.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified String behaviour must include a super call to the String constructor to create and initialize the subclass instance with a [[StringData]] internal slot.

22.1.1.1 String ( value )

This function performs the following steps when called:

  1. If value is not present, let s be the empty String.
  2. Else,
    1. If NewTarget is undefined and value is a Symbol, return SymbolDescriptiveString(value).
    2. Let s be ? ToString(value).
  3. If NewTarget is undefined, return s.
  4. Return StringCreate(s, ? GetPrototypeFromConstructor(NewTarget, "%String.prototype%")).

22.1.2 Properties of the String Constructor

The String constructor:

  • has a [[Prototype]] internal slot whose value is %Function.prototype%.
  • has the following properties:

22.1.2.1 String.fromCharCode ( ...codeUnits )

This function may be called with any number of arguments which form the rest parameter codeUnits.

It performs the following steps when called:

  1. Let result be the empty String.
  2. For each element next of codeUnits, do
    1. Let nextCU be the code unit whose numeric value is ℝ(? ToUint16(next)).
    2. Set result to the string-concatenation of result and nextCU.
  3. Return result.

The "length" property of this function is 1𝔽.

22.1.2.2 String.fromCodePoint ( ...codePoints )

This function may be called with any number of arguments which form the rest parameter codePoints.

It performs the following steps when called:

  1. Let result be the empty String.
  2. For each element next of codePoints, do
    1. Let nextCP be ? ToNumber(next).
    2. If IsIntegralNumber(nextCP) is false, throw a RangeError exception.
    3. If ℝ(nextCP) < 0 or ℝ(nextCP) > 0x10FFFF, throw a RangeError exception.
    4. Set result to the string-concatenation of result and UTF16EncodeCodePoint(ℝ(nextCP)).
  3. Assert: If codePoints is empty, then result is the empty String.
  4. Return result.

The "length" property of this function is 1𝔽.

22.1.2.3 String.prototype

The initial value of String.prototype is the String prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

22.1.2.4 String.raw ( template, ...substitutions )

This function may be called with a variable number of arguments. The first argument is template and the remainder of the arguments form the List substitutions.

It performs the following steps when called:

  1. Let substitutionCount be the number of elements in substitutions.
  2. Let cooked be ? ToObject(template).
  3. Let literals be ? ToObject(? Get(cooked, "raw")).
  4. Let literalCount be ? LengthOfArrayLike(literals).
  5. If literalCount ≤ 0, return the empty String.
  6. Let R be the empty String.
  7. Let nextIndex be 0.
  8. Repeat,
    1. Let nextLiteralVal be ? Get(literals, ! ToString(𝔽(nextIndex))).
    2. Let nextLiteral be ? ToString(nextLiteralVal).
    3. Set R to the string-concatenation of R and nextLiteral.
    4. If nextIndex 1 = literalCount, return R.
    5. If nextIndex < substitutionCount, then
      1. Let nextSubVal be substitutions[nextIndex].
      2. Let nextSub be ? ToString(nextSubVal).
      3. Set R to the string-concatenation of R and nextSub.
    6. Set nextIndex to nextIndex 1.
 Note

This function is intended for use as a tag function of a Tagged Template (13.3.11). When called as such, the first argument will be a well formed template object and the rest parameter will contain the substitution values.

22.1.3 Properties of the String Prototype Object

The String prototype object:

  • is %String.prototype%.
  • is a String exotic object and has the internal methods specified for such objects.
  • has a [[StringData]] internal slot whose value is the empty String.
  • has a "length" property whose initial value is 0𝔽 and whose attributes are { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
  • has a [[Prototype]] internal slot whose value is %Object.prototype%.

Unless explicitly stated otherwise, the methods of the String prototype object defined below are not generic and the this value passed to them must be either a String value or an object that has a [[StringData]] internal slot that has been initialized to a String value.

The abstract operation thisStringValue takes argument value. It performs the following steps when called:

  1. If value is a String, return value.
  2. If value is an Object and value has a [[StringData]] internal slot, then
    1. Let s be value.[[StringData]].
    2. Assert: s is a String.
    3. Return s.
  3. Throw a TypeError exception.

22.1.3.1 String.prototype.at ( index )

  1. Let O be ? RequireObjectCoercible(this value).
  2. Let S be ? ToString(O).
  3. Let len be the length of S.
  4. Let relativeIndex be ? ToIntegerOrInfinity(index).
  5. If relativeIndex ≥ 0, then
    1. Let k be relativeIndex.
  6. Else,
    1. Let k be len relativeIndex.
  7. If k < 0 or k ≥ len, return undefined.
  8. Return the substring of S from k to k 1.

22.1.3.2 String.prototype.charAt ( pos )

Note 1

This method returns a single element String containing the code unit at index pos within the String value resulting from converting this object to a String. If there is no element at that index, the result is the empty String. The result is a String value, not a String object.

If pos is an integral Number, then the result of x.charAt(pos) is equivalent to the result of x.substring(pos, pos 1).

This method performs the following steps when called:

  1. Let O be ? RequireObjectCoercible(this value).
  2. Let S be ? ToString(O).
  3. Let position be ? ToIntegerOrInfinity(pos).
  4. Let size be the length of S.
  5. If position < 0 or position ≥ size, return the empty String.
  6. Return the substring of S from position to position 1.
 Note 2

This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.3 String.prototype.charCodeAt ( pos )

Note 1

This method returns a Number (a non-negative integral Number less than 216) that is the numeric value of the code unit at index pos within the String resulting from converting this object to a String. If there is no element at that index, the result is NaN.

This method performs the following steps when called:

  1. Let O be ? RequireObjectCoercible(this value).
  2. Let S be ? ToString(O).
  3. Let position be ? ToIntegerOrInfinity(pos).
  4. Let size be the length of S.
  5. If position < 0 or position ≥ size, return NaN.
  6. Return the Number value for the numeric value of the code unit at index position within the String S.
 Note 2

This method is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.

22.1.3.4 String.prototype.codePointAt ( pos )

Note 1

This method returns a non-negative integral Number less than or equal to 0x10FFFF𝔽 that is the numeric value of the UTF-16 encoded code point (6.1.4) starting at the string element at index pos within the String resulting from converting this object to a String. If there is no element at that index, the result is undefined. If a valid UTF-16 surrogate pair does not begin at pos, the result is the code unit at pos.

This method performs the following steps when called:

  1. Let O be ? RequireObjectCoercible(this value).
  2. Let S be ? ToString(O).
  3. Let position be ? ToIntegerOrInfinity(pos).
  4. Let size be the length of S.
  5. If position < 0 or position ≥ size, return undefined.
  6. Let cp be CodePointAt(S, position).
  7. Return 𝔽(cp.[[CodePoint]]).
 Note 2

This method is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.

22.1.3.5 String.prototype.concat ( ...args )

Note 1

When this method is called it returns the String value consisting of the code units of the this value (converted to a String) followed by the code units of each of the arguments converted to a String. The result is a String value, not a String object.

This method performs the following steps when called:

  1. Let O be ? RequireObjectCoercible(this value).
  2. Let S be ? ToString(O).
  3. Let R be S.
  4. For each element next of args, do
    1. Let nextString be ? ToString(next).
    2. Set R to the string-concatenation of R and nextString.
  5. Return R.

The "length" property of this method is 1𝔽.

Note 2

This method is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.

22.1.3.6 String.prototype.constructor

The initial value of String.prototype.constructor is %String%.

22.1.3.7 String.prototype.endsWith ( searchString [ , endPosition ] )

This method performs the following steps when called:

  1. Let O be ? RequireObjectCoercible(this value).
  2. Let S be ? ToString(O).
  3. Let isRegExp be ? IsRegExp(searchString).
  4. If isRegExp is true, throw a TypeError exception.
  5. Let searchStr be ? ToString(searchString).
  6. Let len be the length of S.
  7. If endPosition is undefined, let pos be len; else let pos be ? ToIntegerOrInfinity(endPosition).
  8. Let end be the result of clamping pos between 0 and len.
  9. Let searchLength be the length of searchStr.
  10. If searchLength = 0, return true.
  11. Let start be end - searchLength.
  12. If start < 0, return false.
  13. Let substring be the substring of S from start to end.
  14. If substring is searchStr, return true.
  15. Return false.
 Note 1

This method returns true if the sequence of code units of searchString converted to a String is the same as the corresponding code units of this object (converted to a String) starting at endPosition - length(this). Otherwise it returns false.

 Note 2

Throwing an exception if the first argument is a RegExp is specified in order to allow future editions to define extensions that allow such argument values.

 Note 3

This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.8 String.prototype.includes ( searchString [ , position ] )

This method performs the following steps when called:

  1. Let O be ? RequireObjectCoercible(this value).
  2. Let S be ? ToString(O).
  3. Let isRegExp be ? IsRegExp(searchString).
  4. If isRegExp is true, throw a TypeError exception.
  5. Let searchStr be ? ToString(searchString).
  6. Let pos be ? ToIntegerOrInfinity(position).
  7. Assert: If position is undefined, then pos is 0.
  8. Let len be the length of S.
  9. Let start be the result of clamping pos between 0 and len.
  10. Let index be StringIndexOf(S, searchStr, start).
  11. If index ≠ -1, return true.
  12. Return false.
 Note 1

If searchString appears as a substring of the result of converting this object to a String, at one or more indices that are greater than or equal to position, this function returns true; otherwise, it returns false. If position is undefined, 0 is assumed, so as to search all of the String.

 Note 2

Throwing an exception if the first argument is a RegExp is specified in order to allow future editions to define extensions that allow such argument values.

 Note 3

This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.9 String.prototype.indexOf ( searchString [ , position ] )

Note 1

If searchString appears as a substring of the result of converting this object to a String, at one or more indices that are greater than or equal to position, then the smallest such index is returned; otherwise, -1𝔽 is returned. If position is undefined, 0𝔽 is assumed, so as to search all of the String.

This method performs the following steps when called:

  1. Let O be ? RequireObjectCoercible(this value).
  2. Let S be ? ToString(O).
  3. Let searchStr be ? ToString(searchString).
  4. Let pos be ? ToIntegerOrInfinity(position).
  5. Assert: If position is undefined, then pos is 0.
  6. Let len be the length of S.
  7. Let start be the result of clamping pos between 0 and len.
  8. Return 𝔽(StringIndexOf(S, searchStr, start)).
 Note 2

This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.10 String.prototype.lastIndexOf ( searchString [ , position ] )

Note 1

If searchString appears as a substring of the result of converting this object to a String at one or more indices that are smaller than or equal to position, then the greatest such index is returned; otherwise, -1𝔽 is returned. If position is undefined, the length of the String value is assumed, so as to search all of the String.

This method performs the following steps when called:

  1. Let O be ? RequireObjectCoercible(this value).
  2. Let S be ? ToString(O).
  3. Let searchStr be ? ToString(searchString).
  4. Let numPos be ? ToNumber(position).
  5. Assert: If position is undefined, then numPos is NaN.
  6. If numPos is NaN, let pos be ∞; otherwise, let pos be ! ToIntegerOrInfinity(numPos).
  7. Let len be the length of S.
  8. Let searchLen be the length of searchStr.
  9. Let start be the result of clamping pos between 0 and len - searchLen.
  10. If searchStr is the empty String, return 𝔽(start).
  11. For each integer i such that 0 ≤ i ≤ start, in descending order, do
    1. Let candidate be the substring of S from i to i searchLen.
    2. If candidate is searchStr, return 𝔽(i).
  12. Return -1𝔽.
 Note 2

This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.11 String.prototype.localeCompare ( that [ , reserved1 [ , reserved2 ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of this method is used:

This method returns a Number other than NaN representing the result of an implementation-defined locale-sensitive String comparison of the this value (converted to a String S) with that (converted to a String thatValue). The result is intended to correspond with a sort order of String values according to conventions of the host environment's current locale, and will be negative when S is ordered before thatValue, positive when S is ordered after thatValue, and zero in all other cases (representing no relative ordering between S and thatValue).

Before performing the comparisons, this method performs the following steps to prepare the Strings:

  1. Let O be ? RequireObjectCoercible(this value).
  2. Let S be ? ToString(O).
  3. Let thatValue be ? ToString(that).

The meaning of the optional second and third parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not assign any other interpretation to those parameter positions.

The actual return values are implementation-defined to permit encoding additional information in them, but this method, when considered as a method of two arguments, is required to be a consistent comparator defining a total ordering on the set of all Strings. This method is also required to recognize and honour canonical equivalence according to the Unicode Standard, including returning 0𝔽 when comparing distinguishable Strings that are canonically equivalent.

Note 1

This method itself is not directly suitable as an argument to Array.prototype.sort because the latter requires a function of two arguments.

 Note 2

This method may rely on whatever language- and/or locale-sensitive comparison functionality is available to the ECMAScript environment from the host environment, and is intended to compare according to the conventions of the host environment's current locale. However, regardless of comparison capabilities, this method must recognize and honour canonical equivalence according to the Unicode Standard—for example, the following comparisons must all return 0𝔽:

// Å ANGSTROM SIGN vs.
// Å LATIN CAPITAL LETTER A   COMBINING RING ABOVE
"\u212B".localeCompare("A\u030A")

// Ω OHM SIGN vs.
// Ω GREEK CAPITAL LETTER OMEGA
"\u2126".localeCompare("\u03A9")

// ṩ LATIN SMALL LETTER S WITH DOT BELOW AND DOT ABOVE vs.
// ṩ LATIN SMALL LETTER S   COMBINING DOT ABOVE   COMBINING DOT BELOW
"\u1E69".localeCompare("s\u0307\u0323")

// ḍ̇ LATIN SMALL LETTER D WITH DOT ABOVE   COMBINING DOT BELOW vs.
// ḍ̇ LATIN SMALL LETTER D WITH DOT BELOW   COMBINING DOT ABOVE
"\u1E0B\u0323".localeCompare("\u1E0D\u0307")

// 가 HANGUL CHOSEONG KIYEOK   HANGUL JUNGSEONG A vs.
// 가 HANGUL SYLLABLE GA
"\u1100\u1161".localeCompare("\uAC00")

For a definition and discussion of canonical equivalence see the Unicode Standard, chapters 2 and 3, as well as Unicode Standard Annex #15, Unicode Normalization Forms and Unicode Technical Note #5, Canonical Equivalence in Applications. Also see Unicode Technical Standard #10, Unicode Collation Algorithm.

It is recommended that this method should not honour Unicode compatibility equivalents or compatibility decompositions as defined in the Unicode Standard, chapter 3, section 3.7.

 Note 3

This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.12 String.prototype.match ( regexp )

This method performs the following steps when called:

  1. Let O be ? RequireObjectCoercible(this value).
  2. If regexp is neither undefined nor null, then
    1. Let matcher be ? GetMethod(regexp, @@match).
    2. If matcher is not undefined, then
      1. Return ? Call(matcher, regexp, « O »).
  3. Let S be ? ToString(O).
  4. Let rx be ? RegExpCreate(regexp, undefined).
  5. Return ? Invoke(rx, @@match, « S »).
 Note

This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.13 String.prototype.matchAll ( regexp )

This method performs a regular expression match of the String representing the this value against regexp and returns an iterator. Each iteration result's value is an Array containing the results of the match, or null if the String did not match.

It performs the following steps when called:

  1. Let O be ? RequireObjectCoercible(this value).
  2. If regexp is neither undefined nor null, then
    1. Let isRegExp be ? IsRegExp(regexp).
    2. If isRegExp is true, then
      1. Let flags be ? Get(regexp, "flags").
      2. Perform ? RequireObjectCoercible(flags).
      3. If ? ToString(flags) does not contain "g", throw a TypeError exception.
    3. Let matcher be ? GetMethod(regexp, @@matchAll).
    4. If matcher is not undefined, then
      1. Return ? Call(matcher, regexp, « O »).
  3. Let S be ? ToString(O).
  4. Let rx be ? RegExpCreate(regexp, "g").
  5. Return ? Invoke(rx, @@matchAll, « S »).
 Note 1
This method is intentionally generic, it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
 Note 2
Similarly to String.prototype.split, String.prototype.matchAll is designed to typically act without mutating its inputs.

22.1.3.14 String.prototype.normalize ( [ form ] )

This method performs the following steps when called:

  1. Let O be ? RequireObjectCoercible(this value).
  2. Let S be ? ToString(O).
  3. If form is undefined, let f be "NFC".
  4. Else, let f be ? ToString(form).
  5. If f is not one of "NFC", "NFD", "NFKC", or "NFKD", throw a RangeError exception.
  6. Let ns be the String value that is the result of normalizing S into the normalization form named by f as specified in https://unicode.org/reports/tr15/.
  7. Return ns.
 Note

This method is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.

22.1.3.15 String.prototype.padEnd ( maxLength [ , fillString ] )

This method performs the following steps when called:

  1. Let O be ? RequireObjectCoercible(this value).
  2. Return ? StringPad(O, maxLength, fillString, end).

22.1.3.16 String.prototype.padStart ( maxLength [ , fillString ] )

This method performs the following steps when called:

  1. Let O be ? RequireObjectCoercible(this value).
  2. Return ? StringPad(O, maxLength, fillString, start).

22.1.3.16.1 StringPad ( O, maxLength, fillString, placement )

The abstract operation StringPad takes arguments O (an ECMAScript language value), maxLength (an ECMAScript language value), fillString (an ECMAScript language value), and placement (start or end) and returns either a normal completion containing a String or a throw completion. It performs the following steps when called:

  1. Let S be ? ToString(O).
  2. Let intMaxLength be ℝ(? ToLength(maxLength)).
  3. Let stringLength be the length of S.
  4. If intMaxLength ≤ stringLength, return S.
  5. If fillString is undefined, let filler be the String value consisting solely of the code unit 0x0020 (SPACE).
  6. Else, let filler be ? ToString(fillString).
  7. If filler is the empty String, return S.
  8. Let fillLen be intMaxLength - stringLength.
  9. Let truncatedStringFiller be the String value consisting of repeated concatenations of filler truncated to length fillLen.
  10. If placement is start, return the string-concatenation of truncatedStringFiller and S.
  11. Else, return the string-concatenation of S and truncatedStringFiller.
 Note 1

The argument maxLength will be clamped such that it can be no smaller than the length of S.

 Note 2

The argument fillString defaults to " " (the String value consisting of the code unit 0x0020 SPACE).

22.1.3.16.2 ToZeroPaddedDecimalString ( n, minLength )

The abstract operation ToZeroPaddedDecimalString takes arguments n (a non-negative integer) and minLength (a non-negative integer) and returns a String. It performs the following steps when called:

  1. Let S be the String representation of n, formatted as a decimal number.
  2. Return ! StringPad(S, 𝔽(minLength), "0", start).

22.1.3.17 String.prototype.repeat ( count )

This method performs the following steps when called:

  1. Let O be ? RequireObjectCoercible(this value).
  2. Let S be ? ToString(O).
  3. Let n be ? ToIntegerOrInfinity(count).
  4. If n < 0 or n = ∞, throw a RangeError exception.
  5. If n = 0, return the empty String.
  6. Return the String value that is made from n copies of S appended together.
 Note 1

This method creates the String value consisting of the code units of the this value (converted to String) repeated count times.

 Note 2

This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.18 String.prototype.replace ( searchValue, replaceValue )

This method performs the following steps when called:

  1. Let O be ? RequireObjectCoercible(this value).
  2. If searchValue is neither undefined nor null, then
    1. Let replacer be ? GetMethod(searchValue, @@replace).
    2. If replacer is not undefined, then
      1. Return ? Call(replacer, searchValue, « O, replaceValue »).
  3. Let string be ? ToString(O).
  4. Let searchString be ? ToString(searchValue).
  5. Let functionalReplace be IsCallable(replaceValue).
  6. If functionalReplace is false, then
    1. Set replaceValue to ? ToString(replaceValue).
  7. Let searchLength be the length of searchString.
  8. Let position be StringIndexOf(string, searchString, 0).
  9. If position = -1, return string.
  10. Let preceding be the substring of string from 0 to position.
  11. Let following be the substring of string from position searchLength.
  12. If functionalReplace is true, then
    1. Let replacement be ? ToString(? Call(replaceValue, undefined, « searchString, 𝔽(position), string »)).
  13. Else,
    1. Assert: replaceValue is a String.
    2. Let captures be a new empty List.
    3. Let replacement be ! GetSubstitution(searchString, string, position, captures, undefined, replaceValue).
  14. Return the string-concatenation of preceding, replacement, and following.
 Note

This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.18.1 GetSubstitution ( matched, str, position, captures, namedCaptures, replacementTemplate )

The abstract operation GetSubstitution takes arguments matched (a String), str (a String), position (a non-negative integer), captures (a possibly empty List, each of whose elements is a String or undefined), namedCaptures (an Object or undefined), and replacementTemplate (a String) and returns either a normal completion containing a String or a throw completion. For the purposes of this abstract operation, a decimal digit is a code unit in the inclusive interval from 0x0030 (DIGIT ZERO) to 0x0039 (DIGIT NINE). It performs the following steps when called:

  1. Let stringLength be the length of str.
  2. Assert: position ≤ stringLength.
  3. Let result be the empty String.
  4. Let templateRemainder be replacementTemplate.
  5. Repeat, while templateRemainder is not the empty String,
    1. NOTE: The following steps isolate ref (a prefix of templateRemainder), determine refReplacement (its replacement), and then append that replacement to result.
    2. If templateRemainder starts with "$$", then
      1. Let ref be "$$".
      2. Let refReplacement be "$".
    3. Else if templateRemainder starts with "$`", then
      1. Let ref be "$`".
      2. Let refReplacement be the substring of str from 0 to position.
    4. Else if templateRemainder starts with "$&", then
      1. Let ref be "$&".
      2. Let refReplacement be matched.
    5. Else if templateRemainder starts with "$'" (0x0024 (DOLLAR SIGN) followed by 0x0027 (APOSTROPHE)), then
      1. Let ref be "$'".
      2. Let matchLength be the length of matched.
      3. Let tailPos be position matchLength.
      4. Let refReplacement be the substring of str from min(tailPos, stringLength).
      5. NOTE: tailPos can exceed stringLength only if this abstract operation was invoked by a call to the intrinsic @@replace method of %RegExp.prototype% on an object whose "exec" property is not the intrinsic %RegExp.prototype.exec%.
    6. Else if templateRemainder starts with "$" followed by 1 or more decimal digits, then
      1. If templateRemainder starts with "$" followed by 2 or more decimal digits, let digitCount be 2. Otherwise, let digitCount be 1.
      2. Let ref be the substring of templateRemainder from 0 to 1 digitCount.
      3. Let digits be the substring of templateRemainder from 1 to 1 digitCount.
      4. Let index be ℝ(StringToNumber(digits)).
      5. Assert: 0 ≤ index ≤ 99.
      6. Let captureLen be the number of elements in captures.
      7. If 1 ≤ index ≤ captureLen, then
        1. Let capture be captures[index - 1].
        2. If capture is undefined, then
          1. Let refReplacement be the empty String.
        3. Else,
          1. Let refReplacement be capture.
      8. Else,
        1. Let refReplacement be ref.
    7. Else if templateRemainder starts with "$<", then
      1. Let gtPos be StringIndexOf(templateRemainder, ">", 0).
      2. If gtPos = -1 or namedCaptures is undefined, then
        1. Let ref be "$<".
        2. Let refReplacement be ref.
      3. Else,
        1. Let ref be the substring of templateRemainder from 0 to gtPos 1.
        2. Let groupName be the substring of templateRemainder from 2 to gtPos.
        3. Assert: namedCaptures is an Object.
        4. Let capture be ? Get(namedCaptures, groupName).
        5. If capture is undefined, then
          1. Let refReplacement be the empty String.
        6. Else,
          1. Let refReplacement be ? ToString(capture).
    8. Else,
      1. Let ref be the substring of templateRemainder from 0 to 1.
      2. Let refReplacement be ref.
    9. Let refLength be the length of ref.
    10. Set templateRemainder to the substring of templateRemainder from refLength.
    11. Set result to the string-concatenation of result and refReplacement.
  6. Return result.

22.1.3.19 String.prototype.replaceAll ( searchValue, replaceValue )

This method performs the following steps when called:

  1. Let O be ? RequireObjectCoercible(this value).
  2. If searchValue is neither undefined nor null, then
    1. Let isRegExp be ? IsRegExp(searchValue).
    2. If isRegExp is true, then
      1. Let flags be ? Get(searchValue, "flags").
      2. Perform ? RequireObjectCoercible(flags).
      3. If ? ToString(flags) does not contain "g", throw a TypeError exception.
    3. Let replacer be ? GetMethod(searchValue, @@replace).
    4. If replacer is not undefined, then
      1. Return ? Call(replacer, searchValue, « O, replaceValue »).
  3. Let string be ? ToString(O).
  4. Let searchString be ? ToString(searchValue).
  5. Let functionalReplace be IsCallable(replaceValue).
  6. If functionalReplace is false, then
    1. Set replaceValue to ? ToString(replaceValue).
  7. Let searchLength be the length of searchString.
  8. Let advanceBy be max(1, searchLength).
  9. Let matchPositions be a new empty List.
  10. Let position be StringIndexOf(string, searchString, 0).
  11. Repeat, while position ≠ -1,
    1. Append position to matchPositions.
    2. Set position to StringIndexOf(string, searchString, position advanceBy).
  12. Let endOfLastMatch be 0.
  13. Let result be the empty String.
  14. For each element p of matchPositions, do
    1. Let preserved be the substring of string from endOfLastMatch to p.
    2. If functionalReplace is true, then
      1. Let replacement be ? ToString(? Call(replaceValue, undefined, « searchString, 𝔽(p), string »)).
    3. Else,
      1. Assert: replaceValue is a String.
      2. Let captures be a new empty List.
      3. Let replacement be ! GetSubstitution(searchString, string, p, captures, undefined, replaceValue).
    4. Set result to the string-concatenation of result, preserved, and replacement.
    5. Set endOfLastMatch to p searchLength.
  15. If endOfLastMatch < the length of string, then
    1. Set result to the string-concatenation of result and the substring of string from endOfLastMatch.
  16. Return result.

22.1.3.20 String.prototype.search ( regexp )

This method performs the following steps when called:

  1. Let O be ? RequireObjectCoercible(this value).
  2. If regexp is neither undefined nor null, then
    1. Let searcher be ? GetMethod(regexp, @@search).
    2. If searcher is not undefined, then
      1. Return ? Call(searcher, regexp, « O »).
  3. Let string be ? ToString(O).
  4. Let rx be ? RegExpCreate(regexp, undefined).
  5. Return ? Invoke(rx, @@search, « string »).
 Note

This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.21 String.prototype.slice ( start, end )

This method returns a substring of the result of converting this object to a String, starting from index start and running to, but not including, index end (or through the end of the String if end is undefined). If start is negative, it is treated as sourceLength start where sourceLength is the length of the String. If end is negative, it is treated as sourceLength end where sourceLength is the length of the String. The result is a String value, not a String object.

It performs the following steps when called:

  1. Let O be ? RequireObjectCoercible(this value).
  2. Let S be ? ToString(O).
  3. Let len be the length of S.
  4. Let intStart be ? ToIntegerOrInfinity(start).
  5. If intStart = -∞, let from be 0.
  6. Else if intStart < 0, let from be max(len intStart, 0).
  7. Else, let from be min(intStart, len).
  8. If end is undefined, let intEnd be len; else let intEnd be ? ToIntegerOrInfinity(end).
  9. If intEnd = -∞, let to be 0.
  10. Else if intEnd < 0, let to be max(len intEnd, 0).
  11. Else, let to be min(intEnd, len).
  12. If from ≥ to, return the empty String.
  13. Return the substring of S from from to to.
 Note

This method is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.

22.1.3.22 String.prototype.split ( separator, limit )

This method returns an Array into which substrings of the result of converting this object to a String have been stored. The substrings are determined by searching from left to right for occurrences of separator; these occurrences are not part of any String in the returned array, but serve to divide up the String value. The value of separator may be a String of any length or it may be an object, such as a RegExp, that has a @@split method.

It performs the following steps when called:

  1. Let O be ? RequireObjectCoercible(this value).
  2. If separator is neither undefined nor null, then
    1. Let splitter be ? GetMethod(separator, @@split).
    2. If splitter is not undefined, then
      1. Return ? Call(splitter, separator, « O, limit »).
  3. Let S be ? ToString(O).
  4. If limit is undefined, let lim be 232 - 1; else let lim be ℝ(? ToUint32(limit)).
  5. Let R be ? ToString(separator).
  6. If lim = 0, then
    1. Return CreateArrayFromList(« »).
  7. If separator is undefined, then
    1. Return CreateArrayFromList(« S »).
  8. Let separatorLength be the length of R.
  9. If separatorLength = 0, then
    1. Let head be the substring of S from 0 to lim.
    2. Let codeUnits be a List consisting of the sequence of code units that are the elements of head.
    3. Return CreateArrayFromList(codeUnits).
  10. If S is the empty String, return CreateArrayFromList(« S »).
  11. Let substrings be a new empty List.
  12. Let i be 0.
  13. Let j be StringIndexOf(S, R, 0).
  14. Repeat, while j ≠ -1,
    1. Let T be the substring of S from i to j.
    2. Append T to substrings.
    3. If the number of elements in substrings is lim, return CreateArrayFromList(substrings).
    4. Set i to j separatorLength.
    5. Set j to StringIndexOf(S, R, i).
  15. Let T be the substring of S from i.
  16. Append T to substrings.
  17. Return CreateArrayFromList(substrings).
 Note 1

The value of separator may be an empty String. In this case, separator does not match the empty substring at the beginning or end of the input String, nor does it match the empty substring at the end of the previous separator match. If separator is the empty String, the String is split up into individual code unit elements; the length of the result array equals the length of the String, and each substring contains one code unit.

If the this value is (or converts to) the empty String, the result depends on whether separator can match the empty String. If it can, the result array contains no elements. Otherwise, the result array contains one element, which is the empty String.

If separator is undefined, then the result array contains just one String, which is the this value (converted to a String). If limit is not undefined, then the output array is truncated so that it contains no more than limit elements.

 Note 2

This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.23 String.prototype.startsWith ( searchString [ , position ] )

This method performs the following steps when called:

  1. Let O be ? RequireObjectCoercible(this value).
  2. Let S be ? ToString(O).
  3. Let isRegExp be ? IsRegExp(searchString).
  4. If isRegExp is true, throw a TypeError exception.
  5. Let searchStr be ? ToString(searchString).
  6. Let len be the length of S.
  7. If position is undefined, let pos be 0; else let pos be ? ToIntegerOrInfinity(position).
  8. Let start be the result of clamping pos between 0 and len.
  9. Let searchLength be the length of searchStr.
  10. If searchLength = 0, return true.
  11. Let end be start searchLength.
  12. If end > len, return false.
  13. Let substring be the substring of S from start to end.
  14. If substring is searchStr, return true.
  15. Return false.
 Note 1

This method returns true if the sequence of code units of searchString converted to a String is the same as the corresponding code units of this object (converted to a String) starting at index position. Otherwise it returns false.

 Note 2

Throwing an exception if the first argument is a RegExp is specified in order to allow future editions to define extensions that allow such argument values.

 Note 3

This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.24 String.prototype.substring ( start, end )

This method returns a substring of the result of converting this object to a String, starting from index start and running to, but not including, index end of the String (or through the end of the String if end is undefined). The result is a String value, not a String object.

If either argument is NaN or negative, it is replaced with zero; if either argument is strictly greater than the length of the String, it is replaced with the length of the String.

If start is strictly greater than end, they are swapped.

It performs the following steps when called:

  1. Let O be ? RequireObjectCoercible(this value).
  2. Let S be ? ToString(O).
  3. Let len be the length of S.
  4. Let intStart be ? ToIntegerOrInfinity(start).
  5. If end is undefined, let intEnd be len; else let intEnd be ? ToIntegerOrInfinity(end).
  6. Let finalStart be the result of clamping intStart between 0 and len.
  7. Let finalEnd be the result of clamping intEnd between 0 and len.
  8. Let from be min(finalStart, finalEnd).
  9. Let to be max(finalStart, finalEnd).
  10. Return the substring of S from from to to.
 Note

This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.25 String.prototype.toLocaleLowerCase ( [ reserved1 [ , reserved2 ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of this method is used:

This method interprets a String value as a sequence of UTF-16 encoded code points, as described in 6.1.4.

It works exactly the same as toLowerCase except that it is intended to yield a locale-sensitive result corresponding with conventions of the host environment's current locale. There will only be a difference in the few cases (such as Turkish) where the rules for that language conflict with the regular Unicode case mappings.

The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.

Note

This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.26 String.prototype.toLocaleUpperCase ( [ reserved1 [ , reserved2 ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of this method is used:

This method interprets a String value as a sequence of UTF-16 encoded code points, as described in 6.1.4.

It works exactly the same as toUpperCase except that it is intended to yield a locale-sensitive result corresponding with conventions of the host environment's current locale. There will only be a difference in the few cases (such as Turkish) where the rules for that language conflict with the regular Unicode case mappings.

The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.

Note

This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.27 String.prototype.toLowerCase ( )

This method interprets a String value as a sequence of UTF-16 encoded code points, as described in 6.1.4.

It performs the following steps when called:

  1. Let O be ? RequireObjectCoercible(this value).
  2. Let S be ? ToString(O).
  3. Let sText be StringToCodePoints(S).
  4. Let lowerText be the result of toLowercase(sText), according to the Unicode Default Case Conversion algorithm.
  5. Let L be CodePointsToString(lowerText).
  6. Return L.

The result must be derived according to the locale-insensitive case mappings in the Unicode Character Database (this explicitly includes not only the file UnicodeData.txt, but also all locale-insensitive mappings in the file SpecialCasing.txt that accompanies it).

Note 1

The case mapping of some code points may produce multiple code points. In this case the result String may not be the same length as the source String. Because both toUpperCase and toLowerCase have context-sensitive behaviour, the methods are not symmetrical. In other words, s.toUpperCase().toLowerCase() is not necessarily equal to s.toLowerCase().

 Note 2

This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.28 String.prototype.toString ( )

This method performs the following steps when called:

  1. Return ? thisStringValue(this value).
 Note

For a String object, this method happens to return the same thing as the valueOf method.

22.1.3.29 String.prototype.toUpperCase ( )

This method interprets a String value as a sequence of UTF-16 encoded code points, as described in 6.1.4.

It behaves in exactly the same way as String.prototype.toLowerCase, except that the String is mapped using the toUppercase algorithm of the Unicode Default Case Conversion.

Note

This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.30 String.prototype.trim ( )

This method interprets a String value as a sequence of UTF-16 encoded code points, as described in 6.1.4.

It performs the following steps when called:

  1. Let S be the this value.
  2. Return ? TrimString(S, start end).
 Note

This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.30.1 TrimString ( string, where )

The abstract operation TrimString takes arguments string (an ECMAScript language value) and where (start, end, or start end) and returns either a normal completion containing a String or a throw completion. It interprets string as a sequence of UTF-16 encoded code points, as described in 6.1.4. It performs the following steps when called:

  1. Let str be ? RequireObjectCoercible(string).
  2. Let S be ? ToString(str).
  3. If where is start, let T be the String value that is a copy of S with leading white space removed.
  4. Else if where is end, let T be the String value that is a copy of S with trailing white space removed.
  5. Else,
    1. Assert: where is start end.
    2. Let T be the String value that is a copy of S with both leading and trailing white space removed.
  6. Return T.

The definition of white space is the union of WhiteSpace and LineTerminator. When determining whether a Unicode code point is in Unicode general category “Space_Separator” (“Zs”), code unit sequences are interpreted as UTF-16 encoded code point sequences as specified in 6.1.4.

22.1.3.31 String.prototype.trimEnd ( )

This method interprets a String value as a sequence of UTF-16 encoded code points, as described in 6.1.4.

It performs the following steps when called:

  1. Let S be the this value.
  2. Return ? TrimString(S, end).
 Note

This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.32 String.prototype.trimStart ( )

This method interprets a String value as a sequence of UTF-16 encoded code points, as described in 6.1.4.

It performs the following steps when called:

  1. Let S be the this value.
  2. Return ? TrimString(S, start).
 Note

This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.33 String.prototype.valueOf ( )

This method performs the following steps when called:

  1. Return ? thisStringValue(this value).

22.1.3.34 String.prototype [ @@iterator ] ( )

This method returns an Iterator object (27.1.1.2) that iterates over the code points of a String value, returning each code point as a String value.

It performs the following steps when called:

  1. Let O be ? RequireObjectCoercible(this value).
  2. Let s be ? ToString(O).
  3. Let closure be a new Abstract Closure with no parameters that captures s and performs the following steps when called:
    1. Let len be the length of s.
    2. Let position be 0.
    3. Repeat, while position < len,
      1. Let cp be CodePointAt(s, position).
      2. Let nextIndex be position cp.[[CodeUnitCount]].
      3. Let resultString be the substring of s from position to nextIndex.
      4. Set position to nextIndex.
      5. Perform ? GeneratorYield(CreateIterResultObject(resultString, false)).
    4. Return undefined.
  4. Return CreateIteratorFromClosure(closure, "%StringIteratorPrototype%", %StringIteratorPrototype%).

The value of the "name" property of this method is "[Symbol.iterator]".

22.1.4 Properties of String Instances

String instances are String exotic objects and have the internal methods specified for such objects. String instances inherit properties from the String prototype object. String instances also have a [[StringData]] internal slot.

String instances have a "length" property, and a set of enumerable properties with integer-indexed names.

22.1.4.1 length

The number of elements in the String value represented by this String object.

Once a String object is initialized, this property is unchanging. It has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

22.1.5 String Iterator Objects

A String Iterator is an object, that represents a specific iteration over some specific String instance object. There is not a named constructor for String Iterator objects. Instead, String iterator objects are created by calling certain methods of String instance objects.

22.1.5.1 The %StringIteratorPrototype% Object

The %StringIteratorPrototype% object:

  • has properties that are inherited by all String Iterator Objects.
  • is an ordinary object.
  • has a [[Prototype]] internal slot whose value is %IteratorPrototype%.
  • has the following properties:

22.1.5.1.1 %StringIteratorPrototype%.next ( )

  1. Return ? GeneratorResume(this value, empty, "%StringIteratorPrototype%").

22.1.5.1.2 %StringIteratorPrototype% [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "String Iterator".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

22.2 RegExp (Regular Expression) Objects

A RegExp object contains a regular expression and the associated flags.

Note

The form and functionality of regular expressions is modelled after the regular expression facility in the Perl 5 programming language.

22.2.1 Patterns

The RegExp constructor applies the following grammar to the input pattern String. An error occurs if the grammar cannot interpret the String as an expansion of Pattern.

Syntax

Pattern[UnicodeMode, N] :: Disjunction[?UnicodeMode, ?N] Disjunction[UnicodeMode, N] :: Alternative[?UnicodeMode, ?N] Alternative[?UnicodeMode, ?N] | Disjunction[?UnicodeMode, ?N] Alternative[UnicodeMode, N] :: [empty] Alternative[?UnicodeMode, ?N] Term[?UnicodeMode, ?N] Term[UnicodeMode, N] :: Assertion[?UnicodeMode, ?N] Atom[?UnicodeMode, ?N] Atom[?UnicodeMode, ?N] Quantifier Assertion[UnicodeMode, N] :: ^ $ \b \B (?= Disjunction[?UnicodeMode, ?N] ) (?! Disjunction[?UnicodeMode, ?N] ) (?<= Disjunction[?UnicodeMode, ?N] ) (?<! Disjunction[?UnicodeMode, ?N] ) Quantifier :: QuantifierPrefix QuantifierPrefix ? QuantifierPrefix :: * ? { DecimalDigits[~Sep] } { DecimalDigits[~Sep] ,} { DecimalDigits[~Sep] , DecimalDigits[~Sep] } Atom[UnicodeMode, N] :: PatternCharacter . \ AtomEscape[?UnicodeMode, ?N] CharacterClass[?UnicodeMode] ( GroupSpecifier[?UnicodeMode]opt Disjunction[?UnicodeMode, ?N] ) (?: Disjunction[?UnicodeMode, ?N] ) SyntaxCharacter :: one of ^ $ \ . * ? ( ) [ ] { } | PatternCharacter :: SourceCharacter but not SyntaxCharacter AtomEscape[UnicodeMode, N] :: DecimalEscape CharacterClassEscape[?UnicodeMode] CharacterEscape[?UnicodeMode] [ N] k GroupName[?UnicodeMode] CharacterEscape[UnicodeMode] :: ControlEscape c AsciiLetter 0 [lookahead ∉ DecimalDigit] HexEscapeSequence RegExpUnicodeEscapeSequence[?UnicodeMode] IdentityEscape[?UnicodeMode] ControlEscape :: one of f n r t v GroupSpecifier[UnicodeMode] :: ? GroupName[?UnicodeMode] GroupName[UnicodeMode] :: < RegExpIdentifierName[?UnicodeMode] > RegExpIdentifierName[UnicodeMode] :: RegExpIdentifierStart[?UnicodeMode] RegExpIdentifierName[?UnicodeMode] RegExpIdentifierPart[?UnicodeMode] RegExpIdentifierStart[UnicodeMode] :: IdentifierStartChar \ RegExpUnicodeEscapeSequence[ UnicodeMode] [~UnicodeMode] UnicodeLeadSurrogate UnicodeTrailSurrogate RegExpIdentifierPart[UnicodeMode] :: IdentifierPartChar \ RegExpUnicodeEscapeSequence[ UnicodeMode] [~UnicodeMode] UnicodeLeadSurrogate UnicodeTrailSurrogate RegExpUnicodeEscapeSequence[UnicodeMode] :: [ UnicodeMode] u HexLeadSurrogate \u HexTrailSurrogate [ UnicodeMode] u HexLeadSurrogate [ UnicodeMode] u HexTrailSurrogate [ UnicodeMode] u HexNonSurrogate [~UnicodeMode] u Hex4Digits [ UnicodeMode] u{ CodePoint } UnicodeLeadSurrogate :: any Unicode code point in the inclusive interval from U D800 to U DBFF UnicodeTrailSurrogate :: any Unicode code point in the inclusive interval from U DC00 to U DFFF

Each \u HexTrailSurrogate for which the choice of associated u HexLeadSurrogate is ambiguous shall be associated with the nearest possible u HexLeadSurrogate that would otherwise have no corresponding \u HexTrailSurrogate.

HexLeadSurrogate :: Hex4Digits but only if the MV of Hex4Digits is in the inclusive interval from 0xD800 to 0xDBFF HexTrailSurrogate :: Hex4Digits but only if the MV of Hex4Digits is in the inclusive interval from 0xDC00 to 0xDFFF HexNonSurrogate :: Hex4Digits but only if the MV of Hex4Digits is not in the inclusive interval from 0xD800 to 0xDFFF IdentityEscape[UnicodeMode] :: [ UnicodeMode] SyntaxCharacter [ UnicodeMode] / [~UnicodeMode] SourceCharacter but not UnicodeIDContinue DecimalEscape :: NonZeroDigit DecimalDigits[~Sep]opt [lookahead ∉ DecimalDigit] CharacterClassEscape[UnicodeMode] :: d D s S w W [ UnicodeMode] p{ UnicodePropertyValueExpression } [ UnicodeMode] P{ UnicodePropertyValueExpression } UnicodePropertyValueExpression :: UnicodePropertyName = UnicodePropertyValue LoneUnicodePropertyNameOrValue UnicodePropertyName :: UnicodePropertyNameCharacters UnicodePropertyNameCharacters :: UnicodePropertyNameCharacter UnicodePropertyNameCharactersopt UnicodePropertyValue :: UnicodePropertyValueCharacters LoneUnicodePropertyNameOrValue :: UnicodePropertyValueCharacters UnicodePropertyValueCharacters :: UnicodePropertyValueCharacter UnicodePropertyValueCharactersopt UnicodePropertyValueCharacter :: UnicodePropertyNameCharacter DecimalDigit UnicodePropertyNameCharacter :: AsciiLetter _ CharacterClass[UnicodeMode] :: [ [lookahead ≠ ^] ClassRanges[?UnicodeMode] ] [^ ClassRanges[?UnicodeMode] ] ClassRanges[UnicodeMode] :: [empty] NonemptyClassRanges[?UnicodeMode] NonemptyClassRanges[UnicodeMode] :: ClassAtom[?UnicodeMode] ClassAtom[?UnicodeMode] NonemptyClassRangesNoDash[?UnicodeMode] ClassAtom[?UnicodeMode] - ClassAtom[?UnicodeMode] ClassRanges[?UnicodeMode] NonemptyClassRangesNoDash[UnicodeMode] :: ClassAtom[?UnicodeMode] ClassAtomNoDash[?UnicodeMode] NonemptyClassRangesNoDash[?UnicodeMode] ClassAtomNoDash[?UnicodeMode] - ClassAtom[?UnicodeMode] ClassRanges[?UnicodeMode] ClassAtom[UnicodeMode] :: - ClassAtomNoDash[?UnicodeMode] ClassAtomNoDash[UnicodeMode] :: SourceCharacter but not one of \ or ] or - \ ClassEscape[?UnicodeMode] ClassEscape[UnicodeMode] :: b [ UnicodeMode] - CharacterClassEscape[?UnicodeMode] CharacterEscape[?UnicodeMode] Note

A number of productions in this section are given alternative definitions in section B.1.2.

22.2.1.1 Static Semantics: Early Errors

Note

This section is amended in B.1.2.1.

 Pattern :: Disjunction QuantifierPrefix :: { DecimalDigits , DecimalDigits } AtomEscape :: k GroupName AtomEscape :: DecimalEscape NonemptyClassRanges :: ClassAtom - ClassAtom ClassRanges NonemptyClassRangesNoDash :: ClassAtomNoDash - ClassAtom ClassRanges RegExpIdentifierStart :: \ RegExpUnicodeEscapeSequence RegExpIdentifierStart :: UnicodeLeadSurrogate UnicodeTrailSurrogate RegExpIdentifierPart :: \ RegExpUnicodeEscapeSequence RegExpIdentifierPart :: UnicodeLeadSurrogate UnicodeTrailSurrogate UnicodePropertyValueExpression :: UnicodePropertyName = UnicodePropertyValue UnicodePropertyValueExpression :: LoneUnicodePropertyNameOrValue

22.2.1.2 Static Semantics: CountLeftCapturingParensWithin ( node )

The abstract operation CountLeftCapturingParensWithin takes argument node (a Parse Node) and returns a non-negative integer. It returns the number of left-capturing parentheses in node. A left-capturing parenthesis is any ( pattern character that is matched by the ( terminal of the Atom :: ( GroupSpecifieropt Disjunction ) production.

Note

This section is amended in B.1.2.2.

It performs the following steps when called:

  1. Assert: node is an instance of a production in the RegExp Pattern grammar.
  2. Return the number of Atom :: ( GroupSpecifieropt Disjunction ) Parse Nodes contained within node.

22.2.1.3 Static Semantics: CountLeftCapturingParensBefore ( node )

The abstract operation CountLeftCapturingParensBefore takes argument node (a Parse Node) and returns a non-negative integer. It returns the number of left-capturing parentheses within the enclosing pattern that occur to the left of node.

Note

This section is amended in B.1.2.2.

It performs the following steps when called:

  1. Assert: node is an instance of a production in the RegExp Pattern grammar.
  2. Let pattern be the Pattern containing node.
  3. Return the number of Atom :: ( GroupSpecifieropt Disjunction ) Parse Nodes contained within pattern that either occur before node or contain node.

22.2.1.4 Static Semantics: CapturingGroupNumber

The syntax-directed operation CapturingGroupNumber takes no arguments and returns a positive integer.

Note

This section is amended in B.1.2.1.

It is defined piecewise over the following productions:

DecimalEscape :: NonZeroDigit
  1. Return the MV of NonZeroDigit.
 DecimalEscape :: NonZeroDigit DecimalDigits
  1. Let n be the number of code points in DecimalDigits.
  2. Return (the MV of NonZeroDigit × 10n plus the MV of DecimalDigits).

The definitions of “the MV of NonZeroDigit” and “the MV of DecimalDigits” are in 12.9.3.

22.2.1.5 Static Semantics: IsCharacterClass

The syntax-directed operation IsCharacterClass takes no arguments and returns a Boolean.

Note

This section is amended in B.1.2.3.

It is defined piecewise over the following productions:

ClassAtom :: - ClassAtomNoDash :: SourceCharacter but not one of \ or ] or - ClassEscape :: b - CharacterEscape
  1. Return false.
 ClassEscape :: CharacterClassEscape
  1. Return true.

22.2.1.6 Static Semantics: CharacterValue

The syntax-directed operation CharacterValue takes no arguments and returns a non-negative integer.

Note 1

This section is amended in B.1.2.4.

It is defined piecewise over the following productions:

ClassAtom :: -
  1. Return the numeric value of U 002D (HYPHEN-MINUS).
 ClassAtomNoDash :: SourceCharacter but not one of \ or ] or -
  1. Let ch be the code point matched by SourceCharacter.
  2. Return the numeric value of ch.
 ClassEscape :: b
  1. Return the numeric value of U 0008 (BACKSPACE).
 ClassEscape :: -
  1. Return the numeric value of U 002D (HYPHEN-MINUS).
 CharacterEscape :: ControlEscape
  1. Return the numeric value according to Table 63.
Table 63: ControlEscape Code Point Values
ControlEscape Numeric Value Code Point Unicode Name Symbol
t 9 U 0009 CHARACTER TABULATION <HT>
n 10 U 000A LINE FEED (LF) <LF>
v 11 U 000B LINE TABULATION <VT>
f 12 U 000C FORM FEED (FF) <FF>
r 13 U 000D CARRIAGE RETURN (CR) <CR>
CharacterEscape :: c AsciiLetter
  1. Let ch be the code point matched by AsciiLetter.
  2. Let i be the numeric value of ch.
  3. Return the remainder of dividing i by 32.
 CharacterEscape :: 0 [lookahead ∉ DecimalDigit]
  1. Return the numeric value of U 0000 (NULL).
 Note 2

\0 represents the <NUL> character and cannot be followed by a decimal digit.

 CharacterEscape :: HexEscapeSequence
  1. Return the MV of HexEscapeSequence.
 RegExpUnicodeEscapeSequence :: u HexLeadSurrogate \u HexTrailSurrogate
  1. Let lead be the CharacterValue of HexLeadSurrogate.
  2. Let trail be the CharacterValue of HexTrailSurrogate.
  3. Let cp be UTF16SurrogatePairToCodePoint(lead, trail).
  4. Return the numeric value of cp.
 RegExpUnicodeEscapeSequence :: u Hex4Digits
  1. Return the MV of Hex4Digits.
 RegExpUnicodeEscapeSequence :: u{ CodePoint }
  1. Return the MV of CodePoint.
 HexLeadSurrogate :: Hex4Digits HexTrailSurrogate :: Hex4Digits HexNonSurrogate :: Hex4Digits
  1. Return the MV of Hex4Digits.
 CharacterEscape :: IdentityEscape
  1. Let ch be the code point matched by IdentityEscape.
  2. Return the numeric value of ch.

22.2.1.7 Static Semantics: GroupSpecifiersThatMatch ( thisGroupName )

The abstract operation GroupSpecifiersThatMatch takes argument thisGroupName (a GroupName Parse Node) and returns a List of GroupSpecifier Parse Nodes. It performs the following steps when called:

  1. Let name be the CapturingGroupName of thisGroupName.
  2. Let pattern be the Pattern containing thisGroupName.
  3. Let result be a new empty List.
  4. For each GroupSpecifier gs that pattern contains, do
    1. If the CapturingGroupName of gs is name, then
      1. Append gs to result.
  5. Return result.

22.2.1.8 Static Semantics: CapturingGroupName

The syntax-directed operation CapturingGroupName takes no arguments and returns a String. It is defined piecewise over the following productions:

GroupName :: < RegExpIdentifierName >
  1. Let idTextUnescaped be RegExpIdentifierCodePoints of RegExpIdentifierName.
  2. Return CodePointsToString(idTextUnescaped).

22.2.1.9 Static Semantics: RegExpIdentifierCodePoints

The syntax-directed operation RegExpIdentifierCodePoints takes no arguments and returns a List of code points. It is defined piecewise over the following productions:

RegExpIdentifierName :: RegExpIdentifierStart
  1. Let cp be RegExpIdentifierCodePoint of RegExpIdentifierStart.
  2. Return « cp ».
 RegExpIdentifierName :: RegExpIdentifierName RegExpIdentifierPart
  1. Let cps be RegExpIdentifierCodePoints of the derived RegExpIdentifierName.
  2. Let cp be RegExpIdentifierCodePoint of RegExpIdentifierPart.
  3. Return the list-concatenation of cps and « cp ».

22.2.1.10 Static Semantics: RegExpIdentifierCodePoint

The syntax-directed operation RegExpIdentifierCodePoint takes no arguments and returns a code point. It is defined piecewise over the following productions:

RegExpIdentifierStart :: IdentifierStartChar
  1. Return the code point matched by IdentifierStartChar.
 RegExpIdentifierPart :: IdentifierPartChar
  1. Return the code point matched by IdentifierPartChar.
 RegExpIdentifierStart :: \ RegExpUnicodeEscapeSequence RegExpIdentifierPart :: \ RegExpUnicodeEscapeSequence
  1. Return the code point whose numeric value is the CharacterValue of RegExpUnicodeEscapeSequence.
 RegExpIdentifierStart :: UnicodeLeadSurrogate UnicodeTrailSurrogate RegExpIdentifierPart :: UnicodeLeadSurrogate UnicodeTrailSurrogate
  1. Let lead be the code unit whose numeric value is the numeric value of the code point matched by UnicodeLeadSurrogate.
  2. Let trail be the code unit whose numeric value is the numeric value of the code point matched by UnicodeTrailSurrogate.
  3. Return UTF16SurrogatePairToCodePoint(lead, trail).

22.2.2 Pattern Semantics

A regular expression pattern is converted into an Abstract Closure using the process described below. An implementation is encouraged to use more efficient algorithms than the ones listed below, as long as the results are the same. The Abstract Closure is used as the value of a RegExp object's [[RegExpMatcher]] internal slot.

A Pattern is either a BMP pattern or a Unicode pattern depending upon whether or not its associated flags contain a u. A BMP pattern matches against a String interpreted as consisting of a sequence of 16-bit values that are Unicode code points in the range of the Basic Multilingual Plane. A Unicode pattern matches against a String interpreted as consisting of Unicode code points encoded using UTF-16. In the context of describing the behaviour of a BMP pattern “character” means a single 16-bit Unicode BMP code point. In the context of describing the behaviour of a Unicode pattern “character” means a UTF-16 encoded code point (6.1.4). In either context, “character value” means the numeric value of the corresponding non-encoded code point.

The syntax and semantics of Pattern is defined as if the source text for the Pattern was a List of SourceCharacter values where each SourceCharacter corresponds to a Unicode code point. If a BMP pattern contains a non-BMP SourceCharacter the entire pattern is encoded using UTF-16 and the individual code units of that encoding are used as the elements of the List.

Note

For example, consider a pattern expressed in source text as the single non-BMP character U 1D11E (MUSICAL SYMBOL G CLEF). Interpreted as a Unicode pattern, it would be a single element (character) List consisting of the single code point U 1D11E. However, interpreted as a BMP pattern, it is first UTF-16 encoded to produce a two element List consisting of the code units 0xD834 and 0xDD1E.

Patterns are passed to the RegExp constructor as ECMAScript String values in which non-BMP characters are UTF-16 encoded. For example, the single character MUSICAL SYMBOL G CLEF pattern, expressed as a String value, is a String of length 2 whose elements were the code units 0xD834 and 0xDD1E. So no further translation of the string would be necessary to process it as a BMP pattern consisting of two pattern characters. However, to process it as a Unicode pattern UTF16SurrogatePairToCodePoint must be used in producing a List whose sole element is a single pattern character, the code point U 1D11E.

An implementation may not actually perform such translations to or from UTF-16, but the semantics of this specification requires that the result of pattern matching be as if such translations were performed.

22.2.2.1 Notation

The descriptions below use the following internal data structures:

  • A CharSet is a mathematical set of characters. In the context of a Unicode pattern, “all characters” means the CharSet containing all code point values; otherwise “all characters” means the CharSet containing all code unit values.
  • A CaptureRange is an ordered pair (startIndex, endIndex) that represents the range of characters included in a capture, where startIndex is an integer representing the start index (inclusive) of the range within Input, and endIndex is an integer representing the end index (exclusive) of the range within Input. For any CaptureRange, these indices must satisfy the invariant that startIndex ≤ endIndex.
  • A MatchState is an ordered triple (input, endIndex, captures) where input is a List of characters representing the String being matched, endIndex is an integer, and captures is a List of values, one for each left-capturing parenthesis in the pattern. States are used to represent partial match states in the regular expression matching algorithms. The endIndex is one plus the index of the last input character matched so far by the pattern, while captures holds the results of capturing parentheses. The nth element of captures is either a CaptureRange representing the range of characters captured by the nth set of capturing parentheses, or undefined if the nth set of capturing parentheses hasn't been reached yet. Due to backtracking, many States may be in use at any time during the matching process.
  • A MatchResult is either a MatchState or the special token failure that indicates that the match failed.
  • A MatcherContinuation is an Abstract Closure that takes one MatchState argument and returns a MatchResult result. The MatcherContinuation attempts to match the remaining portion (specified by the closure's captured values) of the pattern against Input, starting at the intermediate state given by its MatchState argument. If the match succeeds, the MatcherContinuation returns the final MatchState that it reached; if the match fails, the MatcherContinuation returns failure.
  • A Matcher is an Abstract Closure that takes two arguments—a MatchState and a MatcherContinuation—and returns a MatchResult result. A Matcher attempts to match a middle subpattern (specified by the closure's captured values) of the pattern against the MatchState's input, starting at the intermediate state given by its MatchState argument. The MatcherContinuation argument should be a closure that matches the rest of the pattern. After matching the subpattern of a pattern to obtain a new MatchState, the Matcher then calls MatcherContinuation on that new MatchState to test if the rest of the pattern can match as well. If it can, the Matcher returns the MatchState returned by MatcherContinuation; if not, the Matcher may try different choices at its choice points, repeatedly calling MatcherContinuation until it either succeeds or all possibilities have been exhausted.

22.2.2.1.1 RegExp Records

A RegExp Record is a Record value used to store information about a RegExp that is needed during compilation and possibly during matching.

It has the following fields:

Table 64: RegExp Record Fields
Field Name Value Meaning
[[IgnoreCase]] a Boolean indicates whether "i" appears in the RegExp's flags
[[Multiline]] a Boolean indicates whether "m" appears in the RegExp's flags
[[DotAll]] a Boolean indicates whether "s" appears in the RegExp's flags
[[Unicode]] a Boolean indicates whether "u" appears in the RegExp's flags
[[CapturingGroupsCount]] a non-negative integer the number of left-capturing parentheses in the RegExp's pattern

22.2.2.2 Runtime Semantics: CompilePattern

The syntax-directed operation CompilePattern takes argument rer (a RegExp Record) and returns an Abstract Closure that takes a List of characters and a non-negative integer and returns a MatchResult. It is defined piecewise over the following productions:

Pattern :: Disjunction
  1. Let m be CompileSubpattern of Disjunction with arguments rer and forward.
  2. Return a new Abstract Closure with parameters (Input, index) that captures rer and m and performs the following steps when called:
    1. Assert: Input is a List of characters.
    2. Assert: 0 ≤ index ≤ the number of elements in Input.
    3. Let c be a new MatcherContinuation with parameters (y) that captures nothing and performs the following steps when called:
      1. Assert: y is a MatchState.
      2. Return y.
    4. Let cap be a List of rer.[[CapturingGroupsCount]] undefined values, indexed 1 through rer.[[CapturingGroupsCount]].
    5. Let x be the MatchState (Input, index, cap).
    6. Return m(x, c).
 Note

A Pattern compiles to an Abstract Closure value. RegExpBuiltinExec can then apply this procedure to a List of characters and an offset within that List to determine whether the pattern would match starting at exactly that offset within the List, and, if it does match, what the values of the capturing parentheses would be. The algorithms in 22.2.2 are designed so that compiling a pattern may throw a SyntaxError exception; on the other hand, once the pattern is successfully compiled, applying the resulting Abstract Closure to find a match in a List of characters cannot throw an exception (except for any implementation-defined exceptions that can occur anywhere such as out-of-memory).

22.2.2.3 Runtime Semantics: CompileSubpattern

The syntax-directed operation CompileSubpattern takes arguments rer (a RegExp Record) and direction (forward or backward) and returns a Matcher.

Note 1

This section is amended in B.1.2.5.

It is defined piecewise over the following productions:

Disjunction :: Alternative | Disjunction
  1. Let m1 be CompileSubpattern of Alternative with arguments rer and direction.
  2. Let m2 be CompileSubpattern of Disjunction with arguments rer and direction.
  3. Return a new Matcher with parameters (x, c) that captures m1 and m2 and performs the following steps when called:
    1. Assert: x is a MatchState.
    2. Assert: c is a MatcherContinuation.
    3. Let r be m1(x, c).
    4. If r is not failure, return r.
    5. Return m2(x, c).
 Note 2

The | regular expression operator separates two alternatives. The pattern first tries to match the left Alternative (followed by the sequel of the regular expression); if it fails, it tries to match the right Disjunction (followed by the sequel of the regular expression). If the left Alternative, the right Disjunction, and the sequel all have choice points, all choices in the sequel are tried before moving on to the next choice in the left Alternative. If choices in the left Alternative are exhausted, the right Disjunction is tried instead of the left Alternative. Any capturing parentheses inside a portion of the pattern skipped by | produce undefined values instead of Strings. Thus, for example,

/a|ab/.exec("abc")

returns the result "a" and not "ab". Moreover,

/((a)|(ab))((c)|(bc))/.exec("abc")

returns the array

["abc", "a", "a", undefined, "bc", undefined, "bc"]

and not

["abc", "ab", undefined, "ab", "c", "c", undefined]

The order in which the two alternatives are tried is independent of the value of direction.

 Alternative :: [empty]
  1. Return a new Matcher with parameters (x, c) that captures nothing and performs the following steps when called:
    1. Assert: x is a MatchState.
    2. Assert: c is a MatcherContinuation.
    3. Return c(x).
 Alternative :: Alternative Term
  1. Let m1 be CompileSubpattern of Alternative with arguments rer and direction.
  2. Let m2 be CompileSubpattern of Term with arguments rer and direction.
  3. If direction is forward, then
    1. Return a new Matcher with parameters (x, c) that captures m1 and m2 and performs the following steps when called:
      1. Assert: x is a MatchState.
      2. Assert: c is a MatcherContinuation.
      3. Let d be a new MatcherContinuation with parameters (y) that captures c and m2 and performs the following steps when called:
        1. Assert: y is a MatchState.
        2. Return m2(y, c).
      4. Return m1(x, d).
  4. Else,
    1. Assert: direction is backward.
    2. Return a new Matcher with parameters (x, c) that captures m1 and m2 and performs the following steps when called:
      1. Assert: x is a MatchState.
      2. Assert: c is a MatcherContinuation.
      3. Let d be a new MatcherContinuation with parameters (y) that captures c and m1 and performs the following steps when called:
        1. Assert: y is a MatchState.
        2. Return m1(y, c).
      4. Return m2(x, d).
 Note 3

Consecutive Terms try to simultaneously match consecutive portions of Input. When direction is forward, if the left Alternative, the right Term, and the sequel of the regular expression all have choice points, all choices in the sequel are tried before moving on to the next choice in the right Term, and all choices in the right Term are tried before moving on to the next choice in the left Alternative. When direction is backward, the evaluation order of Alternative and Term are reversed.

 Term :: Assertion
  1. Return CompileAssertion of Assertion with argument rer.
 Note 4

The resulting Matcher is independent of direction.

 Term :: Atom
  1. Return CompileAtom of Atom with arguments rer and direction.
 Term :: Atom Quantifier
  1. Let m be CompileAtom of Atom with arguments rer and direction.
  2. Let q be CompileQuantifier of Quantifier.
  3. Assert: q.[[Min]] ≤ q.[[Max]].
  4. Let parenIndex be CountLeftCapturingParensBefore(Term).
  5. Let parenCount be CountLeftCapturingParensWithin(Atom).
  6. Return a new Matcher with parameters (x, c) that captures m, q, parenIndex, and parenCount and performs the following steps when called:
    1. Assert: x is a MatchState.
    2. Assert: c is a MatcherContinuation.
    3. Return RepeatMatcher(m, q.[[Min]], q.[[Max]], q.[[Greedy]], x, c, parenIndex, parenCount).

22.2.2.3.1 RepeatMatcher ( m, min, max, greedy, x, c, parenIndex, parenCount )

The abstract operation RepeatMatcher takes arguments m (a Matcher), min (a non-negative integer), max (a non-negative integer or ∞), greedy (a Boolean), x (a MatchState), c (a MatcherContinuation), parenIndex (a non-negative integer), and parenCount (a non-negative integer) and returns a MatchResult. It performs the following steps when called:

  1. If max = 0, return c(x).
  2. Let d be a new MatcherContinuation with parameters (y) that captures m, min, max, greedy, x, c, parenIndex, and parenCount and performs the following steps when called:
    1. Assert: y is a MatchState.
    2. If min = 0 and y's endIndex = x's endIndex, return failure.
    3. If min = 0, let min2 be 0; otherwise let min2 be min - 1.
    4. If max = ∞, let max2 be ∞; otherwise let max2 be max - 1.
    5. Return RepeatMatcher(m, min2, max2, greedy, y, c, parenIndex, parenCount).
  3. Let cap be a copy of x's captures List.
  4. For each integer k in the inclusive interval from parenIndex 1 to parenIndex parenCount, set cap[k] to undefined.
  5. Let Input be x's input.
  6. Let e be x's endIndex.
  7. Let xr be the MatchState (Input, e, cap).
  8. If min ≠ 0, return m(xr, d).
  9. If greedy is false, then
    1. Let z be c(x).
    2. If z is not failure, return z.
    3. Return m(xr, d).
  10. Let z be m(xr, d).
  11. If z is not failure, return z.
  12. Return c(x).
 Note 1

An Atom followed by a Quantifier is repeated the number of times specified by the Quantifier. A Quantifier can be non-greedy, in which case the Atom pattern is repeated as few times as possible while still matching the sequel, or it can be greedy, in which case the Atom pattern is repeated as many times as possible while still matching the sequel. The Atom pattern is repeated rather than the input character sequence that it matches, so different repetitions of the Atom can match different input substrings.

 Note 2

If the Atom and the sequel of the regular expression all have choice points, the Atom is first matched as many (or as few, if non-greedy) times as possible. All choices in the sequel are tried before moving on to the next choice in the last repetition of Atom. All choices in the last (nth) repetition of Atom are tried before moving on to the next choice in the next-to-last (n - 1)st repetition of Atom; at which point it may turn out that more or fewer repetitions of Atom are now possible; these are exhausted (again, starting with either as few or as many as possible) before moving on to the next choice in the (n - 1)st repetition of Atom and so on.

Compare

/a[a-z]{2,4}/.exec("abcdefghi")

which returns "abcde" with

/a[a-z]{2,4}?/.exec("abcdefghi")

which returns "abc".

Consider also

/(aa|aabaac|ba|b|c)*/.exec("aabaac")

which, by the choice point ordering above, returns the array

["aaba", "ba"]

and not any of:

["aabaac", "aabaac"]
["aabaac", "c"]

The above ordering of choice points can be used to write a regular expression that calculates the greatest common divisor of two numbers (represented in unary notation). The following example calculates the gcd of 10 and 15:

"aaaaaaaaaa,aaaaaaaaaaaaaaa".replace(/^(a )\1*,\1 $/, "$1")

which returns the gcd in unary notation "aaaaa".

 Note 3

Step 4 of the RepeatMatcher clears Atom's captures each time Atom is repeated. We can see its behaviour in the regular expression

/(z)((a )?(b )?(c))*/.exec("zaacbbbcac")

which returns the array

["zaacbbbcac", "z", "ac", "a", undefined, "c"]

and not

["zaacbbbcac", "z", "ac", "a", "bbb", "c"]

because each iteration of the outermost * clears all captured Strings contained in the quantified Atom, which in this case includes capture Strings numbered 2, 3, 4, and 5.

 Note 4

Step 2.b of the RepeatMatcher states that once the minimum number of repetitions has been satisfied, any more expansions of Atom that match the empty character sequence are not considered for further repetitions. This prevents the regular expression engine from falling into an infinite loop on patterns such as:

/(a*)*/.exec("b")

or the slightly more complicated:

/(a*)b\1 /.exec("baaaac")

which returns the array

["b", ""]

22.2.2.4 Runtime Semantics: CompileAssertion

The syntax-directed operation CompileAssertion takes argument rer (a RegExp Record) and returns a Matcher.

Note 1

This section is amended in B.1.2.6.

It is defined piecewise over the following productions:

Assertion :: ^
  1. Return a new Matcher with parameters (x, c) that captures rer and performs the following steps when called:
    1. Assert: x is a MatchState.
    2. Assert: c is a MatcherContinuation.
    3. Let Input be x's input.
    4. Let e be x's endIndex.
    5. If e = 0, or if rer.[[Multiline]] is true and the character Input[e - 1] is matched by LineTerminator, then
      1. Return c(x).
    6. Return failure.
 Note 2

Even when the y flag is used with a pattern, ^ always matches only at the beginning of Input, or (if rer.[[Multiline]] is true) at the beginning of a line.

 Assertion :: $
  1. Return a new Matcher with parameters (x, c) that captures rer and performs the following steps when called:
    1. Assert: x is a MatchState.
    2. Assert: c is a MatcherContinuation.
    3. Let Input be x's input.
    4. Let e be x's endIndex.
    5. Let InputLength be the number of elements in Input.
    6. If e = InputLength, or if rer.[[Multiline]] is true and the character Input[e] is matched by LineTerminator, then
      1. Return c(x).
    7. Return failure.
 Assertion :: \b
  1. Return a new Matcher with parameters (x, c) that captures rer and performs the following steps when called:
    1. Assert: x is a MatchState.
    2. Assert: c is a MatcherContinuation.
    3. Let Input be x's input.
    4. Let e be x's endIndex.
    5. Let a be IsWordChar(rer, Input, e - 1).
    6. Let b be IsWordChar(rer, Input, e).
    7. If a is true and b is false, or if a is false and b is true, return c(x).
    8. Return failure.
 Assertion :: \B
  1. Return a new Matcher with parameters (x, c) that captures rer and performs the following steps when called:
    1. Assert: x is a MatchState.
    2. Assert: c is a MatcherContinuation.
    3. Let Input be x's input.
    4. Let e be x's endIndex.
    5. Let a be IsWordChar(rer, Input, e - 1).
    6. Let b be IsWordChar(rer, Input, e).
    7. If a is true and b is true, or if a is false and b is false, return c(x).
    8. Return failure.
 Assertion :: (?= Disjunction )
  1. Let m be CompileSubpattern of Disjunction with arguments rer and forward.
  2. Return a new Matcher with parameters (x, c) that captures m and performs the following steps when called:
    1. Assert: x is a MatchState.
    2. Assert: c is a MatcherContinuation.
    3. Let d be a new MatcherContinuation with parameters (y) that captures nothing and performs the following steps when called:
      1. Assert: y is a MatchState.
      2. Return y.
    4. Let r be m(x, d).
    5. If r is failure, return failure.
    6. Let y be r's MatchState.
    7. Let cap be y's captures List.
    8. Let Input be x's input.
    9. Let xe be x's endIndex.
    10. Let z be the MatchState (Input, xe, cap).
    11. Return c(z).
 Note 3

The form (?= Disjunction ) specifies a zero-width positive lookahead. In order for it to succeed, the pattern inside Disjunction must match at the current position, but the current position is not advanced before matching the sequel. If Disjunction can match at the current position in several ways, only the first one is tried. Unlike other regular expression operators, there is no backtracking into a (?= form (this unusual behaviour is inherited from Perl). This only matters when the Disjunction contains capturing parentheses and the sequel of the pattern contains backreferences to those captures.

For example,

/(?=(a ))/.exec("baaabac")

matches the empty String immediately after the first b and therefore returns the array:

["", "aaa"]

To illustrate the lack of backtracking into the lookahead, consider:

/(?=(a ))a*b\1/.exec("baaabac")

This expression returns

["aba", "a"]

and not:

["aaaba", "a"]
 Assertion :: (?! Disjunction )
  1. Let m be CompileSubpattern of Disjunction with arguments rer and forward.
  2. Return a new Matcher with parameters (x, c) that captures m and performs the following steps when called:
    1. Assert: x is a MatchState.
    2. Assert: c is a MatcherContinuation.
    3. Let d be a new MatcherContinuation with parameters (y) that captures nothing and performs the following steps when called:
      1. Assert: y is a MatchState.
      2. Return y.
    4. Let r be m(x, d).
    5. If r is not failure, return failure.
    6. Return c(x).
 Note 4

The form (?! Disjunction ) specifies a zero-width negative lookahead. In order for it to succeed, the pattern inside Disjunction must fail to match at the current position. The current position is not advanced before matching the sequel. Disjunction can contain capturing parentheses, but backreferences to them only make sense from within Disjunction itself. Backreferences to these capturing parentheses from elsewhere in the pattern always return undefined because the negative lookahead must fail for the pattern to succeed. For example,

/(.*?)a(?!(a )b\2c)\2(.*)/.exec("baaabaac")

looks for an a not immediately followed by some positive number n of a's, a b, another n a's (specified by the first \2) and a c. The second \2 is outside the negative lookahead, so it matches against undefined and therefore always succeeds. The whole expression returns the array:

["baaabaac", "ba", undefined, "abaac"]
 Assertion :: (?<= Disjunction )
  1. Let m be CompileSubpattern of Disjunction with arguments rer and backward.
  2. Return a new Matcher with parameters (x, c) that captures m and performs the following steps when called:
    1. Assert: x is a MatchState.
    2. Assert: c is a MatcherContinuation.
    3. Let d be a new MatcherContinuation with parameters (y) that captures nothing and performs the following steps when called:
      1. Assert: y is a MatchState.
      2. Return y.
    4. Let r be m(x, d).
    5. If r is failure, return failure.
    6. Let y be r's MatchState.
    7. Let cap be y's captures List.
    8. Let Input be x's input.
    9. Let xe be x's endIndex.
    10. Let z be the MatchState (Input, xe, cap).
    11. Return c(z).
 Assertion :: (?<! Disjunction )
  1. Let m be CompileSubpattern of Disjunction with arguments rer and backward.
  2. Return a new Matcher with parameters (x, c) that captures m and performs the following steps when called:
    1. Assert: x is a MatchState.
    2. Assert: c is a MatcherContinuation.
    3. Let d be a new MatcherContinuation with parameters (y) that captures nothing and performs the following steps when called:
      1. Assert: y is a MatchState.
      2. Return y.
    4. Let r be m(x, d).
    5. If r is not failure, return failure.
    6. Return c(x).

22.2.2.4.1 IsWordChar ( rer, Input, e )

The abstract operation IsWordChar takes arguments rer (a RegExp Record), Input (a List of characters), and e (an integer) and returns a Boolean. It performs the following steps when called:

  1. Let InputLength be the number of elements in Input.
  2. If e = -1 or e = InputLength, return false.
  3. Let c be the character Input[e].
  4. If WordCharacters(rer) contains c, return true.
  5. Return false.

22.2.2.5 Runtime Semantics: CompileQuantifier

The syntax-directed operation CompileQuantifier takes no arguments and returns a Record with fields [[Min]] (a non-negative integer), [[Max]] (a non-negative integer or ∞), and [[Greedy]] (a Boolean). It is defined piecewise over the following productions:

Quantifier :: QuantifierPrefix
  1. Let qp be CompileQuantifierPrefix of QuantifierPrefix.
  2. Return the Record { [[Min]]: qp.[[Min]], [[Max]]: qp.[[Max]], [[Greedy]]: true }.
 Quantifier :: QuantifierPrefix ?
  1. Let qp be CompileQuantifierPrefix of QuantifierPrefix.
  2. Return the Record { [[Min]]: qp.[[Min]], [[Max]]: qp.[[Max]], [[Greedy]]: false }.

22.2.2.6 Runtime Semantics: CompileQuantifierPrefix

The syntax-directed operation CompileQuantifierPrefix takes no arguments and returns a Record with fields [[Min]] (a non-negative integer) and [[Max]] (a non-negative integer or ∞). It is defined piecewise over the following productions:

QuantifierPrefix :: *
  1. Return the Record { [[Min]]: 0, [[Max]]: ∞ }.
 QuantifierPrefix ::
  1. Return the Record { [[Min]]: 1, [[Max]]: ∞ }.
 QuantifierPrefix :: ?
  1. Return the Record { [[Min]]: 0, [[Max]]: 1 }.
 QuantifierPrefix :: { DecimalDigits }
  1. Let i be the MV of DecimalDigits (see 12.9.3).
  2. Return the Record { [[Min]]: i, [[Max]]: i }.
 QuantifierPrefix :: { DecimalDigits ,}
  1. Let i be the MV of DecimalDigits.
  2. Return the Record { [[Min]]: i, [[Max]]: ∞ }.
 QuantifierPrefix :: { DecimalDigits , DecimalDigits }
  1. Let i be the MV of the first DecimalDigits.
  2. Let j be the MV of the second DecimalDigits.
  3. Return the Record { [[Min]]: i, [[Max]]: j }.

22.2.2.7 Runtime Semantics: CompileAtom

The syntax-directed operation CompileAtom takes arguments rer (a RegExp Record) and direction (forward or backward) and returns a Matcher.

Note 1

This section is amended in B.1.2.7.

It is defined piecewise over the following productions:

Atom :: PatternCharacter
  1. Let ch be the character matched by PatternCharacter.
  2. Let A be a one-element CharSet containing the character ch.
  3. Return CharacterSetMatcher(rer, A, false, direction).
 Atom :: .
  1. Let A be the CharSet of all characters.
  2. If rer.[[DotAll]] is not true, then
    1. Remove from A all characters corresponding to a code point on the right-hand side of the LineTerminator production.
  3. Return CharacterSetMatcher(rer, A, false, direction).
 Atom :: CharacterClass
  1. Let cc be CompileCharacterClass of CharacterClass with argument rer.
  2. Return CharacterSetMatcher(rer, cc.[[CharSet]], cc.[[Invert]], direction).
 Atom :: ( GroupSpecifieropt Disjunction )
  1. Let m be CompileSubpattern of Disjunction with arguments rer and direction.
  2. Let parenIndex be CountLeftCapturingParensBefore(Atom).
  3. Return a new Matcher with parameters (x, c) that captures direction, m, and parenIndex and performs the following steps when called:
    1. Assert: x is a MatchState.
    2. Assert: c is a MatcherContinuation.
    3. Let d be a new MatcherContinuation with parameters (y) that captures x, c, direction, and parenIndex and performs the following steps when called:
      1. Assert: y is a MatchState.
      2. Let cap be a copy of y's captures List.
      3. Let Input be x's input.
      4. Let xe be x's endIndex.
      5. Let ye be y's endIndex.
      6. If direction is forward, then
        1. Assert: xe ≤ ye.
        2. Let r be the CaptureRange (xe, ye).
      7. Else,
        1. Assert: direction is backward.
        2. Assert: ye ≤ xe.
        3. Let r be the CaptureRange (ye, xe).
      8. Set cap[parenIndex 1] to r.
      9. Let z be the MatchState (Input, ye, cap).
      10. Return c(z).
    4. Return m(x, d).
 Note 2

Parentheses of the form ( Disjunction ) serve both to group the components of the Disjunction pattern together and to save the result of the match. The result can be used either in a backreference (\ followed by a non-zero decimal number), referenced in a replace String, or returned as part of an array from the regular expression matching Abstract Closure. To inhibit the capturing behaviour of parentheses, use the form (?: Disjunction ) instead.

 Atom :: (?: Disjunction )
  1. Return CompileSubpattern of Disjunction with arguments rer and direction.
 AtomEscape :: DecimalEscape
  1. Let n be the CapturingGroupNumber of DecimalEscape.
  2. Assert: n ≤ rer.[[CapturingGroupsCount]].
  3. Return BackreferenceMatcher(rer, n, direction).
 Note 3

An escape sequence of the form \ followed by a non-zero decimal number n matches the result of the nth set of capturing parentheses (22.2.2.1). It is an error if the regular expression has fewer than n capturing parentheses. If the regular expression has n or more capturing parentheses but the nth one is undefined because it has not captured anything, then the backreference always succeeds.

 AtomEscape :: CharacterEscape
  1. Let cv be the CharacterValue of CharacterEscape.
  2. Let ch be the character whose character value is cv.
  3. Let A be a one-element CharSet containing the character ch.
  4. Return CharacterSetMatcher(rer, A, false, direction).
 AtomEscape :: CharacterClassEscape
  1. Let A be CompileToCharSet of CharacterClassEscape with argument rer.
  2. Return CharacterSetMatcher(rer, A, false, direction).
 AtomEscape :: k GroupName
  1. Let matchingGroupSpecifiers be GroupSpecifiersThatMatch(GroupName).
  2. Assert: matchingGroupSpecifiers contains a single GroupSpecifier.
  3. Let groupSpecifier be the sole element of matchingGroupSpecifiers.
  4. Let parenIndex be CountLeftCapturingParensBefore(groupSpecifier).
  5. Return BackreferenceMatcher(rer, parenIndex, direction).

22.2.2.7.1 CharacterSetMatcher ( rer, A, invert, direction )

The abstract operation CharacterSetMatcher takes arguments rer (a RegExp Record), A (a CharSet), invert (a Boolean), and direction (forward or backward) and returns a Matcher. It performs the following steps when called:

  1. Return a new Matcher with parameters (x, c) that captures rer, A, invert, and direction and performs the following steps when called:
    1. Assert: x is a MatchState.
    2. Assert: c is a MatcherContinuation.
    3. Let Input be x's input.
    4. Let e be x's endIndex.
    5. If direction is forward, let f be e 1.
    6. Else, let f be e - 1.
    7. Let InputLength be the number of elements in Input.
    8. If f < 0 or f > InputLength, return failure.
    9. Let index be min(e, f).
    10. Let ch be the character Input[index].
    11. Let cc be Canonicalize(rer, ch).
    12. If there exists a member a of A such that Canonicalize(rer, a) is cc, let found be true. Otherwise, let found be false.
    13. If invert is false and found is false, return failure.
    14. If invert is true and found is true, return failure.
    15. Let cap be x's captures List.
    16. Let y be the MatchState (Input, f, cap).
    17. Return c(y).

22.2.2.7.2 BackreferenceMatcher ( rer, n, direction )

The abstract operation BackreferenceMatcher takes arguments rer (a RegExp Record), n (a positive integer), and direction (forward or backward) and returns a Matcher. It performs the following steps when called:

  1. Assert: n ≥ 1.
  2. Return a new Matcher with parameters (x, c) that captures rer, n, and direction and performs the following steps when called:
    1. Assert: x is a MatchState.
    2. Assert: c is a MatcherContinuation.
    3. Let Input be x's input.
    4. Let cap be x's captures List.
    5. Let r be cap[n].
    6. If r is undefined, return c(x).
    7. Let e be x's endIndex.
    8. Let rs be r's startIndex.
    9. Let re be r's endIndex.
    10. Let len be re - rs.
    11. If direction is forward, let f be e len.
    12. Else, let f be e - len.
    13. Let InputLength be the number of elements in Input.
    14. If f < 0 or f > InputLength, return failure.
    15. Let g be min(e, f).
    16. If there exists an integer i in the interval from 0 (inclusive) to len (exclusive) such that Canonicalize(rer, Input[rs i]) is not Canonicalize(rer, Input[g i]), return failure.
    17. Let y be the MatchState (Input, f, cap).
    18. Return c(y).

22.2.2.7.3 Canonicalize ( rer, ch )

The abstract operation Canonicalize takes arguments rer (a RegExp Record) and ch (a character) and returns a character. It performs the following steps when called:

  1. If rer.[[Unicode]] is true and rer.[[IgnoreCase]] is true, then
    1. If the file CaseFolding.txt of the Unicode Character Database provides a simple or common case folding mapping for ch, return the result of applying that mapping to ch.
    2. Return ch.
  2. If rer.[[IgnoreCase]] is false, return ch.
  3. Assert: ch is a UTF-16 code unit.
  4. Let cp be the code point whose numeric value is the numeric value of ch.
  5. Let u be the result of toUppercase(« cp »), according to the Unicode Default Case Conversion algorithm.
  6. Let uStr be CodePointsToString(u).
  7. If the length of uStr ≠ 1, return ch.
  8. Let cu be uStr's single code unit element.
  9. If the numeric value of ch ≥ 128 and the numeric value of cu < 128, return ch.
  10. Return cu.
 Note

In case-insignificant matches when rer.[[Unicode]] is true, all characters are implicitly case-folded using the simple mapping provided by the Unicode Standard immediately before they are compared. The simple mapping always maps to a single code point, so it does not map, for example, ß (U 00DF LATIN SMALL LETTER SHARP S) to ss or SS. It may however map code points outside the Basic Latin block to code points within it—for example, ſ (U 017F LATIN SMALL LETTER LONG S) case-folds to s (U 0073 LATIN SMALL LETTER S) and K (U 212A KELVIN SIGN) case-folds to k (U 006B LATIN SMALL LETTER K). Strings containing those code points are matched by regular expressions such as /[a-z]/ui.

In case-insignificant matches when rer.[[Unicode]] is false, the mapping is based on Unicode Default Case Conversion algorithm toUppercase rather than toCasefold, which results in some subtle differences. For example, Ω (U 2126 OHM SIGN) is mapped by toUppercase to itself but by toCasefold to ω (U 03C9 GREEK SMALL LETTER OMEGA) along with Ω (U 03A9 GREEK CAPITAL LETTER OMEGA), so "\u2126" is matched by /[ω]/ui and /[\u03A9]/ui but not by /[ω]/i or /[\u03A9]/i. Also, no code point outside the Basic Latin block is mapped to a code point within it, so strings such as "\u017F ſ" and "\u212A K" are not matched by /[a-z]/i.

22.2.2.8 Runtime Semantics: CompileCharacterClass

The syntax-directed operation CompileCharacterClass takes argument rer (a RegExp Record) and returns a Record with fields [[CharSet]] (a CharSet) and [[Invert]] (a Boolean). It is defined piecewise over the following productions:

CharacterClass :: [ ClassRanges ]
  1. Let A be CompileToCharSet of ClassRanges with argument rer.
  2. Return the Record { [[CharSet]]: A, [[Invert]]: false }.
 CharacterClass :: [^ ClassRanges ]
  1. Let A be CompileToCharSet of ClassRanges with argument rer.
  2. Return the Record { [[CharSet]]: A, [[Invert]]: true }.

22.2.2.9 Runtime Semantics: CompileToCharSet

The syntax-directed operation CompileToCharSet takes argument rer (a RegExp Record) and returns a CharSet.

Note 1

This section is amended in B.1.2.8.

It is defined piecewise over the following productions:

ClassRanges :: [empty]
  1. Return the empty CharSet.
 NonemptyClassRanges :: ClassAtom NonemptyClassRangesNoDash
  1. Let A be CompileToCharSet of ClassAtom with argument rer.
  2. Let B be CompileToCharSet of NonemptyClassRangesNoDash with argument rer.
  3. Return the union of CharSets A and B.
 NonemptyClassRanges :: ClassAtom - ClassAtom ClassRanges
  1. Let A be CompileToCharSet of the first ClassAtom with argument rer.
  2. Let B be CompileToCharSet of the second ClassAtom with argument rer.
  3. Let C be CompileToCharSet of ClassRanges with argument rer.
  4. Let D be CharacterRange(A, B).
  5. Return the union of D and C.
 NonemptyClassRangesNoDash :: ClassAtomNoDash NonemptyClassRangesNoDash
  1. Let A be CompileToCharSet of ClassAtomNoDash with argument rer.
  2. Let B be CompileToCharSet of NonemptyClassRangesNoDash with argument rer.
  3. Return the union of CharSets A and B.
 NonemptyClassRangesNoDash :: ClassAtomNoDash - ClassAtom ClassRanges
  1. Let A be CompileToCharSet of ClassAtomNoDash with argument rer.
  2. Let B be CompileToCharSet of ClassAtom with argument rer.
  3. Let C be CompileToCharSet of ClassRanges with argument rer.
  4. Let D be CharacterRange(A, B).
  5. Return the union of D and C.
 Note 2

ClassRanges can expand into a single ClassAtom and/or ranges of two ClassAtom separated by dashes. In the latter case the ClassRanges includes all characters between the first ClassAtom and the second ClassAtom, inclusive; an error occurs if either ClassAtom does not represent a single character (for example, if one is \w) or if the first ClassAtom's character value is strictly greater than the second ClassAtom's character value.

 Note 3

Even if the pattern ignores case, the case of the two ends of a range is significant in determining which characters belong to the range. Thus, for example, the pattern /[E-F]/i matches only the letters E, F, e, and f, while the pattern /[E-f]/i matches all uppercase and lowercase letters in the Unicode Basic Latin block as well as the symbols [, \, ], ^, _, and `.

 Note 4

A - character can be treated literally or it can denote a range. It is treated literally if it is the first or last character of ClassRanges, the beginning or end limit of a range specification, or immediately follows a range specification.

 ClassAtom :: -
  1. Return the CharSet containing the single character - U 002D (HYPHEN-MINUS).
 ClassAtomNoDash :: SourceCharacter but not one of \ or ] or -
  1. Return the CharSet containing the character matched by SourceCharacter.
 ClassEscape :: b - CharacterEscape
  1. Let cv be the CharacterValue of this ClassEscape.
  2. Let c be the character whose character value is cv.
  3. Return the CharSet containing the single character c.
 Note 5

A ClassAtom can use any of the escape sequences that are allowed in the rest of the regular expression except for \b, \B, and backreferences. Inside a CharacterClass, \b means the backspace character, while \B and backreferences raise errors. Using a backreference inside a ClassAtom causes an error.

 CharacterClassEscape :: d
  1. Return the ten-element CharSet containing the characters 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9.
 CharacterClassEscape :: D
  1. Return the CharSet containing all characters not in the CharSet returned by CharacterClassEscape :: d .
 CharacterClassEscape :: s
  1. Return the CharSet containing all characters corresponding to a code point on the right-hand side of the WhiteSpace or LineTerminator productions.
 CharacterClassEscape :: S
  1. Return the CharSet containing all characters not in the CharSet returned by CharacterClassEscape :: s .
 CharacterClassEscape :: w
  1. Return WordCharacters(rer).
 CharacterClassEscape :: W
  1. Return the CharSet containing all characters not in the CharSet returned by CharacterClassEscape :: w .
 CharacterClassEscape :: p{ UnicodePropertyValueExpression }
  1. Return the CharSet containing all Unicode code points included in CompileToCharSet of UnicodePropertyValueExpression with argument rer.
 CharacterClassEscape :: P{ UnicodePropertyValueExpression }
  1. Return the CharSet containing all Unicode code points not included in CompileToCharSet of UnicodePropertyValueExpression with argument rer.
 UnicodePropertyValueExpression :: UnicodePropertyName = UnicodePropertyValue
  1. Let ps be the source text matched by UnicodePropertyName.
  2. Let p be UnicodeMatchProperty(ps).
  3. Assert: p is a Unicode property name or property alias listed in the “Property name and aliases” column of Table 65.
  4. Let vs be the source text matched by UnicodePropertyValue.
  5. Let v be UnicodeMatchPropertyValue(p, vs).
  6. Return the CharSet containing all Unicode code points whose character database definition includes the property p with value v.
 UnicodePropertyValueExpression :: LoneUnicodePropertyNameOrValue
  1. Let s be the source text matched by LoneUnicodePropertyNameOrValue.
  2. If UnicodeMatchPropertyValue(General_Category, s) is a Unicode property value or property value alias for the General_Category (gc) property listed in PropertyValueAliases.txt, then
    1. Return the CharSet containing all Unicode code points whose character database definition includes the property “General_Category” with value s.
  3. Let p be UnicodeMatchProperty(s).
  4. Assert: p is a binary Unicode property or binary property alias listed in the “Property name and aliases” column of Table 66.
  5. Return the CharSet containing all Unicode code points whose character database definition includes the property p with value “True”.

22.2.2.9.1 CharacterRange ( A, B )

The abstract operation CharacterRange takes arguments A (a CharSet) and B (a CharSet) and returns a CharSet. It performs the following steps when called:

  1. Assert: A and B each contain exactly one character.
  2. Let a be the one character in CharSet A.
  3. Let b be the one character in CharSet B.
  4. Let i be the character value of character a.
  5. Let j be the character value of character b.
  6. Assert: i ≤ j.
  7. Return the CharSet containing all characters with a character value in the inclusive interval from i to j.

22.2.2.9.2 WordCharacters ( rer )

The abstract operation WordCharacters takes argument rer (a RegExp Record) and returns a CharSet. Returns a CharSet containing the characters considered "word characters" for the purposes of \b, \B, \w, and \W It performs the following steps when called:

  1. Let basicWordChars be the CharSet containing every character in the ASCII word characters.
  2. Let extraWordChars be the CharSet containing all characters c such that c is not in basicWordChars but Canonicalize(rer, c) is in basicWordChars.
  3. Assert: extraWordChars is empty unless rer.[[Unicode]] is true and rer.[[IgnoreCase]] is true.
  4. Return the union of basicWordChars and extraWordChars.

22.2.2.9.3 UnicodeMatchProperty ( p )

The abstract operation UnicodeMatchProperty takes argument p (ECMAScript source text) and returns a Unicode property name. It performs the following steps when called:

  1. Assert: p is a Unicode property name or property alias listed in the “Property name and aliases” column of Table 65 or Table 66.
  2. Let c be the canonical property name of p as given in the “Canonical property name” column of the corresponding row.
  3. Return the List of Unicode code points c.

Implementations must support the Unicode property names and aliases listed in Table 65 and Table 66. To ensure interoperability, implementations must not support any other property names or aliases.

Note 1

For example, Script_Extensions (property name) and scx (property alias) are valid, but script_extensions or Scx aren't.

 Note 2

The listed properties form a superset of what UTS18 RL1.2 requires.

 Note 3

The spellings of entries in these tables (including casing) match the spellings used in the file PropertyAliases.txt in the Unicode Character Database. The precise spellings in that file are guaranteed to be stable.
Table 65: Non-binary Unicode property aliases and their canonical property names
Property name and aliases Canonical property name
General_Category General_Category
gc
Script Script
sc
Script_Extensions Script_Extensions
scx
Table 66: Binary Unicode property aliases and their canonical property names
Property name and aliases Canonical property name
ASCII ASCII
ASCII_Hex_Digit ASCII_Hex_Digit
AHex
Alphabetic Alphabetic
Alpha
Any Any
Assigned Assigned
Bidi_Control Bidi_Control
Bidi_C
Bidi_Mirrored Bidi_Mirrored
Bidi_M
Case_Ignorable Case_Ignorable
CI
Cased Cased
Changes_When_Casefolded Changes_When_Casefolded
CWCF
Changes_When_Casemapped Changes_When_Casemapped
CWCM
Changes_When_Lowercased Changes_When_Lowercased
CWL
Changes_When_NFKC_Casefolded Changes_When_NFKC_Casefolded
CWKCF
Changes_When_Titlecased Changes_When_Titlecased
CWT
Changes_When_Uppercased Changes_When_Uppercased
CWU
Dash Dash
Default_Ignorable_Code_Point Default_Ignorable_Code_Point
DI
Deprecated Deprecated
Dep
Diacritic Diacritic
Dia
Emoji Emoji
Emoji_Component Emoji_Component
EComp
Emoji_Modifier Emoji_Modifier
EMod
Emoji_Modifier_Base Emoji_Modifier_Base
EBase
Emoji_Presentation Emoji_Presentation
EPres
Extended_Pictographic Extended_Pictographic
ExtPict
Extender Extender
Ext
Grapheme_Base Grapheme_Base
Gr_Base
Grapheme_Extend Grapheme_Extend
Gr_Ext
Hex_Digit Hex_Digit
Hex
IDS_Binary_Operator IDS_Binary_Operator
IDSB
IDS_Trinary_Operator IDS_Trinary_Operator
IDST
ID_Continue ID_Continue
IDC
ID_Start ID_Start
IDS
Ideographic Ideographic
Ideo
Join_Control Join_Control
Join_C
Logical_Order_Exception Logical_Order_Exception
LOE
Lowercase Lowercase
Lower
Math Math
Noncharacter_Code_Point Noncharacter_Code_Point
NChar
Pattern_Syntax Pattern_Syntax
Pat_Syn
Pattern_White_Space Pattern_White_Space
Pat_WS
Quotation_Mark Quotation_Mark
QMark
Radical Radical
Regional_Indicator Regional_Indicator
RI
Sentence_Terminal Sentence_Terminal
STerm
Soft_Dotted Soft_Dotted
SD
Terminal_Punctuation Terminal_Punctuation
Term
Unified_Ideograph Unified_Ideograph
UIdeo
Uppercase Uppercase
Upper
Variation_Selector Variation_Selector
VS
White_Space White_Space
space
XID_Continue XID_Continue
XIDC
XID_Start XID_Start
XIDS

22.2.2.9.4 UnicodeMatchPropertyValue ( p, v )

The abstract operation UnicodeMatchPropertyValue takes arguments p (ECMAScript source text) and v (ECMAScript source text) and returns a Unicode property value. It performs the following steps when called:

  1. Assert: p is a canonical, unaliased Unicode property name listed in the “Canonical property name” column of Table 65.
  2. Assert: v is a property value or property value alias for the Unicode property p listed in PropertyValueAliases.txt.
  3. Let value be the canonical property value of v as given in the “Canonical property value” column of the corresponding row.
  4. Return the List of Unicode code points value.

Implementations must support the Unicode property values and property value aliases listed in PropertyValueAliases.txt for the properties listed in Table 65. To ensure interoperability, implementations must not support any other property values or property value aliases.

Note 1

For example, Xpeo and Old_Persian are valid Script_Extensions values, but xpeo and Old Persian aren't.

 Note 2

This algorithm differs from the matching rules for symbolic values listed in UAX44: case, white space, U 002D (HYPHEN-MINUS), and U 005F (LOW LINE) are not ignored, and the Is prefix is not supported.

22.2.3 Abstract Operations for RegExp Creation

22.2.3.1 RegExpCreate ( P, F )

The abstract operation RegExpCreate takes arguments P (an ECMAScript language value) and F (a String or undefined) and returns either a normal completion containing an Object or a throw completion. It performs the following steps when called:

  1. Let obj be ! RegExpAlloc(%RegExp%).
  2. Return ? RegExpInitialize(obj, P, F).

22.2.3.2 RegExpAlloc ( newTarget )

The abstract operation RegExpAlloc takes argument newTarget (a constructor) and returns either a normal completion containing an Object or a throw completion. It performs the following steps when called:

  1. Let obj be ? OrdinaryCreateFromConstructor(newTarget, "%RegExp.prototype%", « [[OriginalSource]], [[OriginalFlags]], [[RegExpRecord]], [[RegExpMatcher]] »).
  2. Perform ! DefinePropertyOrThrow(obj, "lastIndex", PropertyDescriptor { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
  3. Return obj.

22.2.3.3 RegExpInitialize ( obj, pattern, flags )

The abstract operation RegExpInitialize takes arguments obj (an Object), pattern (an ECMAScript language value), and flags (an ECMAScript language value) and returns either a normal completion containing an Object or a throw completion. It performs the following steps when called:

  1. If pattern is undefined, let P be the empty String.
  2. Else, let P be ? ToString(pattern).
  3. If flags is undefined, let F be the empty String.
  4. Else, let F be ? ToString(flags).
  5. If F contains any code unit other than "d", "g", "i", "m", "s", "u", or "y", or if F contains any code unit more than once, throw a SyntaxError exception.
  6. If F contains "i", let i be true; else let i be false.
  7. If F contains "m", let m be true; else let m be false.
  8. If F contains "s", let s be true; else let s be false.
  9. If F contains "u", let u be true; else let u be false.
  10. If u is true, then
    1. Let patternText be StringToCodePoints(P).
  11. Else,
    1. Let patternText be the result of interpreting each of P's 16-bit elements as a Unicode BMP code point. UTF-16 decoding is not applied to the elements.
  12. Let parseResult be ParsePattern(patternText, u).
  13. If parseResult is a non-empty List of SyntaxError objects, throw a SyntaxError exception.
  14. Assert: parseResult is a Pattern Parse Node.
  15. Set obj.[[OriginalSource]] to P.
  16. Set obj.[[OriginalFlags]] to F.
  17. Let capturingGroupsCount be CountLeftCapturingParensWithin(parseResult).
  18. Let rer be the RegExp Record { [[IgnoreCase]]: i, [[Multiline]]: m, [[DotAll]]: s, [[Unicode]]: u, [[CapturingGroupsCount]]: capturingGroupsCount }.
  19. Set obj.[[RegExpRecord]] to rer.
  20. Set obj.[[RegExpMatcher]] to CompilePattern of parseResult with argument rer.
  21. Perform ? Set(obj, "lastIndex", 0𝔽, true).
  22. Return obj.

22.2.3.4 Static Semantics: ParsePattern ( patternText, u )

The abstract operation ParsePattern takes arguments patternText (a sequence of Unicode code points) and u (a Boolean) and returns a Parse Node or a non-empty List of SyntaxError objects.

Note

This section is amended in B.1.2.9.

It performs the following steps when called:

  1. If u is true, then
    1. Let parseResult be ParseText(patternText, Pattern[ UnicodeMode, N]).
  2. Else,
    1. Let parseResult be ParseText(patternText, Pattern[~UnicodeMode, N]).
  3. Return parseResult.

22.2.4 The RegExp Constructor

The RegExp constructor:

  • is %RegExp%.
  • is the initial value of the "RegExp" property of the global object.
  • creates and initializes a new RegExp object when called as a function rather than as a constructor. Thus the function call RegExp(…) is equivalent to the object creation expression new RegExp(…) with the same arguments.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified RegExp behaviour must include a super call to the RegExp constructor to create and initialize subclass instances with the necessary internal slots.

22.2.4.1 RegExp ( pattern, flags )

This function performs the following steps when called:

  1. Let patternIsRegExp be ? IsRegExp(pattern).
  2. If NewTarget is undefined, then
    1. Let newTarget be the active function object.
    2. If patternIsRegExp is true and flags is undefined, then
      1. Let patternConstructor be ? Get(pattern, "constructor").
      2. If SameValue(newTarget, patternConstructor) is true, return pattern.
  3. Else, let newTarget be NewTarget.
  4. If pattern is an Object and pattern has a [[RegExpMatcher]] internal slot, then
    1. Let P be pattern.[[OriginalSource]].
    2. If flags is undefined, let F be pattern.[[OriginalFlags]].
    3. Else, let F be flags.
  5. Else if patternIsRegExp is true, then
    1. Let P be ? Get(pattern, "source").
    2. If flags is undefined, then
      1. Let F be ? Get(pattern, "flags").
    3. Else, let F be flags.
  6. Else,
    1. Let P be pattern.
    2. Let F be flags.
  7. Let O be ? RegExpAlloc(newTarget).
  8. Return ? RegExpInitialize(O, P, F).
 Note

If pattern is supplied using a StringLiteral, the usual escape sequence substitutions are performed before the String is processed by this function. If pattern must contain an escape sequence to be recognized by this function, any U 005C (REVERSE SOLIDUS) code points must be escaped within the StringLiteral to prevent them being removed when the contents of the StringLiteral are formed.

22.2.5 Properties of the RegExp Constructor

The RegExp constructor:

  • has a [[Prototype]] internal slot whose value is %Function.prototype%.
  • has the following properties:

22.2.5.1 RegExp.prototype

The initial value of RegExp.prototype is the RegExp prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

22.2.5.2 get RegExp [ @@species ]

RegExp[@@species] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Return the this value.

The value of the "name" property of this function is "get [Symbol.species]".

Note

RegExp prototype methods normally use their this value's constructor to create a derived object. However, a subclass constructor may over-ride that default behaviour by redefining its @@species property.

22.2.6 Properties of the RegExp Prototype Object

The RegExp prototype object:

  • is %RegExp.prototype%.
  • is an ordinary object.
  • is not a RegExp instance and does not have a [[RegExpMatcher]] internal slot or any of the other internal slots of RegExp instance objects.
  • has a [[Prototype]] internal slot whose value is %Object.prototype%.
Note

The RegExp prototype object does not have a "valueOf" property of its own; however, it inherits the "valueOf" property from the Object prototype object.

22.2.6.1 RegExp.prototype.constructor

The initial value of RegExp.prototype.constructor is %RegExp%.

22.2.6.2 RegExp.prototype.exec ( string )

This method searches string for an occurrence of the regular expression pattern and returns an Array containing the results of the match, or null if string did not match.

It performs the following steps when called:

  1. Let R be the this value.
  2. Perform ? RequireInternalSlot(R, [[RegExpMatcher]]).
  3. Let S be ? ToString(string).
  4. Return ? RegExpBuiltinExec(R, S).

22.2.6.3 get RegExp.prototype.dotAll

RegExp.prototype.dotAll is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Let R be the this value.
  2. Let cu be the code unit 0x0073 (LATIN SMALL LETTER S).
  3. Return ? RegExpHasFlag(R, cu).

22.2.6.4 get RegExp.prototype.flags

RegExp.prototype.flags is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Let R be the this value.
  2. If R is not an Object, throw a TypeError exception.
  3. Let codeUnits be a new empty List.
  4. Let hasIndices be ToBoolean(? Get(R, "hasIndices")).
  5. If hasIndices is true, append the code unit 0x0064 (LATIN SMALL LETTER D) to codeUnits.
  6. Let global be ToBoolean(? Get(R, "global")).
  7. If global is true, append the code unit 0x0067 (LATIN SMALL LETTER G) to codeUnits.
  8. Let ignoreCase be ToBoolean(? Get(R, "ignoreCase")).
  9. If ignoreCase is true, append the code unit 0x0069 (LATIN SMALL LETTER I) to codeUnits.
  10. Let multiline be ToBoolean(? Get(R, "multiline")).
  11. If multiline is true, append the code unit 0x006D (LATIN SMALL LETTER M) to codeUnits.
  12. Let dotAll be ToBoolean(? Get(R, "dotAll")).
  13. If dotAll is true, append the code unit 0x0073 (LATIN SMALL LETTER S) to codeUnits.
  14. Let unicode be ToBoolean(? Get(R, "unicode")).
  15. If unicode is true, append the code unit 0x0075 (LATIN SMALL LETTER U) to codeUnits.
  16. Let sticky be ToBoolean(? Get(R, "sticky")).
  17. If sticky is true, append the code unit 0x0079 (LATIN SMALL LETTER Y) to codeUnits.
  18. Return the String value whose code units are the elements of the List codeUnits. If codeUnits has no elements, the empty String is returned.

22.2.6.4.1 RegExpHasFlag ( R, codeUnit )

The abstract operation RegExpHasFlag takes arguments R (an ECMAScript language value) and codeUnit (a code unit) and returns either a normal completion containing either a Boolean or undefined, or a throw completion. It performs the following steps when called:

  1. If R is not an Object, throw a TypeError exception.
  2. If R does not have an [[OriginalFlags]] internal slot, then
    1. If SameValue(R, %RegExp.prototype%) is true, return undefined.
    2. Otherwise, throw a TypeError exception.
  3. Let flags be R.[[OriginalFlags]].
  4. If flags contains codeUnit, return true.
  5. Return false.

22.2.6.5 get RegExp.prototype.global

RegExp.prototype.global is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Let R be the this value.
  2. Let cu be the code unit 0x0067 (LATIN SMALL LETTER G).
  3. Return ? RegExpHasFlag(R, cu).

22.2.6.6 get RegExp.prototype.hasIndices

RegExp.prototype.hasIndices is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Let R be the this value.
  2. Let cu be the code unit 0x0064 (LATIN SMALL LETTER D).
  3. Return ? RegExpHasFlag(R, cu).

22.2.6.7 get RegExp.prototype.ignoreCase

RegExp.prototype.ignoreCase is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Let R be the this value.
  2. Let cu be the code unit 0x0069 (LATIN SMALL LETTER I).
  3. Return ? RegExpHasFlag(R, cu).

22.2.6.8 RegExp.prototype [ @@match ] ( string )

This method performs the following steps when called:

  1. Let rx be the this value.
  2. If rx is not an Object, throw a TypeError exception.
  3. Let S be ? ToString(string).
  4. Let flags be ? ToString(? Get(rx, "flags")).
  5. If flags does not contain "g", then
    1. Return ? RegExpExec(rx, S).
  6. Else,
    1. If flags contains "u", let fullUnicode be true. Otherwise, let fullUnicode be false.
    2. Perform ? Set(rx, "lastIndex", 0𝔽, true).
    3. Let A be ! ArrayCreate(0).
    4. Let n be 0.
    5. Repeat,
      1. Let result be ? RegExpExec(rx, S).
      2. If result is null, then
        1. If n = 0, return null.
        2. Return A.
      3. Else,
        1. Let matchStr be ? ToString(? Get(result, "0")).
        2. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(n)), matchStr).
        3. If matchStr is the empty String, then
          1. Let thisIndex be ℝ(? ToLength(? Get(rx, "lastIndex"))).
          2. Let nextIndex be AdvanceStringIndex(S, thisIndex, fullUnicode).
          3. Perform ? Set(rx, "lastIndex", 𝔽(nextIndex), true).
        4. Set n to n 1.

The value of the "name" property of this method is "[Symbol.match]".

Note

The @@match property is used by the IsRegExp abstract operation to identify objects that have the basic behaviour of regular expressions. The absence of a @@match property or the existence of such a property whose value does not Boolean coerce to true indicates that the object is not intended to be used as a regular expression object.

22.2.6.9 RegExp.prototype [ @@matchAll ] ( string )

This method performs the following steps when called:

  1. Let R be the this value.
  2. If R is not an Object, throw a TypeError exception.
  3. Let S be ? ToString(string).
  4. Let C be ? SpeciesConstructor(R, %RegExp%).
  5. Let flags be ? ToString(? Get(R, "flags")).
  6. Let matcher be ? Construct(C, « R, flags »).
  7. Let lastIndex be ? ToLength(? Get(R, "lastIndex")).
  8. Perform ? Set(matcher, "lastIndex", lastIndex, true).
  9. If flags contains "g", let global be true.
  10. Else, let global be false.
  11. If flags contains "u", let fullUnicode be true.
  12. Else, let fullUnicode be false.
  13. Return CreateRegExpStringIterator(matcher, S, global, fullUnicode).

The value of the "name" property of this method is "[Symbol.matchAll]".

22.2.6.10 get RegExp.prototype.multiline

RegExp.prototype.multiline is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Let R be the this value.
  2. Let cu be the code unit 0x006D (LATIN SMALL LETTER M).
  3. Return ? RegExpHasFlag(R, cu).

22.2.6.11 RegExp.prototype [ @@replace ] ( string, replaceValue )

This method performs the following steps when called:

  1. Let rx be the this value.
  2. If rx is not an Object, throw a TypeError exception.
  3. Let S be ? ToString(string).
  4. Let lengthS be the length of S.
  5. Let functionalReplace be IsCallable(replaceValue).
  6. If functionalReplace is false, then
    1. Set replaceValue to ? ToString(replaceValue).
  7. Let flags be ? ToString(? Get(rx, "flags")).
  8. If flags contains "g", let global be true. Otherwise, let global be false.
  9. If global is true, then
    1. If flags contains "u", let fullUnicode be true. Otherwise, let fullUnicode be false.
    2. Perform ? Set(rx, "lastIndex", 0𝔽, true).
  10. Let results be a new empty List.
  11. Let done be false.
  12. Repeat, while done is false,
    1. Let result be ? RegExpExec(rx, S).
    2. If result is null, set done to true.
    3. Else,
      1. Append result to results.
      2. If global is false, set done to true.
      3. Else,
        1. Let matchStr be ? ToString(? Get(result, "0")).
        2. If matchStr is the empty String, then
          1. Let thisIndex be ℝ(? ToLength(? Get(rx, "lastIndex"))).
          2. Let nextIndex be AdvanceStringIndex(S, thisIndex, fullUnicode).
          3. Perform ? Set(rx, "lastIndex", 𝔽(nextIndex), true).
  13. Let accumulatedResult be the empty String.
  14. Let nextSourcePosition be 0.
  15. For each element result of results, do
    1. Let resultLength be ? LengthOfArrayLike(result).
    2. Let nCaptures be max(resultLength - 1, 0).
    3. Let matched be ? ToString(? Get(result, "0")).
    4. Let matchLength be the length of matched.
    5. Let position be ? ToIntegerOrInfinity(? Get(result, "index")).
    6. Set position to the result of clamping position between 0 and lengthS.
    7. Let captures be a new empty List.
    8. Let n be 1.
    9. Repeat, while n ≤ nCaptures,
      1. Let capN be ? Get(result, ! ToString(𝔽(n))).
      2. If capN is not undefined, then
        1. Set capN to ? ToString(capN).
      3. Append capN to captures.
      4. NOTE: When n = 1, the preceding step puts the first element into captures (at index 0). More generally, the nth capture (the characters captured by the nth set of capturing parentheses) is at captures[n - 1].
      5. Set n to n 1.
    10. Let namedCaptures be ? Get(result, "groups").
    11. If functionalReplace is true, then
      1. Let replacerArgs be the list-concatenation of « matched », captures, and « 𝔽(position), S ».
      2. If namedCaptures is not undefined, then
        1. Append namedCaptures to replacerArgs.
      3. Let replValue be ? Call(replaceValue, undefined, replacerArgs).
      4. Let replacement be ? ToString(replValue).
    12. Else,
      1. If namedCaptures is not undefined, then
        1. Set namedCaptures to ? ToObject(namedCaptures).
      2. Let replacement be ? GetSubstitution(matched, S, position, captures, namedCaptures, replaceValue).
    13. If position ≥ nextSourcePosition, then
      1. NOTE: position should not normally move backwards. If it does, it is an indication of an ill-behaving RegExp subclass or use of an access triggered side-effect to change the global flag or other characteristics of rx. In such cases, the corresponding substitution is ignored.
      2. Set accumulatedResult to the string-concatenation of accumulatedResult, the substring of S from nextSourcePosition to position, and replacement.
      3. Set nextSourcePosition to position matchLength.
  16. If nextSourcePosition ≥ lengthS, return accumulatedResult.
  17. Return the string-concatenation of accumulatedResult and the substring of S from nextSourcePosition.

The value of the "name" property of this method is "[Symbol.replace]".

22.2.6.12 RegExp.prototype [ @@search ] ( string )

This method performs the following steps when called:

  1. Let rx be the this value.
  2. If rx is not an Object, throw a TypeError exception.
  3. Let S be ? ToString(string).
  4. Let previousLastIndex be ? Get(rx, "lastIndex").
  5. If SameValue(previousLastIndex, 0𝔽) is false, then
    1. Perform ? Set(rx, "lastIndex", 0𝔽, true).
  6. Let result be ? RegExpExec(rx, S).
  7. Let currentLastIndex be ? Get(rx, "lastIndex").
  8. If SameValue(currentLastIndex, previousLastIndex) is false, then
    1. Perform ? Set(rx, "lastIndex", previousLastIndex, true).
  9. If result is null, return -1𝔽.
  10. Return ? Get(result, "index").

The value of the "name" property of this method is "[Symbol.search]".

Note

The "lastIndex" and "global" properties of this RegExp object are ignored when performing the search. The "lastIndex" property is left unchanged.

22.2.6.13 get RegExp.prototype.source

RegExp.prototype.source is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Let R be the this value.
  2. If R is not an Object, throw a TypeError exception.
  3. If R does not have an [[OriginalSource]] internal slot, then
    1. If SameValue(R, %RegExp.prototype%) is true, return "(?:)".
    2. Otherwise, throw a TypeError exception.
  4. Assert: R has an [[OriginalFlags]] internal slot.
  5. Let src be R.[[OriginalSource]].
  6. Let flags be R.[[OriginalFlags]].
  7. Return EscapeRegExpPattern(src, flags).

22.2.6.13.1 EscapeRegExpPattern ( P, F )

The abstract operation EscapeRegExpPattern takes arguments P (a String) and F (a String) and returns a String. It performs the following steps when called:

  1. Let S be a String in the form of a Pattern[~UnicodeMode] (Pattern[ UnicodeMode] if F contains "u") equivalent to P interpreted as UTF-16 encoded Unicode code points (6.1.4), in which certain code points are escaped as described below. S may or may not differ from P; however, the Abstract Closure that would result from evaluating S as a Pattern[~UnicodeMode] (Pattern[ UnicodeMode] if F contains "u") must behave identically to the Abstract Closure given by the constructed object's [[RegExpMatcher]] internal slot. Multiple calls to this abstract operation using the same values for P and F must produce identical results.
  2. The code points / or any LineTerminator occurring in the pattern shall be escaped in S as necessary to ensure that the string-concatenation of "/", S, "/", and F can be parsed (in an appropriate lexical context) as a RegularExpressionLiteral that behaves identically to the constructed regular expression. For example, if P is "/", then S could be "\/" or "\u002F", among other possibilities, but not "/", because /// followed by F would be parsed as a SingleLineComment rather than a RegularExpressionLiteral. If P is the empty String, this specification can be met by letting S be "(?:)".
  3. Return S.

22.2.6.14 RegExp.prototype [ @@split ] ( string, limit )

Note 1

This method returns an Array into which substrings of the result of converting string to a String have been stored. The substrings are determined by searching from left to right for matches of the this value regular expression; these occurrences are not part of any String in the returned array, but serve to divide up the String value.

The this value may be an empty regular expression or a regular expression that can match an empty String. In this case, the regular expression does not match the empty substring at the beginning or end of the input String, nor does it match the empty substring at the end of the previous separator match. (For example, if the regular expression matches the empty String, the String is split up into individual code unit elements; the length of the result array equals the length of the String, and each substring contains one code unit.) Only the first match at a given index of the String is considered, even if backtracking could yield a non-empty substring match at that index. (For example, /a*?/[Symbol.split]("ab") evaluates to the array ["a", "b"], while /a*/[Symbol.split]("ab") evaluates to the array ["","b"].)

If string is (or converts to) the empty String, the result depends on whether the regular expression can match the empty String. If it can, the result array contains no elements. Otherwise, the result array contains one element, which is the empty String.

If the regular expression contains capturing parentheses, then each time separator is matched the results (including any undefined results) of the capturing parentheses are spliced into the output array. For example,

/<(\/)?([^<>] )>/[Symbol.split]("A<B>bold</B>and<CODE>coded</CODE>")

evaluates to the array

["A", undefined, "B", "bold", "/", "B", "and", undefined, "CODE", "coded", "/", "CODE", ""]

If limit is not undefined, then the output array is truncated so that it contains no more than limit elements.

This method performs the following steps when called:

  1. Let rx be the this value.
  2. If rx is not an Object, throw a TypeError exception.
  3. Let S be ? ToString(string).
  4. Let C be ? SpeciesConstructor(rx, %RegExp%).
  5. Let flags be ? ToString(? Get(rx, "flags")).
  6. If flags contains "u", let unicodeMatching be true.
  7. Else, let unicodeMatching be false.
  8. If flags contains "y", let newFlags be flags.
  9. Else, let newFlags be the string-concatenation of flags and "y".
  10. Let splitter be ? Construct(C, « rx, newFlags »).
  11. Let A be ! ArrayCreate(0).
  12. Let lengthA be 0.
  13. If limit is undefined, let lim be 232 - 1; else let lim be ℝ(? ToUint32(limit)).
  14. If lim = 0, return A.
  15. If S is the empty String, then
    1. Let z be ? RegExpExec(splitter, S).
    2. If z is not null, return A.
    3. Perform ! CreateDataPropertyOrThrow(A, "0", S).
    4. Return A.
  16. Let size be the length of S.
  17. Let p be 0.
  18. Let q be p.
  19. Repeat, while q < size,
    1. Perform ? Set(splitter, "lastIndex", 𝔽(q), true).
    2. Let z be ? RegExpExec(splitter, S).
    3. If z is null, set q to AdvanceStringIndex(S, q, unicodeMatching).
    4. Else,
      1. Let e be ℝ(? ToLength(? Get(splitter, "lastIndex"))).
      2. Set e to min(e, size).
      3. If e = p, set q to AdvanceStringIndex(S, q, unicodeMatching).
      4. Else,
        1. Let T be the substring of S from p to q.
        2. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(lengthA)), T).
        3. Set lengthA to lengthA 1.
        4. If lengthA = lim, return A.
        5. Set p to e.
        6. Let numberOfCaptures be ? LengthOfArrayLike(z).
        7. Set numberOfCaptures to max(numberOfCaptures - 1, 0).
        8. Let i be 1.
        9. Repeat, while i ≤ numberOfCaptures,
          1. Let nextCapture be ? Get(z, ! ToString(𝔽(i))).
          2. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(lengthA)), nextCapture).
          3. Set i to i 1.
          4. Set lengthA to lengthA 1.
          5. If lengthA = lim, return A.
        10. Set q to p.
  20. Let T be the substring of S from p to size.
  21. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(lengthA)), T).
  22. Return A.

The value of the "name" property of this method is "[Symbol.split]".

Note 2

This method ignores the value of the "global" and "sticky" properties of this RegExp object.

22.2.6.15 get RegExp.prototype.sticky

RegExp.prototype.sticky is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Let R be the this value.
  2. Let cu be the code unit 0x0079 (LATIN SMALL LETTER Y).
  3. Return ? RegExpHasFlag(R, cu).

22.2.6.16 RegExp.prototype.test ( S )

This method performs the following steps when called:

  1. Let R be the this value.
  2. If R is not an Object, throw a TypeError exception.
  3. Let string be ? ToString(S).
  4. Let match be ? RegExpExec(R, string).
  5. If match is not null, return true; else return false.

22.2.6.17 RegExp.prototype.toString ( )

  1. Let R be the this value.
  2. If R is not an Object, throw a TypeError exception.
  3. Let pattern be ? ToString(? Get(R, "source")).
  4. Let flags be ? ToString(? Get(R, "flags")).
  5. Let result be the string-concatenation of "/", pattern, "/", and flags.
  6. Return result.
 Note

The returned String has the form of a RegularExpressionLiteral that evaluates to another RegExp object with the same behaviour as this object.

22.2.6.18 get RegExp.prototype.unicode

RegExp.prototype.unicode is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Let R be the this value.
  2. Let cu be the code unit 0x0075 (LATIN SMALL LETTER U).
  3. Return ? RegExpHasFlag(R, cu).

22.2.7 Abstract Operations for RegExp Matching

22.2.7.1 RegExpExec ( R, S )

The abstract operation RegExpExec takes arguments R (an Object) and S (a String) and returns either a normal completion containing either an Object or null, or a throw completion. It performs the following steps when called:

  1. Let exec be ? Get(R, "exec").
  2. If IsCallable(exec) is true, then
    1. Let result be ? Call(exec, R, « S »).
    2. If result is not an Object and result is not null, throw a TypeError exception.
    3. Return result.
  3. Perform ? RequireInternalSlot(R, [[RegExpMatcher]]).
  4. Return ? RegExpBuiltinExec(R, S).
 Note

If a callable "exec" property is not found this algorithm falls back to attempting to use the built-in RegExp matching algorithm. This provides compatible behaviour for code written for prior editions where most built-in algorithms that use regular expressions did not perform a dynamic property lookup of "exec".

22.2.7.2 RegExpBuiltinExec ( R, S )

The abstract operation RegExpBuiltinExec takes arguments R (an initialized RegExp instance) and S (a String) and returns either a normal completion containing either an Array exotic object or null, or a throw completion. It performs the following steps when called:

  1. Let length be the length of S.
  2. Let lastIndex be ℝ(? ToLength(? Get(R, "lastIndex"))).
  3. Let flags be R.[[OriginalFlags]].
  4. If flags contains "g", let global be true; else let global be false.
  5. If flags contains "y", let sticky be true; else let sticky be false.
  6. If flags contains "d", let hasIndices be true; else let hasIndices be false.
  7. If global is false and sticky is false, set lastIndex to 0.
  8. Let matcher be R.[[RegExpMatcher]].
  9. If flags contains "u", let fullUnicode be true; else let fullUnicode be false.
  10. Let matchSucceeded be false.
  11. If fullUnicode is true, let input be StringToCodePoints(S). Otherwise, let input be a List whose elements are the code units that are the elements of S.
  12. NOTE: Each element of input is considered to be a character.
  13. Repeat, while matchSucceeded is false,
    1. If lastIndex > length, then
      1. If global is true or sticky is true, then
        1. Perform ? Set(R, "lastIndex", 0𝔽, true).
      2. Return null.
    2. Let inputIndex be the index into input of the character that was obtained from element lastIndex of S.
    3. Let r be matcher(input, inputIndex).
    4. If r is failure, then
      1. If sticky is true, then
        1. Perform ? Set(R, "lastIndex", 0𝔽, true).
        2. Return null.
      2. Set lastIndex to AdvanceStringIndex(S, lastIndex, fullUnicode).
    5. Else,
      1. Assert: r is a MatchState.
      2. Set matchSucceeded to true.
  14. Let e be r's endIndex value.
  15. If fullUnicode is true, set e to GetStringIndex(S, e).
  16. If global is true or sticky is true, then
    1. Perform ? Set(R, "lastIndex", 𝔽(e), true).
  17. Let n be the number of elements in r's captures List.
  18. Assert: n = R.[[RegExpRecord]].[[CapturingGroupsCount]].
  19. Assert: n < 232 - 1.
  20. Let A be ! ArrayCreate(n 1).
  21. Assert: The mathematical value of A's "length" property is n 1.
  22. Perform ! CreateDataPropertyOrThrow(A, "index", 𝔽(lastIndex)).
  23. Perform ! CreateDataPropertyOrThrow(A, "input", S).
  24. Let match be the Match Record { [[StartIndex]]: lastIndex, [[EndIndex]]: e }.
  25. Let indices be a new empty List.
  26. Let groupNames be a new empty List.
  27. Append match to indices.
  28. Let matchedSubstr be GetMatchString(S, match).
  29. Perform ! CreateDataPropertyOrThrow(A, "0", matchedSubstr).
  30. If R contains any GroupName, then
    1. Let groups be OrdinaryObjectCreate(null).
    2. Let hasGroups be true.
  31. Else,
    1. Let groups be undefined.
    2. Let hasGroups be false.
  32. Perform ! CreateDataPropertyOrThrow(A, "groups", groups).
  33. For each integer i such that 1 ≤ i ≤ n, in ascending order, do
    1. Let captureI be ith element of r's captures List.
    2. If captureI is undefined, then
      1. Let capturedValue be undefined.
      2. Append undefined to indices.
    3. Else,
      1. Let captureStart be captureI's startIndex.
      2. Let captureEnd be captureI's endIndex.
      3. If fullUnicode is true, then
        1. Set captureStart to GetStringIndex(S, captureStart).
        2. Set captureEnd to GetStringIndex(S, captureEnd).
      4. Let capture be the Match Record { [[StartIndex]]: captureStart, [[EndIndex]]: captureEnd }.
      5. Let capturedValue be GetMatchString(S, capture).
      6. Append capture to indices.
    4. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(i)), capturedValue).
    5. If the ith capture of R was defined with a GroupName, then
      1. Let s be the CapturingGroupName of that GroupName.
      2. Perform ! CreateDataPropertyOrThrow(groups, s, capturedValue).
      3. Append s to groupNames.
    6. Else,
      1. Append undefined to groupNames.
  34. If hasIndices is true, then
    1. Let indicesArray be MakeMatchIndicesIndexPairArray(S, indices, groupNames, hasGroups).
    2. Perform ! CreateDataPropertyOrThrow(A, "indices", indicesArray).
  35. Return A.

22.2.7.3 AdvanceStringIndex ( S, index, unicode )

The abstract operation AdvanceStringIndex takes arguments S (a String), index (a non-negative integer), and unicode (a Boolean) and returns an integer. It performs the following steps when called:

  1. Assert: index ≤ 253 - 1.
  2. If unicode is false, return index 1.
  3. Let length be the length of S.
  4. If index 1 ≥ length, return index 1.
  5. Let cp be CodePointAt(S, index).
  6. Return index cp.[[CodeUnitCount]].

22.2.7.4 GetStringIndex ( S, codePointIndex )

The abstract operation GetStringIndex takes arguments S (a String) and codePointIndex (a non-negative integer) and returns a non-negative integer. It interprets S as a sequence of UTF-16 encoded code points, as described in 6.1.4, and returns the code unit index corresponding to code point index codePointIndex when such an index exists. Otherwise, it returns the length of S. It performs the following steps when called:

  1. If S is the empty String, return 0.
  2. Let len be the length of S.
  3. Let codeUnitCount be 0.
  4. Let codePointCount be 0.
  5. Repeat, while codeUnitCount < len,
    1. If codePointCount = codePointIndex, return codeUnitCount.
    2. Let cp be CodePointAt(S, codeUnitCount).
    3. Set codeUnitCount to codeUnitCount cp.[[CodeUnitCount]].
    4. Set codePointCount to codePointCount 1.
  6. Return len.

22.2.7.5 Match Records

A Match Record is a Record value used to encapsulate the start and end indices of a regular expression match or capture.

Match Records have the fields listed in Table 67.

Table 67: Match Record Fields
Field Name Value Meaning
[[StartIndex]] a non-negative integer The number of code units from the start of a string at which the match begins (inclusive).
[[EndIndex]] an integer ≥ [[StartIndex]] The number of code units from the start of a string at which the match ends (exclusive).

22.2.7.6 GetMatchString ( S, match )

The abstract operation GetMatchString takes arguments S (a String) and match (a Match Record) and returns a String. It performs the following steps when called:

  1. Assert: match.[[StartIndex]] ≤ match.[[EndIndex]] ≤ the length of S.
  2. Return the substring of S from match.[[StartIndex]] to match.[[EndIndex]].

22.2.7.7 GetMatchIndexPair ( S, match )

The abstract operation GetMatchIndexPair takes arguments S (a String) and match (a Match Record) and returns an Array. It performs the following steps when called:

  1. Assert: match.[[StartIndex]] ≤ match.[[EndIndex]] ≤ the length of S.
  2. Return CreateArrayFromList(« 𝔽(match.[[StartIndex]]), 𝔽(match.[[EndIndex]]) »).

22.2.7.8 MakeMatchIndicesIndexPairArray ( S, indices, groupNames, hasGroups )

The abstract operation MakeMatchIndicesIndexPairArray takes arguments S (a String), indices (a List of either Match Records or undefined), groupNames (a List of either Strings or undefined), and hasGroups (a Boolean) and returns an Array. It performs the following steps when called:

  1. Let n be the number of elements in indices.
  2. Assert: n < 232 - 1.
  3. Assert: groupNames has n - 1 elements.
  4. NOTE: The groupNames List contains elements aligned with the indices List starting at indices[1].
  5. Let A be ! ArrayCreate(n).
  6. If hasGroups is true, then
    1. Let groups be OrdinaryObjectCreate(null).
  7. Else,
    1. Let groups be undefined.
  8. Perform ! CreateDataPropertyOrThrow(A, "groups", groups).
  9. For each integer i such that 0 ≤ i < n, in ascending order, do
    1. Let matchIndices be indices[i].
    2. If matchIndices is not undefined, then
      1. Let matchIndexPair be GetMatchIndexPair(S, matchIndices).
    3. Else,
      1. Let matchIndexPair be undefined.
    4. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(i)), matchIndexPair).
    5. If i > 0 and groupNames[i - 1] is not undefined, then
      1. Assert: groups is not undefined.
      2. Perform ! CreateDataPropertyOrThrow(groups, groupNames[i - 1], matchIndexPair).
  10. Return A.

22.2.8 Properties of RegExp Instances

RegExp instances are ordinary objects that inherit properties from the RegExp prototype object. RegExp instances have internal slots [[OriginalSource]], [[OriginalFlags]], [[RegExpRecord]], and [[RegExpMatcher]]. The value of the [[RegExpMatcher]] internal slot is an Abstract Closure representation of the Pattern of the RegExp object.

Note

Prior to ECMAScript 2015, RegExp instances were specified as having the own data properties "source", "global", "ignoreCase", and "multiline". Those properties are now specified as accessor properties of RegExp.prototype.

RegExp instances also have the following property:

22.2.8.1 lastIndex

The value of the "lastIndex" property specifies the String index at which to start the next match. It is coerced to an integral Number when used (see 22.2.7.2). This property shall have the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.

22.2.9 RegExp String Iterator Objects

A RegExp String Iterator is an object, that represents a specific iteration over some specific String instance object, matching against some specific RegExp instance object. There is not a named constructor for RegExp String Iterator objects. Instead, RegExp String Iterator objects are created by calling certain methods of RegExp instance objects.

22.2.9.1 CreateRegExpStringIterator ( R, S, global, fullUnicode )

The abstract operation CreateRegExpStringIterator takes arguments R (an Object), S (a String), global (a Boolean), and fullUnicode (a Boolean) and returns a Generator. It performs the following steps when called:

  1. Let closure be a new Abstract Closure with no parameters that captures R, S, global, and fullUnicode and performs the following steps when called:
    1. Repeat,
      1. Let match be ? RegExpExec(R, S).
      2. If match is null, return undefined.
      3. If global is false, then
        1. Perform ? GeneratorYield(CreateIterResultObject(match, false)).
        2. Return undefined.
      4. Let matchStr be ? ToString(? Get(match, "0")).
      5. If matchStr is the empty String, then
        1. Let thisIndex be ℝ(? ToLength(? Get(R, "lastIndex"))).
        2. Let nextIndex be AdvanceStringIndex(S, thisIndex, fullUnicode).
        3. Perform ? Set(R, "lastIndex", 𝔽(nextIndex), true).
      6. Perform ? GeneratorYield(CreateIterResultObject(match, false)).
  2. Return CreateIteratorFromClosure(closure, "%RegExpStringIteratorPrototype%", %RegExpStringIteratorPrototype%).

22.2.9.2 The %RegExpStringIteratorPrototype% Object

The %RegExpStringIteratorPrototype% object:

  • has properties that are inherited by all RegExp String Iterator Objects.
  • is an ordinary object.
  • has a [[Prototype]] internal slot whose value is %IteratorPrototype%.
  • has the following properties:

22.2.9.2.1 %RegExpStringIteratorPrototype%.next ( )

  1. Return ? GeneratorResume(this value, empty, "%RegExpStringIteratorPrototype%").

22.2.9.2.2 %RegExpStringIteratorPrototype% [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "RegExp String Iterator".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

23 Indexed Collections

23.1 Array Objects

Arrays are exotic objects that give special treatment to a certain class of property names. See 10.4.2 for a definition of this special treatment.

23.1.1 The Array Constructor

The Array constructor:

  • is %Array%.
  • is the initial value of the "Array" property of the global object.
  • creates and initializes a new Array when called as a constructor.
  • also creates and initializes a new Array when called as a function rather than as a constructor. Thus the function call Array(…) is equivalent to the object creation expression new Array(…) with the same arguments.
  • is a function whose behaviour differs based upon the number and types of its arguments.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the exotic Array behaviour must include a super call to the Array constructor to initialize subclass instances that are Array exotic objects. However, most of the Array.prototype methods are generic methods that are not dependent upon their this value being an Array exotic object.
  • has a "length" property whose value is 1𝔽.

23.1.1.1 Array ( ...values )

This function performs the following steps when called:

  1. If NewTarget is undefined, let newTarget be the active function object; else let newTarget be NewTarget.
  2. Let proto be ? GetPrototypeFromConstructor(newTarget, "%Array.prototype%").
  3. Let numberOfArgs be the number of elements in values.
  4. If numberOfArgs = 0, then
    1. Return ! ArrayCreate(0, proto).
  5. Else if numberOfArgs = 1, then
    1. Let len be values[0].
    2. Let array be ! ArrayCreate(0, proto).
    3. If len is not a Number, then
      1. Perform ! CreateDataPropertyOrThrow(array, "0", len).
      2. Let intLen be 1𝔽.
    4. Else,
      1. Let intLen be ! ToUint32(len).
      2. If SameValueZero(intLen, len) is false, throw a RangeError exception.
    5. Perform ! Set(array, "length", intLen, true).
    6. Return array.
  6. Else,
    1. Assert: numberOfArgs ≥ 2.
    2. Let array be ? ArrayCreate(numberOfArgs, proto).
    3. Let k be 0.
    4. Repeat, while k < numberOfArgs,
      1. Let Pk be ! ToString(𝔽(k)).
      2. Let itemK be values[k].
      3. Perform ! CreateDataPropertyOrThrow(array, Pk, itemK).
      4. Set k to k 1.
    5. Assert: The mathematical value of array's "length" property is numberOfArgs.
    6. Return array.

23.1.2 Properties of the Array Constructor

The Array constructor:

  • has a [[Prototype]] internal slot whose value is %Function.prototype%.
  • has the following properties:

23.1.2.1 Array.from ( items [ , mapfn [ , thisArg ] ] )

This method performs the following steps when called:

  1. Let C be the this value.
  2. If mapfn is undefined, let mapping be false.
  3. Else,
    1. If IsCallable(mapfn) is false, throw a TypeError exception.
    2. Let mapping be true.
  4. Let usingIterator be ? GetMethod(items, @@iterator).
  5. If usingIterator is not undefined, then
    1. If IsConstructor(C) is true, then
      1. Let A be ? Construct(C).
    2. Else,
      1. Let A be ! ArrayCreate(0).
    3. Let iteratorRecord be ? GetIteratorFromMethod(items, usingIterator).
    4. Let k be 0.
    5. Repeat,
      1. If k ≥ 253 - 1, then
        1. Let error be ThrowCompletion(a newly created TypeError object).
        2. Return ? IteratorClose(iteratorRecord, error).
      2. Let Pk be ! ToString(𝔽(k)).
      3. Let next be ? IteratorStep(iteratorRecord).
      4. If next is false, then
        1. Perform ? Set(A, "length", 𝔽(k), true).
        2. Return A.
      5. Let nextValue be ? IteratorValue(next).
      6. If mapping is true, then
        1. Let mappedValue be Completion(Call(mapfn, thisArg, « nextValue, 𝔽(k) »)).
        2. IfAbruptCloseIterator(mappedValue, iteratorRecord).
      7. Else, let mappedValue be nextValue.
      8. Let defineStatus be Completion(CreateDataPropertyOrThrow(A, Pk, mappedValue)).
      9. IfAbruptCloseIterator(defineStatus, iteratorRecord).
      10. Set k to k 1.
  6. NOTE: items is not an Iterable so assume it is an array-like object.
  7. Let arrayLike be ! ToObject(items).
  8. Let len be ? LengthOfArrayLike(arrayLike).
  9. If IsConstructor(C) is true, then
    1. Let A be ? Construct(C, « 𝔽(len) »).
  10. Else,
    1. Let A be ? ArrayCreate(len).
  11. Let k be 0.
  12. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kValue be ? Get(arrayLike, Pk).
    3. If mapping is true, then
      1. Let mappedValue be ? Call(mapfn, thisArg, « kValue, 𝔽(k) »).
    4. Else, let mappedValue be kValue.
    5. Perform ? CreateDataPropertyOrThrow(A, Pk, mappedValue).
    6. Set k to k 1.
  13. Perform ? Set(A, "length", 𝔽(len), true).
  14. Return A.
 Note

This method is an intentionally generic factory method; it does not require that its this value be the Array constructor. Therefore it can be transferred to or inherited by any other constructors that may be called with a single numeric argument.

23.1.2.2 Array.isArray ( arg )

This function performs the following steps when called:

  1. Return ? IsArray(arg).

23.1.2.3 Array.of ( ...items )

This method performs the following steps when called:

  1. Let len be the number of elements in items.
  2. Let lenNumber be 𝔽(len).
  3. Let C be the this value.
  4. If IsConstructor(C) is true, then
    1. Let A be ? Construct(C, « lenNumber »).
  5. Else,
    1. Let A be ? ArrayCreate(len).
  6. Let k be 0.
  7. Repeat, while k < len,
    1. Let kValue be items[k].
    2. Let Pk be ! ToString(𝔽(k)).
    3. Perform ? CreateDataPropertyOrThrow(A, Pk, kValue).
    4. Set k to k 1.
  8. Perform ? Set(A, "length", lenNumber, true).
  9. Return A.
 Note

This method is an intentionally generic factory method; it does not require that its this value be the Array constructor. Therefore it can be transferred to or inherited by other constructors that may be called with a single numeric argument.

23.1.2.4 Array.prototype

The value of Array.prototype is the Array prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

23.1.2.5 get Array [ @@species ]

Array[@@species] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Return the this value.

The value of the "name" property of this function is "get [Symbol.species]".

Note

Array prototype methods normally use their this value's constructor to create a derived object. However, a subclass constructor may over-ride that default behaviour by redefining its @@species property.

23.1.3 Properties of the Array Prototype Object

The Array prototype object:

  • is %Array.prototype%.
  • is an Array exotic object and has the internal methods specified for such objects.
  • has a "length" property whose initial value is 0𝔽 and whose attributes are { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.
  • has a [[Prototype]] internal slot whose value is %Object.prototype%.
Note

The Array prototype object is specified to be an Array exotic object to ensure compatibility with ECMAScript code that was created prior to the ECMAScript 2015 specification.

23.1.3.1 Array.prototype.at ( index )

  1. Let O be ? ToObject(this value).
  2. Let len be ? LengthOfArrayLike(O).
  3. Let relativeIndex be ? ToIntegerOrInfinity(index).
  4. If relativeIndex ≥ 0, then
    1. Let k be relativeIndex.
  5. Else,
    1. Let k be len relativeIndex.
  6. If k < 0 or k ≥ len, return undefined.
  7. Return ? Get(O, ! ToString(𝔽(k))).

23.1.3.2 Array.prototype.concat ( ...items )

This method returns an array containing the array elements of the object followed by the array elements of each argument.

It performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let A be ? ArraySpeciesCreate(O, 0).
  3. Let n be 0.
  4. Prepend O to items.
  5. For each element E of items, do
    1. Let spreadable be ? IsConcatSpreadable(E).
    2. If spreadable is true, then
      1. Let len be ? LengthOfArrayLike(E).
      2. If n len > 253 - 1, throw a TypeError exception.
      3. Let k be 0.
      4. Repeat, while k < len,
        1. Let P be ! ToString(𝔽(k)).
        2. Let exists be ? HasProperty(E, P).
        3. If exists is true, then
          1. Let subElement be ? Get(E, P).
          2. Perform ? CreateDataPropertyOrThrow(A, ! ToString(𝔽(n)), subElement).
        4. Set n to n 1.
        5. Set k to k 1.
    3. Else,
      1. NOTE: E is added as a single item rather than spread.
      2. If n ≥ 253 - 1, throw a TypeError exception.
      3. Perform ? CreateDataPropertyOrThrow(A, ! ToString(𝔽(n)), E).
      4. Set n to n 1.
  6. Perform ? Set(A, "length", 𝔽(n), true).
  7. Return A.

The "length" property of this method is 1𝔽.

Note 1

The explicit setting of the "length" property in step 6 is necessary to ensure that its value is correct in situations where the trailing elements of the result Array are not present.

 Note 2

This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.2.1 IsConcatSpreadable ( O )

The abstract operation IsConcatSpreadable takes argument O (an ECMAScript language value) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:

  1. If O is not an Object, return false.
  2. Let spreadable be ? Get(O, @@isConcatSpreadable).
  3. If spreadable is not undefined, return ToBoolean(spreadable).
  4. Return ? IsArray(O).

23.1.3.3 Array.prototype.constructor

The initial value of Array.prototype.constructor is %Array%.

23.1.3.4 Array.prototype.copyWithin ( target, start [ , end ] )

Note 1

The end argument is optional. If it is not provided, the length of the this value is used.

 Note 2

If target is negative, it is treated as length target where length is the length of the array. If start is negative, it is treated as length start. If end is negative, it is treated as length end.

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let len be ? LengthOfArrayLike(O).
  3. Let relativeTarget be ? ToIntegerOrInfinity(target).
  4. If relativeTarget = -∞, let to be 0.
  5. Else if relativeTarget < 0, let to be max(len relativeTarget, 0).
  6. Else, let to be min(relativeTarget, len).
  7. Let relativeStart be ? ToIntegerOrInfinity(start).
  8. If relativeStart = -∞, let from be 0.
  9. Else if relativeStart < 0, let from be max(len relativeStart, 0).
  10. Else, let from be min(relativeStart, len).
  11. If end is undefined, let relativeEnd be len; else let relativeEnd be ? ToIntegerOrInfinity(end).
  12. If relativeEnd = -∞, let final be 0.
  13. Else if relativeEnd < 0, let final be max(len relativeEnd, 0).
  14. Else, let final be min(relativeEnd, len).
  15. Let count be min(final - from, len - to).
  16. If from < to and to < from count, then
    1. Let direction be -1.
    2. Set from to from count - 1.
    3. Set to to to count - 1.
  17. Else,
    1. Let direction be 1.
  18. Repeat, while count > 0,
    1. Let fromKey be ! ToString(𝔽(from)).
    2. Let toKey be ! ToString(𝔽(to)).
    3. Let fromPresent be ? HasProperty(O, fromKey).
    4. If fromPresent is true, then
      1. Let fromVal be ? Get(O, fromKey).
      2. Perform ? Set(O, toKey, fromVal, true).
    5. Else,
      1. Assert: fromPresent is false.
      2. Perform ? DeletePropertyOrThrow(O, toKey).
    6. Set from to from direction.
    7. Set to to to direction.
    8. Set count to count - 1.
  19. Return O.
 Note 3

This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.5 Array.prototype.entries ( )

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Return CreateArrayIterator(O, key value).

23.1.3.6 Array.prototype.every ( callbackfn [ , thisArg ] )

Note 1

callbackfn should be a function that accepts three arguments and returns a value that is coercible to a Boolean value. every calls callbackfn once for each element present in the array, in ascending order, until it finds one where callbackfn returns false. If such an element is found, every immediately returns false. Otherwise, if callbackfn returned true for all elements, every will return true. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.

If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.

every does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by every is set before the first call to callbackfn. Elements which are appended to the array after the call to every begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time every visits them; elements that are deleted after the call to every begins and before being visited are not visited. every acts like the "for all" quantifier in mathematics. In particular, for an empty array, it returns true.

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let len be ? LengthOfArrayLike(O).
  3. If IsCallable(callbackfn) is false, throw a TypeError exception.
  4. Let k be 0.
  5. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kPresent be ? HasProperty(O, Pk).
    3. If kPresent is true, then
      1. Let kValue be ? Get(O, Pk).
      2. Let testResult be ToBoolean(? Call(callbackfn, thisArg, « kValue, 𝔽(k), O »)).
      3. If testResult is false, return false.
    4. Set k to k 1.
  6. Return true.
 Note 2

This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.7 Array.prototype.fill ( value [ , start [ , end ] ] )

Note 1

The start argument is optional. If it is not provided, 0𝔽 is used.

The end argument is optional. If it is not provided, the length of the this value is used.

 Note 2

If start is negative, it is treated as length start where length is the length of the array. If end is negative, it is treated as length end.

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let len be ? LengthOfArrayLike(O).
  3. Let relativeStart be ? ToIntegerOrInfinity(start).
  4. If relativeStart = -∞, let k be 0.
  5. Else if relativeStart < 0, let k be max(len relativeStart, 0).
  6. Else, let k be min(relativeStart, len).
  7. If end is undefined, let relativeEnd be len; else let relativeEnd be ? ToIntegerOrInfinity(end).
  8. If relativeEnd = -∞, let final be 0.
  9. Else if relativeEnd < 0, let final be max(len relativeEnd, 0).
  10. Else, let final be min(relativeEnd, len).
  11. Repeat, while k < final,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Perform ? Set(O, Pk, value, true).
    3. Set k to k 1.
  12. Return O.
 Note 3

This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.8 Array.prototype.filter ( callbackfn [ , thisArg ] )

Note 1

callbackfn should be a function that accepts three arguments and returns a value that is coercible to a Boolean value. filter calls callbackfn once for each element in the array, in ascending order, and constructs a new array of all the values for which callbackfn returns true. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.

If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.

filter does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by filter is set before the first call to callbackfn. Elements which are appended to the array after the call to filter begins will not be visited by callbackfn. If existing elements of the array are changed their value as passed to callbackfn will be the value at the time filter visits them; elements that are deleted after the call to filter begins and before being visited are not visited.

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let len be ? LengthOfArrayLike(O).
  3. If IsCallable(callbackfn) is false, throw a TypeError exception.
  4. Let A be ? ArraySpeciesCreate(O, 0).
  5. Let k be 0.
  6. Let to be 0.
  7. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kPresent be ? HasProperty(O, Pk).
    3. If kPresent is true, then
      1. Let kValue be ? Get(O, Pk).
      2. Let selected be ToBoolean(? Call(callbackfn, thisArg, « kValue, 𝔽(k), O »)).
      3. If selected is true, then
        1. Perform ? CreateDataPropertyOrThrow(A, ! ToString(𝔽(to)), kValue).
        2. Set to to to 1.
    4. Set k to k 1.
  8. Return A.
 Note 2

This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.9 Array.prototype.find ( predicate [ , thisArg ] )

Note 1

This method calls predicate once for each element of the array, in ascending index order, until it finds one where predicate returns a value that coerces to true. If such an element is found, find immediately returns that element value. Otherwise, find returns undefined.

See FindViaPredicate for additional information.

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let len be ? LengthOfArrayLike(O).
  3. Let findRec be ? FindViaPredicate(O, len, ascending, predicate, thisArg).
  4. Return findRec.[[Value]].
 Note 2

This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.10 Array.prototype.findIndex ( predicate [ , thisArg ] )

Note 1

This method calls predicate once for each element of the array, in ascending index order, until it finds one where predicate returns a value that coerces to true. If such an element is found, findIndex immediately returns the index of that element value. Otherwise, findIndex returns -1.

See FindViaPredicate for additional information.

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let len be ? LengthOfArrayLike(O).
  3. Let findRec be ? FindViaPredicate(O, len, ascending, predicate, thisArg).
  4. Return findRec.[[Index]].
 Note 2

This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.11 Array.prototype.findLast ( predicate [ , thisArg ] )

Note 1

This method calls predicate once for each element of the array, in descending index order, until it finds one where predicate returns a value that coerces to true. If such an element is found, findLast immediately returns that element value. Otherwise, findLast returns undefined.

See FindViaPredicate for additional information.

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let len be ? LengthOfArrayLike(O).
  3. Let findRec be ? FindViaPredicate(O, len, descending, predicate, thisArg).
  4. Return findRec.[[Value]].
 Note 2

This method is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.12 Array.prototype.findLastIndex ( predicate [ , thisArg ] )

Note 1

This method calls predicate once for each element of the array, in descending index order, until it finds one where predicate returns a value that coerces to true. If such an element is found, findLastIndex immediately returns the index of that element value. Otherwise, findLastIndex returns -1.

See FindViaPredicate for additional information.

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let len be ? LengthOfArrayLike(O).
  3. Let findRec be ? FindViaPredicate(O, len, descending, predicate, thisArg).
  4. Return findRec.[[Index]].
 Note 2

This method is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.12.1 FindViaPredicate ( O, len, direction, predicate, thisArg )

The abstract operation FindViaPredicate takes arguments O (an Object), len (a non-negative integer), direction (ascending or descending), predicate (an ECMAScript language value), and thisArg (an ECMAScript language value) and returns either a normal completion containing a Record with fields [[Index]] (an integral Number) and [[Value]] (an ECMAScript language value) or a throw completion.

O should be an array-like object or a TypedArray. This operation calls predicate once for each element of O, in either ascending index order or descending index order (as indicated by direction), until it finds one where predicate returns a value that coerces to true. At that point, this operation returns a Record that gives the index and value of the element found. If no such element is found, this operation returns a Record that specifies -1𝔽 for the index and undefined for the value.

predicate should be a function. When called for an element of the array, it is passed three arguments: the value of the element, the index of the element, and the object being traversed. Its return value will be coerced to a Boolean value.

thisArg will be used as the this value for each invocation of predicate.

This operation does not directly mutate the object on which it is called, but the object may be mutated by the calls to predicate.

The range of elements processed is set before the first call to predicate, just before the traversal begins. Elements that are appended to the array after this will not be visited by predicate. If existing elements of the array are changed, their value as passed to predicate will be the value at the time that this operation visits them. Elements that are deleted after traversal begins and before being visited are still visited and are either looked up from the prototype or are undefined.

It performs the following steps when called:

  1. If IsCallable(predicate) is false, throw a TypeError exception.
  2. If direction is ascending, then
    1. Let indices be a List of the integers in the interval from 0 (inclusive) to len (exclusive), in ascending order.
  3. Else,
    1. Let indices be a List of the integers in the interval from 0 (inclusive) to len (exclusive), in descending order.
  4. For each integer k of indices, do
    1. Let Pk be ! ToString(𝔽(k)).
    2. NOTE: If O is a TypedArray, the following invocation of Get will return a normal completion.
    3. Let kValue be ? Get(O, Pk).
    4. Let testResult be ? Call(predicate, thisArg, « kValue, 𝔽(k), O »).
    5. If ToBoolean(testResult) is true, return the Record { [[Index]]: 𝔽(k), [[Value]]: kValue }.
  5. Return the Record { [[Index]]: -1𝔽, [[Value]]: undefined }.

23.1.3.13 Array.prototype.flat ( [ depth ] )

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let sourceLen be ? LengthOfArrayLike(O).
  3. Let depthNum be 1.
  4. If depth is not undefined, then
    1. Set depthNum to ? ToIntegerOrInfinity(depth).
    2. If depthNum < 0, set depthNum to 0.
  5. Let A be ? ArraySpeciesCreate(O, 0).
  6. Perform ? FlattenIntoArray(A, O, sourceLen, 0, depthNum).
  7. Return A.

23.1.3.13.1 FlattenIntoArray ( target, source, sourceLen, start, depth [ , mapperFunction [ , thisArg ] ] )

The abstract operation FlattenIntoArray takes arguments target (an Object), source (an Object), sourceLen (a non-negative integer), start (a non-negative integer), and depth (a non-negative integer or ∞) and optional arguments mapperFunction (a function object) and thisArg (an ECMAScript language value) and returns either a normal completion containing a non-negative integer or a throw completion. It performs the following steps when called:

  1. Assert: If mapperFunction is present, then IsCallable(mapperFunction) is true, thisArg is present, and depth is 1.
  2. Let targetIndex be start.
  3. Let sourceIndex be 0𝔽.
  4. Repeat, while ℝ(sourceIndex) < sourceLen,
    1. Let P be ! ToString(sourceIndex).
    2. Let exists be ? HasProperty(source, P).
    3. If exists is true, then
      1. Let element be ? Get(source, P).
      2. If mapperFunction is present, then
        1. Set element to ? Call(mapperFunction, thisArg, « element, sourceIndex, source »).
      3. Let shouldFlatten be false.
      4. If depth > 0, then
        1. Set shouldFlatten to ? IsArray(element).
      5. If shouldFlatten is true, then
        1. If depth = ∞, let newDepth be ∞.
        2. Else, let newDepth be depth - 1.
        3. Let elementLen be ? LengthOfArrayLike(element).
        4. Set targetIndex to ? FlattenIntoArray(target, element, elementLen, targetIndex, newDepth).
      6. Else,
        1. If targetIndex ≥ 253 - 1, throw a TypeError exception.
        2. Perform ? CreateDataPropertyOrThrow(target, ! ToString(𝔽(targetIndex)), element).
        3. Set targetIndex to targetIndex 1.
    4. Set sourceIndex to sourceIndex 1𝔽.
  5. Return targetIndex.

23.1.3.14 Array.prototype.flatMap ( mapperFunction [ , thisArg ] )

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let sourceLen be ? LengthOfArrayLike(O).
  3. If IsCallable(mapperFunction) is false, throw a TypeError exception.
  4. Let A be ? ArraySpeciesCreate(O, 0).
  5. Perform ? FlattenIntoArray(A, O, sourceLen, 0, 1, mapperFunction, thisArg).
  6. Return A.

23.1.3.15 Array.prototype.forEach ( callbackfn [ , thisArg ] )

Note 1

callbackfn should be a function that accepts three arguments. forEach calls callbackfn once for each element present in the array, in ascending order. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.

If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.

forEach does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by forEach is set before the first call to callbackfn. Elements which are appended to the array after the call to forEach begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time forEach visits them; elements that are deleted after the call to forEach begins and before being visited are not visited.

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let len be ? LengthOfArrayLike(O).
  3. If IsCallable(callbackfn) is false, throw a TypeError exception.
  4. Let k be 0.
  5. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kPresent be ? HasProperty(O, Pk).
    3. If kPresent is true, then
      1. Let kValue be ? Get(O, Pk).
      2. Perform ? Call(callbackfn, thisArg, « kValue, 𝔽(k), O »).
    4. Set k to k 1.
  6. Return undefined.
 Note 2

This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.16 Array.prototype.includes ( searchElement [ , fromIndex ] )

Note 1

This method compares searchElement to the elements of the array, in ascending order, using the SameValueZero algorithm, and if found at any position, returns true; otherwise, it returns false.

The optional second argument fromIndex defaults to 0𝔽 (i.e. the whole array is searched). If it is greater than or equal to the length of the array, false is returned, i.e. the array will not be searched. If it is less than -0𝔽, it is used as the offset from the end of the array to compute fromIndex. If the computed index is less than or equal to 0𝔽, the whole array will be searched.

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let len be ? LengthOfArrayLike(O).
  3. If len = 0, return false.
  4. Let n be ? ToIntegerOrInfinity(fromIndex).
  5. Assert: If fromIndex is undefined, then n is 0.
  6. If n = ∞, return false.
  7. Else if n = -∞, set n to 0.
  8. If n ≥ 0, then
    1. Let k be n.
  9. Else,
    1. Let k be len n.
    2. If k < 0, set k to 0.
  10. Repeat, while k < len,
    1. Let elementK be ? Get(O, ! ToString(𝔽(k))).
    2. If SameValueZero(searchElement, elementK) is true, return true.
    3. Set k to k 1.
  11. Return false.
 Note 2

This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.

 Note 3

This method intentionally differs from the similar indexOf method in two ways. First, it uses the SameValueZero algorithm, instead of IsStrictlyEqual, allowing it to detect NaN array elements. Second, it does not skip missing array elements, instead treating them as undefined.

23.1.3.17 Array.prototype.indexOf ( searchElement [ , fromIndex ] )

This method compares searchElement to the elements of the array, in ascending order, using the IsStrictlyEqual algorithm, and if found at one or more indices, returns the smallest such index; otherwise, it returns -1𝔽.

Note 1

The optional second argument fromIndex defaults to 0𝔽 (i.e. the whole array is searched). If it is greater than or equal to the length of the array, -1𝔽 is returned, i.e. the array will not be searched. If it is less than -0𝔽, it is used as the offset from the end of the array to compute fromIndex. If the computed index is less than or equal to 0𝔽, the whole array will be searched.

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let len be ? LengthOfArrayLike(O).
  3. If len = 0, return -1𝔽.
  4. Let n be ? ToIntegerOrInfinity(fromIndex).
  5. Assert: If fromIndex is undefined, then n is 0.
  6. If n = ∞, return -1𝔽.
  7. Else if n = -∞, set n to 0.
  8. If n ≥ 0, then
    1. Let k be n.
  9. Else,
    1. Let k be len n.
    2. If k < 0, set k to 0.
  10. Repeat, while k < len,
    1. Let kPresent be ? HasProperty(O, ! ToString(𝔽(k))).
    2. If kPresent is true, then
      1. Let elementK be ? Get(O, ! ToString(𝔽(k))).
      2. If IsStrictlyEqual(searchElement, elementK) is true, return 𝔽(k).
    3. Set k to k 1.
  11. Return -1𝔽.
 Note 2

This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.18 Array.prototype.join ( separator )

This method converts the elements of the array to Strings, and then concatenates these Strings, separated by occurrences of the separator. If no separator is provided, a single comma is used as the separator.

It performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let len be ? LengthOfArrayLike(O).
  3. If separator is undefined, let sep be ",".
  4. Else, let sep be ? ToString(separator).
  5. Let R be the empty String.
  6. Let k be 0.
  7. Repeat, while k < len,
    1. If k > 0, set R to the string-concatenation of R and sep.
    2. Let element be ? Get(O, ! ToString(𝔽(k))).
    3. If element is either undefined or null, let next be the empty String; otherwise, let next be ? ToString(element).
    4. Set R to the string-concatenation of R and next.
    5. Set k to k 1.
  8. Return R.
 Note

This method is intentionally generic; it does not require that its this value be an Array. Therefore, it can be transferred to other kinds of objects for use as a method.

23.1.3.19 Array.prototype.keys ( )

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Return CreateArrayIterator(O, key).

23.1.3.20 Array.prototype.lastIndexOf ( searchElement [ , fromIndex ] )

Note 1

This method compares searchElement to the elements of the array in descending order using the IsStrictlyEqual algorithm, and if found at one or more indices, returns the largest such index; otherwise, it returns -1𝔽.

The optional second argument fromIndex defaults to the array's length minus one (i.e. the whole array is searched). If it is greater than or equal to the length of the array, the whole array will be searched. If it is less than -0𝔽, it is used as the offset from the end of the array to compute fromIndex. If the computed index is less than or equal to 0𝔽, -1𝔽 is returned.

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let len be ? LengthOfArrayLike(O).
  3. If len = 0, return -1𝔽.
  4. If fromIndex is present, let n be ? ToIntegerOrInfinity(fromIndex); else let n be len - 1.
  5. If n = -∞, return -1𝔽.
  6. If n ≥ 0, then
    1. Let k be min(n, len - 1).
  7. Else,
    1. Let k be len n.
  8. Repeat, while k ≥ 0,
    1. Let kPresent be ? HasProperty(O, ! ToString(𝔽(k))).
    2. If kPresent is true, then
      1. Let elementK be ? Get(O, ! ToString(𝔽(k))).
      2. If IsStrictlyEqual(searchElement, elementK) is true, return 𝔽(k).
    3. Set k to k - 1.
  9. Return -1𝔽.
 Note 2

This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.21 Array.prototype.map ( callbackfn [ , thisArg ] )

Note 1

callbackfn should be a function that accepts three arguments. map calls callbackfn once for each element in the array, in ascending order, and constructs a new Array from the results. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.

If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.

map does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by map is set before the first call to callbackfn. Elements which are appended to the array after the call to map begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time map visits them; elements that are deleted after the call to map begins and before being visited are not visited.

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let len be ? LengthOfArrayLike(O).
  3. If IsCallable(callbackfn) is false, throw a TypeError exception.
  4. Let A be ? ArraySpeciesCreate(O, len).
  5. Let k be 0.
  6. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kPresent be ? HasProperty(O, Pk).
    3. If kPresent is true, then
      1. Let kValue be ? Get(O, Pk).
      2. Let mappedValue be ? Call(callbackfn, thisArg, « kValue, 𝔽(k), O »).
      3. Perform ? CreateDataPropertyOrThrow(A, Pk, mappedValue).
    4. Set k to k 1.
  7. Return A.
 Note 2

This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.22 Array.prototype.pop ( )

Note 1

This method removes the last element of the array and returns it.

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let len be ? LengthOfArrayLike(O).
  3. If len = 0, then
    1. Perform ? Set(O, "length", 0𝔽, true).
    2. Return undefined.
  4. Else,
    1. Assert: len > 0.
    2. Let newLen be 𝔽(len - 1).
    3. Let index be ! ToString(newLen).
    4. Let element be ? Get(O, index).
    5. Perform ? DeletePropertyOrThrow(O, index).
    6. Perform ? Set(O, "length", newLen, true).
    7. Return element.
 Note 2

This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.23 Array.prototype.push ( ...items )

Note 1

This method appends the arguments to the end of the array, in the order in which they appear. It returns the new length of the array.

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let len be ? LengthOfArrayLike(O).
  3. Let argCount be the number of elements in items.
  4. If len argCount > 253 - 1, throw a TypeError exception.
  5. For each element E of items, do
    1. Perform ? Set(O, ! ToString(𝔽(len)), E, true).
    2. Set len to len 1.
  6. Perform ? Set(O, "length", 𝔽(len), true).
  7. Return 𝔽(len).

The "length" property of this method is 1𝔽.

Note 2

This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.24 Array.prototype.reduce ( callbackfn [ , initialValue ] )

Note 1

callbackfn should be a function that takes four arguments. reduce calls the callback, as a function, once for each element after the first element present in the array, in ascending order.

callbackfn is called with four arguments: the previousValue (value from the previous call to callbackfn), the currentValue (value of the current element), the currentIndex, and the object being traversed. The first time that callback is called, the previousValue and currentValue can be one of two values. If an initialValue was supplied in the call to reduce, then previousValue will be initialValue and currentValue will be the first value in the array. If no initialValue was supplied, then previousValue will be the first value in the array and currentValue will be the second. It is a TypeError if the array contains no elements and initialValue is not provided.

reduce does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by reduce is set before the first call to callbackfn. Elements that are appended to the array after the call to reduce begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time reduce visits them; elements that are deleted after the call to reduce begins and before being visited are not visited.

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let len be ? LengthOfArrayLike(O).
  3. If IsCallable(callbackfn) is false, throw a TypeError exception.
  4. If len = 0 and initialValue is not present, throw a TypeError exception.
  5. Let k be 0.
  6. Let accumulator be undefined.
  7. If initialValue is present, then
    1. Set accumulator to initialValue.
  8. Else,
    1. Let kPresent be false.
    2. Repeat, while kPresent is false and k < len,
      1. Let Pk be ! ToString(𝔽(k)).
      2. Set kPresent to ? HasProperty(O, Pk).
      3. If kPresent is true, then
        1. Set accumulator to ? Get(O, Pk).
      4. Set k to k 1.
    3. If kPresent is false, throw a TypeError exception.
  9. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kPresent be ? HasProperty(O, Pk).
    3. If kPresent is true, then
      1. Let kValue be ? Get(O, Pk).
      2. Set accumulator to ? Call(callbackfn, undefined, « accumulator, kValue, 𝔽(k), O »).
    4. Set k to k 1.
  10. Return accumulator.
 Note 2

This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.25 Array.prototype.reduceRight ( callbackfn [ , initialValue ] )

Note 1

callbackfn should be a function that takes four arguments. reduceRight calls the callback, as a function, once for each element after the first element present in the array, in descending order.

callbackfn is called with four arguments: the previousValue (value from the previous call to callbackfn), the currentValue (value of the current element), the currentIndex, and the object being traversed. The first time the function is called, the previousValue and currentValue can be one of two values. If an initialValue was supplied in the call to reduceRight, then previousValue will be initialValue and currentValue will be the last value in the array. If no initialValue was supplied, then previousValue will be the last value in the array and currentValue will be the second-to-last value. It is a TypeError if the array contains no elements and initialValue is not provided.

reduceRight does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by reduceRight is set before the first call to callbackfn. Elements that are appended to the array after the call to reduceRight begins will not be visited by callbackfn. If existing elements of the array are changed by callbackfn, their value as passed to callbackfn will be the value at the time reduceRight visits them; elements that are deleted after the call to reduceRight begins and before being visited are not visited.

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let len be ? LengthOfArrayLike(O).
  3. If IsCallable(callbackfn) is false, throw a TypeError exception.
  4. If len = 0 and initialValue is not present, throw a TypeError exception.
  5. Let k be len - 1.
  6. Let accumulator be undefined.
  7. If initialValue is present, then
    1. Set accumulator to initialValue.
  8. Else,
    1. Let kPresent be false.
    2. Repeat, while kPresent is false and k ≥ 0,
      1. Let Pk be ! ToString(𝔽(k)).
      2. Set kPresent to ? HasProperty(O, Pk).
      3. If kPresent is true, then
        1. Set accumulator to ? Get(O, Pk).
      4. Set k to k - 1.
    3. If kPresent is false, throw a TypeError exception.
  9. Repeat, while k ≥ 0,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kPresent be ? HasProperty(O, Pk).
    3. If kPresent is true, then
      1. Let kValue be ? Get(O, Pk).
      2. Set accumulator to ? Call(callbackfn, undefined, « accumulator, kValue, 𝔽(k), O »).
    4. Set k to k - 1.
  10. Return accumulator.
 Note 2

This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.26 Array.prototype.reverse ( )

Note 1

This method rearranges the elements of the array so as to reverse their order. It returns the object as the result of the call.

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let len be ? LengthOfArrayLike(O).
  3. Let middle be floor(len / 2).
  4. Let lower be 0.
  5. Repeat, while lower ≠ middle,
    1. Let upper be len - lower - 1.
    2. Let upperP be ! ToString(𝔽(upper)).
    3. Let lowerP be ! ToString(𝔽(lower)).
    4. Let lowerExists be ? HasProperty(O, lowerP).
    5. If lowerExists is true, then
      1. Let lowerValue be ? Get(O, lowerP).
    6. Let upperExists be ? HasProperty(O, upperP).
    7. If upperExists is true, then
      1. Let upperValue be ? Get(O, upperP).
    8. If lowerExists is true and upperExists is true, then
      1. Perform ? Set(O, lowerP, upperValue, true).
      2. Perform ? Set(O, upperP, lowerValue, true).
    9. Else if lowerExists is false and upperExists is true, then
      1. Perform ? Set(O, lowerP, upperValue, true).
      2. Perform ? DeletePropertyOrThrow(O, upperP).
    10. Else if lowerExists is true and upperExists is false, then
      1. Perform ? DeletePropertyOrThrow(O, lowerP).
      2. Perform ? Set(O, upperP, lowerValue, true).
    11. Else,
      1. Assert: lowerExists and upperExists are both false.
      2. NOTE: No action is required.
    12. Set lower to lower 1.
  6. Return O.
 Note 2

This method is intentionally generic; it does not require that its this value be an Array. Therefore, it can be transferred to other kinds of objects for use as a method.

23.1.3.27 Array.prototype.shift ( )

This method removes the first element of the array and returns it.

It performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let len be ? LengthOfArrayLike(O).
  3. If len = 0, then
    1. Perform ? Set(O, "length", 0𝔽, true).
    2. Return undefined.
  4. Let first be ? Get(O, "0").
  5. Let k be 1.
  6. Repeat, while k < len,
    1. Let from be ! ToString(𝔽(k)).
    2. Let to be ! ToString(𝔽(k - 1)).
    3. Let fromPresent be ? HasProperty(O, from).
    4. If fromPresent is true, then
      1. Let fromVal be ? Get(O, from).
      2. Perform ? Set(O, to, fromVal, true).
    5. Else,
      1. Assert: fromPresent is false.
      2. Perform ? DeletePropertyOrThrow(O, to).
    6. Set k to k 1.
  7. Perform ? DeletePropertyOrThrow(O, ! ToString(𝔽(len - 1))).
  8. Perform ? Set(O, "length", 𝔽(len - 1), true).
  9. Return first.
 Note

This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.28 Array.prototype.slice ( start, end )

This method returns an array containing the elements of the array from element start up to, but not including, element end (or through the end of the array if end is undefined). If start is negative, it is treated as length start where length is the length of the array. If end is negative, it is treated as length end where length is the length of the array.

It performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let len be ? LengthOfArrayLike(O).
  3. Let relativeStart be ? ToIntegerOrInfinity(start).
  4. If relativeStart = -∞, let k be 0.
  5. Else if relativeStart < 0, let k be max(len relativeStart, 0).
  6. Else, let k be min(relativeStart, len).
  7. If end is undefined, let relativeEnd be len; else let relativeEnd be ? ToIntegerOrInfinity(end).
  8. If relativeEnd = -∞, let final be 0.
  9. Else if relativeEnd < 0, let final be max(len relativeEnd, 0).
  10. Else, let final be min(relativeEnd, len).
  11. Let count be max(final - k, 0).
  12. Let A be ? ArraySpeciesCreate(O, count).
  13. Let n be 0.
  14. Repeat, while k < final,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kPresent be ? HasProperty(O, Pk).
    3. If kPresent is true, then
      1. Let kValue be ? Get(O, Pk).
      2. Perform ? CreateDataPropertyOrThrow(A, ! ToString(𝔽(n)), kValue).
    4. Set k to k 1.
    5. Set n to n 1.
  15. Perform ? Set(A, "length", 𝔽(n), true).
  16. Return A.
 Note 1

The explicit setting of the "length" property of the result Array in step 15 was necessary in previous editions of ECMAScript to ensure that its length was correct in situations where the trailing elements of the result Array were not present. Setting "length" became unnecessary starting in ES2015 when the result Array was initialized to its proper length rather than an empty Array but is carried forward to preserve backward compatibility.

 Note 2

This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.29 Array.prototype.some ( callbackfn [ , thisArg ] )

Note 1

callbackfn should be a function that accepts three arguments and returns a value that is coercible to a Boolean value. some calls callbackfn once for each element present in the array, in ascending order, until it finds one where callbackfn returns true. If such an element is found, some immediately returns true. Otherwise, some returns false. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.

If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.

some does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by some is set before the first call to callbackfn. Elements that are appended to the array after the call to some begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time that some visits them; elements that are deleted after the call to some begins and before being visited are not visited. some acts like the "exists" quantifier in mathematics. In particular, for an empty array, it returns false.

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let len be ? LengthOfArrayLike(O).
  3. If IsCallable(callbackfn) is false, throw a TypeError exception.
  4. Let k be 0.
  5. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kPresent be ? HasProperty(O, Pk).
    3. If kPresent is true, then
      1. Let kValue be ? Get(O, Pk).
      2. Let testResult be ToBoolean(? Call(callbackfn, thisArg, « kValue, 𝔽(k), O »)).
      3. If testResult is true, return true.
    4. Set k to k 1.
  6. Return false.
 Note 2

This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.30 Array.prototype.sort ( comparefn )

This method sorts the elements of this array. The sort must be stable (that is, elements that compare equal must remain in their original order). If comparefn is not undefined, it should be a function that accepts two arguments x and y and returns a negative Number if x < y, a positive Number if x > y, or a zero otherwise.

It performs the following steps when called:

  1. If comparefn is not undefined and IsCallable(comparefn) is false, throw a TypeError exception.
  2. Let obj be ? ToObject(this value).
  3. Let len be ? LengthOfArrayLike(obj).
  4. Let SortCompare be a new Abstract Closure with parameters (x, y) that captures comparefn and performs the following steps when called:
    1. Return ? CompareArrayElements(x, y, comparefn).
  5. Let sortedList be ? SortIndexedProperties(obj, len, SortCompare, skip-holes).
  6. Let itemCount be the number of elements in sortedList.
  7. Let j be 0.
  8. Repeat, while j < itemCount,
    1. Perform ? Set(obj, ! ToString(𝔽(j)), sortedList[j], true).
    2. Set j to j 1.
  9. NOTE: The call to SortIndexedProperties in step 5 uses skip-holes. The remaining indices are deleted to preserve the number of holes that were detected and excluded from the sort.
  10. Repeat, while j < len,
    1. Perform ? DeletePropertyOrThrow(obj, ! ToString(𝔽(j))).
    2. Set j to j 1.
  11. Return obj.
 Note 1

Because non-existent property values always compare greater than undefined property values, and undefined always compares greater than any other value (see CompareArrayElements), undefined property values always sort to the end of the result, followed by non-existent property values.

 Note 2

Method calls performed by the ToString abstract operations in steps 5 and 6 have the potential to cause SortCompare to not behave as a consistent comparator.

 Note 3

This method is intentionally generic; it does not require that its this value be an Array. Therefore, it can be transferred to other kinds of objects for use as a method.

23.1.3.30.1 SortIndexedProperties ( obj, len, SortCompare, holes )

The abstract operation SortIndexedProperties takes arguments obj (an Object), len (a non-negative integer), SortCompare (an Abstract Closure with two parameters), and holes (skip-holes or read-through-holes) and returns either a normal completion containing a List of ECMAScript language values or a throw completion. It performs the following steps when called:

  1. Let items be a new empty List.
  2. Let k be 0.
  3. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. If holes is skip-holes, then
      1. Let kRead be ? HasProperty(obj, Pk).
    3. Else,
      1. Assert: holes is read-through-holes.
      2. Let kRead be true.
    4. If kRead is true, then
      1. Let kValue be ? Get(obj, Pk).
      2. Append kValue to items.
    5. Set k to k 1.
  4. Sort items using an implementation-defined sequence of calls to SortCompare. If any such call returns an abrupt completion, stop before performing any further calls to SortCompare and return that Completion Record.
  5. Return items.

The sort order is the ordering of items after completion of step 4 of the algorithm above. The sort order is implementation-defined if SortCompare is not a consistent comparator for the elements of items. When SortIndexedProperties is invoked by Array.prototype.sort, the sort order is also implementation-defined if comparefn is undefined, and all applications of ToString, to any specific value passed as an argument to SortCompare, do not produce the same result.

Unless the sort order is specified to be implementation-defined, it must satisfy all of the following conditions:

  • There must be some mathematical permutation π of the non-negative integers less than itemCount, such that for every non-negative integer j less than itemCount, the element old[j] is exactly the same as new[π(j)].
  • Then for all non-negative integers j and k, each less than itemCount, if ℝ(SortCompare(old[j], old[k])) < 0, then π(j) < π(k).

Here the notation old[j] is used to refer to items[j] before step 4 is executed, and the notation new[j] to refer to items[j] after step 4 has been executed.

An abstract closure or function comparator is a consistent comparator for a set of values S if all of the requirements below are met for all values a, b, and c (possibly the same value) in the set S: The notation a <C b means ℝ(comparator(a, b)) < 0; a =C b means ℝ(comparator(a, b)) = 0; and a >C b means ℝ(comparator(a, b)) > 0.

  • Calling comparator(a, b) always returns the same value v when given a specific pair of values a and b as its two arguments. Furthermore, v is a Number, and v is not NaN. Note that this implies that exactly one of a <C b, a =C b, and a >C b will be true for a given pair of a and b.
  • Calling comparator(a, b) does not modify obj or any object on obj's prototype chain.
  • a =C a (reflexivity)
  • If a =C b, then b =C a (symmetry)
  • If a =C b and b =C c, then a =C c (transitivity of =C)
  • If a <C b and b <C c, then a <C c (transitivity of <C)
  • If a >C b and b >C c, then a >C c (transitivity of >C)
Note

The above conditions are necessary and sufficient to ensure that comparator divides the set S into equivalence classes and that these equivalence classes are totally ordered.

23.1.3.30.2 CompareArrayElements ( x, y, comparefn )

The abstract operation CompareArrayElements takes arguments x (an ECMAScript language value), y (an ECMAScript language value), and comparefn (a function object or undefined) and returns either a normal completion containing a Number or an abrupt completion. It performs the following steps when called:

  1. If x and y are both undefined, return 0𝔽.
  2. If x is undefined, return 1𝔽.
  3. If y is undefined, return -1𝔽.
  4. If comparefn is not undefined, then
    1. Let v be ? ToNumber(? Call(comparefn, undefined, « x, y »)).
    2. If v is NaN, return 0𝔽.
    3. Return v.
  5. Let xString be ? ToString(x).
  6. Let yString be ? ToString(y).
  7. Let xSmaller be ! IsLessThan(xString, yString, true).
  8. If xSmaller is true, return -1𝔽.
  9. Let ySmaller be ! IsLessThan(yString, xString, true).
  10. If ySmaller is true, return 1𝔽.
  11. Return 0𝔽.

23.1.3.31 Array.prototype.splice ( start, deleteCount, ...items )

Note 1

This method deletes the deleteCount elements of the array starting at integer index start and replaces them with the elements of items. It returns an Array containing the deleted elements (if any).

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let len be ? LengthOfArrayLike(O).
  3. Let relativeStart be ? ToIntegerOrInfinity(start).
  4. If relativeStart = -∞, let actualStart be 0.
  5. Else if relativeStart < 0, let actualStart be max(len relativeStart, 0).
  6. Else, let actualStart be min(relativeStart, len).
  7. Let itemCount be the number of elements in items.
  8. If start is not present, then
    1. Let actualDeleteCount be 0.
  9. Else if deleteCount is not present, then
    1. Let actualDeleteCount be len - actualStart.
  10. Else,
    1. Let dc be ? ToIntegerOrInfinity(deleteCount).
    2. Let actualDeleteCount be the result of clamping dc between 0 and len - actualStart.
  11. If len itemCount - actualDeleteCount > 253 - 1, throw a TypeError exception.
  12. Let A be ? ArraySpeciesCreate(O, actualDeleteCount).
  13. Let k be 0.
  14. Repeat, while k < actualDeleteCount,
    1. Let from be ! ToString(𝔽(actualStart k)).
    2. If ? HasProperty(O, from) is true, then
      1. Let fromValue be ? Get(O, from).
      2. Perform ? CreateDataPropertyOrThrow(A, ! ToString(𝔽(k)), fromValue).
    3. Set k to k 1.
  15. Perform ? Set(A, "length", 𝔽(actualDeleteCount), true).
  16. If itemCount < actualDeleteCount, then
    1. Set k to actualStart.
    2. Repeat, while k < (len - actualDeleteCount),
      1. Let from be ! ToString(𝔽(k actualDeleteCount)).
      2. Let to be ! ToString(𝔽(k itemCount)).
      3. If ? HasProperty(O, from) is true, then
        1. Let fromValue be ? Get(O, from).
        2. Perform ? Set(O, to, fromValue, true).
      4. Else,
        1. Perform ? DeletePropertyOrThrow(O, to).
      5. Set k to k 1.
    3. Set k to len.
    4. Repeat, while k > (len - actualDeleteCount itemCount),
      1. Perform ? DeletePropertyOrThrow(O, ! ToString(𝔽(k - 1))).
      2. Set k to k - 1.
  17. Else if itemCount > actualDeleteCount, then
    1. Set k to (len - actualDeleteCount).
    2. Repeat, while k > actualStart,
      1. Let from be ! ToString(𝔽(k actualDeleteCount - 1)).
      2. Let to be ! ToString(𝔽(k itemCount - 1)).
      3. If ? HasProperty(O, from) is true, then
        1. Let fromValue be ? Get(O, from).
        2. Perform ? Set(O, to, fromValue, true).
      4. Else,
        1. Perform ? DeletePropertyOrThrow(O, to).
      5. Set k to k - 1.
  18. Set k to actualStart.
  19. For each element E of items, do
    1. Perform ? Set(O, ! ToString(𝔽(k)), E, true).
    2. Set k to k 1.
  20. Perform ? Set(O, "length", 𝔽(len - actualDeleteCount itemCount), true).
  21. Return A.
 Note 2

The explicit setting of the "length" property of the result Array in step 20 was necessary in previous editions of ECMAScript to ensure that its length was correct in situations where the trailing elements of the result Array were not present. Setting "length" became unnecessary starting in ES2015 when the result Array was initialized to its proper length rather than an empty Array but is carried forward to preserve backward compatibility.

 Note 3

This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.32 Array.prototype.toLocaleString ( [ reserved1 [ , reserved2 ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of this method is used.

Note 1

The first edition of ECMA-402 did not include a replacement specification for this method.

The meanings of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.

This method performs the following steps when called:

  1. Let array be ? ToObject(this value).
  2. Let len be ? LengthOfArrayLike(array).
  3. Let separator be the implementation-defined list-separator String value appropriate for the host environment's current locale (such as ", ").
  4. Let R be the empty String.
  5. Let k be 0.
  6. Repeat, while k < len,
    1. If k > 0, then
      1. Set R to the string-concatenation of R and separator.
    2. Let nextElement be ? Get(array, ! ToString(𝔽(k))).
    3. If nextElement is neither undefined nor null, then
      1. Let S be ? ToString(? Invoke(nextElement, "toLocaleString")).
      2. Set R to the string-concatenation of R and S.
    4. Set k to k 1.
  7. Return R.
 Note 2

This method converts the elements of the array to Strings using their toLocaleString methods, and then concatenates these Strings, separated by occurrences of an implementation-defined locale-sensitive separator String. This method is analogous to toString except that it is intended to yield a locale-sensitive result corresponding with conventions of the host environment's current locale.

 Note 3

This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.33 Array.prototype.toReversed ( )

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let len be ? LengthOfArrayLike(O).
  3. Let A be ? ArrayCreate(len).
  4. Let k be 0.
  5. Repeat, while k < len,
    1. Let from be ! ToString(𝔽(len - k - 1)).
    2. Let Pk be ! ToString(𝔽(k)).
    3. Let fromValue be ? Get(O, from).
    4. Perform ! CreateDataPropertyOrThrow(A, Pk, fromValue).
    5. Set k to k 1.
  6. Return A.

23.1.3.34 Array.prototype.toSorted ( comparefn )

This method performs the following steps when called:

  1. If comparefn is not undefined and IsCallable(comparefn) is false, throw a TypeError exception.
  2. Let O be ? ToObject(this value).
  3. Let len be ? LengthOfArrayLike(O).
  4. Let A be ? ArrayCreate(len).
  5. Let SortCompare be a new Abstract Closure with parameters (x, y) that captures comparefn and performs the following steps when called:
    1. Return ? CompareArrayElements(x, y, comparefn).
  6. Let sortedList be ? SortIndexedProperties(O, len, SortCompare, read-through-holes).
  7. Let j be 0.
  8. Repeat, while j < len,
    1. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(j)), sortedList[j]).
    2. Set j to j 1.
  9. Return A.

23.1.3.35 Array.prototype.toSpliced ( start, skipCount, ...items )

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let len be ? LengthOfArrayLike(O).
  3. Let relativeStart be ? ToIntegerOrInfinity(start).
  4. If relativeStart is -∞, let actualStart be 0.
  5. Else if relativeStart < 0, let actualStart be max(len relativeStart, 0).
  6. Else, let actualStart be min(relativeStart, len).
  7. Let insertCount be the number of elements in items.
  8. If start is not present, then
    1. Let actualSkipCount be 0.
  9. Else if skipCount is not present, then
    1. Let actualSkipCount be len - actualStart.
  10. Else,
    1. Let sc be ? ToIntegerOrInfinity(skipCount).
    2. Let actualSkipCount be the result of clamping sc between 0 and len - actualStart.
  11. Let newLen be len insertCount - actualSkipCount.
  12. If newLen > 253 - 1, throw a TypeError exception.
  13. Let A be ? ArrayCreate(newLen).
  14. Let i be 0.
  15. Let r be actualStart actualSkipCount.
  16. Repeat, while i < actualStart,
    1. Let Pi be ! ToString(𝔽(i)).
    2. Let iValue be ? Get(O, Pi).
    3. Perform ! CreateDataPropertyOrThrow(A, Pi, iValue).
    4. Set i to i 1.
  17. For each element E of items, do
    1. Let Pi be ! ToString(𝔽(i)).
    2. Perform ! CreateDataPropertyOrThrow(A, Pi, E).
    3. Set i to i 1.
  18. Repeat, while i < newLen,
    1. Let Pi be ! ToString(𝔽(i)).
    2. Let from be ! ToString(𝔽(r)).
    3. Let fromValue be ? Get(O, from).
    4. Perform ! CreateDataPropertyOrThrow(A, Pi, fromValue).
    5. Set i to i 1.
    6. Set r to r 1.
  19. Return A.

23.1.3.36 Array.prototype.toString ( )

This method performs the following steps when called:

  1. Let array be ? ToObject(this value).
  2. Let func be ? Get(array, "join").
  3. If IsCallable(func) is false, set func to the intrinsic function %Object.prototype.toString%.
  4. Return ? Call(func, array).
 Note

This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.37 Array.prototype.unshift ( ...items )

This method prepends the arguments to the start of the array, such that their order within the array is the same as the order in which they appear in the argument list.

It performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let len be ? LengthOfArrayLike(O).
  3. Let argCount be the number of elements in items.
  4. If argCount > 0, then
    1. If len argCount > 253 - 1, throw a TypeError exception.
    2. Let k be len.
    3. Repeat, while k > 0,
      1. Let from be ! ToString(𝔽(k - 1)).
      2. Let to be ! ToString(𝔽(k argCount - 1)).
      3. Let fromPresent be ? HasProperty(O, from).
      4. If fromPresent is true, then
        1. Let fromValue be ? Get(O, from).
        2. Perform ? Set(O, to, fromValue, true).
      5. Else,
        1. Assert: fromPresent is false.
        2. Perform ? DeletePropertyOrThrow(O, to).
      6. Set k to k - 1.
    4. Let j be 0𝔽.
    5. For each element E of items, do
      1. Perform ? Set(O, ! ToString(j), E, true).
      2. Set j to j 1𝔽.
  5. Perform ? Set(O, "length", 𝔽(len argCount), true).
  6. Return 𝔽(len argCount).

The "length" property of this method is 1𝔽.

Note

This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.38 Array.prototype.values ( )

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Return CreateArrayIterator(O, value).

23.1.3.39 Array.prototype.with ( index, value )

This method performs the following steps when called:

  1. Let O be ? ToObject(this value).
  2. Let len be ? LengthOfArrayLike(O).
  3. Let relativeIndex be ? ToIntegerOrInfinity(index).
  4. If relativeIndex ≥ 0, let actualIndex be relativeIndex.
  5. Else, let actualIndex be len relativeIndex.
  6. If actualIndex ≥ len or actualIndex < 0, throw a RangeError exception.
  7. Let A be ? ArrayCreate(len).
  8. Let k be 0.
  9. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. If k is actualIndex, let fromValue be value.
    3. Else, let fromValue be ? Get(O, Pk).
    4. Perform ! CreateDataPropertyOrThrow(A, Pk, fromValue).
    5. Set k to k 1.
  10. Return A.

23.1.3.40 Array.prototype [ @@iterator ] ( )

The initial value of the @@iterator property is %Array.prototype.values%, defined in 23.1.3.38.

23.1.3.41 Array.prototype [ @@unscopables ]

The initial value of the @@unscopables data property is an object created by the following steps:

  1. Let unscopableList be OrdinaryObjectCreate(null).
  2. Perform ! CreateDataPropertyOrThrow(unscopableList, "at", true).
  3. Perform ! CreateDataPropertyOrThrow(unscopableList, "copyWithin", true).
  4. Perform ! CreateDataPropertyOrThrow(unscopableList, "entries", true).
  5. Perform ! CreateDataPropertyOrThrow(unscopableList, "fill", true).
  6. Perform ! CreateDataPropertyOrThrow(unscopableList, "find", true).
  7. Perform ! CreateDataPropertyOrThrow(unscopableList, "findIndex", true).
  8. Perform ! CreateDataPropertyOrThrow(unscopableList, "findLast", true).
  9. Perform ! CreateDataPropertyOrThrow(unscopableList, "findLastIndex", true).
  10. Perform ! CreateDataPropertyOrThrow(unscopableList, "flat", true).
  11. Perform ! CreateDataPropertyOrThrow(unscopableList, "flatMap", true).
  12. Perform ! CreateDataPropertyOrThrow(unscopableList, "includes", true).
  13. Perform ! CreateDataPropertyOrThrow(unscopableList, "keys", true).
  14. Perform ! CreateDataPropertyOrThrow(unscopableList, "toReversed", true).
  15. Perform ! CreateDataPropertyOrThrow(unscopableList, "toSorted", true).
  16. Perform ! CreateDataPropertyOrThrow(unscopableList, "toSpliced", true).
  17. Perform ! CreateDataPropertyOrThrow(unscopableList, "values", true).
  18. Return unscopableList.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

Note

The own property names of this object are property names that were not included as standard properties of Array.prototype prior to the ECMAScript 2015 specification. These names are ignored for with statement binding purposes in order to preserve the behaviour of existing code that might use one of these names as a binding in an outer scope that is shadowed by a with statement whose binding object is an Array.

The reason that "with" is not included in the unscopableList is because it is already a reserved word.

23.1.4 Properties of Array Instances

Array instances are Array exotic objects and have the internal methods specified for such objects. Array instances inherit properties from the Array prototype object.

Array instances have a "length" property, and a set of enumerable properties with array index names.

23.1.4.1 length

The "length" property of an Array instance is a data property whose value is always numerically greater than the name of every configurable own property whose name is an array index.

The "length" property initially has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.

Note

Reducing the value of the "length" property has the side-effect of deleting own array elements whose array index is between the old and new length values. However, non-configurable properties can not be deleted. Attempting to set the "length" property of an Array to a value that is numerically less than or equal to the largest numeric own property name of an existing non-configurable array-indexed property of the array will result in the length being set to a numeric value that is one greater than that non-configurable numeric own property name. See 10.4.2.1.

23.1.5 Array Iterator Objects

An Array Iterator is an object, that represents a specific iteration over some specific Array instance object. There is not a named constructor for Array Iterator objects. Instead, Array iterator objects are created by calling certain methods of Array instance objects.

23.1.5.1 CreateArrayIterator ( array, kind )

The abstract operation CreateArrayIterator takes arguments array (an Object) and kind (key value, key, or value) and returns a Generator. It is used to create iterator objects for Array methods that return such iterators. It performs the following steps when called:

  1. Let closure be a new Abstract Closure with no parameters that captures kind and array and performs the following steps when called:
    1. Let index be 0.
    2. Repeat,
      1. If array has a [[TypedArrayName]] internal slot, then
        1. If IsDetachedBuffer(array.[[ViewedArrayBuffer]]) is true, throw a TypeError exception.
        2. Let len be array.[[ArrayLength]].
      2. Else,
        1. Let len be ? LengthOfArrayLike(array).
      3. If index ≥ len, return NormalCompletion(undefined).
      4. If kind is key, perform ? GeneratorYield(CreateIterResultObject(𝔽(index), false)).
      5. Else,
        1. Let elementKey be ! ToString(𝔽(index)).
        2. Let elementValue be ? Get(array, elementKey).
        3. If kind is value, perform ? GeneratorYield(CreateIterResultObject(elementValue, false)).
        4. Else,
          1. Assert: kind is key value.
          2. Let result be CreateArrayFromList(« 𝔽(index), elementValue »).
          3. Perform ? GeneratorYield(CreateIterResultObject(result, false)).
      6. Set index to index 1.
  2. Return CreateIteratorFromClosure(closure, "%ArrayIteratorPrototype%", %ArrayIteratorPrototype%).

23.1.5.2 The %ArrayIteratorPrototype% Object

The %ArrayIteratorPrototype% object:

  • has properties that are inherited by all Array Iterator Objects.
  • is an ordinary object.
  • has a [[Prototype]] internal slot whose value is %IteratorPrototype%.
  • has the following properties:

23.1.5.2.1 %ArrayIteratorPrototype%.next ( )

  1. Return ? GeneratorResume(this value, empty, "%ArrayIteratorPrototype%").

23.1.5.2.2 %ArrayIteratorPrototype% [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "Array Iterator".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

23.2 TypedArray Objects

A TypedArray presents an array-like view of an underlying binary data buffer (25.1). A TypedArray element type is the underlying binary scalar data type that all elements of a TypedArray instance have. There is a distinct TypedArray constructor, listed in Table 68, for each of the supported element types. Each constructor in Table 68 has a corresponding distinct prototype object.

Table 68: The TypedArray Constructors
Constructor Name and Intrinsic Element Type Element Size Conversion Operation Description
Int8Array
%Int8Array% 		Int8 1 ToInt8 8-bit two's complement signed integer
Uint8Array
%Uint8Array% 		Uint8 1 ToUint8 8-bit unsigned integer
Uint8ClampedArray
%Uint8ClampedArray% 		Uint8C 1 ToUint8Clamp 8-bit unsigned integer (clamped conversion)
Int16Array
%Int16Array% 		Int16 2 ToInt16 16-bit two's complement signed integer
Uint16Array
%Uint16Array% 		Uint16 2 ToUint16 16-bit unsigned integer
Int32Array
%Int32Array% 		Int32 4 ToInt32 32-bit two's complement signed integer
Uint32Array
%Uint32Array% 		Uint32 4 ToUint32 32-bit unsigned integer
BigInt64Array
%BigInt64Array% 		BigInt64 8 ToBigInt64 64-bit two's complement signed integer
BigUint64Array
%BigUint64Array% 		BigUint64 8 ToBigUint64 64-bit unsigned integer
Float32Array
%Float32Array% 		Float32 4 32-bit IEEE floating point
Float64Array
%Float64Array% 		Float64 8 64-bit IEEE floating point

In the definitions below, references to TypedArray should be replaced with the appropriate constructor name from the above table.

23.2.1 The %TypedArray% Intrinsic Object

The %TypedArray% intrinsic object:

  • is a constructor function object that all of the TypedArray constructor objects inherit from.
  • along with its corresponding prototype object, provides common properties that are inherited by all TypedArray constructors and their instances.
  • does not have a global name or appear as a property of the global object.
  • acts as the abstract superclass of the various TypedArray constructors.
  • will throw an error when invoked, because it is an abstract class constructor. The TypedArray constructors do not perform a super call to it.

23.2.1.1 %TypedArray% ( )

This function performs the following steps when called:

  1. Throw a TypeError exception.

The "length" property of this function is 0𝔽.

23.2.2 Properties of the %TypedArray% Intrinsic Object

The %TypedArray% intrinsic object:

  • has a [[Prototype]] internal slot whose value is %Function.prototype%.
  • has a "name" property whose value is "TypedArray".
  • has the following properties:

23.2.2.1 %TypedArray%.from ( source [ , mapfn [ , thisArg ] ] )

This method performs the following steps when called:

  1. Let C be the this value.
  2. If IsConstructor(C) is false, throw a TypeError exception.
  3. If mapfn is undefined, let mapping be false.
  4. Else,
    1. If IsCallable(mapfn) is false, throw a TypeError exception.
    2. Let mapping be true.
  5. Let usingIterator be ? GetMethod(source, @@iterator).
  6. If usingIterator is not undefined, then
    1. Let values be ? IteratorToList(? GetIteratorFromMethod(source, usingIterator)).
    2. Let len be the number of elements in values.
    3. Let targetObj be ? TypedArrayCreate(C, « 𝔽(len) »).
    4. Let k be 0.
    5. Repeat, while k < len,
      1. Let Pk be ! ToString(𝔽(k)).
      2. Let kValue be the first element of values.
      3. Remove the first element from values.
      4. If mapping is true, then
        1. Let mappedValue be ? Call(mapfn, thisArg, « kValue, 𝔽(k) »).
      5. Else, let mappedValue be kValue.
      6. Perform ? Set(targetObj, Pk, mappedValue, true).
      7. Set k to k 1.
    6. Assert: values is now an empty List.
    7. Return targetObj.
  7. NOTE: source is not an Iterable so assume it is already an array-like object.
  8. Let arrayLike be ! ToObject(source).
  9. Let len be ? LengthOfArrayLike(arrayLike).
  10. Let targetObj be ? TypedArrayCreate(C, « 𝔽(len) »).
  11. Let k be 0.
  12. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kValue be ? Get(arrayLike, Pk).
    3. If mapping is true, then
      1. Let mappedValue be ? Call(mapfn, thisArg, « kValue, 𝔽(k) »).
    4. Else, let mappedValue be kValue.
    5. Perform ? Set(targetObj, Pk, mappedValue, true).
    6. Set k to k 1.
  13. Return targetObj.

23.2.2.2 %TypedArray%.of ( ...items )

This method performs the following steps when called:

  1. Let len be the number of elements in items.
  2. Let C be the this value.
  3. If IsConstructor(C) is false, throw a TypeError exception.
  4. Let newObj be ? TypedArrayCreate(C, « 𝔽(len) »).
  5. Let k be 0.
  6. Repeat, while k < len,
    1. Let kValue be items[k].
    2. Let Pk be ! ToString(𝔽(k)).
    3. Perform ? Set(newObj, Pk, kValue, true).
    4. Set k to k 1.
  7. Return newObj.

23.2.2.3 %TypedArray%.prototype

The initial value of %TypedArray%.prototype is the %TypedArray% prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

23.2.2.4 get %TypedArray% [ @@species ]

%TypedArray%[@@species] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Return the this value.

The value of the "name" property of this function is "get [Symbol.species]".

Note

%TypedArray.prototype% methods normally use their this value's constructor to create a derived object. However, a subclass constructor may over-ride that default behaviour by redefining its @@species property.

23.2.3 Properties of the %TypedArray% Prototype Object

The %TypedArray% prototype object:

  • has a [[Prototype]] internal slot whose value is %Object.prototype%.
  • is %TypedArray.prototype%.
  • is an ordinary object.
  • does not have a [[ViewedArrayBuffer]] or any other of the internal slots that are specific to TypedArray instance objects.

23.2.3.1 %TypedArray%.prototype.at ( index )

  1. Let O be the this value.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. Let relativeIndex be ? ToIntegerOrInfinity(index).
  5. If relativeIndex ≥ 0, then
    1. Let k be relativeIndex.
  6. Else,
    1. Let k be len relativeIndex.
  7. If k < 0 or k ≥ len, return undefined.
  8. Return ! Get(O, ! ToString(𝔽(k))).

23.2.3.2 get %TypedArray%.prototype.buffer

%TypedArray%.prototype.buffer is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? RequireInternalSlot(O, [[TypedArrayName]]).
  3. Assert: O has a [[ViewedArrayBuffer]] internal slot.
  4. Let buffer be O.[[ViewedArrayBuffer]].
  5. Return buffer.

23.2.3.3 get %TypedArray%.prototype.byteLength

%TypedArray%.prototype.byteLength is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? RequireInternalSlot(O, [[TypedArrayName]]).
  3. Assert: O has a [[ViewedArrayBuffer]] internal slot.
  4. Let buffer be O.[[ViewedArrayBuffer]].
  5. If IsDetachedBuffer(buffer) is true, return 0𝔽.
  6. Let size be O.[[ByteLength]].
  7. Return 𝔽(size).

23.2.3.4 get %TypedArray%.prototype.byteOffset

%TypedArray%.prototype.byteOffset is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? RequireInternalSlot(O, [[TypedArrayName]]).
  3. Assert: O has a [[ViewedArrayBuffer]] internal slot.
  4. Let buffer be O.[[ViewedArrayBuffer]].
  5. If IsDetachedBuffer(buffer) is true, return 0𝔽.
  6. Let offset be O.[[ByteOffset]].
  7. Return 𝔽(offset).

23.2.3.5 %TypedArray%.prototype.constructor

The initial value of %TypedArray%.prototype.constructor is %TypedArray%.

23.2.3.6 %TypedArray%.prototype.copyWithin ( target, start [ , end ] )

The interpretation and use of the arguments of this method are the same as for Array.prototype.copyWithin as defined in 23.1.3.4.

This method performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. Let relativeTarget be ? ToIntegerOrInfinity(target).
  5. If relativeTarget = -∞, let to be 0.
  6. Else if relativeTarget < 0, let to be max(len relativeTarget, 0).
  7. Else, let to be min(relativeTarget, len).
  8. Let relativeStart be ? ToIntegerOrInfinity(start).
  9. If relativeStart = -∞, let from be 0.
  10. Else if relativeStart < 0, let from be max(len relativeStart, 0).
  11. Else, let from be min(relativeStart, len).
  12. If end is undefined, let relativeEnd be len; else let relativeEnd be ? ToIntegerOrInfinity(end).
  13. If relativeEnd = -∞, let final be 0.
  14. Else if relativeEnd < 0, let final be max(len relativeEnd, 0).
  15. Else, let final be min(relativeEnd, len).
  16. Let count be min(final - from, len - to).
  17. If count > 0, then
    1. NOTE: The copying must be performed in a manner that preserves the bit-level encoding of the source data.
    2. Let buffer be O.[[ViewedArrayBuffer]].
    3. If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
    4. Let elementSize be TypedArrayElementSize(O).
    5. Let byteOffset be O.[[ByteOffset]].
    6. Let toByteIndex be to × elementSize byteOffset.
    7. Let fromByteIndex be from × elementSize byteOffset.
    8. Let countBytes be count × elementSize.
    9. If fromByteIndex < toByteIndex and toByteIndex < fromByteIndex countBytes, then
      1. Let direction be -1.
      2. Set fromByteIndex to fromByteIndex countBytes - 1.
      3. Set toByteIndex to toByteIndex countBytes - 1.
    10. Else,
      1. Let direction be 1.
    11. Repeat, while countBytes > 0,
      1. Let value be GetValueFromBuffer(buffer, fromByteIndex, Uint8, true, Unordered).
      2. Perform SetValueInBuffer(buffer, toByteIndex, Uint8, value, true, Unordered).
      3. Set fromByteIndex to fromByteIndex direction.
      4. Set toByteIndex to toByteIndex direction.
      5. Set countBytes to countBytes - 1.
  18. Return O.

23.2.3.7 %TypedArray%.prototype.entries ( )

This method performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? ValidateTypedArray(O).
  3. Return CreateArrayIterator(O, key value).

23.2.3.8 %TypedArray%.prototype.every ( callbackfn [ , thisArg ] )

The interpretation and use of the arguments of this method are the same as for Array.prototype.every as defined in 23.1.3.6.

This method performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. If IsCallable(callbackfn) is false, throw a TypeError exception.
  5. Let k be 0.
  6. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kValue be ! Get(O, Pk).
    3. Let testResult be ToBoolean(? Call(callbackfn, thisArg, « kValue, 𝔽(k), O »)).
    4. If testResult is false, return false.
    5. Set k to k 1.
  7. Return true.

This method is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.

23.2.3.9 %TypedArray%.prototype.fill ( value [ , start [ , end ] ] )

The interpretation and use of the arguments of this method are the same as for Array.prototype.fill as defined in 23.1.3.7.

This method performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. If O.[[ContentType]] is BigInt, set value to ? ToBigInt(value).
  5. Otherwise, set value to ? ToNumber(value).
  6. Let relativeStart be ? ToIntegerOrInfinity(start).
  7. If relativeStart = -∞, let k be 0.
  8. Else if relativeStart < 0, let k be max(len relativeStart, 0).
  9. Else, let k be min(relativeStart, len).
  10. If end is undefined, let relativeEnd be len; else let relativeEnd be ? ToIntegerOrInfinity(end).
  11. If relativeEnd = -∞, let final be 0.
  12. Else if relativeEnd < 0, let final be max(len relativeEnd, 0).
  13. Else, let final be min(relativeEnd, len).
  14. If IsDetachedBuffer(O.[[ViewedArrayBuffer]]) is true, throw a TypeError exception.
  15. Repeat, while k < final,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Perform ! Set(O, Pk, value, true).
    3. Set k to k 1.
  16. Return O.

23.2.3.10 %TypedArray%.prototype.filter ( callbackfn [ , thisArg ] )

The interpretation and use of the arguments of this method are the same as for Array.prototype.filter as defined in 23.1.3.8.

This method performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. If IsCallable(callbackfn) is false, throw a TypeError exception.
  5. Let kept be a new empty List.
  6. Let captured be 0.
  7. Let k be 0.
  8. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kValue be ! Get(O, Pk).
    3. Let selected be ToBoolean(? Call(callbackfn, thisArg, « kValue, 𝔽(k), O »)).
    4. If selected is true, then
      1. Append kValue to kept.
      2. Set captured to captured 1.
    5. Set k to k 1.
  9. Let A be ? TypedArraySpeciesCreate(O, « 𝔽(captured) »).
  10. Let n be 0.
  11. For each element e of kept, do
    1. Perform ! Set(A, ! ToString(𝔽(n)), e, true).
    2. Set n to n 1.
  12. Return A.

This method is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.

23.2.3.11 %TypedArray%.prototype.find ( predicate [ , thisArg ] )

The interpretation and use of the arguments of this method are the same as for Array.prototype.find as defined in 23.1.3.9.

This method performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. Let findRec be ? FindViaPredicate(O, len, ascending, predicate, thisArg).
  5. Return findRec.[[Value]].

This method is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.

23.2.3.12 %TypedArray%.prototype.findIndex ( predicate [ , thisArg ] )

The interpretation and use of the arguments of this method are the same as for Array.prototype.findIndex as defined in 23.1.3.10.

This method performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. Let findRec be ? FindViaPredicate(O, len, ascending, predicate, thisArg).
  5. Return findRec.[[Index]].

This method is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.

23.2.3.13 %TypedArray%.prototype.findLast ( predicate [ , thisArg ] )

The interpretation and use of the arguments of this method are the same as for Array.prototype.findLast as defined in 23.1.3.11.

This method performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. Let findRec be ? FindViaPredicate(O, len, descending, predicate, thisArg).
  5. Return findRec.[[Value]].

This method is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.

23.2.3.14 %TypedArray%.prototype.findLastIndex ( predicate [ , thisArg ] )

The interpretation and use of the arguments of this method are the same as for Array.prototype.findLastIndex as defined in 23.1.3.12.

This method performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. Let findRec be ? FindViaPredicate(O, len, descending, predicate, thisArg).
  5. Return findRec.[[Index]].

This method is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.

23.2.3.15 %TypedArray%.prototype.forEach ( callbackfn [ , thisArg ] )

The interpretation and use of the arguments of this method are the same as for Array.prototype.forEach as defined in 23.1.3.15.

This method performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. If IsCallable(callbackfn) is false, throw a TypeError exception.
  5. Let k be 0.
  6. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kValue be ! Get(O, Pk).
    3. Perform ? Call(callbackfn, thisArg, « kValue, 𝔽(k), O »).
    4. Set k to k 1.
  7. Return undefined.

This method is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.

23.2.3.16 %TypedArray%.prototype.includes ( searchElement [ , fromIndex ] )

The interpretation and use of the arguments of this method are the same as for Array.prototype.includes as defined in 23.1.3.16.

This method performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. If len = 0, return false.
  5. Let n be ? ToIntegerOrInfinity(fromIndex).
  6. Assert: If fromIndex is undefined, then n is 0.
  7. If n = ∞, return false.
  8. Else if n = -∞, set n to 0.
  9. If n ≥ 0, then
    1. Let k be n.
  10. Else,
    1. Let k be len n.
    2. If k < 0, set k to 0.
  11. Repeat, while k < len,
    1. Let elementK be ! Get(O, ! ToString(𝔽(k))).
    2. If SameValueZero(searchElement, elementK) is true, return true.
    3. Set k to k 1.
  12. Return false.

This method is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.

23.2.3.17 %TypedArray%.prototype.indexOf ( searchElement [ , fromIndex ] )

The interpretation and use of the arguments of this method are the same as for Array.prototype.indexOf as defined in 23.1.3.17.

This method performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. If len = 0, return -1𝔽.
  5. Let n be ? ToIntegerOrInfinity(fromIndex).
  6. Assert: If fromIndex is undefined, then n is 0.
  7. If n = ∞, return -1𝔽.
  8. Else if n = -∞, set n to 0.
  9. If n ≥ 0, then
    1. Let k be n.
  10. Else,
    1. Let k be len n.
    2. If k < 0, set k to 0.
  11. Repeat, while k < len,
    1. Let kPresent be ! HasProperty(O, ! ToString(𝔽(k))).
    2. If kPresent is true, then
      1. Let elementK be ! Get(O, ! ToString(𝔽(k))).
      2. If IsStrictlyEqual(searchElement, elementK) is true, return 𝔽(k).
    3. Set k to k 1.
  12. Return -1𝔽.

This method is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.

23.2.3.18 %TypedArray%.prototype.join ( separator )

The interpretation and use of the arguments of this method are the same as for Array.prototype.join as defined in 23.1.3.18.

This method performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. If separator is undefined, let sep be ",".
  5. Else, let sep be ? ToString(separator).
  6. Let R be the empty String.
  7. Let k be 0.
  8. Repeat, while k < len,
    1. If k > 0, set R to the string-concatenation of R and sep.
    2. Let element be ! Get(O, ! ToString(𝔽(k))).
    3. If element is undefined, let next be the empty String; otherwise, let next be ! ToString(element).
    4. Set R to the string-concatenation of R and next.
    5. Set k to k 1.
  9. Return R.

This method is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.

23.2.3.19 %TypedArray%.prototype.keys ( )

This method performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? ValidateTypedArray(O).
  3. Return CreateArrayIterator(O, key).

23.2.3.20 %TypedArray%.prototype.lastIndexOf ( searchElement [ , fromIndex ] )

The interpretation and use of the arguments of this method are the same as for Array.prototype.lastIndexOf as defined in 23.1.3.20.

This method performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. If len = 0, return -1𝔽.
  5. If fromIndex is present, let n be ? ToIntegerOrInfinity(fromIndex); else let n be len - 1.
  6. If n = -∞, return -1𝔽.
  7. If n ≥ 0, then
    1. Let k be min(n, len - 1).
  8. Else,
    1. Let k be len n.
  9. Repeat, while k ≥ 0,
    1. Let kPresent be ! HasProperty(O, ! ToString(𝔽(k))).
    2. If kPresent is true, then
      1. Let elementK be ! Get(O, ! ToString(𝔽(k))).
      2. If IsStrictlyEqual(searchElement, elementK) is true, return 𝔽(k).
    3. Set k to k - 1.
  10. Return -1𝔽.

This method is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.

23.2.3.21 get %TypedArray%.prototype.length

%TypedArray%.prototype.length is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? RequireInternalSlot(O, [[TypedArrayName]]).
  3. Assert: O has [[ViewedArrayBuffer]] and [[ArrayLength]] internal slots.
  4. Let buffer be O.[[ViewedArrayBuffer]].
  5. If IsDetachedBuffer(buffer) is true, return 0𝔽.
  6. Let length be O.[[ArrayLength]].
  7. Return 𝔽(length).

This function is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.

23.2.3.22 %TypedArray%.prototype.map ( callbackfn [ , thisArg ] )

The interpretation and use of the arguments of this method are the same as for Array.prototype.map as defined in 23.1.3.21.

This method performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. If IsCallable(callbackfn) is false, throw a TypeError exception.
  5. Let A be ? TypedArraySpeciesCreate(O, « 𝔽(len) »).
  6. Let k be 0.
  7. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kValue be ! Get(O, Pk).
    3. Let mappedValue be ? Call(callbackfn, thisArg, « kValue, 𝔽(k), O »).
    4. Perform ? Set(A, Pk, mappedValue, true).
    5. Set k to k 1.
  8. Return A.

This method is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.

23.2.3.23 %TypedArray%.prototype.reduce ( callbackfn [ , initialValue ] )

The interpretation and use of the arguments of this method are the same as for Array.prototype.reduce as defined in 23.1.3.24.

This method performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. If IsCallable(callbackfn) is false, throw a TypeError exception.
  5. If len = 0 and initialValue is not present, throw a TypeError exception.
  6. Let k be 0.
  7. Let accumulator be undefined.
  8. If initialValue is present, then
    1. Set accumulator to initialValue.
  9. Else,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Set accumulator to ! Get(O, Pk).
    3. Set k to k 1.
  10. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kValue be ! Get(O, Pk).
    3. Set accumulator to ? Call(callbackfn, undefined, « accumulator, kValue, 𝔽(k), O »).
    4. Set k to k 1.
  11. Return accumulator.

This method is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.

23.2.3.24 %TypedArray%.prototype.reduceRight ( callbackfn [ , initialValue ] )

The interpretation and use of the arguments of this method are the same as for Array.prototype.reduceRight as defined in 23.1.3.25.

This method performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. If IsCallable(callbackfn) is false, throw a TypeError exception.
  5. If len = 0 and initialValue is not present, throw a TypeError exception.
  6. Let k be len - 1.
  7. Let accumulator be undefined.
  8. If initialValue is present, then
    1. Set accumulator to initialValue.
  9. Else,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Set accumulator to ! Get(O, Pk).
    3. Set k to k - 1.
  10. Repeat, while k ≥ 0,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kValue be ! Get(O, Pk).
    3. Set accumulator to ? Call(callbackfn, undefined, « accumulator, kValue, 𝔽(k), O »).
    4. Set k to k - 1.
  11. Return accumulator.

This method is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.

23.2.3.25 %TypedArray%.prototype.reverse ( )

The interpretation and use of the arguments of this method are the same as for Array.prototype.reverse as defined in 23.1.3.26.

This method performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. Let middle be floor(len / 2).
  5. Let lower be 0.
  6. Repeat, while lower ≠ middle,
    1. Let upper be len - lower - 1.
    2. Let upperP be ! ToString(𝔽(upper)).
    3. Let lowerP be ! ToString(𝔽(lower)).
    4. Let lowerValue be ! Get(O, lowerP).
    5. Let upperValue be ! Get(O, upperP).
    6. Perform ! Set(O, lowerP, upperValue, true).
    7. Perform ! Set(O, upperP, lowerValue, true).
    8. Set lower to lower 1.
  7. Return O.

This method is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.

23.2.3.26 %TypedArray%.prototype.set ( source [ , offset ] )

This method sets multiple values in this TypedArray, reading the values from source. The details differ based upon the type of source. The optional offset value indicates the first element index in this TypedArray where values are written. If omitted, it is assumed to be 0.

It performs the following steps when called:

  1. Let target be the this value.
  2. Perform ? RequireInternalSlot(target, [[TypedArrayName]]).
  3. Assert: target has a [[ViewedArrayBuffer]] internal slot.
  4. Let targetOffset be ? ToIntegerOrInfinity(offset).
  5. If targetOffset < 0, throw a RangeError exception.
  6. If source is an Object that has a [[TypedArrayName]] internal slot, then
    1. Perform ? SetTypedArrayFromTypedArray(target, targetOffset, source).
  7. Else,
    1. Perform ? SetTypedArrayFromArrayLike(target, targetOffset, source).
  8. Return undefined.

This method is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.

23.2.3.26.1 SetTypedArrayFromTypedArray ( target, targetOffset, source )

The abstract operation SetTypedArrayFromTypedArray takes arguments target (a TypedArray), targetOffset (a non-negative integer or ∞), and source (a TypedArray) and returns either a normal completion containing unused or a throw completion. It sets multiple values in target, starting at index targetOffset, reading the values from source. It performs the following steps when called:

  1. Let targetBuffer be target.[[ViewedArrayBuffer]].
  2. If IsDetachedBuffer(targetBuffer) is true, throw a TypeError exception.
  3. Let targetLength be target.[[ArrayLength]].
  4. Let srcBuffer be source.[[ViewedArrayBuffer]].
  5. If IsDetachedBuffer(srcBuffer) is true, throw a TypeError exception.
  6. Let targetType be TypedArrayElementType(target).
  7. Let targetElementSize be TypedArrayElementSize(target).
  8. Let targetByteOffset be target.[[ByteOffset]].
  9. Let srcType be TypedArrayElementType(source).
  10. Let srcElementSize be TypedArrayElementSize(source).
  11. Let srcLength be source.[[ArrayLength]].
  12. Let srcByteOffset be source.[[ByteOffset]].
  13. If targetOffset = ∞, throw a RangeError exception.
  14. If srcLength targetOffset > targetLength, throw a RangeError exception.
  15. If target.[[ContentType]] is not source.[[ContentType]], throw a TypeError exception.
  16. If IsSharedArrayBuffer(srcBuffer) is true, IsSharedArrayBuffer(targetBuffer) is true, and srcBuffer.[[ArrayBufferData]] is targetBuffer.[[ArrayBufferData]], let sameSharedArrayBuffer be true; otherwise, let sameSharedArrayBuffer be false.
  17. If SameValue(srcBuffer, targetBuffer) is true or sameSharedArrayBuffer is true, then
    1. Let srcByteLength be source.[[ByteLength]].
    2. Set srcBuffer to ? CloneArrayBuffer(srcBuffer, srcByteOffset, srcByteLength).
    3. Let srcByteIndex be 0.
  18. Else, let srcByteIndex be srcByteOffset.
  19. Let targetByteIndex be targetOffset × targetElementSize targetByteOffset.
  20. Let limit be targetByteIndex targetElementSize × srcLength.
  21. If srcType is targetType, then
    1. NOTE: The transfer must be performed in a manner that preserves the bit-level encoding of the source data.
    2. Repeat, while targetByteIndex < limit,
      1. Let value be GetValueFromBuffer(srcBuffer, srcByteIndex, Uint8, true, Unordered).
      2. Perform SetValueInBuffer(targetBuffer, targetByteIndex, Uint8, value, true, Unordered).
      3. Set srcByteIndex to srcByteIndex 1.
      4. Set targetByteIndex to targetByteIndex 1.
  22. Else,
    1. Repeat, while targetByteIndex < limit,
      1. Let value be GetValueFromBuffer(srcBuffer, srcByteIndex, srcType, true, Unordered).
      2. Perform SetValueInBuffer(targetBuffer, targetByteIndex, targetType, value, true, Unordered).
      3. Set srcByteIndex to srcByteIndex srcElementSize.
      4. Set targetByteIndex to targetByteIndex targetElementSize.
  23. Return unused.

23.2.3.26.2 SetTypedArrayFromArrayLike ( target, targetOffset, source )

The abstract operation SetTypedArrayFromArrayLike takes arguments target (a TypedArray), targetOffset (a non-negative integer or ∞), and source (an ECMAScript language value, but not a TypedArray) and returns either a normal completion containing unused or a throw completion. It sets multiple values in target, starting at index targetOffset, reading the values from source. It performs the following steps when called:

  1. Let targetBuffer be target.[[ViewedArrayBuffer]].
  2. If IsDetachedBuffer(targetBuffer) is true, throw a TypeError exception.
  3. Let targetLength be target.[[ArrayLength]].
  4. Let src be ? ToObject(source).
  5. Let srcLength be ? LengthOfArrayLike(src).
  6. If targetOffset = ∞, throw a RangeError exception.
  7. If srcLength targetOffset > targetLength, throw a RangeError exception.
  8. Let k be 0.
  9. Repeat, while k < srcLength,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let value be ? Get(src, Pk).
    3. Let targetIndex be 𝔽(targetOffset k).
    4. Perform ? IntegerIndexedElementSet(target, targetIndex, value).
    5. Set k to k 1.
  10. Return unused.

23.2.3.27 %TypedArray%.prototype.slice ( start, end )

The interpretation and use of the arguments of this method are the same as for Array.prototype.slice as defined in 23.1.3.28.

This method performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. Let relativeStart be ? ToIntegerOrInfinity(start).
  5. If relativeStart = -∞, let k be 0.
  6. Else if relativeStart < 0, let k be max(len relativeStart, 0).
  7. Else, let k be min(relativeStart, len).
  8. If end is undefined, let relativeEnd be len; else let relativeEnd be ? ToIntegerOrInfinity(end).
  9. If relativeEnd = -∞, let final be 0.
  10. Else if relativeEnd < 0, let final be max(len relativeEnd, 0).
  11. Else, let final be min(relativeEnd, len).
  12. Let count be max(final - k, 0).
  13. Let A be ? TypedArraySpeciesCreate(O, « 𝔽(count) »).
  14. If count > 0, then
    1. If IsDetachedBuffer(O.[[ViewedArrayBuffer]]) is true, throw a TypeError exception.
    2. Let srcType be TypedArrayElementType(O).
    3. Let targetType be TypedArrayElementType(A).
    4. If srcType is targetType, then
      1. NOTE: The transfer must be performed in a manner that preserves the bit-level encoding of the source data.
      2. Let srcBuffer be O.[[ViewedArrayBuffer]].
      3. Let targetBuffer be A.[[ViewedArrayBuffer]].
      4. Let elementSize be TypedArrayElementSize(O).
      5. Let srcByteOffset be O.[[ByteOffset]].
      6. Let srcByteIndex be (k × elementSize) srcByteOffset.
      7. Let targetByteIndex be A.[[ByteOffset]].
      8. Let limit be targetByteIndex count × elementSize.
      9. Repeat, while targetByteIndex < limit,
        1. Let value be GetValueFromBuffer(srcBuffer, srcByteIndex, Uint8, true, Unordered).
        2. Perform SetValueInBuffer(targetBuffer, targetByteIndex, Uint8, value, true, Unordered).
        3. Set srcByteIndex to srcByteIndex 1.
        4. Set targetByteIndex to targetByteIndex 1.
    5. Else,
      1. Let n be 0.
      2. Repeat, while k < final,
        1. Let Pk be ! ToString(𝔽(k)).
        2. Let kValue be ! Get(O, Pk).
        3. Perform ! Set(A, ! ToString(𝔽(n)), kValue, true).
        4. Set k to k 1.
        5. Set n to n 1.
  15. Return A.

This method is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.

23.2.3.28 %TypedArray%.prototype.some ( callbackfn [ , thisArg ] )

The interpretation and use of the arguments of this method are the same as for Array.prototype.some as defined in 23.1.3.29.

This method performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. If IsCallable(callbackfn) is false, throw a TypeError exception.
  5. Let k be 0.
  6. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kValue be ! Get(O, Pk).
    3. Let testResult be ToBoolean(? Call(callbackfn, thisArg, « kValue, 𝔽(k), O »)).
    4. If testResult is true, return true.
    5. Set k to k 1.
  7. Return false.

This method is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.

23.2.3.29 %TypedArray%.prototype.sort ( comparefn )

This is a distinct method that, except as described below, implements the same requirements as those of Array.prototype.sort as defined in 23.1.3.30. The implementation of this method may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer-indexed properties are not sparse.

This method is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.

It performs the following steps when called:

  1. If comparefn is not undefined and IsCallable(comparefn) is false, throw a TypeError exception.
  2. Let obj be the this value.
  3. Perform ? ValidateTypedArray(obj).
  4. Let len be obj.[[ArrayLength]].
  5. NOTE: The following closure performs a numeric comparison rather than the string comparison used in 23.1.3.30.
  6. Let SortCompare be a new Abstract Closure with parameters (x, y) that captures comparefn and performs the following steps when called:
    1. Return ? CompareTypedArrayElements(x, y, comparefn).
  7. Let sortedList be ? SortIndexedProperties(obj, len, SortCompare, read-through-holes).
  8. Let j be 0.
  9. Repeat, while j < len,
    1. Perform ! Set(obj, ! ToString(𝔽(j)), sortedList[j], true).
    2. Set j to j 1.
  10. Return obj.
 Note

Because NaN always compares greater than any other value (see CompareTypedArrayElements), NaN property values always sort to the end of the result when comparefn is not provided.

23.2.3.30 %TypedArray%.prototype.subarray ( begin, end )

This method returns a new TypedArray whose element type is the element type of this TypedArray and whose ArrayBuffer is the ArrayBuffer of this TypedArray, referencing the elements in the interval from begin (inclusive) to end (exclusive). If either begin or end is negative, it refers to an index from the end of the array, as opposed to from the beginning.

It performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? RequireInternalSlot(O, [[TypedArrayName]]).
  3. Assert: O has a [[ViewedArrayBuffer]] internal slot.
  4. Let buffer be O.[[ViewedArrayBuffer]].
  5. Let srcLength be O.[[ArrayLength]].
  6. Let relativeBegin be ? ToIntegerOrInfinity(begin).
  7. If relativeBegin = -∞, let beginIndex be 0.
  8. Else if relativeBegin < 0, let beginIndex be max(srcLength relativeBegin, 0).
  9. Else, let beginIndex be min(relativeBegin, srcLength).
  10. If end is undefined, let relativeEnd be srcLength; else let relativeEnd be ? ToIntegerOrInfinity(end).
  11. If relativeEnd = -∞, let endIndex be 0.
  12. Else if relativeEnd < 0, let endIndex be max(srcLength relativeEnd, 0).
  13. Else, let endIndex be min(relativeEnd, srcLength).
  14. Let newLength be max(endIndex - beginIndex, 0).
  15. Let elementSize be TypedArrayElementSize(O).
  16. Let srcByteOffset be O.[[ByteOffset]].
  17. Let beginByteOffset be srcByteOffset beginIndex × elementSize.
  18. Let argumentsList be « buffer, 𝔽(beginByteOffset), 𝔽(newLength) ».
  19. Return ? TypedArraySpeciesCreate(O, argumentsList).

This method is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.

23.2.3.31 %TypedArray%.prototype.toLocaleString ( [ reserved1 [ , reserved2 ] ] )

This is a distinct method that implements the same algorithm as Array.prototype.toLocaleString as defined in 23.1.3.32 except that the this value's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer-indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm.

This method is not generic. ValidateTypedArray is applied to the this value prior to evaluating the algorithm. If its result is an abrupt completion that exception is thrown instead of evaluating the algorithm.

Note

If the ECMAScript implementation includes the ECMA-402 Internationalization API this method is based upon the algorithm for Array.prototype.toLocaleString that is in the ECMA-402 specification.

23.2.3.32 %TypedArray%.prototype.toReversed ( )

This method performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? ValidateTypedArray(O).
  3. Let length be O.[[ArrayLength]].
  4. Let A be ? TypedArrayCreateSameType(O, « 𝔽(length) »).
  5. Let k be 0.
  6. Repeat, while k < length,
    1. Let from be ! ToString(𝔽(length - k - 1)).
    2. Let Pk be ! ToString(𝔽(k)).
    3. Let fromValue be ! Get(O, from).
    4. Perform ! Set(A, Pk, fromValue, true).
    5. Set k to k 1.
  7. Return A.

23.2.3.33 %TypedArray%.prototype.toSorted ( comparefn )

This method performs the following steps when called:

  1. If comparefn is not undefined and IsCallable(comparefn) is false, throw a TypeError exception.
  2. Let O be the this value.
  3. Perform ? ValidateTypedArray(O).
  4. Let len be O.[[ArrayLength]].
  5. Let A be ? TypedArrayCreateSameType(O, « 𝔽(len) »).
  6. NOTE: The following closure performs a numeric comparison rather than the string comparison used in 23.1.3.34.
  7. Let SortCompare be a new Abstract Closure with parameters (x, y) that captures comparefn and performs the following steps when called:
    1. Return ? CompareTypedArrayElements(x, y, comparefn).
  8. Let sortedList be ? SortIndexedProperties(O, len, SortCompare, read-through-holes).
  9. Let j be 0.
  10. Repeat, while j < len,
    1. Perform ! Set(A, ! ToString(𝔽(j)), sortedList[j], true).
    2. Set j to j 1.
  11. Return A.

23.2.3.34 %TypedArray%.prototype.toString ( )

The initial value of the "toString" property is %Array.prototype.toString%, defined in 23.1.3.36.

23.2.3.35 %TypedArray%.prototype.values ( )

This method performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? ValidateTypedArray(O).
  3. Return CreateArrayIterator(O, value).

23.2.3.36 %TypedArray%.prototype.with ( index, value )

This method performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. Let relativeIndex be ? ToIntegerOrInfinity(index).
  5. If relativeIndex ≥ 0, let actualIndex be relativeIndex.
  6. Else, let actualIndex be len relativeIndex.
  7. If O.[[ContentType]] is BigInt, let numericValue be ? ToBigInt(value).
  8. Else, let numericValue be ? ToNumber(value).
  9. If IsValidIntegerIndex(O, 𝔽(actualIndex)) is false, throw a RangeError exception.
  10. Let A be ? TypedArrayCreateSameType(O, « 𝔽(len) »).
  11. Let k be 0.
  12. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. If k is actualIndex, let fromValue be numericValue.
    3. Else, let fromValue be ! Get(O, Pk).
    4. Perform ! Set(A, Pk, fromValue, true).
    5. Set k to k 1.
  13. Return A.

23.2.3.37 %TypedArray%.prototype [ @@iterator ] ( )

The initial value of the @@iterator property is %TypedArray.prototype.values%, defined in 23.2.3.35.

23.2.3.38 get %TypedArray%.prototype [ @@toStringTag ]

%TypedArray%.prototype[@@toStringTag] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Let O be the this value.
  2. If O is not an Object, return undefined.
  3. If O does not have a [[TypedArrayName]] internal slot, return undefined.
  4. Let name be O.[[TypedArrayName]].
  5. Assert: name is a String.
  6. Return name.

This property has the attributes { [[Enumerable]]: false, [[Configurable]]: true }.

The initial value of the "name" property of this function is "get [Symbol.toStringTag]".

23.2.4 Abstract Operations for TypedArray Objects

23.2.4.1 TypedArraySpeciesCreate ( exemplar, argumentList )

The abstract operation TypedArraySpeciesCreate takes arguments exemplar (a TypedArray) and argumentList (a List of ECMAScript language values) and returns either a normal completion containing a TypedArray or a throw completion. It is used to specify the creation of a new TypedArray using a constructor function that is derived from exemplar. Unlike ArraySpeciesCreate, which can create non-Array objects through the use of @@species, this operation enforces that the constructor function creates an actual TypedArray. It performs the following steps when called:

  1. Let defaultConstructor be the intrinsic object associated with the constructor name exemplar.[[TypedArrayName]] in Table 68.
  2. Let constructor be ? SpeciesConstructor(exemplar, defaultConstructor).
  3. Let result be ? TypedArrayCreate(constructor, argumentList).
  4. Assert: result has [[TypedArrayName]] and [[ContentType]] internal slots.
  5. If result.[[ContentType]] is not exemplar.[[ContentType]], throw a TypeError exception.
  6. Return result.

23.2.4.2 TypedArrayCreate ( constructor, argumentList )

The abstract operation TypedArrayCreate takes arguments constructor (a constructor) and argumentList (a List of ECMAScript language values) and returns either a normal completion containing a TypedArray or a throw completion. It is used to specify the creation of a new TypedArray using a constructor function. It performs the following steps when called:

  1. Let newTypedArray be ? Construct(constructor, argumentList).
  2. Perform ? ValidateTypedArray(newTypedArray).
  3. If the number of elements in argumentList is 1 and argumentList[0] is a Number, then
    1. If newTypedArray.[[ArrayLength]] < ℝ(argumentList[0]), throw a TypeError exception.
  4. Return newTypedArray.

23.2.4.3 TypedArrayCreateSameType ( exemplar, argumentList )

The abstract operation TypedArrayCreateSameType takes arguments exemplar (a TypedArray) and argumentList (a List of ECMAScript language values) and returns either a normal completion containing a TypedArray or a throw completion. It is used to specify the creation of a new TypedArray using a constructor function that is derived from exemplar. Unlike TypedArraySpeciesCreate, which can construct custom TypedArray subclasses through the use of @@species, this operation always uses one of the built-in TypedArray constructors. It performs the following steps when called:

  1. Let constructor be the intrinsic object associated with the constructor name exemplar.[[TypedArrayName]] in Table 68.
  2. Let result be ? TypedArrayCreate(constructor, argumentList).
  3. Assert: result has [[TypedArrayName]] and [[ContentType]] internal slots.
  4. Assert: result.[[ContentType]] is exemplar.[[ContentType]].
  5. Return result.

23.2.4.4 ValidateTypedArray ( O )

The abstract operation ValidateTypedArray takes argument O (an ECMAScript language value) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

  1. Perform ? RequireInternalSlot(O, [[TypedArrayName]]).
  2. Assert: O has a [[ViewedArrayBuffer]] internal slot.
  3. Let buffer be O.[[ViewedArrayBuffer]].
  4. If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
  5. Return unused.

23.2.4.5 TypedArrayElementSize ( O )

The abstract operation TypedArrayElementSize takes argument O (a TypedArray) and returns a non-negative integer. It performs the following steps when called:

  1. Return the Element Size value specified in Table 68 for O.[[TypedArrayName]].

23.2.4.6 TypedArrayElementType ( O )

The abstract operation TypedArrayElementType takes argument O (a TypedArray) and returns a TypedArray element type. It performs the following steps when called:

  1. Return the Element Type value specified in Table 68 for O.[[TypedArrayName]].

23.2.4.7 CompareTypedArrayElements ( x, y, comparefn )

The abstract operation CompareTypedArrayElements takes arguments x (a Number or a BigInt), y (a Number or a BigInt), and comparefn (a function object or undefined) and returns either a normal completion containing a Number or an abrupt completion. It performs the following steps when called:

  1. Assert: x is a Number and y is a Number, or x is a BigInt and y is a BigInt.
  2. If comparefn is not undefined, then
    1. Let v be ? ToNumber(? Call(comparefn, undefined, « x, y »)).
    2. If v is NaN, return 0𝔽.
    3. Return v.
  3. If x and y are both NaN, return 0𝔽.
  4. If x is NaN, return 1𝔽.
  5. If y is NaN, return -1𝔽.
  6. If x < y, return -1𝔽.
  7. If x > y, return 1𝔽.
  8. If x is -0𝔽 and y is 0𝔽, return -1𝔽.
  9. If x is 0𝔽 and y is -0𝔽, return 1𝔽.
  10. Return 0𝔽.
 Note
This performs a numeric comparison rather than the string comparison used in 23.1.3.30.2.

23.2.5 The TypedArray Constructors

Each TypedArray constructor:

  • is an intrinsic object that has the structure described below, differing only in the name used as the constructor name instead of TypedArray, in Table 68.
  • is a function whose behaviour differs based upon the number and types of its arguments. The actual behaviour of a call of TypedArray depends upon the number and kind of arguments that are passed to it.
  • is not intended to be called as a function and will throw an exception when called in that manner.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified TypedArray behaviour must include a super call to the TypedArray constructor to create and initialize the subclass instance with the internal state necessary to support the %TypedArray%.prototype built-in methods.
  • has a "length" property whose value is 3𝔽.

23.2.5.1 TypedArray ( ...args )

Each TypedArray constructor performs the following steps when called:

  1. If NewTarget is undefined, throw a TypeError exception.
  2. Let constructorName be the String value of the Constructor Name value specified in Table 68 for this TypedArray constructor.
  3. Let proto be "%TypedArray.prototype%".
  4. Let numberOfArgs be the number of elements in args.
  5. If numberOfArgs = 0, then
    1. Return ? AllocateTypedArray(constructorName, NewTarget, proto, 0).
  6. Else,
    1. Let firstArgument be args[0].
    2. If firstArgument is an Object, then
      1. Let O be ? AllocateTypedArray(constructorName, NewTarget, proto).
      2. If firstArgument has a [[TypedArrayName]] internal slot, then
        1. Perform ? InitializeTypedArrayFromTypedArray(O, firstArgument).
      3. Else if firstArgument has an [[ArrayBufferData]] internal slot, then
        1. If numberOfArgs > 1, let byteOffset be args[1]; else let byteOffset be undefined.
        2. If numberOfArgs > 2, let length be args[2]; else let length be undefined.
        3. Perform ? InitializeTypedArrayFromArrayBuffer(O, firstArgument, byteOffset, length).
      4. Else,
        1. Assert: firstArgument is an Object and firstArgument does not have either a [[TypedArrayName]] or an [[ArrayBufferData]] internal slot.
        2. Let usingIterator be ? GetMethod(firstArgument, @@iterator).
        3. If usingIterator is not undefined, then
          1. Let values be ? IteratorToList(? GetIteratorFromMethod(firstArgument, usingIterator)).
          2. Perform ? InitializeTypedArrayFromList(O, values).
        4. Else,
          1. NOTE: firstArgument is not an Iterable so assume it is already an array-like object.
          2. Perform ? InitializeTypedArrayFromArrayLike(O, firstArgument).
      5. Return O.
    3. Else,
      1. Assert: firstArgument is not an Object.
      2. Let elementLength be ? ToIndex(firstArgument).
      3. Return ? AllocateTypedArray(constructorName, NewTarget, proto, elementLength).

23.2.5.1.1 AllocateTypedArray ( constructorName, newTarget, defaultProto [ , length ] )

The abstract operation AllocateTypedArray takes arguments constructorName (a String which is the name of a TypedArray constructor in Table 68), newTarget (a constructor), and defaultProto (a String) and optional argument length (a non-negative integer) and returns either a normal completion containing a TypedArray or a throw completion. It is used to validate and create an instance of a TypedArray constructor. If the length argument is passed, an ArrayBuffer of that length is also allocated and associated with the new TypedArray instance. AllocateTypedArray provides common semantics that is used by TypedArray. It performs the following steps when called:

  1. Let proto be ? GetPrototypeFromConstructor(newTarget, defaultProto).
  2. Let obj be IntegerIndexedObjectCreate(proto).
  3. Assert: obj.[[ViewedArrayBuffer]] is undefined.
  4. Set obj.[[TypedArrayName]] to constructorName.
  5. If constructorName is either "BigInt64Array" or "BigUint64Array", set obj.[[ContentType]] to BigInt.
  6. Otherwise, set obj.[[ContentType]] to Number.
  7. If length is not present, then
    1. Set obj.[[ByteLength]] to 0.
    2. Set obj.[[ByteOffset]] to 0.
    3. Set obj.[[ArrayLength]] to 0.
  8. Else,
    1. Perform ? AllocateTypedArrayBuffer(obj, length).
  9. Return obj.

23.2.5.1.2 InitializeTypedArrayFromTypedArray ( O, srcArray )

The abstract operation InitializeTypedArrayFromTypedArray takes arguments O (a TypedArray) and srcArray (a TypedArray) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

  1. Let srcData be srcArray.[[ViewedArrayBuffer]].
  2. If IsDetachedBuffer(srcData) is true, throw a TypeError exception.
  3. Let elementType be TypedArrayElementType(O).
  4. Let elementSize be TypedArrayElementSize(O).
  5. Let srcType be TypedArrayElementType(srcArray).
  6. Let srcElementSize be TypedArrayElementSize(srcArray).
  7. Let srcByteOffset be srcArray.[[ByteOffset]].
  8. Let elementLength be srcArray.[[ArrayLength]].
  9. Let byteLength be elementSize × elementLength.
  10. If elementType is srcType, then
    1. Let data be ? CloneArrayBuffer(srcData, srcByteOffset, byteLength).
  11. Else,
    1. Let data be ? AllocateArrayBuffer(%ArrayBuffer%, byteLength).
    2. If srcArray.[[ContentType]] is not O.[[ContentType]], throw a TypeError exception.
    3. Let srcByteIndex be srcByteOffset.
    4. Let targetByteIndex be 0.
    5. Let count be elementLength.
    6. Repeat, while count > 0,
      1. Let value be GetValueFromBuffer(srcData, srcByteIndex, srcType, true, Unordered).
      2. Perform SetValueInBuffer(data, targetByteIndex, elementType, value, true, Unordered).
      3. Set srcByteIndex to srcByteIndex srcElementSize.
      4. Set targetByteIndex to targetByteIndex elementSize.
      5. Set count to count - 1.
  12. Set O.[[ViewedArrayBuffer]] to data.
  13. Set O.[[ByteLength]] to byteLength.
  14. Set O.[[ByteOffset]] to 0.
  15. Set O.[[ArrayLength]] to elementLength.
  16. Return unused.

23.2.5.1.3 InitializeTypedArrayFromArrayBuffer ( O, buffer, byteOffset, length )

The abstract operation InitializeTypedArrayFromArrayBuffer takes arguments O (a TypedArray), buffer (an ArrayBuffer or a SharedArrayBuffer), byteOffset (an ECMAScript language value), and length (an ECMAScript language value) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

  1. Let elementSize be TypedArrayElementSize(O).
  2. Let offset be ? ToIndex(byteOffset).
  3. If offset modulo elementSize ≠ 0, throw a RangeError exception.
  4. If length is not undefined, then
    1. Let newLength be ? ToIndex(length).
  5. If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
  6. Let bufferByteLength be buffer.[[ArrayBufferByteLength]].
  7. If length is undefined, then
    1. If bufferByteLength modulo elementSize ≠ 0, throw a RangeError exception.
    2. Let newByteLength be bufferByteLength - offset.
    3. If newByteLength < 0, throw a RangeError exception.
  8. Else,
    1. Let newByteLength be newLength × elementSize.
    2. If offset newByteLength > bufferByteLength, throw a RangeError exception.
  9. Set O.[[ViewedArrayBuffer]] to buffer.
  10. Set O.[[ByteLength]] to newByteLength.
  11. Set O.[[ByteOffset]] to offset.
  12. Set O.[[ArrayLength]] to newByteLength / elementSize.
  13. Return unused.

23.2.5.1.4 InitializeTypedArrayFromList ( O, values )

The abstract operation InitializeTypedArrayFromList takes arguments O (a TypedArray) and values (a List of ECMAScript language values) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

  1. Let len be the number of elements in values.
  2. Perform ? AllocateTypedArrayBuffer(O, len).
  3. Let k be 0.
  4. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kValue be the first element of values.
    3. Remove the first element from values.
    4. Perform ? Set(O, Pk, kValue, true).
    5. Set k to k 1.
  5. Assert: values is now an empty List.
  6. Return unused.

23.2.5.1.5 InitializeTypedArrayFromArrayLike ( O, arrayLike )

The abstract operation InitializeTypedArrayFromArrayLike takes arguments O (a TypedArray) and arrayLike (an Object, but not a TypedArray or an ArrayBuffer) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

  1. Let len be ? LengthOfArrayLike(arrayLike).
  2. Perform ? AllocateTypedArrayBuffer(O, len).
  3. Let k be 0.
  4. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kValue be ? Get(arrayLike, Pk).
    3. Perform ? Set(O, Pk, kValue, true).
    4. Set k to k 1.
  5. Return unused.

23.2.5.1.6 AllocateTypedArrayBuffer ( O, length )

The abstract operation AllocateTypedArrayBuffer takes arguments O (a TypedArray) and length (a non-negative integer) and returns either a normal completion containing unused or a throw completion. It allocates and associates an ArrayBuffer with O. It performs the following steps when called:

  1. Assert: O.[[ViewedArrayBuffer]] is undefined.
  2. Let elementSize be TypedArrayElementSize(O).
  3. Let byteLength be elementSize × length.
  4. Let data be ? AllocateArrayBuffer(%ArrayBuffer%, byteLength).
  5. Set O.[[ViewedArrayBuffer]] to data.
  6. Set O.[[ByteLength]] to byteLength.
  7. Set O.[[ByteOffset]] to 0.
  8. Set O.[[ArrayLength]] to length.
  9. Return unused.

23.2.6 Properties of the TypedArray Constructors

Each TypedArray constructor:

  • has a [[Prototype]] internal slot whose value is %TypedArray%.
  • has a "name" property whose value is the String value of the constructor name specified for it in Table 68.
  • has the following properties:

23.2.6.1 TypedArray.BYTES_PER_ELEMENT

The value of TypedArray.BYTES_PER_ELEMENT is the Element Size value specified in Table 68 for TypedArray.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

23.2.6.2 TypedArray.prototype

The initial value of TypedArray.prototype is the corresponding TypedArray prototype intrinsic object (23.2.7).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

23.2.7 Properties of the TypedArray Prototype Objects

Each TypedArray prototype object:

  • has a [[Prototype]] internal slot whose value is %TypedArray.prototype%.
  • is an ordinary object.
  • does not have a [[ViewedArrayBuffer]] or any other of the internal slots that are specific to TypedArray instance objects.

23.2.7.1 TypedArray.prototype.BYTES_PER_ELEMENT

The value of TypedArray.prototype.BYTES_PER_ELEMENT is the Element Size value specified in Table 68 for TypedArray.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

23.2.7.2 TypedArray.prototype.constructor

The initial value of a TypedArray.prototype.constructor is the corresponding %TypedArray% intrinsic object.

23.2.8 Properties of TypedArray Instances

TypedArray instances are Integer-Indexed exotic objects. Each TypedArray instance inherits properties from the corresponding TypedArray prototype object. Each TypedArray instance has the following internal slots: [[TypedArrayName]], [[ViewedArrayBuffer]], [[ByteLength]], [[ByteOffset]], and [[ArrayLength]].

24 Keyed Collections

24.1 Map Objects

Maps are collections of key/value pairs where both the keys and values may be arbitrary ECMAScript language values. A distinct key value may only occur in one key/value pair within the Map's collection. Distinct key values are discriminated using the SameValueZero comparison algorithm.

Maps must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of elements in the collection. The data structure used in this specification is only intended to describe the required observable semantics of Maps. It is not intended to be a viable implementation model.

24.1.1 The Map Constructor

The Map constructor:

  • is %Map%.
  • is the initial value of the "Map" property of the global object.
  • creates and initializes a new Map when called as a constructor.
  • is not intended to be called as a function and will throw an exception when called in that manner.
  • may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified Map behaviour must include a super call to the Map constructor to create and initialize the subclass instance with the internal state necessary to support the Map.prototype built-in methods.

24.1.1.1 Map ( [ iterable ] )

This function performs the following steps when called:

  1. If NewTarget is undefined, throw a TypeError exception.
  2. Let map be ? OrdinaryCreateFromConstructor(NewTarget, "%Map.prototype%", « [[MapData]] »).
  3. Set map.[[MapData]] to a new empty List.
  4. If iterable is either undefined or null, return map.
  5. Let adder be ? Get(map, "set").
  6. If IsCallable(adder) is false, throw a TypeError exception.
  7. Return ? AddEntriesFromIterable(map, iterable, adder).
 Note

If the parameter iterable is present, it is expected to be an object that implements an @@iterator method that returns an iterator object that produces a two element array-like object whose first element is a value that will be used as a Map key and whose second element is the value to associate with that key.

24.1.1.2 AddEntriesFromIterable ( target, iterable, adder )

The abstract operation AddEntriesFromIterable takes arguments target (an Object), iterable (an ECMAScript language value, but not undefined or null), and adder (a function object) and returns either a normal completion containing an ECMAScript language value or a throw completion. adder will be invoked, with target as the receiver. It performs the following steps when called:

  1. Let iteratorRecord be ? GetIterator(iterable, sync).
  2. Repeat,
    1. Let next be ? IteratorStep(iteratorRecord).
    2. If next is false, return target.
    3. Let nextItem be ? IteratorValue(next).
    4. If nextItem is not an Object, then
      1. Let error be ThrowCompletion(a newly created TypeError object).
      2. Return ? IteratorClose(iteratorRecord, error).
    5. Let k be Completion(Get(nextItem, "0")).
    6. IfAbruptCloseIterator(k, iteratorRecord).
    7. Let v be Completion(Get(nextItem, "1")).
    8. IfAbruptCloseIterator(v, iteratorRecord).
    9. Let status be Completion(Call(adder, target, « k, v »)).
    10. IfAbruptCloseIterator(status, iteratorRecord).
 Note

The parameter iterable is expected to be an object that implements an @@iterator method that returns an iterator object that produces a two element array-like object whose first element is a value that will be used as a Map key and whose second element is the value to associate with that key.

24.1.2 Properties of the Map Constructor

The Map constructor:

  • has a [[Prototype]] internal slot whose value is %Function.prototype%.
  • has the following properties:

24.1.2.1 Map.prototype

The initial value of Map.prototype is the Map prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

24.1.2.2 get Map [ @@species ]

Map[@@species] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Return the this value.

The value of the "name" property of this function is "get [Symbol.species]".

Note

Methods that create derived collection objects should call @@species to determine the constructor to use to create the derived objects. Subclass constructor may over-ride @@species to change the default constructor assignment.

24.1.3 Properties of the Map Prototype Object

The Map prototype object:

24.1.3.1 Map.prototype.clear ( )

This method performs the following steps when called:

  1. Let M be the this value.
  2. Perform ? RequireInternalSlot(M, [[MapData]]).
  3. For each Record { [[Key]], [[Value]] } p of M.[[MapData]], do
    1. Set p.[[Key]] to empty.
    2. Set p.[[Value]] to empty.
  4. Return undefined.
 Note

The existing [[MapData]] List is preserved because there may be existing Map Iterator objects that are suspended midway through iterating over that List.

24.1.3.2 Map.prototype.constructor

The initial value of Map.prototype.constructor is %Map%.

24.1.3.3 Map.prototype.delete ( key )

This method performs the following steps when called:

  1. Let M be the this value.
  2. Perform ? RequireInternalSlot(M, [[MapData]]).
  3. For each Record { [[Key]], [[Value]] } p of M.[[MapData]], do
    1. If p.[[Key]] is not empty and SameValueZero(p.[[Key]], key) is true, then
      1. Set p.[[Key]] to empty.
      2. Set p.[[Value]] to empty.
      3. Return true.
  4. Return false.
 Note

The value empty is used as a specification device to indicate that an entry has been deleted. Actual implementations may take other actions such as physically removing the entry from internal data structures.

24.1.3.4 Map.prototype.entries ( )

This method performs the following steps when called:

  1. Let M be the this value.
  2. Return ? CreateMapIterator(M, key value).

24.1.3.5 Map.prototype.forEach ( callbackfn [ , thisArg ] )

This method performs the following steps when called:

  1. Let M be the this value.
  2. Perform ? RequireInternalSlot(M, [[MapData]]).
  3. If IsCallable(callbackfn) is false, throw a TypeError exception.
  4. Let entries be M.[[MapData]].
  5. Let numEntries be the number of elements in entries.
  6. Let index be 0.
  7. Repeat, while index < numEntries,
    1. Let e be entries[index].
    2. Set index to index 1.
    3. If e.[[Key]] is not empty, then
      1. Perform ? Call(callbackfn, thisArg, « e.[[Value]], e.[[Key]], M »).
      2. NOTE: The number of elements in entries may have increased during execution of callbackfn.
      3. Set numEntries to the number of elements in entries.
  8. Return undefined.
 Note

callbackfn should be a function that accepts three arguments. forEach calls callbackfn once for each key/value pair present in the Map, in key insertion order. callbackfn is called only for keys of the Map which actually exist; it is not called for keys that have been deleted from the Map.

If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the item, the key of the item, and the Map being traversed.

forEach does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn. Each entry of a map's [[MapData]] is only visited once. New keys added after the call to forEach begins are visited. A key will be revisited if it is deleted after it has been visited and then re-added before the forEach call completes. Keys that are deleted after the call to forEach begins and before being visited are not visited unless the key is added again before the forEach call completes.

24.1.3.6 Map.prototype.get ( key )

This method performs the following steps when called:

  1. Let M be the this value.
  2. Perform ? RequireInternalSlot(M, [[MapData]]).
  3. For each Record { [[Key]], [[Value]] } p of M.[[MapData]], do
    1. If p.[[Key]] is not empty and SameValueZero(p.[[Key]], key) is true, return p.[[Value]].
  4. Return undefined.

24.1.3.7 Map.prototype.has ( key )

This method performs the following steps when called:

  1. Let M be the this value.
  2. Perform ? RequireInternalSlot(M, [[MapData]]).
  3. For each Record { [[Key]], [[Value]] } p of M.[[MapData]], do
    1. If p.[[Key]] is not empty and SameValueZero(p.[[Key]], key) is true, return true.
  4. Return false.

24.1.3.8 Map.prototype.keys ( )

This method performs the following steps when called:

  1. Let M be the this value.
  2. Return ? CreateMapIterator(M, key).

24.1.3.9 Map.prototype.set ( key, value )

This method performs the following steps when called:

  1. Let M be the this value.
  2. Perform ? RequireInternalSlot(M, [[MapData]]).
  3. For each Record { [[Key]], [[Value]] } p of M.[[MapData]], do
    1. If p.[[Key]] is not empty and SameValueZero(p.[[Key]], key) is true, then
      1. Set p.[[Value]] to value.
      2. Return M.
  4. If key is -0𝔽, set key to 0𝔽.
  5. Let p be the Record { [[Key]]: key, [[Value]]: value }.
  6. Append p to M.[[MapData]].
  7. Return M.

24.1.3.10 get Map.prototype.size

Map.prototype.size is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Let M be the this value.
  2. Perform ? RequireInternalSlot(M, [[MapData]]).
  3. Let count be 0.
  4. For each Record { [[Key]], [[Value]] } p of M.[[MapData]], do
    1. If p.[[Key]] is not empty, set count to count 1.
  5. Return 𝔽(count).

24.1.3.11 Map.prototype.values ( )

This method performs the following steps when called:

  1. Let M be the this value.
  2. Return ? CreateMapIterator(M, value).

24.1.3.12 Map.prototype [ @@iterator ] ( )

The initial value of the @@iterator property is %Map.prototype.entries%, defined in 24.1.3.4.

24.1.3.13 Map.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "Map".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

24.1.4 Properties of Map Instances

Map instances are ordinary objects that inherit properties from the Map prototype. Map instances also have a [[MapData]] internal slot.

24.1.5 Map Iterator Objects

A Map Iterator is an object, that represents a specific iteration over some specific Map instance object. There is not a named constructor for Map Iterator objects. Instead, map iterator objects are created by calling certain methods of Map instance objects.

24.1.5.1 CreateMapIterator ( map, kind )

The abstract operation CreateMapIterator takes arguments map (an ECMAScript language value) and kind (key value, key, or value) and returns either a normal completion containing a Generator or a throw completion. It is used to create iterator objects for Map methods that return such iterators. It performs the following steps when called:

  1. Perform ? RequireInternalSlot(map, [[MapData]]).
  2. Let closure be a new Abstract Closure with no parameters that captures map and kind and performs the following steps when called:
    1. Let entries be map.[[MapData]].
    2. Let index be 0.
    3. Let numEntries be the number of elements in entries.
    4. Repeat, while index < numEntries,
      1. Let e be entries[index].
      2. Set index to index 1.
      3. If e.[[Key]] is not empty, then
        1. If kind is key, let result be e.[[Key]].
        2. Else if kind is value, let result be e.[[Value]].
        3. Else,
          1. Assert: kind is key value.
          2. Let result be CreateArrayFromList(« e.[[Key]], e.[[Value]] »).
        4. Perform ? GeneratorYield(CreateIterResultObject(result, false)).
        5. NOTE: The number of elements in entries may have increased while execution of this abstract operation was paused by Yield.
        6. Set numEntries to the number of elements in entries.
    5. Return undefined.
  3. Return CreateIteratorFromClosure(closure, "%MapIteratorPrototype%", %MapIteratorPrototype%).

24.1.5.2 The %MapIteratorPrototype% Object

The %MapIteratorPrototype% object:

  • has properties that are inherited by all Map Iterator Objects.
  • is an ordinary object.
  • has a [[Prototype]] internal slot whose value is %IteratorPrototype%.
  • has the following properties:

24.1.5.2.1 %MapIteratorPrototype%.next ( )

  1. Return ? GeneratorResume(this value, empty, "%MapIteratorPrototype%").

24.1.5.2.2 %MapIteratorPrototype% [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "Map Iterator".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

24.2 Set Objects

Set objects are collections of ECMAScript language values. A distinct value may only occur once as an element of a Set's collection. Distinct values are discriminated using the SameValueZero comparison algorithm.

Set objects must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of elements in the collection. The data structure used in this specification is only intended to describe the required observable semantics of Set objects. It is not intended to be a viable implementation model.

24.2.1 The Set Constructor

The Set constructor:

  • is %Set%.
  • is the initial value of the "Set" property of the global object.
  • creates and initializes a new Set object when called as a constructor.
  • is not intended to be called as a function and will throw an exception when called in that manner.
  • may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified Set behaviour must include a super call to the Set constructor to create and initialize the subclass instance with the internal state necessary to support the Set.prototype built-in methods.

24.2.1.1 Set ( [ iterable ] )

This function performs the following steps when called:

  1. If NewTarget is undefined, throw a TypeError exception.
  2. Let set be ? OrdinaryCreateFromConstructor(NewTarget, "%Set.prototype%", « [[SetData]] »).
  3. Set set.[[SetData]] to a new empty List.
  4. If iterable is either undefined or null, return set.
  5. Let adder be ? Get(set, "add").
  6. If IsCallable(adder) is false, throw a TypeError exception.
  7. Let iteratorRecord be ? GetIterator(iterable, sync).
  8. Repeat,
    1. Let next be ? IteratorStep(iteratorRecord).
    2. If next is false, return set.
    3. Let nextValue be ? IteratorValue(next).
    4. Let status be Completion(Call(adder, set, « nextValue »)).
    5. IfAbruptCloseIterator(status, iteratorRecord).

24.2.2 Properties of the Set Constructor

The Set constructor:

  • has a [[Prototype]] internal slot whose value is %Function.prototype%.
  • has the following properties:

24.2.2.1 Set.prototype

The initial value of Set.prototype is the Set prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

24.2.2.2 get Set [ @@species ]

Set[@@species] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Return the this value.

The value of the "name" property of this function is "get [Symbol.species]".

Note

Methods that create derived collection objects should call @@species to determine the constructor to use to create the derived objects. Subclass constructor may over-ride @@species to change the default constructor assignment.

24.2.3 Properties of the Set Prototype Object

The Set prototype object:

24.2.3.1 Set.prototype.add ( value )

This method performs the following steps when called:

  1. Let S be the this value.
  2. Perform ? RequireInternalSlot(S, [[SetData]]).
  3. For each element e of S.[[SetData]], do
    1. If e is not empty and SameValueZero(e, value) is true, then
      1. Return S.
  4. If value is -0𝔽, set value to 0𝔽.
  5. Append value to S.[[SetData]].
  6. Return S.

24.2.3.2 Set.prototype.clear ( )

This method performs the following steps when called:

  1. Let S be the this value.
  2. Perform ? RequireInternalSlot(S, [[SetData]]).
  3. For each element e of S.[[SetData]], do
    1. Replace the element of S.[[SetData]] whose value is e with an element whose value is empty.
  4. Return undefined.
 Note

The existing [[SetData]] List is preserved because there may be existing Set Iterator objects that are suspended midway through iterating over that List.

24.2.3.3 Set.prototype.constructor

The initial value of Set.prototype.constructor is %Set%.

24.2.3.4 Set.prototype.delete ( value )

This method performs the following steps when called:

  1. Let S be the this value.
  2. Perform ? RequireInternalSlot(S, [[SetData]]).
  3. For each element e of S.[[SetData]], do
    1. If e is not empty and SameValueZero(e, value) is true, then
      1. Replace the element of S.[[SetData]] whose value is e with an element whose value is empty.
      2. Return true.
  4. Return false.
 Note

The value empty is used as a specification device to indicate that an entry has been deleted. Actual implementations may take other actions such as physically removing the entry from internal data structures.

24.2.3.5 Set.prototype.entries ( )

This method performs the following steps when called:

  1. Let S be the this value.
  2. Return ? CreateSetIterator(S, key value).
 Note

For iteration purposes, a Set appears similar to a Map where each entry has the same value for its key and value.

24.2.3.6 Set.prototype.forEach ( callbackfn [ , thisArg ] )

This method performs the following steps when called:

  1. Let S be the this value.
  2. Perform ? RequireInternalSlot(S, [[SetData]]).
  3. If IsCallable(callbackfn) is false, throw a TypeError exception.
  4. Let entries be S.[[SetData]].
  5. Let numEntries be the number of elements in entries.
  6. Let index be 0.
  7. Repeat, while index < numEntries,
    1. Let e be entries[index].
    2. Set index to index 1.
    3. If e is not empty, then
      1. Perform ? Call(callbackfn, thisArg, « e, e, S »).
      2. NOTE: The number of elements in entries may have increased during execution of callbackfn.
      3. Set numEntries to the number of elements in entries.
  8. Return undefined.
 Note

callbackfn should be a function that accepts three arguments. forEach calls callbackfn once for each value present in the Set object, in value insertion order. callbackfn is called only for values of the Set which actually exist; it is not called for keys that have been deleted from the set.

If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the first two arguments are a value contained in the Set. The same value is passed for both arguments. The Set object being traversed is passed as the third argument.

The callbackfn is called with three arguments to be consistent with the call back functions used by forEach methods for Map and Array. For Sets, each item value is considered to be both the key and the value.

forEach does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

Each value is normally visited only once. However, a value will be revisited if it is deleted after it has been visited and then re-added before the forEach call completes. Values that are deleted after the call to forEach begins and before being visited are not visited unless the value is added again before the forEach call completes. New values added after the call to forEach begins are visited.

24.2.3.7 Set.prototype.has ( value )

This method performs the following steps when called:

  1. Let S be the this value.
  2. Perform ? RequireInternalSlot(S, [[SetData]]).
  3. For each element e of S.[[SetData]], do
    1. If e is not empty and SameValueZero(e, value) is true, return true.
  4. Return false.

24.2.3.8 Set.prototype.keys ( )

The initial value of the "keys" property is %Set.prototype.values%, defined in 24.2.3.10.

Note

For iteration purposes, a Set appears similar to a Map where each entry has the same value for its key and value.

24.2.3.9 get Set.prototype.size

Set.prototype.size is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Let S be the this value.
  2. Perform ? RequireInternalSlot(S, [[SetData]]).
  3. Let count be 0.
  4. For each element e of S.[[SetData]], do
    1. If e is not empty, set count to count 1.
  5. Return 𝔽(count).

24.2.3.10 Set.prototype.values ( )

This method performs the following steps when called:

  1. Let S be the this value.
  2. Return ? CreateSetIterator(S, value).

24.2.3.11 Set.prototype [ @@iterator ] ( )

The initial value of the @@iterator property is %Set.prototype.values%, defined in 24.2.3.10.

24.2.3.12 Set.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "Set".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

24.2.4 Properties of Set Instances

Set instances are ordinary objects that inherit properties from the Set prototype. Set instances also have a [[SetData]] internal slot.

24.2.5 Set Iterator Objects

A Set Iterator is an ordinary object, with the structure defined below, that represents a specific iteration over some specific Set instance object. There is not a named constructor for Set Iterator objects. Instead, set iterator objects are created by calling certain methods of Set instance objects.

24.2.5.1 CreateSetIterator ( set, kind )

The abstract operation CreateSetIterator takes arguments set (an ECMAScript language value) and kind (key value or value) and returns either a normal completion containing a Generator or a throw completion. It is used to create iterator objects for Set methods that return such iterators. It performs the following steps when called:

  1. Perform ? RequireInternalSlot(set, [[SetData]]).
  2. Let closure be a new Abstract Closure with no parameters that captures set and kind and performs the following steps when called:
    1. Let index be 0.
    2. Let entries be set.[[SetData]].
    3. Let numEntries be the number of elements in entries.
    4. Repeat, while index < numEntries,
      1. Let e be entries[index].
      2. Set index to index 1.
      3. If e is not empty, then
        1. If kind is key value, then
          1. Let result be CreateArrayFromList(« e, e »).
          2. Perform ? GeneratorYield(CreateIterResultObject(result, false)).
        2. Else,
          1. Assert: kind is value.
          2. Perform ? GeneratorYield(CreateIterResultObject(e, false)).
        3. NOTE: The number of elements in entries may have increased while execution of this abstract operation was paused by Yield.
        4. Set numEntries to the number of elements in entries.
    5. Return undefined.
  3. Return CreateIteratorFromClosure(closure, "%SetIteratorPrototype%", %SetIteratorPrototype%).

24.2.5.2 The %SetIteratorPrototype% Object

The %SetIteratorPrototype% object:

  • has properties that are inherited by all Set Iterator Objects.
  • is an ordinary object.
  • has a [[Prototype]] internal slot whose value is %IteratorPrototype%.
  • has the following properties:

24.2.5.2.1 %SetIteratorPrototype%.next ( )

  1. Return ? GeneratorResume(this value, empty, "%SetIteratorPrototype%").

24.2.5.2.2 %SetIteratorPrototype% [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "Set Iterator".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

24.3 WeakMap Objects

WeakMaps are collections of key/value pairs where the keys are objects and/or symbols and values may be arbitrary ECMAScript language values. A WeakMap may be queried to see if it contains a key/value pair with a specific key, but no mechanism is provided for enumerating the values it holds as keys. In certain conditions, values which are not live are removed as WeakMap keys, as described in 9.10.3.

An implementation may impose an arbitrarily determined latency between the time a key/value pair of a WeakMap becomes inaccessible and the time when the key/value pair is removed from the WeakMap. If this latency was observable to ECMAScript program, it would be a source of indeterminacy that could impact program execution. For that reason, an ECMAScript implementation must not provide any means to observe a key of a WeakMap that does not require the observer to present the observed key.

WeakMaps must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of key/value pairs in the collection. The data structure used in this specification is only intended to describe the required observable semantics of WeakMaps. It is not intended to be a viable implementation model.

Note

WeakMap and WeakSet are intended to provide mechanisms for dynamically associating state with an object or symbol in a manner that does not “leak” memory resources if, in the absence of the WeakMap or WeakSet instance, the object or symbol otherwise became inaccessible and subject to resource reclamation by the implementation's garbage collection mechanisms. This characteristic can be achieved by using an inverted per-object/symbol mapping of WeakMap or WeakSet instances to keys. Alternatively, each WeakMap or WeakSet instance may internally store its key and value data, but this approach requires coordination between the WeakMap or WeakSet implementation and the garbage collector. The following references describe mechanism that may be useful to implementations of WeakMap and WeakSet:

Barry Hayes. 1997. Ephemerons: a new finalization mechanism. In Proceedings of the 12th ACM SIGPLAN conference on Object-oriented programming, systems, languages, and applications (OOPSLA '97), A. Michael Berman (Ed.). ACM, New York, NY, USA, 176-183, http://doi.acm.org/10.1145/263698.263733.

Alexandra Barros, Roberto Ierusalimschy, Eliminating Cycles in Weak Tables. Journal of Universal Computer Science - J.UCS, vol. 14, no. 21, pp. 3481-3497, 2008, http://www.jucs.org/jucs_14_21/eliminating_cycles_in_weak

24.3.1 The WeakMap Constructor

The WeakMap constructor:

  • is %WeakMap%.
  • is the initial value of the "WeakMap" property of the global object.
  • creates and initializes a new WeakMap when called as a constructor.
  • is not intended to be called as a function and will throw an exception when called in that manner.
  • may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified WeakMap behaviour must include a super call to the WeakMap constructor to create and initialize the subclass instance with the internal state necessary to support the WeakMap.prototype built-in methods.

24.3.1.1 WeakMap ( [ iterable ] )

This function performs the following steps when called:

  1. If NewTarget is undefined, throw a TypeError exception.
  2. Let map be ? OrdinaryCreateFromConstructor(NewTarget, "%WeakMap.prototype%", « [[WeakMapData]] »).
  3. Set map.[[WeakMapData]] to a new empty List.
  4. If iterable is either undefined or null, return map.
  5. Let adder be ? Get(map, "set").
  6. If IsCallable(adder) is false, throw a TypeError exception.
  7. Return ? AddEntriesFromIterable(map, iterable, adder).
 Note

If the parameter iterable is present, it is expected to be an object that implements an @@iterator method that returns an iterator object that produces a two element array-like object whose first element is a value that will be used as a WeakMap key and whose second element is the value to associate with that key.

24.3.2 Properties of the WeakMap Constructor

The WeakMap constructor:

  • has a [[Prototype]] internal slot whose value is %Function.prototype%.
  • has the following properties:

24.3.2.1 WeakMap.prototype

The initial value of WeakMap.prototype is the WeakMap prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

24.3.3 Properties of the WeakMap Prototype Object

The WeakMap prototype object:

24.3.3.1 WeakMap.prototype.constructor

The initial value of WeakMap.prototype.constructor is %WeakMap%.

24.3.3.2 WeakMap.prototype.delete ( key )

This method performs the following steps when called:

  1. Let M be the this value.
  2. Perform ? RequireInternalSlot(M, [[WeakMapData]]).
  3. If CanBeHeldWeakly(key) is false, return false.
  4. For each Record { [[Key]], [[Value]] } p of M.[[WeakMapData]], do
    1. If p.[[Key]] is not empty and SameValue(p.[[Key]], key) is true, then
      1. Set p.[[Key]] to empty.
      2. Set p.[[Value]] to empty.
      3. Return true.
  5. Return false.
 Note

The value empty is used as a specification device to indicate that an entry has been deleted. Actual implementations may take other actions such as physically removing the entry from internal data structures.

24.3.3.3 WeakMap.prototype.get ( key )

This method performs the following steps when called:

  1. Let M be the this value.
  2. Perform ? RequireInternalSlot(M, [[WeakMapData]]).
  3. If CanBeHeldWeakly(key) is false, return undefined.
  4. For each Record { [[Key]], [[Value]] } p of M.[[WeakMapData]], do
    1. If p.[[Key]] is not empty and SameValue(p.[[Key]], key) is true, return p.[[Value]].
  5. Return undefined.

24.3.3.4 WeakMap.prototype.has ( key )

This method performs the following steps when called:

  1. Let M be the this value.
  2. Perform ? RequireInternalSlot(M, [[WeakMapData]]).
  3. If CanBeHeldWeakly(key) is false, return false.
  4. For each Record { [[Key]], [[Value]] } p of M.[[WeakMapData]], do
    1. If p.[[Key]] is not empty and SameValue(p.[[Key]], key) is true, return true.
  5. Return false.

24.3.3.5 WeakMap.prototype.set ( key, value )

This method performs the following steps when called:

  1. Let M be the this value.
  2. Perform ? RequireInternalSlot(M, [[WeakMapData]]).
  3. If CanBeHeldWeakly(key) is false, throw a TypeError exception.
  4. For each Record { [[Key]], [[Value]] } p of M.[[WeakMapData]], do
    1. If p.[[Key]] is not empty and SameValue(p.[[Key]], key) is true, then
      1. Set p.[[Value]] to value.
      2. Return M.
  5. Let p be the Record { [[Key]]: key, [[Value]]: value }.
  6. Append p to M.[[WeakMapData]].
  7. Return M.

24.3.3.6 WeakMap.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "WeakMap".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

24.3.4 Properties of WeakMap Instances

WeakMap instances are ordinary objects that inherit properties from the WeakMap prototype. WeakMap instances also have a [[WeakMapData]] internal slot.

24.4 WeakSet Objects

WeakSets are collections of objects and/or symbols. A distinct object or symbol may only occur once as an element of a WeakSet's collection. A WeakSet may be queried to see if it contains a specific value, but no mechanism is provided for enumerating the values it holds. In certain conditions, values which are not live are removed as WeakSet elements, as described in 9.10.3.

An implementation may impose an arbitrarily determined latency between the time a value contained in a WeakSet becomes inaccessible and the time when the value is removed from the WeakSet. If this latency was observable to ECMAScript program, it would be a source of indeterminacy that could impact program execution. For that reason, an ECMAScript implementation must not provide any means to determine if a WeakSet contains a particular value that does not require the observer to present the observed value.

WeakSets must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of elements in the collection. The data structure used in this specification is only intended to describe the required observable semantics of WeakSets. It is not intended to be a viable implementation model.

Note

See the NOTE in 24.3.

24.4.1 The WeakSet Constructor

The WeakSet constructor:

  • is %WeakSet%.
  • is the initial value of the "WeakSet" property of the global object.
  • creates and initializes a new WeakSet when called as a constructor.
  • is not intended to be called as a function and will throw an exception when called in that manner.
  • may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified WeakSet behaviour must include a super call to the WeakSet constructor to create and initialize the subclass instance with the internal state necessary to support the WeakSet.prototype built-in methods.

24.4.1.1 WeakSet ( [ iterable ] )

This function performs the following steps when called:

  1. If NewTarget is undefined, throw a TypeError exception.
  2. Let set be ? OrdinaryCreateFromConstructor(NewTarget, "%WeakSet.prototype%", « [[WeakSetData]] »).
  3. Set set.[[WeakSetData]] to a new empty List.
  4. If iterable is either undefined or null, return set.
  5. Let adder be ? Get(set, "add").
  6. If IsCallable(adder) is false, throw a TypeError exception.
  7. Let iteratorRecord be ? GetIterator(iterable, sync).
  8. Repeat,
    1. Let next be ? IteratorStep(iteratorRecord).
    2. If next is false, return set.
    3. Let nextValue be ? IteratorValue(next).
    4. Let status be Completion(Call(adder, set, « nextValue »)).
    5. IfAbruptCloseIterator(status, iteratorRecord).

24.4.2 Properties of the WeakSet Constructor

The WeakSet constructor:

  • has a [[Prototype]] internal slot whose value is %Function.prototype%.
  • has the following properties:

24.4.2.1 WeakSet.prototype

The initial value of WeakSet.prototype is the WeakSet prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

24.4.3 Properties of the WeakSet Prototype Object

The WeakSet prototype object:

24.4.3.1 WeakSet.prototype.add ( value )

This method performs the following steps when called:

  1. Let S be the this value.
  2. Perform ? RequireInternalSlot(S, [[WeakSetData]]).
  3. If CanBeHeldWeakly(value) is false, throw a TypeError exception.
  4. For each element e of S.[[WeakSetData]], do
    1. If e is not empty and SameValue(e, value) is true, then
      1. Return S.
  5. Append value to S.[[WeakSetData]].
  6. Return S.

24.4.3.2 WeakSet.prototype.constructor

The initial value of WeakSet.prototype.constructor is %WeakSet%.

24.4.3.3 WeakSet.prototype.delete ( value )

This method performs the following steps when called:

  1. Let S be the this value.
  2. Perform ? RequireInternalSlot(S, [[WeakSetData]]).
  3. If CanBeHeldWeakly(value) is false, return false.
  4. For each element e of S.[[WeakSetData]], do
    1. If e is not empty and SameValue(e, value) is true, then
      1. Replace the element of S.[[WeakSetData]] whose value is e with an element whose value is empty.
      2. Return true.
  5. Return false.
 Note

The value empty is used as a specification device to indicate that an entry has been deleted. Actual implementations may take other actions such as physically removing the entry from internal data structures.

24.4.3.4 WeakSet.prototype.has ( value )

This method performs the following steps when called:

  1. Let S be the this value.
  2. Perform ? RequireInternalSlot(S, [[WeakSetData]]).
  3. If CanBeHeldWeakly(value) is false, return false.
  4. For each element e of S.[[WeakSetData]], do
    1. If e is not empty and SameValue(e, value) is true, return true.
  5. Return false.

24.4.3.5 WeakSet.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "WeakSet".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

24.4.4 Properties of WeakSet Instances

WeakSet instances are ordinary objects that inherit properties from the WeakSet prototype. WeakSet instances also have a [[WeakSetData]] internal slot.

25 Structured Data

25.1 ArrayBuffer Objects

25.1.1 Notation

The descriptions below in this section, 25.4, and 29 use the read-modify-write modification function internal data structure.

A read-modify-write modification function is a mathematical function that is notationally represented as an abstract closure that takes two Lists of byte values as arguments and returns a List of byte values. These abstract closures satisfy all of the following properties:

  • They perform all their algorithm steps atomically.
  • Their individual algorithm steps are not observable.
Note

To aid verifying that a read-modify-write modification function's algorithm steps constitute a pure, mathematical function, the following editorial conventions are recommended:

  • They do not access, directly or transitively via invoked abstract operations and abstract closures, any language or specification values except their parameters and captured values.
  • They do not return Completion Records.

25.1.2 Abstract Operations For ArrayBuffer Objects

25.1.2.1 AllocateArrayBuffer ( constructor, byteLength )

The abstract operation AllocateArrayBuffer takes arguments constructor (a constructor) and byteLength (a non-negative integer) and returns either a normal completion containing an ArrayBuffer or a throw completion. It is used to create an ArrayBuffer. It performs the following steps when called:

  1. Let obj be ? OrdinaryCreateFromConstructor(constructor, "%ArrayBuffer.prototype%", « [[ArrayBufferData]], [[ArrayBufferByteLength]], [[ArrayBufferDetachKey]] »).
  2. Let block be ? CreateByteDataBlock(byteLength).
  3. Set obj.[[ArrayBufferData]] to block.
  4. Set obj.[[ArrayBufferByteLength]] to byteLength.
  5. Return obj.

25.1.2.2 IsDetachedBuffer ( arrayBuffer )

The abstract operation IsDetachedBuffer takes argument arrayBuffer (an ArrayBuffer or a SharedArrayBuffer) and returns a Boolean. It performs the following steps when called:

  1. If arrayBuffer.[[ArrayBufferData]] is null, return true.
  2. Return false.

25.1.2.3 DetachArrayBuffer ( arrayBuffer [ , key ] )

The abstract operation DetachArrayBuffer takes argument arrayBuffer (an ArrayBuffer) and optional argument key (anything) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

  1. Assert: IsSharedArrayBuffer(arrayBuffer) is false.
  2. If key is not present, set key to undefined.
  3. If arrayBuffer.[[ArrayBufferDetachKey]] is not key, throw a TypeError exception.
  4. Set arrayBuffer.[[ArrayBufferData]] to null.
  5. Set arrayBuffer.[[ArrayBufferByteLength]] to 0.
  6. Return unused.
 Note

Detaching an ArrayBuffer instance disassociates the Data Block used as its backing store from the instance and sets the byte length of the buffer to 0. No operations defined by this specification use the DetachArrayBuffer abstract operation. However, an ECMAScript host or implementation may define such operations.

25.1.2.4 CloneArrayBuffer ( srcBuffer, srcByteOffset, srcLength )

The abstract operation CloneArrayBuffer takes arguments srcBuffer (an ArrayBuffer or a SharedArrayBuffer), srcByteOffset (a non-negative integer), and srcLength (a non-negative integer) and returns either a normal completion containing an ArrayBuffer or a throw completion. It creates a new ArrayBuffer whose data is a copy of srcBuffer's data over the range starting at srcByteOffset and continuing for srcLength bytes. It performs the following steps when called:

  1. Assert: IsDetachedBuffer(srcBuffer) is false.
  2. Let targetBuffer be ? AllocateArrayBuffer(%ArrayBuffer%, srcLength).
  3. Let srcBlock be srcBuffer.[[ArrayBufferData]].
  4. Let targetBlock be targetBuffer.[[ArrayBufferData]].
  5. Perform CopyDataBlockBytes(targetBlock, 0, srcBlock, srcByteOffset, srcLength).
  6. Return targetBuffer.

25.1.2.5 IsUnsignedElementType ( type )

The abstract operation IsUnsignedElementType takes argument type (a TypedArray element type) and returns a Boolean. It verifies if the argument type is an unsigned TypedArray element type. It performs the following steps when called:

  1. If type is one of Uint8, Uint8C, Uint16, Uint32, or BigUint64, return true.
  2. Return false.

25.1.2.6 IsUnclampedIntegerElementType ( type )

The abstract operation IsUnclampedIntegerElementType takes argument type (a TypedArray element type) and returns a Boolean. It verifies if the argument type is an Integer TypedArray element type not including Uint8C. It performs the following steps when called:

  1. If type is one of Int8, Uint8, Int16, Uint16, Int32, or Uint32, return true.
  2. Return false.

25.1.2.7 IsBigIntElementType ( type )

The abstract operation IsBigIntElementType takes argument type (a TypedArray element type) and returns a Boolean. It verifies if the argument type is a BigInt TypedArray element type. It performs the following steps when called:

  1. If type is either BigUint64 or BigInt64, return true.
  2. Return false.

25.1.2.8 IsNoTearConfiguration ( type, order )

The abstract operation IsNoTearConfiguration takes arguments type (a TypedArray element type) and order (SeqCst, Unordered, or Init) and returns a Boolean. It performs the following steps when called:

  1. If IsUnclampedIntegerElementType(type) is true, return true.
  2. If IsBigIntElementType(type) is true and order is neither Init nor Unordered, return true.
  3. Return false.

25.1.2.9 RawBytesToNumeric ( type, rawBytes, isLittleEndian )

The abstract operation RawBytesToNumeric takes arguments type (a TypedArray element type), rawBytes (a List of byte values), and isLittleEndian (a Boolean) and returns a Number or a BigInt. It performs the following steps when called:

  1. Let elementSize be the Element Size value specified in Table 68 for Element Type type.
  2. If isLittleEndian is false, reverse the order of the elements of rawBytes.
  3. If type is Float32, then
    1. Let value be the byte elements of rawBytes concatenated and interpreted as a little-endian bit string encoding of an IEEE 754-2019 binary32 value.
    2. If value is an IEEE 754-2019 binary32 NaN value, return the NaN Number value.
    3. Return the Number value that corresponds to value.
  4. If type is Float64, then
    1. Let value be the byte elements of rawBytes concatenated and interpreted as a little-endian bit string encoding of an IEEE 754-2019 binary64 value.
    2. If value is an IEEE 754-2019 binary64 NaN value, return the NaN Number value.
    3. Return the Number value that corresponds to value.
  5. If IsUnsignedElementType(type) is true, then
    1. Let intValue be the byte elements of rawBytes concatenated and interpreted as a bit string encoding of an unsigned little-endian binary number.
  6. Else,
    1. Let intValue be the byte elements of rawBytes concatenated and interpreted as a bit string encoding of a binary little-endian two's complement number of bit length elementSize × 8.
  7. If IsBigIntElementType(type) is true, return the BigInt value that corresponds to intValue.
  8. Otherwise, return the Number value that corresponds to intValue.

25.1.2.10 GetValueFromBuffer ( arrayBuffer, byteIndex, type, isTypedArray, order [ , isLittleEndian ] )

The abstract operation GetValueFromBuffer takes arguments arrayBuffer (an ArrayBuffer or SharedArrayBuffer), byteIndex (a non-negative integer), type (a TypedArray element type), isTypedArray (a Boolean), and order (SeqCst or Unordered) and optional argument isLittleEndian (a Boolean) and returns a Number or a BigInt. It performs the following steps when called:

  1. Assert: IsDetachedBuffer(arrayBuffer) is false.
  2. Assert: There are sufficient bytes in arrayBuffer starting at byteIndex to represent a value of type.
  3. Let block be arrayBuffer.[[ArrayBufferData]].
  4. Let elementSize be the Element Size value specified in Table 68 for Element Type type.
  5. If IsSharedArrayBuffer(arrayBuffer) is true, then
    1. Let execution be the [[CandidateExecution]] field of the surrounding agent's Agent Record.
    2. Let eventsRecord be the Agent Events Record of execution.[[EventsRecords]] whose [[AgentSignifier]] is AgentSignifier().
    3. If isTypedArray is true and IsNoTearConfiguration(type, order) is true, let noTear be true; otherwise let noTear be false.
    4. Let rawValue be a List of length elementSize whose elements are nondeterministically chosen byte values.
    5. NOTE: In implementations, rawValue is the result of a non-atomic or atomic read instruction on the underlying hardware. The nondeterminism is a semantic prescription of the memory model to describe observable behaviour of hardware with weak consistency.
    6. Let readEvent be ReadSharedMemory { [[Order]]: order, [[NoTear]]: noTear, [[Block]]: block, [[ByteIndex]]: byteIndex, [[ElementSize]]: elementSize }.
    7. Append readEvent to eventsRecord.[[EventList]].
    8. Append Chosen Value Record { [[Event]]: readEvent, [[ChosenValue]]: rawValue } to execution.[[ChosenValues]].
  6. Else, let rawValue be a List whose elements are bytes from block at indices in the interval from byteIndex (inclusive) to byteIndex elementSize (exclusive).
  7. Assert: The number of elements in rawValue is elementSize.
  8. If isLittleEndian is not present, set isLittleEndian to the value of the [[LittleEndian]] field of the surrounding agent's Agent Record.
  9. Return RawBytesToNumeric(type, rawValue, isLittleEndian).

25.1.2.11 NumericToRawBytes ( type, value, isLittleEndian )

The abstract operation NumericToRawBytes takes arguments type (a TypedArray element type), value (a Number or a BigInt), and isLittleEndian (a Boolean) and returns a List of byte values. It performs the following steps when called:

  1. If type is Float32, then
    1. Let rawBytes be a List whose elements are the 4 bytes that are the result of converting value to IEEE 754-2019 binary32 format using roundTiesToEven mode. The bytes are arranged in little endian order. If value is NaN, rawBytes may be set to any implementation chosen IEEE 754-2019 binary32 format Not-a-Number encoding. An implementation must always choose the same encoding for each implementation distinguishable NaN value.
  2. Else if type is Float64, then
    1. Let rawBytes be a List whose elements are the 8 bytes that are the IEEE 754-2019 binary64 format encoding of value. The bytes are arranged in little endian order. If value is NaN, rawBytes may be set to any implementation chosen IEEE 754-2019 binary64 format Not-a-Number encoding. An implementation must always choose the same encoding for each implementation distinguishable NaN value.
  3. Else,
    1. Let n be the Element Size value specified in Table 68 for Element Type type.
    2. Let convOp be the abstract operation named in the Conversion Operation column in Table 68 for Element Type type.
    3. Let intValue be ℝ(convOp(value)).
    4. If intValue ≥ 0, then
      1. Let rawBytes be a List whose elements are the n-byte binary encoding of intValue. The bytes are ordered in little endian order.
    5. Else,
      1. Let rawBytes be a List whose elements are the n-byte binary two's complement encoding of intValue. The bytes are ordered in little endian order.
  4. If isLittleEndian is false, reverse the order of the elements of rawBytes.
  5. Return rawBytes.

25.1.2.12 SetValueInBuffer ( arrayBuffer, byteIndex, type, value, isTypedArray, order [ , isLittleEndian ] )

The abstract operation SetValueInBuffer takes arguments arrayBuffer (an ArrayBuffer or SharedArrayBuffer), byteIndex (a non-negative integer), type (a TypedArray element type), value (a Number or a BigInt), isTypedArray (a Boolean), and order (SeqCst, Unordered, or Init) and optional argument isLittleEndian (a Boolean) and returns unused. It performs the following steps when called:

  1. Assert: IsDetachedBuffer(arrayBuffer) is false.
  2. Assert: There are sufficient bytes in arrayBuffer starting at byteIndex to represent a value of type.
  3. Assert: value is a BigInt if IsBigIntElementType(type) is true; otherwise, value is a Number.
  4. Let block be arrayBuffer.[[ArrayBufferData]].
  5. Let elementSize be the Element Size value specified in Table 68 for Element Type type.
  6. If isLittleEndian is not present, set isLittleEndian to the value of the [[LittleEndian]] field of the surrounding agent's Agent Record.
  7. Let rawBytes be NumericToRawBytes(type, value, isLittleEndian).
  8. If IsSharedArrayBuffer(arrayBuffer) is true, then
    1. Let execution be the [[CandidateExecution]] field of the surrounding agent's Agent Record.
    2. Let eventsRecord be the Agent Events Record of execution.[[EventsRecords]] whose [[AgentSignifier]] is AgentSignifier().
    3. If isTypedArray is true and IsNoTearConfiguration(type, order) is true, let noTear be true; otherwise let noTear be false.
    4. Append WriteSharedMemory { [[Order]]: order, [[NoTear]]: noTear, [[Block]]: block, [[ByteIndex]]: byteIndex, [[ElementSize]]: elementSize, [[Payload]]: rawBytes } to eventsRecord.[[EventList]].
  9. Else, store the individual bytes of rawBytes into block, starting at block[byteIndex].
  10. Return unused.

25.1.2.13 GetModifySetValueInBuffer ( arrayBuffer, byteIndex, type, value, op [ , isLittleEndian ] )

The abstract operation GetModifySetValueInBuffer takes arguments arrayBuffer (an ArrayBuffer or a SharedArrayBuffer), byteIndex (a non-negative integer), type (a TypedArray element type), value (a Number or a BigInt), and op (a read-modify-write modification function) and optional argument isLittleEndian (a Boolean) and returns a Number or a BigInt. It performs the following steps when called:

  1. Assert: IsDetachedBuffer(arrayBuffer) is false.
  2. Assert: There are sufficient bytes in arrayBuffer starting at byteIndex to represent a value of type.
  3. Assert: value is a BigInt if IsBigIntElementType(type) is true; otherwise, value is a Number.
  4. Let block be arrayBuffer.[[ArrayBufferData]].
  5. Let elementSize be the Element Size value specified in Table 68 for Element Type type.
  6. If isLittleEndian is not present, set isLittleEndian to the value of the [[LittleEndian]] field of the surrounding agent's Agent Record.
  7. Let rawBytes be NumericToRawBytes(type, value, isLittleEndian).
  8. If IsSharedArrayBuffer(arrayBuffer) is true, then
    1. Let execution be the [[CandidateExecution]] field of the surrounding agent's Agent Record.
    2. Let eventsRecord be the Agent Events Record of execution.[[EventsRecords]] whose [[AgentSignifier]] is AgentSignifier().
    3. Let rawBytesRead be a List of length elementSize whose elements are nondeterministically chosen byte values.
    4. NOTE: In implementations, rawBytesRead is the result of a load-link, of a load-exclusive, or of an operand of a read-modify-write instruction on the underlying hardware. The nondeterminism is a semantic prescription of the memory model to describe observable behaviour of hardware with weak consistency.
    5. Let rmwEvent be ReadModifyWriteSharedMemory { [[Order]]: SeqCst, [[NoTear]]: true, [[Block]]: block, [[ByteIndex]]: byteIndex, [[ElementSize]]: elementSize, [[Payload]]: rawBytes, [[ModifyOp]]: op }.
    6. Append rmwEvent to eventsRecord.[[EventList]].
    7. Append Chosen Value Record { [[Event]]: rmwEvent, [[ChosenValue]]: rawBytesRead } to execution.[[ChosenValues]].
  9. Else,
    1. Let rawBytesRead be a List of length elementSize whose elements are the sequence of elementSize bytes starting with block[byteIndex].
    2. Let rawBytesModified be op(rawBytesRead, rawBytes).
    3. Store the individual bytes of rawBytesModified into block, starting at block[byteIndex].
  10. Return RawBytesToNumeric(type, rawBytesRead, isLittleEndian).

25.1.3 The ArrayBuffer Constructor

The ArrayBuffer constructor:

  • is %ArrayBuffer%.
  • is the initial value of the "ArrayBuffer" property of the global object.
  • creates and initializes a new ArrayBuffer when called as a constructor.
  • is not intended to be called as a function and will throw an exception when called in that manner.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified ArrayBuffer behaviour must include a super call to the ArrayBuffer constructor to create and initialize subclass instances with the internal state necessary to support the ArrayBuffer.prototype built-in methods.

25.1.3.1 ArrayBuffer ( length )

This function performs the following steps when called:

  1. If NewTarget is undefined, throw a TypeError exception.
  2. Let byteLength be ? ToIndex(length).
  3. Return ? AllocateArrayBuffer(NewTarget, byteLength).

25.1.4 Properties of the ArrayBuffer Constructor

The ArrayBuffer constructor:

  • has a [[Prototype]] internal slot whose value is %Function.prototype%.
  • has the following properties:

25.1.4.1 ArrayBuffer.isView ( arg )

This function performs the following steps when called:

  1. If arg is not an Object, return false.
  2. If arg has a [[ViewedArrayBuffer]] internal slot, return true.
  3. Return false.

25.1.4.2 ArrayBuffer.prototype

The initial value of ArrayBuffer.prototype is the ArrayBuffer prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

25.1.4.3 get ArrayBuffer [ @@species ]

ArrayBuffer[@@species] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Return the this value.

The value of the "name" property of this function is "get [Symbol.species]".

Note

ArrayBuffer prototype methods normally use their this value's constructor to create a derived object. However, a subclass constructor may over-ride that default behaviour by redefining its @@species property.

25.1.5 Properties of the ArrayBuffer Prototype Object

The ArrayBuffer prototype object:

  • is %ArrayBuffer.prototype%.
  • has a [[Prototype]] internal slot whose value is %Object.prototype%.
  • is an ordinary object.
  • does not have an [[ArrayBufferData]] or [[ArrayBufferByteLength]] internal slot.

25.1.5.1 get ArrayBuffer.prototype.byteLength

ArrayBuffer.prototype.byteLength is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? RequireInternalSlot(O, [[ArrayBufferData]]).
  3. If IsSharedArrayBuffer(O) is true, throw a TypeError exception.
  4. If IsDetachedBuffer(O) is true, return 0𝔽.
  5. Let length be O.[[ArrayBufferByteLength]].
  6. Return 𝔽(length).

25.1.5.2 ArrayBuffer.prototype.constructor

The initial value of ArrayBuffer.prototype.constructor is %ArrayBuffer%.

25.1.5.3 ArrayBuffer.prototype.slice ( start, end )

This method performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? RequireInternalSlot(O, [[ArrayBufferData]]).
  3. If IsSharedArrayBuffer(O) is true, throw a TypeError exception.
  4. If IsDetachedBuffer(O) is true, throw a TypeError exception.
  5. Let len be O.[[ArrayBufferByteLength]].
  6. Let relativeStart be ? ToIntegerOrInfinity(start).
  7. If relativeStart = -∞, let first be 0.
  8. Else if relativeStart < 0, let first be max(len relativeStart, 0).
  9. Else, let first be min(relativeStart, len).
  10. If end is undefined, let relativeEnd be len; else let relativeEnd be ? ToIntegerOrInfinity(end).
  11. If relativeEnd = -∞, let final be 0.
  12. Else if relativeEnd < 0, let final be max(len relativeEnd, 0).
  13. Else, let final be min(relativeEnd, len).
  14. Let newLen be max(final - first, 0).
  15. Let ctor be ? SpeciesConstructor(O, %ArrayBuffer%).
  16. Let new be ? Construct(ctor, « 𝔽(newLen) »).
  17. Perform ? RequireInternalSlot(new, [[ArrayBufferData]]).
  18. If IsSharedArrayBuffer(new) is true, throw a TypeError exception.
  19. If IsDetachedBuffer(new) is true, throw a TypeError exception.
  20. If SameValue(new, O) is true, throw a TypeError exception.
  21. If new.[[ArrayBufferByteLength]] < newLen, throw a TypeError exception.
  22. NOTE: Side-effects of the above steps may have detached O.
  23. If IsDetachedBuffer(O) is true, throw a TypeError exception.
  24. Let fromBuf be O.[[ArrayBufferData]].
  25. Let toBuf be new.[[ArrayBufferData]].
  26. Perform CopyDataBlockBytes(toBuf, 0, fromBuf, first, newLen).
  27. Return new.

25.1.5.4 ArrayBuffer.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "ArrayBuffer".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

25.1.6 Properties of ArrayBuffer Instances

ArrayBuffer instances inherit properties from the ArrayBuffer prototype object. ArrayBuffer instances each have an [[ArrayBufferData]] internal slot, an [[ArrayBufferByteLength]] internal slot, and an [[ArrayBufferDetachKey]] internal slot.

ArrayBuffer instances whose [[ArrayBufferData]] is null are considered to be detached and all operators to access or modify data contained in the ArrayBuffer instance will fail.

ArrayBuffer instances whose [[ArrayBufferDetachKey]] is set to a value other than undefined need to have all DetachArrayBuffer calls passing that same "detach key" as an argument, otherwise a TypeError will result. This internal slot is only ever set by certain embedding environments, not by algorithms in this specification.

25.2 SharedArrayBuffer Objects

25.2.1 Abstract Operations for SharedArrayBuffer Objects

25.2.1.1 AllocateSharedArrayBuffer ( constructor, byteLength )

The abstract operation AllocateSharedArrayBuffer takes arguments constructor (a constructor) and byteLength (a non-negative integer) and returns either a normal completion containing a SharedArrayBuffer or a throw completion. It is used to create a SharedArrayBuffer. It performs the following steps when called:

  1. Let obj be ? OrdinaryCreateFromConstructor(constructor, "%SharedArrayBuffer.prototype%", « [[ArrayBufferData]], [[ArrayBufferByteLength]] »).
  2. Let block be ? CreateSharedByteDataBlock(byteLength).
  3. Set obj.[[ArrayBufferData]] to block.
  4. Set obj.[[ArrayBufferByteLength]] to byteLength.
  5. Return obj.

25.2.1.2 IsSharedArrayBuffer ( obj )

The abstract operation IsSharedArrayBuffer takes argument obj (an ArrayBuffer or a SharedArrayBuffer) and returns a Boolean. It tests whether an object is an ArrayBuffer, a SharedArrayBuffer, or a subtype of either. It performs the following steps when called:

  1. Let bufferData be obj.[[ArrayBufferData]].
  2. If bufferData is null, return false.
  3. If bufferData is a Data Block, return false.
  4. Assert: bufferData is a Shared Data Block.
  5. Return true.

25.2.2 The SharedArrayBuffer Constructor

The SharedArrayBuffer constructor:

  • is %SharedArrayBuffer%.
  • is the initial value of the "SharedArrayBuffer" property of the global object, if that property is present (see below).
  • creates and initializes a new SharedArrayBuffer when called as a constructor.
  • is not intended to be called as a function and will throw an exception when called in that manner.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified SharedArrayBuffer behaviour must include a super call to the SharedArrayBuffer constructor to create and initialize subclass instances with the internal state necessary to support the SharedArrayBuffer.prototype built-in methods.

Whenever a host does not provide concurrent access to SharedArrayBuffers it may omit the "SharedArrayBuffer" property of the global object.

Note

Unlike an ArrayBuffer, a SharedArrayBuffer cannot become detached, and its internal [[ArrayBufferData]] slot is never null.

25.2.2.1 SharedArrayBuffer ( length )

This function performs the following steps when called:

  1. If NewTarget is undefined, throw a TypeError exception.
  2. Let byteLength be ? ToIndex(length).
  3. Return ? AllocateSharedArrayBuffer(NewTarget, byteLength).

25.2.3 Properties of the SharedArrayBuffer Constructor

The SharedArrayBuffer constructor:

  • has a [[Prototype]] internal slot whose value is %Function.prototype%.
  • has the following properties:

25.2.3.1 SharedArrayBuffer.prototype

The initial value of SharedArrayBuffer.prototype is the SharedArrayBuffer prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

25.2.3.2 get SharedArrayBuffer [ @@species ]

SharedArrayBuffer[@@species] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Return the this value.

The value of the "name" property of this function is "get [Symbol.species]".

25.2.4 Properties of the SharedArrayBuffer Prototype Object

The SharedArrayBuffer prototype object:

  • is %SharedArrayBuffer.prototype%.
  • has a [[Prototype]] internal slot whose value is %Object.prototype%.
  • is an ordinary object.
  • does not have an [[ArrayBufferData]] or [[ArrayBufferByteLength]] internal slot.

25.2.4.1 get SharedArrayBuffer.prototype.byteLength

SharedArrayBuffer.prototype.byteLength is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? RequireInternalSlot(O, [[ArrayBufferData]]).
  3. If IsSharedArrayBuffer(O) is false, throw a TypeError exception.
  4. Let length be O.[[ArrayBufferByteLength]].
  5. Return 𝔽(length).

25.2.4.2 SharedArrayBuffer.prototype.constructor

The initial value of SharedArrayBuffer.prototype.constructor is %SharedArrayBuffer%.

25.2.4.3 SharedArrayBuffer.prototype.slice ( start, end )

This method performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? RequireInternalSlot(O, [[ArrayBufferData]]).
  3. If IsSharedArrayBuffer(O) is false, throw a TypeError exception.
  4. Let len be O.[[ArrayBufferByteLength]].
  5. Let relativeStart be ? ToIntegerOrInfinity(start).
  6. If relativeStart = -∞, let first be 0.
  7. Else if relativeStart < 0, let first be max(len relativeStart, 0).
  8. Else, let first be min(relativeStart, len).
  9. If end is undefined, let relativeEnd be len; else let relativeEnd be ? ToIntegerOrInfinity(end).
  10. If relativeEnd = -∞, let final be 0.
  11. Else if relativeEnd < 0, let final be max(len relativeEnd, 0).
  12. Else, let final be min(relativeEnd, len).
  13. Let newLen be max(final - first, 0).
  14. Let ctor be ? SpeciesConstructor(O, %SharedArrayBuffer%).
  15. Let new be ? Construct(ctor, « 𝔽(newLen) »).
  16. Perform ? RequireInternalSlot(new, [[ArrayBufferData]]).
  17. If IsSharedArrayBuffer(new) is false, throw a TypeError exception.
  18. If new.[[ArrayBufferData]] is O.[[ArrayBufferData]], throw a TypeError exception.
  19. If new.[[ArrayBufferByteLength]] < newLen, throw a TypeError exception.
  20. Let fromBuf be O.[[ArrayBufferData]].
  21. Let toBuf be new.[[ArrayBufferData]].
  22. Perform CopyDataBlockBytes(toBuf, 0, fromBuf, first, newLen).
  23. Return new.

25.2.4.4 SharedArrayBuffer.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "SharedArrayBuffer".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

25.2.5 Properties of SharedArrayBuffer Instances

SharedArrayBuffer instances inherit properties from the SharedArrayBuffer prototype object. SharedArrayBuffer instances each have an [[ArrayBufferData]] internal slot and an [[ArrayBufferByteLength]] internal slot.

Note

SharedArrayBuffer instances, unlike ArrayBuffer instances, are never detached.

25.3 DataView Objects

25.3.1 Abstract Operations For DataView Objects

25.3.1.1 GetViewValue ( view, requestIndex, isLittleEndian, type )

The abstract operation GetViewValue takes arguments view (an ECMAScript language value), requestIndex (an ECMAScript language value), isLittleEndian (an ECMAScript language value), and type (a TypedArray element type) and returns either a normal completion containing either a Number or a BigInt, or a throw completion. It is used by functions on DataView instances to retrieve values from the view's buffer. It performs the following steps when called:

  1. Perform ? RequireInternalSlot(view, [[DataView]]).
  2. Assert: view has a [[ViewedArrayBuffer]] internal slot.
  3. Let getIndex be ? ToIndex(requestIndex).
  4. Set isLittleEndian to ToBoolean(isLittleEndian).
  5. Let buffer be view.[[ViewedArrayBuffer]].
  6. If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
  7. Let viewOffset be view.[[ByteOffset]].
  8. Let viewSize be view.[[ByteLength]].
  9. Let elementSize be the Element Size value specified in Table 68 for Element Type type.
  10. If getIndex elementSize > viewSize, throw a RangeError exception.
  11. Let bufferIndex be getIndex viewOffset.
  12. Return GetValueFromBuffer(buffer, bufferIndex, type, false, Unordered, isLittleEndian).

25.3.1.2 SetViewValue ( view, requestIndex, isLittleEndian, type, value )

The abstract operation SetViewValue takes arguments view (an ECMAScript language value), requestIndex (an ECMAScript language value), isLittleEndian (an ECMAScript language value), type (a TypedArray element type), and value (an ECMAScript language value) and returns either a normal completion containing undefined or a throw completion. It is used by functions on DataView instances to store values into the view's buffer. It performs the following steps when called:

  1. Perform ? RequireInternalSlot(view, [[DataView]]).
  2. Assert: view has a [[ViewedArrayBuffer]] internal slot.
  3. Let getIndex be ? ToIndex(requestIndex).
  4. If IsBigIntElementType(type) is true, let numberValue be ? ToBigInt(value).
  5. Otherwise, let numberValue be ? ToNumber(value).
  6. Set isLittleEndian to ToBoolean(isLittleEndian).
  7. Let buffer be view.[[ViewedArrayBuffer]].
  8. If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
  9. Let viewOffset be view.[[ByteOffset]].
  10. Let viewSize be view.[[ByteLength]].
  11. Let elementSize be the Element Size value specified in Table 68 for Element Type type.
  12. If getIndex elementSize > viewSize, throw a RangeError exception.
  13. Let bufferIndex be getIndex viewOffset.
  14. Perform SetValueInBuffer(buffer, bufferIndex, type, numberValue, false, Unordered, isLittleEndian).
  15. Return undefined.

25.3.2 The DataView Constructor

The DataView constructor:

  • is �taView%.
  • is the initial value of the "DataView" property of the global object.
  • creates and initializes a new DataView when called as a constructor.
  • is not intended to be called as a function and will throw an exception when called in that manner.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified DataView behaviour must include a super call to the DataView constructor to create and initialize subclass instances with the internal state necessary to support the DataView.prototype built-in methods.

25.3.2.1 DataView ( buffer [ , byteOffset [ , byteLength ] ] )

This function performs the following steps when called:

  1. If NewTarget is undefined, throw a TypeError exception.
  2. Perform ? RequireInternalSlot(buffer, [[ArrayBufferData]]).
  3. Let offset be ? ToIndex(byteOffset).
  4. If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
  5. Let bufferByteLength be buffer.[[ArrayBufferByteLength]].
  6. If offset > bufferByteLength, throw a RangeError exception.
  7. If byteLength is undefined, then
    1. Let viewByteLength be bufferByteLength - offset.
  8. Else,
    1. Let viewByteLength be ? ToIndex(byteLength).
    2. If offset viewByteLength > bufferByteLength, throw a RangeError exception.
  9. Let O be ? OrdinaryCreateFromConstructor(NewTarget, "�taView.prototype%", « [[DataView]], [[ViewedArrayBuffer]], [[ByteLength]], [[ByteOffset]] »).
  10. If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
  11. Set O.[[ViewedArrayBuffer]] to buffer.
  12. Set O.[[ByteLength]] to viewByteLength.
  13. Set O.[[ByteOffset]] to offset.
  14. Return O.

25.3.3 Properties of the DataView Constructor

The DataView constructor:

  • has a [[Prototype]] internal slot whose value is %Function.prototype%.
  • has the following properties:

25.3.3.1 DataView.prototype

The initial value of DataView.prototype is the DataView prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

25.3.4 Properties of the DataView Prototype Object

The DataView prototype object:

  • is �taView.prototype%.
  • has a [[Prototype]] internal slot whose value is %Object.prototype%.
  • is an ordinary object.
  • does not have a [[DataView]], [[ViewedArrayBuffer]], [[ByteLength]], or [[ByteOffset]] internal slot.

25.3.4.1 get DataView.prototype.buffer

DataView.prototype.buffer is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? RequireInternalSlot(O, [[DataView]]).
  3. Assert: O has a [[ViewedArrayBuffer]] internal slot.
  4. Let buffer be O.[[ViewedArrayBuffer]].
  5. Return buffer.

25.3.4.2 get DataView.prototype.byteLength

DataView.prototype.byteLength is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? RequireInternalSlot(O, [[DataView]]).
  3. Assert: O has a [[ViewedArrayBuffer]] internal slot.
  4. Let buffer be O.[[ViewedArrayBuffer]].
  5. If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
  6. Let size be O.[[ByteLength]].
  7. Return 𝔽(size).

25.3.4.3 get DataView.prototype.byteOffset

DataView.prototype.byteOffset is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? RequireInternalSlot(O, [[DataView]]).
  3. Assert: O has a [[ViewedArrayBuffer]] internal slot.
  4. Let buffer be O.[[ViewedArrayBuffer]].
  5. If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
  6. Let offset be O.[[ByteOffset]].
  7. Return 𝔽(offset).

25.3.4.4 DataView.prototype.constructor

The initial value of DataView.prototype.constructor is �taView%.

25.3.4.5 DataView.prototype.getBigInt64 ( byteOffset [ , littleEndian ] )

This method performs the following steps when called:

  1. Let v be the this value.
  2. Return ? GetViewValue(v, byteOffset, littleEndian, BigInt64).

25.3.4.6 DataView.prototype.getBigUint64 ( byteOffset [ , littleEndian ] )

This method performs the following steps when called:

  1. Let v be the this value.
  2. Return ? GetViewValue(v, byteOffset, littleEndian, BigUint64).

25.3.4.7 DataView.prototype.getFloat32 ( byteOffset [ , littleEndian ] )

This method performs the following steps when called:

  1. Let v be the this value.
  2. If littleEndian is not present, set littleEndian to false.
  3. Return ? GetViewValue(v, byteOffset, littleEndian, Float32).

25.3.4.8 DataView.prototype.getFloat64 ( byteOffset [ , littleEndian ] )

This method performs the following steps when called:

  1. Let v be the this value.
  2. If littleEndian is not present, set littleEndian to false.
  3. Return ? GetViewValue(v, byteOffset, littleEndian, Float64).

25.3.4.9 DataView.prototype.getInt8 ( byteOffset )

This method performs the following steps when called:

  1. Let v be the this value.
  2. Return ? GetViewValue(v, byteOffset, true, Int8).

25.3.4.10 DataView.prototype.getInt16 ( byteOffset [ , littleEndian ] )

This method performs the following steps when called:

  1. Let v be the this value.
  2. If littleEndian is not present, set littleEndian to false.
  3. Return ? GetViewValue(v, byteOffset, littleEndian, Int16).

25.3.4.11 DataView.prototype.getInt32 ( byteOffset [ , littleEndian ] )

This method performs the following steps when called:

  1. Let v be the this value.
  2. If littleEndian is not present, set littleEndian to false.
  3. Return ? GetViewValue(v, byteOffset, littleEndian, Int32).

25.3.4.12 DataView.prototype.getUint8 ( byteOffset )

This method performs the following steps when called:

  1. Let v be the this value.
  2. Return ? GetViewValue(v, byteOffset, true, Uint8).

25.3.4.13 DataView.prototype.getUint16 ( byteOffset [ , littleEndian ] )

This method performs the following steps when called:

  1. Let v be the this value.
  2. If littleEndian is not present, set littleEndian to false.
  3. Return ? GetViewValue(v, byteOffset, littleEndian, Uint16).

25.3.4.14 DataView.prototype.getUint32 ( byteOffset [ , littleEndian ] )

This method performs the following steps when called:

  1. Let v be the this value.
  2. If littleEndian is not present, set littleEndian to false.
  3. Return ? GetViewValue(v, byteOffset, littleEndian, Uint32).

25.3.4.15 DataView.prototype.setBigInt64 ( byteOffset, value [ , littleEndian ] )

This method performs the following steps when called:

  1. Let v be the this value.
  2. Return ? SetViewValue(v, byteOffset, littleEndian, BigInt64, value).

25.3.4.16 DataView.prototype.setBigUint64 ( byteOffset, value [ , littleEndian ] )

This method performs the following steps when called:

  1. Let v be the this value.
  2. Return ? SetViewValue(v, byteOffset, littleEndian, BigUint64, value).

25.3.4.17 DataView.prototype.setFloat32 ( byteOffset, value [ , littleEndian ] )

This method performs the following steps when called:

  1. Let v be the this value.
  2. If littleEndian is not present, set littleEndian to false.
  3. Return ? SetViewValue(v, byteOffset, littleEndian, Float32, value).

25.3.4.18 DataView.prototype.setFloat64 ( byteOffset, value [ , littleEndian ] )

This method performs the following steps when called:

  1. Let v be the this value.
  2. If littleEndian is not present, set littleEndian to false.
  3. Return ? SetViewValue(v, byteOffset, littleEndian, Float64, value).

25.3.4.19 DataView.prototype.setInt8 ( byteOffset, value )

This method performs the following steps when called:

  1. Let v be the this value.
  2. Return ? SetViewValue(v, byteOffset, true, Int8, value).

25.3.4.20 DataView.prototype.setInt16 ( byteOffset, value [ , littleEndian ] )

This method performs the following steps when called:

  1. Let v be the this value.
  2. If littleEndian is not present, set littleEndian to false.
  3. Return ? SetViewValue(v, byteOffset, littleEndian, Int16, value).

25.3.4.21 DataView.prototype.setInt32 ( byteOffset, value [ , littleEndian ] )

This method performs the following steps when called:

  1. Let v be the this value.
  2. If littleEndian is not present, set littleEndian to false.
  3. Return ? SetViewValue(v, byteOffset, littleEndian, Int32, value).

25.3.4.22 DataView.prototype.setUint8 ( byteOffset, value )

This method performs the following steps when called:

  1. Let v be the this value.
  2. Return ? SetViewValue(v, byteOffset, true, Uint8, value).

25.3.4.23 DataView.prototype.setUint16 ( byteOffset, value [ , littleEndian ] )

This method performs the following steps when called:

  1. Let v be the this value.
  2. If littleEndian is not present, set littleEndian to false.
  3. Return ? SetViewValue(v, byteOffset, littleEndian, Uint16, value).

25.3.4.24 DataView.prototype.setUint32 ( byteOffset, value [ , littleEndian ] )

This method performs the following steps when called:

  1. Let v be the this value.
  2. If littleEndian is not present, set littleEndian to false.
  3. Return ? SetViewValue(v, byteOffset, littleEndian, Uint32, value).

25.3.4.25 DataView.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "DataView".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

25.3.5 Properties of DataView Instances

DataView instances are ordinary objects that inherit properties from the DataView prototype object. DataView instances each have [[DataView]], [[ViewedArrayBuffer]], [[ByteLength]], and [[ByteOffset]] internal slots.

Note

The value of the [[DataView]] internal slot is not used within this specification. The simple presence of that internal slot is used within the specification to identify objects created using the DataView constructor.

25.4 The Atomics Object

The Atomics object:

  • is %Atomics%.
  • is the initial value of the "Atomics" property of the global object.
  • is an ordinary object.
  • has a [[Prototype]] internal slot whose value is %Object.prototype%.
  • does not have a [[Construct]] internal method; it cannot be used as a constructor with the new operator.
  • does not have a [[Call]] internal method; it cannot be invoked as a function.

The Atomics object provides functions that operate indivisibly (atomically) on shared memory array cells as well as functions that let agents wait for and dispatch primitive events. When used with discipline, the Atomics functions allow multi-agent programs that communicate through shared memory to execute in a well-understood order even on parallel CPUs. The rules that govern shared-memory communication are provided by the memory model, defined below.

Note

For informative guidelines for programming and implementing shared memory in ECMAScript, please see the notes at the end of the memory model section.

25.4.1 WaiterList Objects

A WaiterList is a semantic object that contains an ordered list of agent signifiers for those agents that are waiting on a location (block, i) in shared memory; block is a Shared Data Block and i a byte offset into the memory of block. A WaiterList object also optionally contains a Synchronize event denoting the previous leaving of its critical section.

Initially a WaiterList object has an empty list and no Synchronize event.

The agent cluster has a store of WaiterList objects; the store is indexed by (block, i). WaiterLists are agent-independent: a lookup in the store of WaiterLists by (block, i) will result in the same WaiterList object in any agent in the agent cluster.

Each WaiterList has a critical section that controls exclusive access to that WaiterList during evaluation. Only a single agent may enter a WaiterList's critical section at one time. Entering and leaving a WaiterList's critical section is controlled by the abstract operations EnterCriticalSection and LeaveCriticalSection. Operations on a WaiterList—adding and removing waiting agents, traversing the list of agents, suspending and notifying agents on the list, setting and retrieving the Synchronize event—may only be performed by agents that have entered the WaiterList's critical section.

25.4.2 Abstract Operations for Atomics

25.4.2.1 ValidateIntegerTypedArray ( typedArray [ , waitable ] )

The abstract operation ValidateIntegerTypedArray takes argument typedArray (an ECMAScript language value) and optional argument waitable (a Boolean) and returns either a normal completion containing either an ArrayBuffer or a SharedArrayBuffer, or a throw completion. It performs the following steps when called:

  1. If waitable is not present, set waitable to false.
  2. Perform ? ValidateTypedArray(typedArray).
  3. Let buffer be typedArray.[[ViewedArrayBuffer]].
  4. If waitable is true, then
    1. If typedArray.[[TypedArrayName]] is neither "Int32Array" nor "BigInt64Array", throw a TypeError exception.
  5. Else,
    1. Let type be TypedArrayElementType(typedArray).
    2. If IsUnclampedIntegerElementType(type) is false and IsBigIntElementType(type) is false, throw a TypeError exception.
  6. Return buffer.

25.4.2.2 ValidateAtomicAccess ( typedArray, requestIndex )

The abstract operation ValidateAtomicAccess takes arguments typedArray (a TypedArray) and requestIndex (an ECMAScript language value) and returns either a normal completion containing an integer or a throw completion. It performs the following steps when called:

  1. Let length be typedArray.[[ArrayLength]].
  2. Let accessIndex be ? ToIndex(requestIndex).
  3. Assert: accessIndex ≥ 0.
  4. If accessIndex ≥ length, throw a RangeError exception.
  5. Let elementSize be TypedArrayElementSize(typedArray).
  6. Let offset be typedArray.[[ByteOffset]].
  7. Return (accessIndex × elementSize) offset.

25.4.2.3 GetWaiterList ( block, i )

The abstract operation GetWaiterList takes arguments block (a Shared Data Block) and i (a non-negative integer that is evenly divisible by 4) and returns a WaiterList. It performs the following steps when called:

  1. Assert: i and i 3 are valid byte offsets within the memory of block.
  2. Return the WaiterList that is referenced by the pair (block, i).

25.4.2.4 EnterCriticalSection ( WL )

The abstract operation EnterCriticalSection takes argument WL (a WaiterList) and returns unused. It performs the following steps when called:

  1. Assert: The surrounding agent is not in the critical section for any WaiterList.
  2. Wait until no agent is in the critical section for WL, then enter the critical section for WL (without allowing any other agent to enter).
  3. If WL has a Synchronize event, then
    1. NOTE: A WL whose critical section has been entered at least once has a Synchronize event set by LeaveCriticalSection.
    2. Let execution be the [[CandidateExecution]] field of the surrounding agent's Agent Record.
    3. Let eventsRecord be the Agent Events Record of execution.[[EventsRecords]] whose [[AgentSignifier]] is AgentSignifier().
    4. Let enterEvent be a new Synchronize event.
    5. Append enterEvent to eventsRecord.[[EventList]].
    6. Let leaveEvent be the Synchronize event in WL.
    7. Append (leaveEvent, enterEvent) to eventsRecord.[[AgentSynchronizesWith]].
  4. Return unused.

EnterCriticalSection has contention when an agent attempting to enter the critical section must wait for another agent to leave it. When there is no contention, FIFO order of EnterCriticalSection calls is observable. When there is contention, an implementation may choose an arbitrary order but may not cause an agent to wait indefinitely.

25.4.2.5 LeaveCriticalSection ( WL )

The abstract operation LeaveCriticalSection takes argument WL (a WaiterList) and returns unused. It performs the following steps when called:

  1. Assert: The surrounding agent is in the critical section for WL.
  2. Let execution be the [[CandidateExecution]] field of the surrounding agent's Agent Record.
  3. Let eventsRecord be the Agent Events Record of execution.[[EventsRecords]] whose [[AgentSignifier]] is AgentSignifier().
  4. Let leaveEvent be a new Synchronize event.
  5. Append leaveEvent to eventsRecord.[[EventList]].
  6. Set the Synchronize event in WL to leaveEvent.
  7. Leave the critical section for WL.
  8. Return unused.

25.4.2.6 AddWaiter ( WL, W )

The abstract operation AddWaiter takes arguments WL (a WaiterList) and W (an agent signifier) and returns unused. It performs the following steps when called:

  1. Assert: The surrounding agent is in the critical section for WL.
  2. Assert: W is not on the list of waiters in any WaiterList.
  3. Append W to WL.
  4. Return unused.

25.4.2.7 RemoveWaiter ( WL, W )

The abstract operation RemoveWaiter takes arguments WL (a WaiterList) and W (an agent signifier) and returns unused. It performs the following steps when called:

  1. Assert: The surrounding agent is in the critical section for WL.
  2. Assert: W is on the list of waiters in WL.
  3. Remove W from the list of waiters in WL.
  4. Return unused.

25.4.2.8 RemoveWaiters ( WL, c )

The abstract operation RemoveWaiters takes arguments WL (a WaiterList) and c (a non-negative integer or ∞) and returns a List of agent signifiers. It performs the following steps when called:

  1. Assert: The surrounding agent is in the critical section for WL.
  2. Let S be a reference to the list of waiters in WL.
  3. Let len be the number of elements in S.
  4. Let n be min(c, len).
  5. Let L be a List whose elements are the first n elements of S.
  6. Remove the first n elements of S.
  7. Return L.

25.4.2.9 SuspendAgent ( WL, W, minimumTimeout )

The abstract operation SuspendAgent takes arguments WL (a WaiterList), W (an agent signifier), and minimumTimeout (a non-negative extended mathematical value) and returns a Boolean. It performs the following steps when called:

  1. Assert: The surrounding agent is in the critical section for WL.
  2. Assert: W is AgentSignifier().
  3. Assert: W is on the list of waiters in WL.
  4. Assert: AgentCanSuspend() is true.
  5. Let additionalTimeout be an implementation-defined non-negative mathematical value.
  6. Let timeout be minimumTimeout additionalTimeout.
  7. NOTE: When minimumTimeout is ∞, timeout is also ∞ and the following step can terminate only by another agent calling NotifyWaiter.
  8. Perform LeaveCriticalSection(WL) and suspend W for up to timeout milliseconds, performing the combined operation in such a way that a notification that arrives after the critical section is exited but before the suspension takes effect is not lost. W can wake from suspension either because the timeout expired or because it was notified explicitly by another agent calling NotifyWaiter with arguments WL and W, and not for any other reasons at all.
  9. Perform EnterCriticalSection(WL).
  10. If W was notified explicitly by another agent calling NotifyWaiter with arguments WL and W, return true.
  11. Return false.
 Note

additionalTimeout allows implementations to pad timeouts as necessary, such as for reducing power consumption or coarsening timer resolution to mitigate timing attacks. This value may differ from call to call of SuspendAgent.

25.4.2.10 NotifyWaiter ( WL, W )

The abstract operation NotifyWaiter takes arguments WL (a WaiterList) and W (an agent signifier) and returns unused. It performs the following steps when called:

  1. Assert: The surrounding agent is in the critical section for WL.
  2. Notify the agent W.
  3. Return unused.
 Note

The embedding may delay notifying W, e.g. for resource management reasons, but W must eventually be notified in order to guarantee forward progress.

25.4.2.11 AtomicReadModifyWrite ( typedArray, index, value, op )

The abstract operation AtomicReadModifyWrite takes arguments typedArray (an ECMAScript language value), index (an ECMAScript language value), value (an ECMAScript language value), and op (a read-modify-write modification function) and returns either a normal completion containing either a Number or a BigInt, or a throw completion. op takes two List of byte values arguments and returns a List of byte values. This operation atomically loads a value, combines it with another value, and stores the result of the combination. It returns the loaded value. It performs the following steps when called:

  1. Let buffer be ? ValidateIntegerTypedArray(typedArray).
  2. Let indexedPosition be ? ValidateAtomicAccess(typedArray, index).
  3. If typedArray.[[ContentType]] is BigInt, let v be ? ToBigInt(value).
  4. Otherwise, let v be 𝔽(? ToIntegerOrInfinity(value)).
  5. If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
  6. NOTE: The above check is not redundant with the check in ValidateIntegerTypedArray because the call to ToBigInt or ToIntegerOrInfinity on the preceding lines can have arbitrary side effects, which could cause the buffer to become detached.
  7. Let elementType be TypedArrayElementType(typedArray).
  8. Return GetModifySetValueInBuffer(buffer, indexedPosition, elementType, v, op).

25.4.2.12 ByteListBitwiseOp ( op, xBytes, yBytes )

The abstract operation ByteListBitwiseOp takes arguments op (&, ^, or |), xBytes (a List of byte values), and yBytes (a List of byte values) and returns a List of byte values. The operation atomically performs a bitwise operation on all byte values of the arguments and returns a List of byte values. It performs the following steps when called:

  1. Assert: xBytes and yBytes have the same number of elements.
  2. Let result be a new empty List.
  3. Let i be 0.
  4. For each element xByte of xBytes, do
    1. Let yByte be yBytes[i].
    2. If op is &, let resultByte be the result of applying the bitwise AND operation to xByte and yByte.
    3. Else if op is ^, let resultByte be the result of applying the bitwise exclusive OR (XOR) operation to xByte and yByte.
    4. Else, op is |. Let resultByte be the result of applying the bitwise inclusive OR operation to xByte and yByte.
    5. Set i to i 1.
    6. Append resultByte to result.
  5. Return result.

25.4.2.13 ByteListEqual ( xBytes, yBytes )

The abstract operation ByteListEqual takes arguments xBytes (a List of byte values) and yBytes (a List of byte values) and returns a Boolean. It performs the following steps when called:

  1. If xBytes and yBytes do not have the same number of elements, return false.
  2. Let i be 0.
  3. For each element xByte of xBytes, do
    1. Let yByte be yBytes[i].
    2. If xByte ≠ yByte, return false.
    3. Set i to i 1.
  4. Return true.

25.4.3 Atomics.add ( typedArray, index, value )

This function performs the following steps when called:

  1. Let type be TypedArrayElementType(typedArray).
  2. Let isLittleEndian be the value of the [[LittleEndian]] field of the surrounding agent's Agent Record.
  3. Let add be a new read-modify-write modification function with parameters (xBytes, yBytes) that captures type and isLittleEndian and performs the following steps atomically when called:
    1. Let x be RawBytesToNumeric(type, xBytes, isLittleEndian).
    2. Let y be RawBytesToNumeric(type, yBytes, isLittleEndian).
    3. If x is a Number, then
      1. Let sum be Number::add(x, y).
    4. Else,
      1. Assert: x is a BigInt.
      2. Let sum be BigInt::add(x, y).
    5. Let sumBytes be NumericToRawBytes(type, sum, isLittleEndian).
    6. Assert: sumBytes, xBytes, and yBytes have the same number of elements.
    7. Return sumBytes.
  4. Return ? AtomicReadModifyWrite(typedArray, index, value, add).

25.4.4 Atomics.and ( typedArray, index, value )

This function performs the following steps when called:

  1. Let and be a new read-modify-write modification function with parameters (xBytes, yBytes) that captures nothing and performs the following steps atomically when called:
    1. Return ByteListBitwiseOp(&, xBytes, yBytes).
  2. Return ? AtomicReadModifyWrite(typedArray, index, value, and).

25.4.5 Atomics.compareExchange ( typedArray, index, expectedValue, replacementValue )

This function performs the following steps when called:

  1. Let buffer be ? ValidateIntegerTypedArray(typedArray).
  2. Let block be buffer.[[ArrayBufferData]].
  3. Let indexedPosition be ? ValidateAtomicAccess(typedArray, index).
  4. If typedArray.[[ContentType]] is BigInt, then
    1. Let expected be ? ToBigInt(expectedValue).
    2. Let replacement be ? ToBigInt(replacementValue).
  5. Else,
    1. Let expected be 𝔽(? ToIntegerOrInfinity(expectedValue)).
    2. Let replacement be 𝔽(? ToIntegerOrInfinity(replacementValue)).
  6. If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
  7. NOTE: The above check is not redundant with the check in ValidateIntegerTypedArray because the call to ToBigInt or ToIntegerOrInfinity on the preceding lines can have arbitrary side effects, which could cause the buffer to become detached.
  8. Let elementType be TypedArrayElementType(typedArray).
  9. Let elementSize be TypedArrayElementSize(typedArray).
  10. Let isLittleEndian be the value of the [[LittleEndian]] field of the surrounding agent's Agent Record.
  11. Let expectedBytes be NumericToRawBytes(elementType, expected, isLittleEndian).
  12. Let replacementBytes be NumericToRawBytes(elementType, replacement, isLittleEndian).
  13. If IsSharedArrayBuffer(buffer) is true, then
    1. Let execution be the [[CandidateExecution]] field of the surrounding agent's Agent Record.
    2. Let eventsRecord be the Agent Events Record of execution.[[EventsRecords]] whose [[AgentSignifier]] is AgentSignifier().
    3. Let rawBytesRead be a List of length elementSize whose elements are nondeterministically chosen byte values.
    4. NOTE: In implementations, rawBytesRead is the result of a load-link, of a load-exclusive, or of an operand of a read-modify-write instruction on the underlying hardware. The nondeterminism is a semantic prescription of the memory model to describe observable behaviour of hardware with weak consistency.
    5. NOTE: The comparison of the expected value and the read value is performed outside of the read-modify-write modification function to avoid needlessly strong synchronization when the expected value is not equal to the read value.
    6. If ByteListEqual(rawBytesRead, expectedBytes) is true, then
      1. Let second be a new read-modify-write modification function with parameters (oldBytes, newBytes) that captures nothing and performs the following steps atomically when called:
        1. Return newBytes.
      2. Let event be ReadModifyWriteSharedMemory { [[Order]]: SeqCst, [[NoTear]]: true, [[Block]]: block, [[ByteIndex]]: indexedPosition, [[ElementSize]]: elementSize, [[Payload]]: replacementBytes, [[ModifyOp]]: second }.
    7. Else,
      1. Let event be ReadSharedMemory { [[Order]]: SeqCst, [[NoTear]]: true, [[Block]]: block, [[ByteIndex]]: indexedPosition, [[ElementSize]]: elementSize }.
    8. Append event to eventsRecord.[[EventList]].
    9. Append Chosen Value Record { [[Event]]: event, [[ChosenValue]]: rawBytesRead } to execution.[[ChosenValues]].
  14. Else,
    1. Let rawBytesRead be a List of length elementSize whose elements are the sequence of elementSize bytes starting with block[indexedPosition].
    2. If ByteListEqual(rawBytesRead, expectedBytes) is true, then
      1. Store the individual bytes of replacementBytes into block, starting at block[indexedPosition].
  15. Return RawBytesToNumeric(elementType, rawBytesRead, isLittleEndian).

25.4.6 Atomics.exchange ( typedArray, index, value )

This function performs the following steps when called:

  1. Let second be a new read-modify-write modification function with parameters (oldBytes, newBytes) that captures nothing and performs the following steps atomically when called:
    1. Return newBytes.
  2. Return ? AtomicReadModifyWrite(typedArray, index, value, second).

25.4.7 Atomics.isLockFree ( size )

This function performs the following steps when called:

  1. Let n be ? ToIntegerOrInfinity(size).
  2. Let AR be the Agent Record of the surrounding agent.
  3. If n = 1, return AR.[[IsLockFree1]].
  4. If n = 2, return AR.[[IsLockFree2]].
  5. If n = 4, return true.
  6. If n = 8, return AR.[[IsLockFree8]].
  7. Return false.
 Note

This function is an optimization primitive. The intuition is that if the atomic step of an atomic primitive (compareExchange, load, store, add, sub, and, or, xor, or exchange) on a datum of size n bytes will be performed without the surrounding agent acquiring a lock outside the n bytes comprising the datum, then Atomics.isLockFree(n) will return true. High-performance algorithms will use this function to determine whether to use locks or atomic operations in critical sections. If an atomic primitive is not lock-free then it is often more efficient for an algorithm to provide its own locking.

Atomics.isLockFree(4) always returns true as that can be supported on all known relevant hardware. Being able to assume this will generally simplify programs.

Regardless of the value returned by this function, all atomic operations are guaranteed to be atomic. For example, they will never have a visible operation take place in the middle of the operation (e.g., "tearing").

25.4.8 Atomics.load ( typedArray, index )

This function performs the following steps when called:

  1. Let buffer be ? ValidateIntegerTypedArray(typedArray).
  2. Let indexedPosition be ? ValidateAtomicAccess(typedArray, index).
  3. If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
  4. NOTE: The above check is not redundant with the check in ValidateIntegerTypedArray because the call to ValidateAtomicAccess on the preceding line can have arbitrary side effects, which could cause the buffer to become detached.
  5. Let elementType be TypedArrayElementType(typedArray).
  6. Return GetValueFromBuffer(buffer, indexedPosition, elementType, true, SeqCst).

25.4.9 Atomics.or ( typedArray, index, value )

This function performs the following steps when called:

  1. Let or be a new read-modify-write modification function with parameters (xBytes, yBytes) that captures nothing and performs the following steps atomically when called:
    1. Return ByteListBitwiseOp(|, xBytes, yBytes).
  2. Return ? AtomicReadModifyWrite(typedArray, index, value, or).

25.4.10 Atomics.store ( typedArray, index, value )

This function performs the following steps when called:

  1. Let buffer be ? ValidateIntegerTypedArray(typedArray).
  2. Let indexedPosition be ? ValidateAtomicAccess(typedArray, index).
  3. If typedArray.[[ContentType]] is BigInt, let v be ? ToBigInt(value).
  4. Otherwise, let v be 𝔽(? ToIntegerOrInfinity(value)).
  5. If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
  6. NOTE: The above check is not redundant with the check in ValidateIntegerTypedArray because the call to ToBigInt or ToIntegerOrInfinity on the preceding lines can have arbitrary side effects, which could cause the buffer to become detached.
  7. Let elementType be TypedArrayElementType(typedArray).
  8. Perform SetValueInBuffer(buffer, indexedPosition, elementType, v, true, SeqCst).
  9. Return v.

25.4.11 Atomics.sub ( typedArray, index, value )

This function performs the following steps when called:

  1. Let type be TypedArrayElementType(typedArray).
  2. Let isLittleEndian be the value of the [[LittleEndian]] field of the surrounding agent's Agent Record.
  3. Let subtract be a new read-modify-write modification function with parameters (xBytes, yBytes) that captures type and isLittleEndian and performs the following steps atomically when called:
    1. Let x be RawBytesToNumeric(type, xBytes, isLittleEndian).
    2. Let y be RawBytesToNumeric(type, yBytes, isLittleEndian).
    3. If x is a Number, then
      1. Let difference be Number::subtract(x, y).
    4. Else,
      1. Assert: x is a BigInt.
      2. Let difference be BigInt::subtract(x, y).
    5. Let differenceBytes be NumericToRawBytes(type, difference, isLittleEndian).
    6. Assert: differenceBytes, xBytes, and yBytes have the same number of elements.
    7. Return differenceBytes.
  4. Return ? AtomicReadModifyWrite(typedArray, index, value, subtract).

25.4.12 Atomics.wait ( typedArray, index, value, timeout )

This function puts the surrounding agent in a wait queue and suspends it until notified or until the wait times out, returning a String differentiating those cases.

It performs the following steps when called:

  1. Let buffer be ? ValidateIntegerTypedArray(typedArray, true).
  2. If IsSharedArrayBuffer(buffer) is false, throw a TypeError exception.
  3. Let indexedPosition be ? ValidateAtomicAccess(typedArray, index).
  4. If typedArray.[[TypedArrayName]] is "BigInt64Array", let v be ? ToBigInt64(value).
  5. Otherwise, let v be ? ToInt32(value).
  6. Let q be ? ToNumber(timeout).
  7. If q is either NaN or ∞𝔽, let t be ∞; else if q is -∞𝔽, let t be 0; else let t be max(ℝ(q), 0).
  8. Let B be AgentCanSuspend().
  9. If B is false, throw a TypeError exception.
  10. Let block be buffer.[[ArrayBufferData]].
  11. Let WL be GetWaiterList(block, indexedPosition).
  12. Perform EnterCriticalSection(WL).
  13. Let elementType be TypedArrayElementType(typedArray).
  14. Let w be GetValueFromBuffer(buffer, indexedPosition, elementType, true, SeqCst).
  15. If v ≠ w, then
    1. Perform LeaveCriticalSection(WL).
    2. Return "not-equal".
  16. Let W be AgentSignifier().
  17. Perform AddWaiter(WL, W).
  18. Let notified be SuspendAgent(WL, W, t).
  19. If notified is true, then
    1. Assert: W is not on the list of waiters in WL.
  20. Else,
    1. Perform RemoveWaiter(WL, W).
  21. Perform LeaveCriticalSection(WL).
  22. If notified is true, return "ok".
  23. Return "timed-out".

25.4.13 Atomics.notify ( typedArray, index, count )

This function notifies some agents that are sleeping in the wait queue.

It performs the following steps when called:

  1. Let buffer be ? ValidateIntegerTypedArray(typedArray, true).
  2. Let indexedPosition be ? ValidateAtomicAccess(typedArray, index).
  3. If count is undefined, let c be ∞.
  4. Else,
    1. Let intCount be ? ToIntegerOrInfinity(count).
    2. Let c be max(intCount, 0).
  5. Let block be buffer.[[ArrayBufferData]].
  6. If IsSharedArrayBuffer(buffer) is false, return 0𝔽.
  7. Let WL be GetWaiterList(block, indexedPosition).
  8. Perform EnterCriticalSection(WL).
  9. Let S be RemoveWaiters(WL, c).
  10. For each element W of S, do
    1. Perform NotifyWaiter(WL, W).
  11. Perform LeaveCriticalSection(WL).
  12. Let n be the number of elements in S.
  13. Return 𝔽(n).

25.4.14 Atomics.xor ( typedArray, index, value )

This function performs the following steps when called:

  1. Let xor be a new read-modify-write modification function with parameters (xBytes, yBytes) that captures nothing and performs the following steps atomically when called:
    1. Return ByteListBitwiseOp(^, xBytes, yBytes).
  2. Return ? AtomicReadModifyWrite(typedArray, index, value, xor).

25.4.15 Atomics [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "Atomics".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

25.5 The JSON Object

The JSON object:

  • is %JSON%.
  • is the initial value of the "JSON" property of the global object.
  • is an ordinary object.
  • contains two functions, parse and stringify, that are used to parse and construct JSON texts.
  • has a [[Prototype]] internal slot whose value is %Object.prototype%.
  • does not have a [[Construct]] internal method; it cannot be used as a constructor with the new operator.
  • does not have a [[Call]] internal method; it cannot be invoked as a function.

The JSON Data Interchange Format is defined in ECMA-404. The JSON interchange format used in this specification is exactly that described by ECMA-404. Conforming implementations of JSON.parse and JSON.stringify must support the exact interchange format described in the ECMA-404 specification without any deletions or extensions to the format.

25.5.1 JSON.parse ( text [ , reviver ] )

This function parses a JSON text (a JSON-formatted String) and produces an ECMAScript language value. The JSON format represents literals, arrays, and objects with a syntax similar to the syntax for ECMAScript literals, Array Initializers, and Object Initializers. After parsing, JSON objects are realized as ECMAScript objects. JSON arrays are realized as ECMAScript Array instances. JSON strings, numbers, booleans, and null are realized as ECMAScript Strings, Numbers, Booleans, and null.

The optional reviver parameter is a function that takes two parameters, key and value. It can filter and transform the results. It is called with each of the key/value pairs produced by the parse, and its return value is used instead of the original value. If it returns what it received, the structure is not modified. If it returns undefined then the property is deleted from the result.

  1. Let jsonString be ? ToString(text).
  2. Parse StringToCodePoints(jsonString) as a JSON text as specified in ECMA-404. Throw a SyntaxError exception if it is not a valid JSON text as defined in that specification.
  3. Let scriptString be the string-concatenation of "(", jsonString, and ");".
  4. Let script be ParseText(StringToCodePoints(scriptString), Script).
  5. NOTE: The early error rules defined in 13.2.5.1 have special handling for the above invocation of ParseText.
  6. Assert: script is a Parse Node.
  7. Let completion be Completion(Evaluation of script).
  8. NOTE: The PropertyDefinitionEvaluation semantics defined in 13.2.5.5 have special handling for the above evaluation.
  9. Let unfiltered be completion.[[Value]].
  10. Assert: unfiltered is either a String, a Number, a Boolean, an Object that is defined by either an ArrayLiteral or an ObjectLiteral, or null.
  11. If IsCallable(reviver) is true, then
    1. Let root be OrdinaryObjectCreate(%Object.prototype%).
    2. Let rootName be the empty String.
    3. Perform ! CreateDataPropertyOrThrow(root, rootName, unfiltered).
    4. Return ? InternalizeJSONProperty(root, rootName, reviver).
  12. Else,
    1. Return unfiltered.

The "length" property of this function is 2𝔽.

Note

Valid JSON text is a subset of the ECMAScript PrimaryExpression syntax. Step 2 verifies that jsonString conforms to that subset, and step 10 asserts that that parsing and evaluation returns a value of an appropriate type.

However, because 13.2.5.5 behaves differently during JSON.parse, the same source text can produce different results when evaluated as a PrimaryExpression rather than as JSON. Furthermore, the Early Error for duplicate "__proto__" properties in object literals, which likewise does not apply during JSON.parse, means that not all texts accepted by JSON.parse are valid as a PrimaryExpression, despite matching the grammar.

25.5.1.1 InternalizeJSONProperty ( holder, name, reviver )

The abstract operation InternalizeJSONProperty takes arguments holder (an Object), name (a String), and reviver (a function object) and returns either a normal completion containing an ECMAScript language value or a throw completion.

Note 1

This algorithm intentionally does not throw an exception if either [[Delete]] or CreateDataProperty return false.

It performs the following steps when called:

  1. Let val be ? Get(holder, name).
  2. If val is an Object, then
    1. Let isArray be ? IsArray(val).
    2. If isArray is true, then
      1. Let len be ? LengthOfArrayLike(val).
      2. Let I be 0.
      3. Repeat, while I < len,
        1. Let prop be ! ToString(𝔽(I)).
        2. Let newElement be ? InternalizeJSONProperty(val, prop, reviver).
        3. If newElement is undefined, then
          1. Perform ? val.[[Delete]](prop).
        4. Else,
          1. Perform ? CreateDataProperty(val, prop, newElement).
        5. Set I to I 1.
    3. Else,
      1. Let keys be ? EnumerableOwnProperties(val, key).
      2. For each String P of keys, do
        1. Let newElement be ? InternalizeJSONProperty(val, P, reviver).
        2. If newElement is undefined, then
          1. Perform ? val.[[Delete]](P).
        3. Else,
          1. Perform ? CreateDataProperty(val, P, newElement).
  3. Return ? Call(reviver, holder, « name, val »).

It is not permitted for a conforming implementation of JSON.parse to extend the JSON grammars. If an implementation wishes to support a modified or extended JSON interchange format it must do so by defining a different parse function.

Note 2

In the case where there are duplicate name Strings within an object, lexically preceding values for the same key shall be overwritten.

25.5.2 JSON.stringify ( value [ , replacer [ , space ] ] )

This function returns a String in UTF-16 encoded JSON format representing an ECMAScript language value, or undefined. It can take three parameters. The value parameter is an ECMAScript language value, which is usually an object or array, although it can also be a String, Boolean, Number or null. The optional replacer parameter is either a function that alters the way objects and arrays are stringified, or an array of Strings and Numbers that acts as an inclusion list for selecting the object properties that will be stringified. The optional space parameter is a String or Number that allows the result to have white space injected into it to improve human readability.

It performs the following steps when called:

  1. Let stack be a new empty List.
  2. Let indent be the empty String.
  3. Let PropertyList be undefined.
  4. Let ReplacerFunction be undefined.
  5. If replacer is an Object, then
    1. If IsCallable(replacer) is true, then
      1. Set ReplacerFunction to replacer.
    2. Else,
      1. Let isArray be ? IsArray(replacer).
      2. If isArray is true, then
        1. Set PropertyList to a new empty List.
        2. Let len be ? LengthOfArrayLike(replacer).
        3. Let k be 0.
        4. Repeat, while k < len,
          1. Let prop be ! ToString(𝔽(k)).
          2. Let v be ? Get(replacer, prop).
          3. Let item be undefined.
          4. If v is a String, set item to v.
          5. Else if v is a Number, set item to ! ToString(v).
          6. Else if v is an Object, then
            1. If v has a [[StringData]] or [[NumberData]] internal slot, set item to ? ToString(v).
          7. If item is not undefined and PropertyList does not contain item, then
            1. Append item to PropertyList.
          8. Set k to k 1.
  6. If space is an Object, then
    1. If space has a [[NumberData]] internal slot, then
      1. Set space to ? ToNumber(space).
    2. Else if space has a [[StringData]] internal slot, then
      1. Set space to ? ToString(space).
  7. If space is a Number, then
    1. Let spaceMV be ! ToIntegerOrInfinity(space).
    2. Set spaceMV to min(10, spaceMV).
    3. If spaceMV < 1, let gap be the empty String; otherwise let gap be the String value containing spaceMV occurrences of the code unit 0x0020 (SPACE).
  8. Else if space is a String, then
    1. If the length of space ≤ 10, let gap be space; otherwise let gap be the substring of space from 0 to 10.
  9. Else,
    1. Let gap be the empty String.
  10. Let wrapper be OrdinaryObjectCreate(%Object.prototype%).
  11. Perform ! CreateDataPropertyOrThrow(wrapper, the empty String, value).
  12. Let state be the JSON Serialization Record { [[ReplacerFunction]]: ReplacerFunction, [[Stack]]: stack, [[Indent]]: indent, [[Gap]]: gap, [[PropertyList]]: PropertyList }.
  13. Return ? SerializeJSONProperty(state, the empty String, wrapper).

The "length" property of this function is 3𝔽.

Note 1

JSON structures are allowed to be nested to any depth, but they must be acyclic. If value is or contains a cyclic structure, then this function must throw a TypeError exception. This is an example of a value that cannot be stringified:

a = [];
a[0] = a;
my_text = JSON.stringify(a); // This must throw a TypeError.
 Note 2

Symbolic primitive values are rendered as follows:

  • The null value is rendered in JSON text as the String value "null".
  • The undefined value is not rendered.
  • The true value is rendered in JSON text as the String value "true".
  • The false value is rendered in JSON text as the String value "false".
Note 3

String values are wrapped in QUOTATION MARK (") code units. The code units " and \ are escaped with \ prefixes. Control characters code units are replaced with escape sequences \uHHHH, or with the shorter forms, \b (BACKSPACE), \f (FORM FEED), \n (LINE FEED), \r (CARRIAGE RETURN), \t (CHARACTER TABULATION).

 Note 4

Finite numbers are stringified as if by calling ToString(number). NaN and Infinity regardless of sign are represented as the String value "null".

 Note 5

Values that do not have a JSON representation (such as undefined and functions) do not produce a String. Instead they produce the undefined value. In arrays these values are represented as the String value "null". In objects an unrepresentable value causes the property to be excluded from stringification.

 Note 6

An object is rendered as U 007B (LEFT CURLY BRACKET) followed by zero or more properties, separated with a U 002C (COMMA), closed with a U 007D (RIGHT CURLY BRACKET). A property is a quoted String representing the property name, a U 003A (COLON), and then the stringified property value. An array is rendered as an opening U 005B (LEFT SQUARE BRACKET) followed by zero or more values, separated with a U 002C (COMMA), closed with a U 005D (RIGHT SQUARE BRACKET).

25.5.2.1 JSON Serialization Record

A JSON Serialization Record is a Record value used to enable serialization to the JSON format.

JSON Serialization Records have the fields listed in Table 69.

Table 69: JSON Serialization Record Fields
Field Name Value Meaning
[[ReplacerFunction]] a function object or undefined A function that can supply replacement values for object properties (from JSON.stringify's replacer parameter).
[[PropertyList]] either a List of Strings or undefined The names of properties to include when serializing a non-array object (from JSON.stringify's replacer parameter).
[[Gap]] a String The unit of indentation (from JSON.stringify's space parameter).
[[Stack]] a List of Objects The set of nested objects that are in the process of being serialized. Used to detect cyclic structures.
[[Indent]] a String The current indentation.

25.5.2.2 SerializeJSONProperty ( state, key, holder )

The abstract operation SerializeJSONProperty takes arguments state (a JSON Serialization Record), key (a String), and holder (an Object) and returns either a normal completion containing either a String or undefined, or a throw completion. It performs the following steps when called:

  1. Let value be ? Get(holder, key).
  2. If value is an Object or value is a BigInt, then
    1. Let toJSON be ? GetV(value, "toJSON").
    2. If IsCallable(toJSON) is true, then
      1. Set value to ? Call(toJSON, value, « key »).
  3. If state.[[ReplacerFunction]] is not undefined, then
    1. Set value to ? Call(state.[[ReplacerFunction]], holder, « key, value »).
  4. If value is an Object, then
    1. If value has a [[NumberData]] internal slot, then
      1. Set value to ? ToNumber(value).
    2. Else if value has a [[StringData]] internal slot, then
      1. Set value to ? ToString(value).
    3. Else if value has a [[BooleanData]] internal slot, then
      1. Set value to value.[[BooleanData]].
    4. Else if value has a [[BigIntData]] internal slot, then
      1. Set value to value.[[BigIntData]].
  5. If value is null, return "null".
  6. If value is true, return "true".
  7. If value is false, return "false".
  8. If value is a String, return QuoteJSONString(value).
  9. If value is a Number, then
    1. If value is finite, return ! ToString(value).
    2. Return "null".
  10. If value is a BigInt, throw a TypeError exception.
  11. If value is an Object and IsCallable(value) is false, then
    1. Let isArray be ? IsArray(value).
    2. If isArray is true, return ? SerializeJSONArray(state, value).
    3. Return ? SerializeJSONObject(state, value).
  12. Return undefined.

25.5.2.3 QuoteJSONString ( value )

The abstract operation QuoteJSONString takes argument value (a String) and returns a String. It wraps value in 0x0022 (QUOTATION MARK) code units and escapes certain other code units within it. This operation interprets value as a sequence of UTF-16 encoded code points, as described in 6.1.4. It performs the following steps when called:

  1. Let product be the String value consisting solely of the code unit 0x0022 (QUOTATION MARK).
  2. For each code point C of StringToCodePoints(value), do
    1. If C is listed in the “Code Point” column of Table 70, then
      1. Set product to the string-concatenation of product and the escape sequence for C as specified in the “Escape Sequence” column of the corresponding row.
    2. Else if C has a numeric value less than 0x0020 (SPACE) or C has the same numeric value as a leading surrogate or trailing surrogate, then
      1. Let unit be the code unit whose numeric value is the numeric value of C.
      2. Set product to the string-concatenation of product and UnicodeEscape(unit).
    3. Else,
      1. Set product to the string-concatenation of product and UTF16EncodeCodePoint(C).
  3. Set product to the string-concatenation of product and the code unit 0x0022 (QUOTATION MARK).
  4. Return product.
Table 70: JSON Single Character Escape Sequences
Code Point Unicode Character Name Escape Sequence
U 0008 BACKSPACE \b
U 0009 CHARACTER TABULATION \t
U 000A LINE FEED (LF) \n
U 000C FORM FEED (FF) \f
U 000D CARRIAGE RETURN (CR) \r
U 0022 QUOTATION MARK \"
U 005C REVERSE SOLIDUS \\

25.5.2.4 UnicodeEscape ( C )

The abstract operation UnicodeEscape takes argument C (a code unit) and returns a String. It represents C as a Unicode escape sequence. It performs the following steps when called:

  1. Let n be the numeric value of C.
  2. Assert: n ≤ 0xFFFF.
  3. Let hex be the String representation of n, formatted as a lowercase hexadecimal number.
  4. Return the string-concatenation of the code unit 0x005C (REVERSE SOLIDUS), "u", and ! StringPad(hex, 4𝔽, "0", start).

25.5.2.5 SerializeJSONObject ( state, value )

The abstract operation SerializeJSONObject takes arguments state (a JSON Serialization Record) and value (an Object) and returns either a normal completion containing a String or a throw completion. It serializes an object. It performs the following steps when called:

  1. If state.[[Stack]] contains value, throw a TypeError exception because the structure is cyclical.
  2. Append value to state.[[Stack]].
  3. Let stepback be state.[[Indent]].
  4. Set state.[[Indent]] to the string-concatenation of state.[[Indent]] and state.[[Gap]].
  5. If state.[[PropertyList]] is not undefined, then
    1. Let K be state.[[PropertyList]].
  6. Else,
    1. Let K be ? EnumerableOwnProperties(value, key).
  7. Let partial be a new empty List.
  8. For each element P of K, do
    1. Let strP be ? SerializeJSONProperty(state, P, value).
    2. If strP is not undefined, then
      1. Let member be QuoteJSONString(P).
      2. Set member to the string-concatenation of member and ":".
      3. If state.[[Gap]] is not the empty String, then
        1. Set member to the string-concatenation of member and the code unit 0x0020 (SPACE).
      4. Set member to the string-concatenation of member and strP.
      5. Append member to partial.
  9. If partial is empty, then
    1. Let final be "{}".
  10. Else,
    1. If state.[[Gap]] is the empty String, then
      1. Let properties be the String value formed by concatenating all the element Strings of partial with each adjacent pair of Strings separated with the code unit 0x002C (COMMA). A comma is not inserted either before the first String or after the last String.
      2. Let final be the string-concatenation of "{", properties, and "}".
    2. Else,
      1. Let separator be the string-concatenation of the code unit 0x002C (COMMA), the code unit 0x000A (LINE FEED), and state.[[Indent]].
      2. Let properties be the String value formed by concatenating all the element Strings of partial with each adjacent pair of Strings separated with separator. The separator String is not inserted either before the first String or after the last String.
      3. Let final be the string-concatenation of "{", the code unit 0x000A (LINE FEED), state.[[Indent]], properties, the code unit 0x000A (LINE FEED), stepback, and "}".
  11. Remove the last element of state.[[Stack]].
  12. Set state.[[Indent]] to stepback.
  13. Return final.

25.5.2.6 SerializeJSONArray ( state, value )

The abstract operation SerializeJSONArray takes arguments state (a JSON Serialization Record) and value (an ECMAScript language value) and returns either a normal completion containing a String or a throw completion. It serializes an array. It performs the following steps when called:

  1. If state.[[Stack]] contains value, throw a TypeError exception because the structure is cyclical.
  2. Append value to state.[[Stack]].
  3. Let stepback be state.[[Indent]].
  4. Set state.[[Indent]] to the string-concatenation of state.[[Indent]] and state.[[Gap]].
  5. Let partial be a new empty List.
  6. Let len be ? LengthOfArrayLike(value).
  7. Let index be 0.
  8. Repeat, while index < len,
    1. Let strP be ? SerializeJSONProperty(state, ! ToString(𝔽(index)), value).
    2. If strP is undefined, then
      1. Append "null" to partial.
    3. Else,
      1. Append strP to partial.
    4. Set index to index 1.
  9. If partial is empty, then
    1. Let final be "[]".
  10. Else,
    1. If state.[[Gap]] is the empty String, then
      1. Let properties be the String value formed by concatenating all the element Strings of partial with each adjacent pair of Strings separated with the code unit 0x002C (COMMA). A comma is not inserted either before the first String or after the last String.
      2. Let final be the string-concatenation of "[", properties, and "]".
    2. Else,
      1. Let separator be the string-concatenation of the code unit 0x002C (COMMA), the code unit 0x000A (LINE FEED), and state.[[Indent]].
      2. Let properties be the String value formed by concatenating all the element Strings of partial with each adjacent pair of Strings separated with separator. The separator String is not inserted either before the first String or after the last String.
      3. Let final be the string-concatenation of "[", the code unit 0x000A (LINE FEED), state.[[Indent]], properties, the code unit 0x000A (LINE FEED), stepback, and "]".
  11. Remove the last element of state.[[Stack]].
  12. Set state.[[Indent]] to stepback.
  13. Return final.
 Note

The representation of arrays includes only the elements in the interval from 0𝔽 (inclusive) to array.length (exclusive). Properties whose keys are not array indices are excluded from the stringification. An array is stringified as an opening LEFT SQUARE BRACKET, elements separated by COMMA, and a closing RIGHT SQUARE BRACKET.

25.5.3 JSON [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "JSON".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

26 Managing Memory

26.1 WeakRef Objects

A WeakRef is an object that is used to refer to a target object or symbol without preserving it from garbage collection. WeakRefs can be dereferenced to allow access to the target value, if the target hasn't been reclaimed by garbage collection.

26.1.1 The WeakRef Constructor

The WeakRef constructor:

  • is %WeakRef%.
  • is the initial value of the "WeakRef" property of the global object.
  • creates and initializes a new WeakRef when called as a constructor.
  • is not intended to be called as a function and will throw an exception when called in that manner.
  • may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified WeakRef behaviour must include a super call to the WeakRef constructor to create and initialize the subclass instance with the internal state necessary to support the WeakRef.prototype built-in methods.

26.1.1.1 WeakRef ( target )

This function performs the following steps when called:

  1. If NewTarget is undefined, throw a TypeError exception.
  2. If CanBeHeldWeakly(target) is false, throw a TypeError exception.
  3. Let weakRef be ? OrdinaryCreateFromConstructor(NewTarget, "%WeakRef.prototype%", « [[WeakRefTarget]] »).
  4. Perform AddToKeptObjects(target).
  5. Set weakRef.[[WeakRefTarget]] to target.
  6. Return weakRef.

26.1.2 Properties of the WeakRef Constructor

The WeakRef constructor:

  • has a [[Prototype]] internal slot whose value is %Function.prototype%.
  • has the following properties:

26.1.2.1 WeakRef.prototype

The initial value of WeakRef.prototype is the WeakRef prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

26.1.3 Properties of the WeakRef Prototype Object

The WeakRef prototype object:

  • is %WeakRef.prototype%.
  • has a [[Prototype]] internal slot whose value is %Object.prototype%.
  • is an ordinary object.
  • does not have a [[WeakRefTarget]] internal slot.
Normative Optional

26.1.3.1 WeakRef.prototype.constructor

The initial value of WeakRef.prototype.constructor is %WeakRef%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

26.1.3.2 WeakRef.prototype.deref ( )

This method performs the following steps when called:

  1. Let weakRef be the this value.
  2. Perform ? RequireInternalSlot(weakRef, [[WeakRefTarget]]).
  3. Return WeakRefDeref(weakRef).
 Note

If the WeakRef returns a target value that is not undefined, then this target value should not be garbage collected until the current execution of ECMAScript code has completed. The AddToKeptObjects operation makes sure read consistency is maintained.

let target = { foo() {} };
let weakRef = new WeakRef(target);

// ... later ...

if (weakRef.deref()) {
  weakRef.deref().foo();
}

In the above example, if the first deref does not evaluate to undefined then the second deref cannot either.

26.1.3.3 WeakRef.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "WeakRef".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

26.1.4 WeakRef Abstract Operations

26.1.4.1 WeakRefDeref ( weakRef )

The abstract operation WeakRefDeref takes argument weakRef (a WeakRef) and returns an ECMAScript language value. It performs the following steps when called:

  1. Let target be weakRef.[[WeakRefTarget]].
  2. If target is not empty, then
    1. Perform AddToKeptObjects(target).
    2. Return target.
  3. Return undefined.
 Note

This abstract operation is defined separately from WeakRef.prototype.deref strictly to make it possible to succinctly define liveness.

26.1.5 Properties of WeakRef Instances

WeakRef instances are ordinary objects that inherit properties from the WeakRef prototype. WeakRef instances also have a [[WeakRefTarget]] internal slot.

26.2 FinalizationRegistry Objects

A FinalizationRegistry is an object that manages registration and unregistration of cleanup operations that are performed when target objects and symbols are garbage collected.

26.2.1 The FinalizationRegistry Constructor

The FinalizationRegistry constructor:

  • is %FinalizationRegistry%.
  • is the initial value of the "FinalizationRegistry" property of the global object.
  • creates and initializes a new FinalizationRegistry when called as a constructor.
  • is not intended to be called as a function and will throw an exception when called in that manner.
  • may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified FinalizationRegistry behaviour must include a super call to the FinalizationRegistry constructor to create and initialize the subclass instance with the internal state necessary to support the FinalizationRegistry.prototype built-in methods.

26.2.1.1 FinalizationRegistry ( cleanupCallback )

This function performs the following steps when called:

  1. If NewTarget is undefined, throw a TypeError exception.
  2. If IsCallable(cleanupCallback) is false, throw a TypeError exception.
  3. Let finalizationRegistry be ? OrdinaryCreateFromConstructor(NewTarget, "%FinalizationRegistry.prototype%", « [[Realm]], [[CleanupCallback]], [[Cells]] »).
  4. Let fn be the active function object.
  5. Set finalizationRegistry.[[Realm]] to fn.[[Realm]].
  6. Set finalizationRegistry.[[CleanupCallback]] to HostMakeJobCallback(cleanupCallback).
  7. Set finalizationRegistry.[[Cells]] to a new empty List.
  8. Return finalizationRegistry.

26.2.2 Properties of the FinalizationRegistry Constructor

The FinalizationRegistry constructor:

  • has a [[Prototype]] internal slot whose value is %Function.prototype%.
  • has the following properties:

26.2.2.1 FinalizationRegistry.prototype

The initial value of FinalizationRegistry.prototype is the FinalizationRegistry prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

26.2.3 Properties of the FinalizationRegistry Prototype Object

The FinalizationRegistry prototype object:

  • is %FinalizationRegistry.prototype%.
  • has a [[Prototype]] internal slot whose value is %Object.prototype%.
  • is an ordinary object.
  • does not have [[Cells]] and [[CleanupCallback]] internal slots.

26.2.3.1 FinalizationRegistry.prototype.constructor

The initial value of FinalizationRegistry.prototype.constructor is %FinalizationRegistry%.

26.2.3.2 FinalizationRegistry.prototype.register ( target, heldValue [ , unregisterToken ] )

This method performs the following steps when called:

  1. Let finalizationRegistry be the this value.
  2. Perform ? RequireInternalSlot(finalizationRegistry, [[Cells]]).
  3. If CanBeHeldWeakly(target) is false, throw a TypeError exception.
  4. If SameValue(target, heldValue) is true, throw a TypeError exception.
  5. If CanBeHeldWeakly(unregisterToken) is false, then
    1. If unregisterToken is not undefined, throw a TypeError exception.
    2. Set unregisterToken to empty.
  6. Let cell be the Record { [[WeakRefTarget]]: target, [[HeldValue]]: heldValue, [[UnregisterToken]]: unregisterToken }.
  7. Append cell to finalizationRegistry.[[Cells]].
  8. Return undefined.
 Note

Based on the algorithms and definitions in this specification, cell.[[HeldValue]] is live when finalizationRegistry.[[Cells]] contains cell; however, this does not necessarily mean that cell.[[UnregisterToken]] or cell.[[Target]] are live. For example, registering an object with itself as its unregister token would not keep the object alive forever.

26.2.3.3 FinalizationRegistry.prototype.unregister ( unregisterToken )

This method performs the following steps when called:

  1. Let finalizationRegistry be the this value.
  2. Perform ? RequireInternalSlot(finalizationRegistry, [[Cells]]).
  3. If CanBeHeldWeakly(unregisterToken) is false, throw a TypeError exception.
  4. Let removed be false.
  5. For each Record { [[WeakRefTarget]], [[HeldValue]], [[UnregisterToken]] } cell of finalizationRegistry.[[Cells]], do
    1. If cell.[[UnregisterToken]] is not empty and SameValue(cell.[[UnregisterToken]], unregisterToken) is true, then
      1. Remove cell from finalizationRegistry.[[Cells]].
      2. Set removed to true.
  6. Return removed.

26.2.3.4 FinalizationRegistry.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "FinalizationRegistry".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

26.2.4 Properties of FinalizationRegistry Instances

FinalizationRegistry instances are ordinary objects that inherit properties from the FinalizationRegistry prototype. FinalizationRegistry instances also have [[Cells]] and [[CleanupCallback]] internal slots.

27 Control Abstraction Objects

27.1 Iteration

27.1.1 Common Iteration Interfaces

An interface is a set of property keys whose associated values match a specific specification. Any object that provides all the properties as described by an interface's specification conforms to that interface. An interface is not represented by a distinct object. There may be many separately implemented objects that conform to any interface. An individual object may conform to multiple interfaces.

27.1.1.1 The Iterable Interface

The Iterable interface includes the property described in Table 71:

Table 71: Iterable Interface Required Properties
Property Value Requirements
@@iterator a function that returns an Iterator object The returned object must conform to the Iterator interface.

27.1.1.2 The Iterator Interface

An object that implements the Iterator interface must include the property in Table 72. Such objects may also implement the properties in Table 73.

Table 72: Iterator Interface Required Properties
Property Value Requirements
"next" a function that returns an IteratorResult object The returned object must conform to the IteratorResult interface. If a previous call to the next method of an Iterator has returned an IteratorResult object whose "done" property is true, then all subsequent calls to the next method of that object should also return an IteratorResult object whose "done" property is true. However, this requirement is not enforced.
Note 1

Arguments may be passed to the next function but their interpretation and validity is dependent upon the target Iterator. The for-of statement and other common users of Iterators do not pass any arguments, so Iterator objects that expect to be used in such a manner must be prepared to deal with being called with no arguments.
Table 73: Iterator Interface Optional Properties
Property Value Requirements
"return" a function that returns an IteratorResult object The returned object must conform to the IteratorResult interface. Invoking this method notifies the Iterator object that the caller does not intend to make any more next method calls to the Iterator. The returned IteratorResult object will typically have a "done" property whose value is true, and a "value" property with the value passed as the argument of the return method. However, this requirement is not enforced.
"throw" a function that returns an IteratorResult object The returned object must conform to the IteratorResult interface. Invoking this method notifies the Iterator object that the caller has detected an error condition. The argument may be used to identify the error condition and typically will be an exception object. A typical response is to throw the value passed as the argument. If the method does not throw, the returned IteratorResult object will typically have a "done" property whose value is true.
Note 2

Typically callers of these methods should check for their existence before invoking them. Certain ECMAScript language features including for-of, yield*, and array destructuring call these methods after performing an existence check. Most ECMAScript library functions that accept Iterable objects as arguments also conditionally call them.

27.1.1.3 The AsyncIterable Interface

The AsyncIterable interface includes the properties described in Table 74:

Table 74: AsyncIterable Interface Required Properties
Property Value Requirements
@@asyncIterator a function that returns an AsyncIterator object The returned object must conform to the AsyncIterator interface.

27.1.1.4 The AsyncIterator Interface

An object that implements the AsyncIterator interface must include the properties in Table 75. Such objects may also implement the properties in Table 76.

Table 75: AsyncIterator Interface Required Properties
Property Value Requirements
"next" a function that returns a promise for an IteratorResult object

The returned promise, when fulfilled, must fulfill with an object that conforms to the IteratorResult interface. If a previous call to the next method of an AsyncIterator has returned a promise for an IteratorResult object whose "done" property is true, then all subsequent calls to the next method of that object should also return a promise for an IteratorResult object whose "done" property is true. However, this requirement is not enforced.

Additionally, the IteratorResult object that serves as a fulfillment value should have a "value" property whose value is not a promise (or "thenable"). However, this requirement is also not enforced.
Note 1

Arguments may be passed to the next function but their interpretation and validity is dependent upon the target AsyncIterator. The for-await-of statement and other common users of AsyncIterators do not pass any arguments, so AsyncIterator objects that expect to be used in such a manner must be prepared to deal with being called with no arguments.
Table 76: AsyncIterator Interface Optional Properties
Property Value Requirements
"return" a function that returns a promise for an IteratorResult object

The returned promise, when fulfilled, must fulfill with an object that conforms to the IteratorResult interface. Invoking this method notifies the AsyncIterator object that the caller does not intend to make any more next method calls to the AsyncIterator. The returned promise will fulfill with an IteratorResult object which will typically have a "done" property whose value is true, and a "value" property with the value passed as the argument of the return method. However, this requirement is not enforced.

Additionally, the IteratorResult object that serves as a fulfillment value should have a "value" property whose value is not a promise (or "thenable"). If the argument value is used in the typical manner, then if it is a rejected promise, a promise rejected with the same reason should be returned; if it is a fulfilled promise, then its fulfillment value should be used as the "value" property of the returned promise's IteratorResult object fulfillment value. However, these requirements are also not enforced.
"throw" a function that returns a promise for an IteratorResult object

The returned promise, when fulfilled, must fulfill with an object that conforms to the IteratorResult interface. Invoking this method notifies the AsyncIterator object that the caller has detected an error condition. The argument may be used to identify the error condition and typically will be an exception object. A typical response is to return a rejected promise which rejects with the value passed as the argument.

If the returned promise is fulfilled, the IteratorResult fulfillment value will typically have a "done" property whose value is true. Additionally, it should have a "value" property whose value is not a promise (or "thenable"), but this requirement is not enforced.
Note 2

Typically callers of these methods should check for their existence before invoking them. Certain ECMAScript language features including for-await-of and yield* call these methods after performing an existence check.

27.1.1.5 The IteratorResult Interface

The IteratorResult interface includes the properties listed in Table 77:

Table 77: IteratorResult Interface Properties
Property Value Requirements
"done" a Boolean This is the result status of an iterator next method call. If the end of the iterator was reached "done" is true. If the end was not reached "done" is false and a value is available. If a "done" property (either own or inherited) does not exist, it is considered to have the value false.
"value" an ECMAScript language value If done is false, this is the current iteration element value. If done is true, this is the return value of the iterator, if it supplied one. If the iterator does not have a return value, "value" is undefined. In that case, the "value" property may be absent from the conforming object if it does not inherit an explicit "value" property.

27.1.2 The %IteratorPrototype% Object

The %IteratorPrototype% object:

Note

All objects defined in this specification that implement the Iterator interface also inherit from %IteratorPrototype%. ECMAScript code may also define objects that inherit from %IteratorPrototype%. The %IteratorPrototype% object provides a place where additional methods that are applicable to all iterator objects may be added.

The following expression is one way that ECMAScript code can access the %IteratorPrototype% object:

Object.getPrototypeOf(Object.getPrototypeOf([][Symbol.iterator]()))

27.1.2.1 %IteratorPrototype% [ @@iterator ] ( )

This function performs the following steps when called:

  1. Return the this value.

The value of the "name" property of this function is "[Symbol.iterator]".

27.1.3 The %AsyncIteratorPrototype% Object

The %AsyncIteratorPrototype% object:

Note

All objects defined in this specification that implement the AsyncIterator interface also inherit from %AsyncIteratorPrototype%. ECMAScript code may also define objects that inherit from %AsyncIteratorPrototype%. The %AsyncIteratorPrototype% object provides a place where additional methods that are applicable to all async iterator objects may be added.

27.1.3.1 %AsyncIteratorPrototype% [ @@asyncIterator ] ( )

This function performs the following steps when called:

  1. Return the this value.

The value of the "name" property of this function is "[Symbol.asyncIterator]".

27.1.4 Async-from-Sync Iterator Objects

An Async-from-Sync Iterator object is an async iterator that adapts a specific synchronous iterator. There is not a named constructor for Async-from-Sync Iterator objects. Instead, Async-from-Sync iterator objects are created by the CreateAsyncFromSyncIterator abstract operation as needed.

27.1.4.1 CreateAsyncFromSyncIterator ( syncIteratorRecord )

The abstract operation CreateAsyncFromSyncIterator takes argument syncIteratorRecord (an Iterator Record) and returns an Iterator Record. It is used to create an async Iterator Record from a synchronous Iterator Record. It performs the following steps when called:

  1. Let asyncIterator be OrdinaryObjectCreate(%AsyncFromSyncIteratorPrototype%, « [[SyncIteratorRecord]] »).
  2. Set asyncIterator.[[SyncIteratorRecord]] to syncIteratorRecord.
  3. Let nextMethod be ! Get(asyncIterator, "next").
  4. Let iteratorRecord be the Iterator Record { [[Iterator]]: asyncIterator, [[NextMethod]]: nextMethod, [[Done]]: false }.
  5. Return iteratorRecord.

27.1.4.2 The %AsyncFromSyncIteratorPrototype% Object

The %AsyncFromSyncIteratorPrototype% object:

  • has properties that are inherited by all Async-from-Sync Iterator Objects.
  • is an ordinary object.
  • has a [[Prototype]] internal slot whose value is %AsyncIteratorPrototype%.
  • has the following properties:

27.1.4.2.1 %AsyncFromSyncIteratorPrototype%.next ( [ value ] )

  1. Let O be the this value.
  2. Assert: O is an Object that has a [[SyncIteratorRecord]] internal slot.
  3. Let promiseCapability be ! NewPromiseCapability(%Promise%).
  4. Let syncIteratorRecord be O.[[SyncIteratorRecord]].
  5. If value is present, then
    1. Let result be Completion(IteratorNext(syncIteratorRecord, value)).
  6. Else,
    1. Let result be Completion(IteratorNext(syncIteratorRecord)).
  7. IfAbruptRejectPromise(result, promiseCapability).
  8. Return AsyncFromSyncIteratorContinuation(result, promiseCapability).

27.1.4.2.2 %AsyncFromSyncIteratorPrototype%.return ( [ value ] )

  1. Let O be the this value.
  2. Assert: O is an Object that has a [[SyncIteratorRecord]] internal slot.
  3. Let promiseCapability be ! NewPromiseCapability(%Promise%).
  4. Let syncIterator be O.[[SyncIteratorRecord]].[[Iterator]].
  5. Let return be Completion(GetMethod(syncIterator, "return")).
  6. IfAbruptRejectPromise(return, promiseCapability).
  7. If return is undefined, then
    1. Let iterResult be CreateIterResultObject(value, true).
    2. Perform ! Call(promiseCapability.[[Resolve]], undefined, « iterResult »).
    3. Return promiseCapability.[[Promise]].
  8. If value is present, then
    1. Let result be Completion(Call(return, syncIterator, « value »)).
  9. Else,
    1. Let result be Completion(Call(return, syncIterator)).
  10. IfAbruptRejectPromise(result, promiseCapability).
  11. If result is not an Object, then
    1. Perform ! Call(promiseCapability.[[Reject]], undefined, « a newly created TypeError object »).
    2. Return promiseCapability.[[Promise]].
  12. Return AsyncFromSyncIteratorContinuation(result, promiseCapability).

27.1.4.2.3 %AsyncFromSyncIteratorPrototype%.throw ( [ value ] )

Note
In this specification, value is always provided, but is left optional for consistency with %AsyncFromSyncIteratorPrototype%.return ( [ value ] ).
  1. Let O be the this value.
  2. Assert: O is an Object that has a [[SyncIteratorRecord]] internal slot.
  3. Let promiseCapability be ! NewPromiseCapability(%Promise%).
  4. Let syncIterator be O.[[SyncIteratorRecord]].[[Iterator]].
  5. Let throw be Completion(GetMethod(syncIterator, "throw")).
  6. IfAbruptRejectPromise(throw, promiseCapability).
  7. If throw is undefined, then
    1. Perform ! Call(promiseCapability.[[Reject]], undefined, « value »).
    2. Return promiseCapability.[[Promise]].
  8. If value is present, then
    1. Let result be Completion(Call(throw, syncIterator, « value »)).
  9. Else,
    1. Let result be Completion(Call(throw, syncIterator)).
  10. IfAbruptRejectPromise(result, promiseCapability).
  11. If result is not an Object, then
    1. Perform ! Call(promiseCapability.[[Reject]], undefined, « a newly created TypeError object »).
    2. Return promiseCapability.[[Promise]].
  12. Return AsyncFromSyncIteratorContinuation(result, promiseCapability).

27.1.4.3 Properties of Async-from-Sync Iterator Instances

Async-from-Sync Iterator instances are ordinary objects that inherit properties from the %AsyncFromSyncIteratorPrototype% intrinsic object. Async-from-Sync Iterator instances are initially created with the internal slots listed in Table 78. Async-from-Sync Iterator instances are not directly observable from ECMAScript code.

Table 78: Internal Slots of Async-from-Sync Iterator Instances
Internal Slot Type Description
[[SyncIteratorRecord]] an Iterator Record Represents the original synchronous iterator which is being adapted.

27.1.4.4 AsyncFromSyncIteratorContinuation ( result, promiseCapability )

The abstract operation AsyncFromSyncIteratorContinuation takes arguments result (an Object) and promiseCapability (a PromiseCapability Record for an intrinsic %Promise%) and returns a Promise. It performs the following steps when called:

  1. NOTE: Because promiseCapability is derived from the intrinsic %Promise%, the calls to promiseCapability.[[Reject]] entailed by the use IfAbruptRejectPromise below are guaranteed not to throw.
  2. Let done be Completion(IteratorComplete(result)).
  3. IfAbruptRejectPromise(done, promiseCapability).
  4. Let value be Completion(IteratorValue(result)).
  5. IfAbruptRejectPromise(value, promiseCapability).
  6. Let valueWrapper be Completion(PromiseResolve(%Promise%, value)).
  7. IfAbruptRejectPromise(valueWrapper, promiseCapability).
  8. Let unwrap be a new Abstract Closure with parameters (v) that captures done and performs the following steps when called:
    1. Return CreateIterResultObject(v, done).
  9. Let onFulfilled be CreateBuiltinFunction(unwrap, 1, "", « »).
  10. NOTE: onFulfilled is used when processing the "value" property of an IteratorResult object in order to wait for its value if it is a promise and re-package the result in a new "unwrapped" IteratorResult object.
  11. Perform PerformPromiseThen(valueWrapper, onFulfilled, undefined, promiseCapability).
  12. Return promiseCapability.[[Promise]].

27.2 Promise Objects

A Promise is an object that is used as a placeholder for the eventual results of a deferred (and possibly asynchronous) computation.

Any Promise is in one of three mutually exclusive states: fulfilled, rejected, and pending:

  • A promise p is fulfilled if p.then(f, r) will immediately enqueue a Job to call the function f.
  • A promise p is rejected if p.then(f, r) will immediately enqueue a Job to call the function r.
  • A promise is pending if it is neither fulfilled nor rejected.

A promise is said to be settled if it is not pending, i.e. if it is either fulfilled or rejected.

A promise is resolved if it is settled or if it has been “locked in” to match the state of another promise. Attempting to resolve or reject a resolved promise has no effect. A promise is unresolved if it is not resolved. An unresolved promise is always in the pending state. A resolved promise may be pending, fulfilled or rejected.

27.2.1 Promise Abstract Operations

27.2.1.1 PromiseCapability Records

A PromiseCapability Record is a Record value used to encapsulate a Promise or promise-like object along with the functions that are capable of resolving or rejecting that promise. PromiseCapability Records are produced by the NewPromiseCapability abstract operation.

PromiseCapability Records have the fields listed in Table 79.

Table 79: PromiseCapability Record Fields
Field Name Value Meaning
[[Promise]] an Object An object that is usable as a promise.
[[Resolve]] a function object The function that is used to resolve the given promise.
[[Reject]] a function object The function that is used to reject the given promise.

27.2.1.1.1 IfAbruptRejectPromise ( value, capability )

IfAbruptRejectPromise is a shorthand for a sequence of algorithm steps that use a PromiseCapability Record. An algorithm step of the form:

  1. IfAbruptRejectPromise(value, capability).

means the same thing as:

  1. Assert: value is a Completion Record.
  2. If value is an abrupt completion, then
    1. Perform ? Call(capability.[[Reject]], undefined, « value.[[Value]] »).
    2. Return capability.[[Promise]].
  3. Else, set value to value.[[Value]].

27.2.1.2 PromiseReaction Records

The PromiseReaction is a Record value used to store information about how a promise should react when it becomes resolved or rejected with a given value. PromiseReaction records are created by the PerformPromiseThen abstract operation, and are used by the Abstract Closure returned by NewPromiseReactionJob.

PromiseReaction records have the fields listed in Table 80.

Table 80: PromiseReaction Record Fields
Field Name Value Meaning
[[Capability]] a PromiseCapability Record or undefined The capabilities of the promise for which this record provides a reaction handler.
[[Type]] Fulfill or Reject The [[Type]] is used when [[Handler]] is empty to allow for behaviour specific to the settlement type.
[[Handler]] a JobCallback Record or empty The function that should be applied to the incoming value, and whose return value will govern what happens to the derived promise. If [[Handler]] is empty, a function that depends on the value of [[Type]] will be used instead.

27.2.1.3 CreateResolvingFunctions ( promise )

The abstract operation CreateResolvingFunctions takes argument promise (a Promise) and returns a Record with fields [[Resolve]] (a function object) and [[Reject]] (a function object). It performs the following steps when called:

  1. Let alreadyResolved be the Record { [[Value]]: false }.
  2. Let stepsResolve be the algorithm steps defined in Promise Resolve Functions.
  3. Let lengthResolve be the number of non-optional parameters of the function definition in Promise Resolve Functions.
  4. Let resolve be CreateBuiltinFunction(stepsResolve, lengthResolve, "", « [[Promise]], [[AlreadyResolved]] »).
  5. Set resolve.[[Promise]] to promise.
  6. Set resolve.[[AlreadyResolved]] to alreadyResolved.
  7. Let stepsReject be the algorithm steps defined in Promise Reject Functions.
  8. Let lengthReject be the number of non-optional parameters of the function definition in Promise Reject Functions.
  9. Let reject be CreateBuiltinFunction(stepsReject, lengthReject, "", « [[Promise]], [[AlreadyResolved]] »).
  10. Set reject.[[Promise]] to promise.
  11. Set reject.[[AlreadyResolved]] to alreadyResolved.
  12. Return the Record { [[Resolve]]: resolve, [[Reject]]: reject }.

27.2.1.3.1 Promise Reject Functions

A promise reject function is an anonymous built-in function that has [[Promise]] and [[AlreadyResolved]] internal slots.

When a promise reject function is called with argument reason, the following steps are taken:

  1. Let F be the active function object.
  2. Assert: F has a [[Promise]] internal slot whose value is an Object.
  3. Let promise be F.[[Promise]].
  4. Let alreadyResolved be F.[[AlreadyResolved]].
  5. If alreadyResolved.[[Value]] is true, return undefined.
  6. Set alreadyResolved.[[Value]] to true.
  7. Perform RejectPromise(promise, reason).
  8. Return undefined.

The "length" property of a promise reject function is 1𝔽.

27.2.1.3.2 Promise Resolve Functions

A promise resolve function is an anonymous built-in function that has [[Promise]] and [[AlreadyResolved]] internal slots.

When a promise resolve function is called with argument resolution, the following steps are taken:

  1. Let F be the active function object.
  2. Assert: F has a [[Promise]] internal slot whose value is an Object.
  3. Let promise be F.[[Promise]].
  4. Let alreadyResolved be F.[[AlreadyResolved]].
  5. If alreadyResolved.[[Value]] is true, return undefined.
  6. Set alreadyResolved.[[Value]] to true.
  7. If SameValue(resolution, promise) is true, then
    1. Let selfResolutionError be a newly created TypeError object.
    2. Perform RejectPromise(promise, selfResolutionError).
    3. Return undefined.
  8. If resolution is not an Object, then
    1. Perform FulfillPromise(promise, resolution).
    2. Return undefined.
  9. Let then be Completion(Get(resolution, "then")).
  10. If then is an abrupt completion, then
    1. Perform RejectPromise(promise, then.[[Value]]).
    2. Return undefined.
  11. Let thenAction be then.[[Value]].
  12. If IsCallable(thenAction) is false, then
    1. Perform FulfillPromise(promise, resolution).
    2. Return undefined.
  13. Let thenJobCallback be HostMakeJobCallback(thenAction).
  14. Let job be NewPromiseResolveThenableJob(promise, resolution, thenJobCallback).
  15. Perform HostEnqueuePromiseJob(job.[[Job]], job.[[Realm]]).
  16. Return undefined.

The "length" property of a promise resolve function is 1𝔽.

27.2.1.4 FulfillPromise ( promise, value )

The abstract operation FulfillPromise takes arguments promise (a Promise) and value (an ECMAScript language value) and returns unused. It performs the following steps when called:

  1. Assert: The value of promise.[[PromiseState]] is pending.
  2. Let reactions be promise.[[PromiseFulfillReactions]].
  3. Set promise.[[PromiseResult]] to value.
  4. Set promise.[[PromiseFulfillReactions]] to undefined.
  5. Set promise.[[PromiseRejectReactions]] to undefined.
  6. Set promise.[[PromiseState]] to fulfilled.
  7. Perform TriggerPromiseReactions(reactions, value).
  8. Return unused.

27.2.1.5 NewPromiseCapability ( C )

The abstract operation NewPromiseCapability takes argument C (an ECMAScript language value) and returns either a normal completion containing a PromiseCapability Record or a throw completion. It attempts to use C as a constructor in the fashion of the built-in Promise constructor to create a promise and extract its resolve and reject functions. The promise plus the resolve and reject functions are used to initialize a new PromiseCapability Record. It performs the following steps when called:

  1. If IsConstructor(C) is false, throw a TypeError exception.
  2. NOTE: C is assumed to be a constructor function that supports the parameter conventions of the Promise constructor (see 27.2.3.1).
  3. Let resolvingFunctions be the Record { [[Resolve]]: undefined, [[Reject]]: undefined }.
  4. Let executorClosure be a new Abstract Closure with parameters (resolve, reject) that captures resolvingFunctions and performs the following steps when called:
    1. If resolvingFunctions.[[Resolve]] is not undefined, throw a TypeError exception.
    2. If resolvingFunctions.[[Reject]] is not undefined, throw a TypeError exception.
    3. Set resolvingFunctions.[[Resolve]] to resolve.
    4. Set resolvingFunctions.[[Reject]] to reject.
    5. Return undefined.
  5. Let executor be CreateBuiltinFunction(executorClosure, 2, "", « »).
  6. Let promise be ? Construct(C, « executor »).
  7. If IsCallable(resolvingFunctions.[[Resolve]]) is false, throw a TypeError exception.
  8. If IsCallable(resolvingFunctions.[[Reject]]) is false, throw a TypeError exception.
  9. Return the PromiseCapability Record { [[Promise]]: promise, [[Resolve]]: resolvingFunctions.[[Resolve]], [[Reject]]: resolvingFunctions.[[Reject]] }.
 Note

This abstract operation supports Promise subclassing, as it is generic on any constructor that calls a passed executor function argument in the same way as the Promise constructor. It is used to generalize static methods of the Promise constructor to any subclass.

27.2.1.6 IsPromise ( x )

The abstract operation IsPromise takes argument x (an ECMAScript language value) and returns a Boolean. It checks for the promise brand on an object. It performs the following steps when called:

  1. If x is not an Object, return false.
  2. If x does not have a [[PromiseState]] internal slot, return false.
  3. Return true.

27.2.1.7 RejectPromise ( promise, reason )

The abstract operation RejectPromise takes arguments promise (a Promise) and reason (an ECMAScript language value) and returns unused. It performs the following steps when called:

  1. Assert: The value of promise.[[PromiseState]] is pending.
  2. Let reactions be promise.[[PromiseRejectReactions]].
  3. Set promise.[[PromiseResult]] to reason.
  4. Set promise.[[PromiseFulfillReactions]] to undefined.
  5. Set promise.[[PromiseRejectReactions]] to undefined.
  6. Set promise.[[PromiseState]] to rejected.
  7. If promise.[[PromiseIsHandled]] is false, perform HostPromiseRejectionTracker(promise, "reject").
  8. Perform TriggerPromiseReactions(reactions, reason).
  9. Return unused.

27.2.1.8 TriggerPromiseReactions ( reactions, argument )

The abstract operation TriggerPromiseReactions takes arguments reactions (a List of PromiseReaction Records) and argument (an ECMAScript language value) and returns unused. It enqueues a new Job for each record in reactions. Each such Job processes the [[Type]] and [[Handler]] of the PromiseReaction Record, and if the [[Handler]] is not empty, calls it passing the given argument. If the [[Handler]] is empty, the behaviour is determined by the [[Type]]. It performs the following steps when called:

  1. For each element reaction of reactions, do
    1. Let job be NewPromiseReactionJob(reaction, argument).
    2. Perform HostEnqueuePromiseJob(job.[[Job]], job.[[Realm]]).
  2. Return unused.

27.2.1.9 HostPromiseRejectionTracker ( promise, operation )

The host-defined abstract operation HostPromiseRejectionTracker takes arguments promise (a Promise) and operation ("reject" or "handle") and returns unused. It allows host environments to track promise rejections.

An implementation of HostPromiseRejectionTracker must conform to the following requirements:

The default implementation of HostPromiseRejectionTracker is to return unused.

Note 1

HostPromiseRejectionTracker is called in two scenarios:

  • When a promise is rejected without any handlers, it is called with its operation argument set to "reject".
  • When a handler is added to a rejected promise for the first time, it is called with its operation argument set to "handle".

A typical implementation of HostPromiseRejectionTracker might try to notify developers of unhandled rejections, while also being careful to notify them if such previous notifications are later invalidated by new handlers being attached.

 Note 2

If operation is "handle", an implementation should not hold a reference to promise in a way that would interfere with garbage collection. An implementation may hold a reference to promise if operation is "reject", since it is expected that rejections will be rare and not on hot code paths.

27.2.2 Promise Jobs

27.2.2.1 NewPromiseReactionJob ( reaction, argument )

The abstract operation NewPromiseReactionJob takes arguments reaction (a PromiseReaction Record) and argument (an ECMAScript language value) and returns a Record with fields [[Job]] (a Job Abstract Closure) and [[Realm]] (a Realm Record or null). It returns a new Job Abstract Closure that applies the appropriate handler to the incoming value, and uses the handler's return value to resolve or reject the derived promise associated with that handler. It performs the following steps when called:

  1. Let job be a new Job Abstract Closure with no parameters that captures reaction and argument and performs the following steps when called:
    1. Let promiseCapability be reaction.[[Capability]].
    2. Let type be reaction.[[Type]].
    3. Let handler be reaction.[[Handler]].
    4. If handler is empty, then
      1. If type is Fulfill, let handlerResult be NormalCompletion(argument).
      2. Else,
        1. Assert: type is Reject.
        2. Let handlerResult be ThrowCompletion(argument).
    5. Else, let handlerResult be Completion(HostCallJobCallback(handler, undefined, « argument »)).
    6. If promiseCapability is undefined, then
      1. Assert: handlerResult is not an abrupt completion.
      2. Return empty.
    7. Assert: promiseCapability is a PromiseCapability Record.
    8. If handlerResult is an abrupt completion, then
      1. Return ? Call(promiseCapability.[[Reject]], undefined, « handlerResult.[[Value]] »).
    9. Else,
      1. Return ? Call(promiseCapability.[[Resolve]], undefined, « handlerResult.[[Value]] »).
  2. Let handlerRealm be null.
  3. If reaction.[[Handler]] is not empty, then
    1. Let getHandlerRealmResult be Completion(GetFunctionRealm(reaction.[[Handler]].[[Callback]])).
    2. If getHandlerRealmResult is a normal completion, set handlerRealm to getHandlerRealmResult.[[Value]].
    3. Else, set handlerRealm to the current Realm Record.
    4. NOTE: handlerRealm is never null unless the handler is undefined. When the handler is a revoked Proxy and no ECMAScript code runs, handlerRealm is used to create error objects.
  4. Return the Record { [[Job]]: job, [[Realm]]: handlerRealm }.

27.2.2.2 NewPromiseResolveThenableJob ( promiseToResolve, thenable, then )

The abstract operation NewPromiseResolveThenableJob takes arguments promiseToResolve (a Promise), thenable (an Object), and then (a JobCallback Record) and returns a Record with fields [[Job]] (a Job Abstract Closure) and [[Realm]] (a Realm Record). It performs the following steps when called:

  1. Let job be a new Job Abstract Closure with no parameters that captures promiseToResolve, thenable, and then and performs the following steps when called:
    1. Let resolvingFunctions be CreateResolvingFunctions(promiseToResolve).
    2. Let thenCallResult be Completion(HostCallJobCallback(then, thenable, « resolvingFunctions.[[Resolve]], resolvingFunctions.[[Reject]] »)).
    3. If thenCallResult is an abrupt completion, then
      1. Return ? Call(resolvingFunctions.[[Reject]], undefined, « thenCallResult.[[Value]] »).
    4. Return ? thenCallResult.
  2. Let getThenRealmResult be Completion(GetFunctionRealm(then.[[Callback]])).
  3. If getThenRealmResult is a normal completion, let thenRealm be getThenRealmResult.[[Value]].
  4. Else, let thenRealm be the current Realm Record.
  5. NOTE: thenRealm is never null. When then.[[Callback]] is a revoked Proxy and no code runs, thenRealm is used to create error objects.
  6. Return the Record { [[Job]]: job, [[Realm]]: thenRealm }.
 Note

This Job uses the supplied thenable and its then method to resolve the given promise. This process must take place as a Job to ensure that the evaluation of the then method occurs after evaluation of any surrounding code has completed.

27.2.3 The Promise Constructor

The Promise constructor:

  • is %Promise%.
  • is the initial value of the "Promise" property of the global object.
  • creates and initializes a new Promise when called as a constructor.
  • is not intended to be called as a function and will throw an exception when called in that manner.
  • may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified Promise behaviour must include a super call to the Promise constructor to create and initialize the subclass instance with the internal state necessary to support the Promise and Promise.prototype built-in methods.

27.2.3.1 Promise ( executor )

This function performs the following steps when called:

  1. If NewTarget is undefined, throw a TypeError exception.
  2. If IsCallable(executor) is false, throw a TypeError exception.
  3. Let promise be ? OrdinaryCreateFromConstructor(NewTarget, "%Promise.prototype%", « [[PromiseState]], [[PromiseResult]], [[PromiseFulfillReactions]], [[PromiseRejectReactions]], [[PromiseIsHandled]] »).
  4. Set promise.[[PromiseState]] to pending.
  5. Set promise.[[PromiseFulfillReactions]] to a new empty List.
  6. Set promise.[[PromiseRejectReactions]] to a new empty List.
  7. Set promise.[[PromiseIsHandled]] to false.
  8. Let resolvingFunctions be CreateResolvingFunctions(promise).
  9. Let completion be Completion(Call(executor, undefined, « resolvingFunctions.[[Resolve]], resolvingFunctions.[[Reject]] »)).
  10. If completion is an abrupt completion, then
    1. Perform ? Call(resolvingFunctions.[[Reject]], undefined, « completion.[[Value]] »).
  11. Return promise.
 Note

The executor argument must be a function object. It is called for initiating and reporting completion of the possibly deferred action represented by this Promise. The executor is called with two arguments: resolve and reject. These are functions that may be used by the executor function to report eventual completion or failure of the deferred computation. Returning from the executor function does not mean that the deferred action has been completed but only that the request to eventually perform the deferred action has been accepted.

The resolve function that is passed to an executor function accepts a single argument. The executor code may eventually call the resolve function to indicate that it wishes to resolve the associated Promise. The argument passed to the resolve function represents the eventual value of the deferred action and can be either the actual fulfillment value or another promise which will provide the value if it is fulfilled.

The reject function that is passed to an executor function accepts a single argument. The executor code may eventually call the reject function to indicate that the associated Promise is rejected and will never be fulfilled. The argument passed to the reject function is used as the rejection value of the promise. Typically it will be an Error object.

The resolve and reject functions passed to an executor function by the Promise constructor have the capability to actually resolve and reject the associated promise. Subclasses may have different constructor behaviour that passes in customized values for resolve and reject.

27.2.4 Properties of the Promise Constructor

The Promise constructor:

  • has a [[Prototype]] internal slot whose value is %Function.prototype%.
  • has the following properties:

27.2.4.1 Promise.all ( iterable )

This function returns a new promise which is fulfilled with an array of fulfillment values for the passed promises, or rejects with the reason of the first passed promise that rejects. It resolves all elements of the passed iterable to promises as it runs this algorithm.

  1. Let C be the this value.
  2. Let promiseCapability be ? NewPromiseCapability(C).
  3. Let promiseResolve be Completion(GetPromiseResolve(C)).
  4. IfAbruptRejectPromise(promiseResolve, promiseCapability).
  5. Let iteratorRecord be Completion(GetIterator(iterable, sync)).
  6. IfAbruptRejectPromise(iteratorRecord, promiseCapability).
  7. Let result be Completion(PerformPromiseAll(iteratorRecord, C, promiseCapability, promiseResolve)).
  8. If result is an abrupt completion, then
    1. If iteratorRecord.[[Done]] is false, set result to Completion(IteratorClose(iteratorRecord, result)).
    2. IfAbruptRejectPromise(result, promiseCapability).
  9. Return ? result.
 Note

This function requires its this value to be a constructor function that supports the parameter conventions of the Promise constructor.

27.2.4.1.1 GetPromiseResolve ( promiseConstructor )

The abstract operation GetPromiseResolve takes argument promiseConstructor (a constructor) and returns either a normal completion containing a function object or a throw completion. It performs the following steps when called:

  1. Let promiseResolve be ? Get(promiseConstructor, "resolve").
  2. If IsCallable(promiseResolve) is false, throw a TypeError exception.
  3. Return promiseResolve.

27.2.4.1.2 PerformPromiseAll ( iteratorRecord, constructor, resultCapability, promiseResolve )

The abstract operation PerformPromiseAll takes arguments iteratorRecord (an Iterator Record), constructor (a constructor), resultCapability (a PromiseCapability Record), and promiseResolve (a function object) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

  1. Let values be a new empty List.
  2. Let remainingElementsCount be the Record { [[Value]]: 1 }.
  3. Let index be 0.
  4. Repeat,
    1. Let next be Completion(IteratorStep(iteratorRecord)).
    2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
    3. ReturnIfAbrupt(next).
    4. If next is false, then
      1. Set iteratorRecord.[[Done]] to true.
      2. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
      3. If remainingElementsCount.[[Value]] = 0, then
        1. Let valuesArray be CreateArrayFromList(values).
        2. Perform ? Call(resultCapability.[[Resolve]], undefined, « valuesArray »).
      4. Return resultCapability.[[Promise]].
    5. Let nextValue be Completion(IteratorValue(next)).
    6. If nextValue is an abrupt completion, set iteratorRecord.[[Done]] to true.
    7. ReturnIfAbrupt(nextValue).
    8. Append undefined to values.
    9. Let nextPromise be ? Call(promiseResolve, constructor, « nextValue »).
    10. Let steps be the algorithm steps defined in Promise.all Resolve Element Functions.
    11. Let length be the number of non-optional parameters of the function definition in Promise.all Resolve Element Functions.
    12. Let onFulfilled be CreateBuiltinFunction(steps, length, "", « [[AlreadyCalled]], [[Index]], [[Values]], [[Capability]], [[RemainingElements]] »).
    13. Set onFulfilled.[[AlreadyCalled]] to false.
    14. Set onFulfilled.[[Index]] to index.
    15. Set onFulfilled.[[Values]] to values.
    16. Set onFulfilled.[[Capability]] to resultCapability.
    17. Set onFulfilled.[[RemainingElements]] to remainingElementsCount.
    18. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] 1.
    19. Perform ? Invoke(nextPromise, "then", « onFulfilled, resultCapability.[[Reject]] »).
    20. Set index to index 1.

27.2.4.1.3 Promise.all Resolve Element Functions

A Promise.all resolve element function is an anonymous built-in function that is used to resolve a specific Promise.all element. Each Promise.all resolve element function has [[Index]], [[Values]], [[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.

When a Promise.all resolve element function is called with argument x, the following steps are taken:

  1. Let F be the active function object.
  2. If F.[[AlreadyCalled]] is true, return undefined.
  3. Set F.[[AlreadyCalled]] to true.
  4. Let index be F.[[Index]].
  5. Let values be F.[[Values]].
  6. Let promiseCapability be F.[[Capability]].
  7. Let remainingElementsCount be F.[[RemainingElements]].
  8. Set values[index] to x.
  9. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
  10. If remainingElementsCount.[[Value]] = 0, then
    1. Let valuesArray be CreateArrayFromList(values).
    2. Return ? Call(promiseCapability.[[Resolve]], undefined, « valuesArray »).
  11. Return undefined.

The "length" property of a Promise.all resolve element function is 1𝔽.

27.2.4.2 Promise.allSettled ( iterable )

This function returns a promise that is fulfilled with an array of promise state snapshots, but only after all the original promises have settled, i.e. become either fulfilled or rejected. It resolves all elements of the passed iterable to promises as it runs this algorithm.

  1. Let C be the this value.
  2. Let promiseCapability be ? NewPromiseCapability(C).
  3. Let promiseResolve be Completion(GetPromiseResolve(C)).
  4. IfAbruptRejectPromise(promiseResolve, promiseCapability).
  5. Let iteratorRecord be Completion(GetIterator(iterable, sync)).
  6. IfAbruptRejectPromise(iteratorRecord, promiseCapability).
  7. Let result be Completion(PerformPromiseAllSettled(iteratorRecord, C, promiseCapability, promiseResolve)).
  8. If result is an abrupt completion, then
    1. If iteratorRecord.[[Done]] is false, set result to Completion(IteratorClose(iteratorRecord, result)).
    2. IfAbruptRejectPromise(result, promiseCapability).
  9. Return ? result.
 Note

This function requires its this value to be a constructor function that supports the parameter conventions of the Promise constructor.

27.2.4.2.1 PerformPromiseAllSettled ( iteratorRecord, constructor, resultCapability, promiseResolve )

The abstract operation PerformPromiseAllSettled takes arguments iteratorRecord (an Iterator Record), constructor (a constructor), resultCapability (a PromiseCapability Record), and promiseResolve (a function object) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

  1. Let values be a new empty List.
  2. Let remainingElementsCount be the Record { [[Value]]: 1 }.
  3. Let index be 0.
  4. Repeat,
    1. Let next be Completion(IteratorStep(iteratorRecord)).
    2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
    3. ReturnIfAbrupt(next).
    4. If next is false, then
      1. Set iteratorRecord.[[Done]] to true.
      2. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
      3. If remainingElementsCount.[[Value]] = 0, then
        1. Let valuesArray be CreateArrayFromList(values).
        2. Perform ? Call(resultCapability.[[Resolve]], undefined, « valuesArray »).
      4. Return resultCapability.[[Promise]].
    5. Let nextValue be Completion(IteratorValue(next)).
    6. If nextValue is an abrupt completion, set iteratorRecord.[[Done]] to true.
    7. ReturnIfAbrupt(nextValue).
    8. Append undefined to values.
    9. Let nextPromise be ? Call(promiseResolve, constructor, « nextValue »).
    10. Let stepsFulfilled be the algorithm steps defined in Promise.allSettled Resolve Element Functions.
    11. Let lengthFulfilled be the number of non-optional parameters of the function definition in Promise.allSettled Resolve Element Functions.
    12. Let onFulfilled be CreateBuiltinFunction(stepsFulfilled, lengthFulfilled, "", « [[AlreadyCalled]], [[Index]], [[Values]], [[Capability]], [[RemainingElements]] »).
    13. Let alreadyCalled be the Record { [[Value]]: false }.
    14. Set onFulfilled.[[AlreadyCalled]] to alreadyCalled.
    15. Set onFulfilled.[[Index]] to index.
    16. Set onFulfilled.[[Values]] to values.
    17. Set onFulfilled.[[Capability]] to resultCapability.
    18. Set onFulfilled.[[RemainingElements]] to remainingElementsCount.
    19. Let stepsRejected be the algorithm steps defined in Promise.allSettled Reject Element Functions.
    20. Let lengthRejected be the number of non-optional parameters of the function definition in Promise.allSettled Reject Element Functions.
    21. Let onRejected be CreateBuiltinFunction(stepsRejected, lengthRejected, "", « [[AlreadyCalled]], [[Index]], [[Values]], [[Capability]], [[RemainingElements]] »).
    22. Set onRejected.[[AlreadyCalled]] to alreadyCalled.
    23. Set onRejected.[[Index]] to index.
    24. Set onRejected.[[Values]] to values.
    25. Set onRejected.[[Capability]] to resultCapability.
    26. Set onRejected.[[RemainingElements]] to remainingElementsCount.
    27. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] 1.
    28. Perform ? Invoke(nextPromise, "then", « onFulfilled, onRejected »).
    29. Set index to index 1.

27.2.4.2.2 Promise.allSettled Resolve Element Functions

A Promise.allSettled resolve element function is an anonymous built-in function that is used to resolve a specific Promise.allSettled element. Each Promise.allSettled resolve element function has [[Index]], [[Values]], [[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.

When a Promise.allSettled resolve element function is called with argument x, the following steps are taken:

  1. Let F be the active function object.
  2. Let alreadyCalled be F.[[AlreadyCalled]].
  3. If alreadyCalled.[[Value]] is true, return undefined.
  4. Set alreadyCalled.[[Value]] to true.
  5. Let index be F.[[Index]].
  6. Let values be F.[[Values]].
  7. Let promiseCapability be F.[[Capability]].
  8. Let remainingElementsCount be F.[[RemainingElements]].
  9. Let obj be OrdinaryObjectCreate(%Object.prototype%).
  10. Perform ! CreateDataPropertyOrThrow(obj, "status", "fulfilled").
  11. Perform ! CreateDataPropertyOrThrow(obj, "value", x).
  12. Set values[index] to obj.
  13. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
  14. If remainingElementsCount.[[Value]] = 0, then
    1. Let valuesArray be CreateArrayFromList(values).
    2. Return ? Call(promiseCapability.[[Resolve]], undefined, « valuesArray »).
  15. Return undefined.

The "length" property of a Promise.allSettled resolve element function is 1𝔽.

27.2.4.2.3 Promise.allSettled Reject Element Functions

A Promise.allSettled reject element function is an anonymous built-in function that is used to reject a specific Promise.allSettled element. Each Promise.allSettled reject element function has [[Index]], [[Values]], [[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.

When a Promise.allSettled reject element function is called with argument x, the following steps are taken:

  1. Let F be the active function object.
  2. Let alreadyCalled be F.[[AlreadyCalled]].
  3. If alreadyCalled.[[Value]] is true, return undefined.
  4. Set alreadyCalled.[[Value]] to true.
  5. Let index be F.[[Index]].
  6. Let values be F.[[Values]].
  7. Let promiseCapability be F.[[Capability]].
  8. Let remainingElementsCount be F.[[RemainingElements]].
  9. Let obj be OrdinaryObjectCreate(%Object.prototype%).
  10. Perform ! CreateDataPropertyOrThrow(obj, "status", "rejected").
  11. Perform ! CreateDataPropertyOrThrow(obj, "reason", x).
  12. Set values[index] to obj.
  13. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
  14. If remainingElementsCount.[[Value]] = 0, then
    1. Let valuesArray be CreateArrayFromList(values).
    2. Return ? Call(promiseCapability.[[Resolve]], undefined, « valuesArray »).
  15. Return undefined.

The "length" property of a Promise.allSettled reject element function is 1𝔽.

27.2.4.3 Promise.any ( iterable )

This function returns a promise that is fulfilled by the first given promise to be fulfilled, or rejected with an AggregateError holding the rejection reasons if all of the given promises are rejected. It resolves all elements of the passed iterable to promises as it runs this algorithm.

  1. Let C be the this value.
  2. Let promiseCapability be ? NewPromiseCapability(C).
  3. Let promiseResolve be Completion(GetPromiseResolve(C)).
  4. IfAbruptRejectPromise(promiseResolve, promiseCapability).
  5. Let iteratorRecord be Completion(GetIterator(iterable, sync)).
  6. IfAbruptRejectPromise(iteratorRecord, promiseCapability).
  7. Let result be Completion(PerformPromiseAny(iteratorRecord, C, promiseCapability, promiseResolve)).
  8. If result is an abrupt completion, then
    1. If iteratorRecord.[[Done]] is false, set result to Completion(IteratorClose(iteratorRecord, result)).
    2. IfAbruptRejectPromise(result, promiseCapability).
  9. Return ? result.
 Note

This function requires its this value to be a constructor function that supports the parameter conventions of the Promise constructor.

27.2.4.3.1 PerformPromiseAny ( iteratorRecord, constructor, resultCapability, promiseResolve )

The abstract operation PerformPromiseAny takes arguments iteratorRecord (an Iterator Record), constructor (a constructor), resultCapability (a PromiseCapability Record), and promiseResolve (a function object) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

  1. Let errors be a new empty List.
  2. Let remainingElementsCount be the Record { [[Value]]: 1 }.
  3. Let index be 0.
  4. Repeat,
    1. Let next be Completion(IteratorStep(iteratorRecord)).
    2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
    3. ReturnIfAbrupt(next).
    4. If next is false, then
      1. Set iteratorRecord.[[Done]] to true.
      2. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
      3. If remainingElementsCount.[[Value]] = 0, then
        1. Let error be a newly created AggregateError object.
        2. Perform ! DefinePropertyOrThrow(error, "errors", PropertyDescriptor { [[Configurable]]: true, [[Enumerable]]: false, [[Writable]]: true, [[Value]]: CreateArrayFromList(errors) }).
        3. Return ThrowCompletion(error).
      4. Return resultCapability.[[Promise]].
    5. Let nextValue be Completion(IteratorValue(next)).
    6. If nextValue is an abrupt completion, set iteratorRecord.[[Done]] to true.
    7. ReturnIfAbrupt(nextValue).
    8. Append undefined to errors.
    9. Let nextPromise be ? Call(promiseResolve, constructor, « nextValue »).
    10. Let stepsRejected be the algorithm steps defined in Promise.any Reject Element Functions.
    11. Let lengthRejected be the number of non-optional parameters of the function definition in Promise.any Reject Element Functions.
    12. Let onRejected be CreateBuiltinFunction(stepsRejected, lengthRejected, "", « [[AlreadyCalled]], [[Index]], [[Errors]], [[Capability]], [[RemainingElements]] »).
    13. Set onRejected.[[AlreadyCalled]] to false.
    14. Set onRejected.[[Index]] to index.
    15. Set onRejected.[[Errors]] to errors.
    16. Set onRejected.[[Capability]] to resultCapability.
    17. Set onRejected.[[RemainingElements]] to remainingElementsCount.
    18. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] 1.
    19. Perform ? Invoke(nextPromise, "then", « resultCapability.[[Resolve]], onRejected »).
    20. Set index to index 1.

27.2.4.3.2 Promise.any Reject Element Functions

A Promise.any reject element function is an anonymous built-in function that is used to reject a specific Promise.any element. Each Promise.any reject element function has [[Index]], [[Errors]], [[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.

When a Promise.any reject element function is called with argument x, the following steps are taken:

  1. Let F be the active function object.
  2. If F.[[AlreadyCalled]] is true, return undefined.
  3. Set F.[[AlreadyCalled]] to true.
  4. Let index be F.[[Index]].
  5. Let errors be F.[[Errors]].
  6. Let promiseCapability be F.[[Capability]].
  7. Let remainingElementsCount be F.[[RemainingElements]].
  8. Set errors[index] to x.
  9. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
  10. If remainingElementsCount.[[Value]] = 0, then
    1. Let error be a newly created AggregateError object.
    2. Perform ! DefinePropertyOrThrow(error, "errors", PropertyDescriptor { [[Configurable]]: true, [[Enumerable]]: false, [[Writable]]: true, [[Value]]: CreateArrayFromList(errors) }).
    3. Return ? Call(promiseCapability.[[Reject]], undefined, « error »).
  11. Return undefined.

The "length" property of a Promise.any reject element function is 1𝔽.

27.2.4.4 Promise.prototype

The initial value of Promise.prototype is the Promise prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

27.2.4.5 Promise.race ( iterable )

This function returns a new promise which is settled in the same way as the first passed promise to settle. It resolves all elements of the passed iterable to promises as it runs this algorithm.

  1. Let C be the this value.
  2. Let promiseCapability be ? NewPromiseCapability(C).
  3. Let promiseResolve be Completion(GetPromiseResolve(C)).
  4. IfAbruptRejectPromise(promiseResolve, promiseCapability).
  5. Let iteratorRecord be Completion(GetIterator(iterable, sync)).
  6. IfAbruptRejectPromise(iteratorRecord, promiseCapability).
  7. Let result be Completion(PerformPromiseRace(iteratorRecord, C, promiseCapability, promiseResolve)).
  8. If result is an abrupt completion, then
    1. If iteratorRecord.[[Done]] is false, set result to Completion(IteratorClose(iteratorRecord, result)).
    2. IfAbruptRejectPromise(result, promiseCapability).
  9. Return ? result.
 Note 1

If the iterable argument yields no values or if none of the promises yielded by iterable ever settle, then the pending promise returned by this method will never be settled.

 Note 2

This function expects its this value to be a constructor function that supports the parameter conventions of the Promise constructor. It also expects that its this value provides a resolve method.

27.2.4.5.1 PerformPromiseRace ( iteratorRecord, constructor, resultCapability, promiseResolve )

The abstract operation PerformPromiseRace takes arguments iteratorRecord (an Iterator Record), constructor (a constructor), resultCapability (a PromiseCapability Record), and promiseResolve (a function object) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

  1. Repeat,
    1. Let next be Completion(IteratorStep(iteratorRecord)).
    2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
    3. ReturnIfAbrupt(next).
    4. If next is false, then
      1. Set iteratorRecord.[[Done]] to true.
      2. Return resultCapability.[[Promise]].
    5. Let nextValue be Completion(IteratorValue(next)).
    6. If nextValue is an abrupt completion, set iteratorRecord.[[Done]] to true.
    7. ReturnIfAbrupt(nextValue).
    8. Let nextPromise be ? Call(promiseResolve, constructor, « nextValue »).
    9. Perform ? Invoke(nextPromise, "then", « resultCapability.[[Resolve]], resultCapability.[[Reject]] »).

27.2.4.6 Promise.reject ( r )

This function returns a new promise rejected with the passed argument.

  1. Let C be the this value.
  2. Let promiseCapability be ? NewPromiseCapability(C).
  3. Perform ? Call(promiseCapability.[[Reject]], undefined, « r »).
  4. Return promiseCapability.[[Promise]].
 Note

This function expects its this value to be a constructor function that supports the parameter conventions of the Promise constructor.

27.2.4.7 Promise.resolve ( x )

This function returns either a new promise resolved with the passed argument, or the argument itself if the argument is a promise produced by this constructor.

  1. Let C be the this value.
  2. If C is not an Object, throw a TypeError exception.
  3. Return ? PromiseResolve(C, x).
 Note

This function expects its this value to be a constructor function that supports the parameter conventions of the Promise constructor.

27.2.4.7.1 PromiseResolve ( C, x )

The abstract operation PromiseResolve takes arguments C (a constructor) and x (an ECMAScript language value) and returns either a normal completion containing an ECMAScript language value or a throw completion. It returns a new promise resolved with x. It performs the following steps when called:

  1. If IsPromise(x) is true, then
    1. Let xConstructor be ? Get(x, "constructor").
    2. If SameValue(xConstructor, C) is true, return x.
  2. Let promiseCapability be ? NewPromiseCapability(C).
  3. Perform ? Call(promiseCapability.[[Resolve]], undefined, « x »).
  4. Return promiseCapability.[[Promise]].

27.2.4.8 get Promise [ @@species ]

Promise[@@species] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Return the this value.

The value of the "name" property of this function is "get [Symbol.species]".

Note

Promise prototype methods normally use their this value's constructor to create a derived object. However, a subclass constructor may over-ride that default behaviour by redefining its @@species property.

27.2.5 Properties of the Promise Prototype Object

The Promise prototype object:

  • is %Promise.prototype%.
  • has a [[Prototype]] internal slot whose value is %Object.prototype%.
  • is an ordinary object.
  • does not have a [[PromiseState]] internal slot or any of the other internal slots of Promise instances.

27.2.5.1 Promise.prototype.catch ( onRejected )

This method performs the following steps when called:

  1. Let promise be the this value.
  2. Return ? Invoke(promise, "then", « undefined, onRejected »).

27.2.5.2 Promise.prototype.constructor

The initial value of Promise.prototype.constructor is %Promise%.

27.2.5.3 Promise.prototype.finally ( onFinally )

This method performs the following steps when called:

  1. Let promise be the this value.
  2. If promise is not an Object, throw a TypeError exception.
  3. Let C be ? SpeciesConstructor(promise, %Promise%).
  4. Assert: IsConstructor(C) is true.
  5. If IsCallable(onFinally) is false, then
    1. Let thenFinally be onFinally.
    2. Let catchFinally be onFinally.
  6. Else,
    1. Let thenFinallyClosure be a new Abstract Closure with parameters (value) that captures onFinally and C and performs the following steps when called:
      1. Let result be ? Call(onFinally, undefined).
      2. Let p be ? PromiseResolve(C, result).
      3. Let returnValue be a new Abstract Closure with no parameters that captures value and performs the following steps when called:
        1. Return value.
      4. Let valueThunk be CreateBuiltinFunction(returnValue, 0, "", « »).
      5. Return ? Invoke(p, "then", « valueThunk »).
    2. Let thenFinally be CreateBuiltinFunction(thenFinallyClosure, 1, "", « »).
    3. Let catchFinallyClosure be a new Abstract Closure with parameters (reason) that captures onFinally and C and performs the following steps when called:
      1. Let result be ? Call(onFinally, undefined).
      2. Let p be ? PromiseResolve(C, result).
      3. Let throwReason be a new Abstract Closure with no parameters that captures reason and performs the following steps when called:
        1. Return ThrowCompletion(reason).
      4. Let thrower be CreateBuiltinFunction(throwReason, 0, "", « »).
      5. Return ? Invoke(p, "then", « thrower »).
    4. Let catchFinally be CreateBuiltinFunction(catchFinallyClosure, 1, "", « »).
  7. Return ? Invoke(promise, "then", « thenFinally, catchFinally »).

27.2.5.4 Promise.prototype.then ( onFulfilled, onRejected )

This method performs the following steps when called:

  1. Let promise be the this value.
  2. If IsPromise(promise) is false, throw a TypeError exception.
  3. Let C be ? SpeciesConstructor(promise, %Promise%).
  4. Let resultCapability be ? NewPromiseCapability(C).
  5. Return PerformPromiseThen(promise, onFulfilled, onRejected, resultCapability).

27.2.5.4.1 PerformPromiseThen ( promise, onFulfilled, onRejected [ , resultCapability ] )

The abstract operation PerformPromiseThen takes arguments promise (a Promise), onFulfilled (an ECMAScript language value), and onRejected (an ECMAScript language value) and optional argument resultCapability (a PromiseCapability Record) and returns an ECMAScript language value. It performs the “then” operation on promise using onFulfilled and onRejected as its settlement actions. If resultCapability is passed, the result is stored by updating resultCapability's promise. If it is not passed, then PerformPromiseThen is being called by a specification-internal operation where the result does not matter. It performs the following steps when called:

  1. Assert: IsPromise(promise) is true.
  2. If resultCapability is not present, then
    1. Set resultCapability to undefined.
  3. If IsCallable(onFulfilled) is false, then
    1. Let onFulfilledJobCallback be empty.
  4. Else,
    1. Let onFulfilledJobCallback be HostMakeJobCallback(onFulfilled).
  5. If IsCallable(onRejected) is false, then
    1. Let onRejectedJobCallback be empty.
  6. Else,
    1. Let onRejectedJobCallback be HostMakeJobCallback(onRejected).
  7. Let fulfillReaction be the PromiseReaction { [[Capability]]: resultCapability, [[Type]]: Fulfill, [[Handler]]: onFulfilledJobCallback }.
  8. Let rejectReaction be the PromiseReaction { [[Capability]]: resultCapability, [[Type]]: Reject, [[Handler]]: onRejectedJobCallback }.
  9. If promise.[[PromiseState]] is pending, then
    1. Append fulfillReaction to promise.[[PromiseFulfillReactions]].
    2. Append rejectReaction to promise.[[PromiseRejectReactions]].
  10. Else if promise.[[PromiseState]] is fulfilled, then
    1. Let value be promise.[[PromiseResult]].
    2. Let fulfillJob be NewPromiseReactionJob(fulfillReaction, value).
    3. Perform HostEnqueuePromiseJob(fulfillJob.[[Job]], fulfillJob.[[Realm]]).
  11. Else,
    1. Assert: The value of promise.[[PromiseState]] is rejected.
    2. Let reason be promise.[[PromiseResult]].
    3. If promise.[[PromiseIsHandled]] is false, perform HostPromiseRejectionTracker(promise, "handle").
    4. Let rejectJob be NewPromiseReactionJob(rejectReaction, reason).
    5. Perform HostEnqueuePromiseJob(rejectJob.[[Job]], rejectJob.[[Realm]]).
  12. Set promise.[[PromiseIsHandled]] to true.
  13. If resultCapability is undefined, then
    1. Return undefined.
  14. Else,
    1. Return resultCapability.[[Promise]].

27.2.5.5 Promise.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "Promise".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

27.2.6 Properties of Promise Instances

Promise instances are ordinary objects that inherit properties from the Promise prototype object (the intrinsic, %Promise.prototype%). Promise instances are initially created with the internal slots described in Table 81.

Table 81: Internal Slots of Promise Instances
Internal Slot Type Description
[[PromiseState]] pending, fulfilled, or rejected Governs how a promise will react to incoming calls to its then method.
[[PromiseResult]] an ECMAScript language value The value with which the promise has been fulfilled or rejected, if any. Only meaningful if [[PromiseState]] is not pending.
[[PromiseFulfillReactions]] a List of PromiseReaction Records Records to be processed when/if the promise transitions from the pending state to the fulfilled state.
[[PromiseRejectReactions]] a List of PromiseReaction Records Records to be processed when/if the promise transitions from the pending state to the rejected state.
[[PromiseIsHandled]] a Boolean Indicates whether the promise has ever had a fulfillment or rejection handler; used in unhandled rejection tracking.

27.3 GeneratorFunction Objects

GeneratorFunctions are functions that are usually created by evaluating GeneratorDeclarations, GeneratorExpressions, and GeneratorMethods. They may also be created by calling the %GeneratorFunction% intrinsic.

Figure 6 (Informative): Generator Objects Relationships
A staggering variety of boxes and arrows.

27.3.1 The GeneratorFunction Constructor

The GeneratorFunction constructor:

  • is %GeneratorFunction%.
  • is a subclass of Function.
  • creates and initializes a new GeneratorFunction when called as a function rather than as a constructor. Thus the function call GeneratorFunction (…) is equivalent to the object creation expression new GeneratorFunction (…) with the same arguments.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified GeneratorFunction behaviour must include a super call to the GeneratorFunction constructor to create and initialize subclass instances with the internal slots necessary for built-in GeneratorFunction behaviour. All ECMAScript syntactic forms for defining generator function objects create direct instances of GeneratorFunction. There is no syntactic means to create instances of GeneratorFunction subclasses.

27.3.1.1 GeneratorFunction ( ...parameterArgs, bodyArg )

The last argument (if any) specifies the body (executable code) of a generator function; any preceding arguments specify formal parameters.

This function performs the following steps when called:

  1. Let C be the active function object.
  2. If bodyArg is not present, set bodyArg to the empty String.
  3. Return ? CreateDynamicFunction(C, NewTarget, generator, parameterArgs, bodyArg).
 Note

See NOTE for 20.2.1.1.

27.3.2 Properties of the GeneratorFunction Constructor

The GeneratorFunction constructor:

  • is a standard built-in function object that inherits from the Function constructor.
  • has a [[Prototype]] internal slot whose value is %Function%.
  • has a "name" property whose value is "GeneratorFunction".
  • has the following properties:

27.3.2.1 GeneratorFunction.length

This is a data property with a value of 1. This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

27.3.2.2 GeneratorFunction.prototype

The initial value of GeneratorFunction.prototype is the GeneratorFunction prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

27.3.3 Properties of the GeneratorFunction Prototype Object

The GeneratorFunction prototype object:

27.3.3.1 GeneratorFunction.prototype.constructor

The initial value of GeneratorFunction.prototype.constructor is %GeneratorFunction%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

27.3.3.2 GeneratorFunction.prototype.prototype

The initial value of GeneratorFunction.prototype.prototype is the Generator prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

27.3.3.3 GeneratorFunction.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "GeneratorFunction".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

27.3.4 GeneratorFunction Instances

Every GeneratorFunction instance is an ECMAScript function object and has the internal slots listed in Table 30. The value of the [[IsClassConstructor]] internal slot for all such instances is false.

Each GeneratorFunction instance has the following own properties:

27.3.4.1 length

The specification for the "length" property of Function instances given in 20.2.4.1 also applies to GeneratorFunction instances.

27.3.4.2 name

The specification for the "name" property of Function instances given in 20.2.4.2 also applies to GeneratorFunction instances.

27.3.4.3 prototype

Whenever a GeneratorFunction instance is created another ordinary object is also created and is the initial value of the generator function's "prototype" property. The value of the prototype property is used to initialize the [[Prototype]] internal slot of a newly created Generator when the generator function object is invoked using [[Call]].

This property has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.

Note

Unlike Function instances, the object that is the value of a GeneratorFunction's "prototype" property does not have a "constructor" property whose value is the GeneratorFunction instance.

27.4 AsyncGeneratorFunction Objects

AsyncGeneratorFunctions are functions that are usually created by evaluating AsyncGeneratorDeclaration, AsyncGeneratorExpression, and AsyncGeneratorMethod syntactic productions. They may also be created by calling the %AsyncGeneratorFunction% intrinsic.

27.4.1 The AsyncGeneratorFunction Constructor

The AsyncGeneratorFunction constructor:

  • is %AsyncGeneratorFunction%.
  • is a subclass of Function.
  • creates and initializes a new AsyncGeneratorFunction when called as a function rather than as a constructor. Thus the function call AsyncGeneratorFunction (...) is equivalent to the object creation expression new AsyncGeneratorFunction (...) with the same arguments.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified AsyncGeneratorFunction behaviour must include a super call to the AsyncGeneratorFunction constructor to create and initialize subclass instances with the internal slots necessary for built-in AsyncGeneratorFunction behaviour. All ECMAScript syntactic forms for defining async generator function objects create direct instances of AsyncGeneratorFunction. There is no syntactic means to create instances of AsyncGeneratorFunction subclasses.

27.4.1.1 AsyncGeneratorFunction ( ...parameterArgs, bodyArg )

The last argument (if any) specifies the body (executable code) of an async generator function; any preceding arguments specify formal parameters.

This function performs the following steps when called:

  1. Let C be the active function object.
  2. If bodyArg is not present, set bodyArg to the empty String.
  3. Return ? CreateDynamicFunction(C, NewTarget, asyncGenerator, parameterArgs, bodyArg).
 Note

See NOTE for 20.2.1.1.

27.4.2 Properties of the AsyncGeneratorFunction Constructor

The AsyncGeneratorFunction constructor:

  • is a standard built-in function object that inherits from the Function constructor.
  • has a [[Prototype]] internal slot whose value is %Function%.
  • has a "name" property whose value is "AsyncGeneratorFunction".
  • has the following properties:

27.4.2.1 AsyncGeneratorFunction.length

This is a data property with a value of 1. This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

27.4.2.2 AsyncGeneratorFunction.prototype

The initial value of AsyncGeneratorFunction.prototype is the AsyncGeneratorFunction prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

27.4.3 Properties of the AsyncGeneratorFunction Prototype Object

The AsyncGeneratorFunction prototype object:

27.4.3.1 AsyncGeneratorFunction.prototype.constructor

The initial value of AsyncGeneratorFunction.prototype.constructor is %AsyncGeneratorFunction%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

27.4.3.2 AsyncGeneratorFunction.prototype.prototype

The initial value of AsyncGeneratorFunction.prototype.prototype is the AsyncGenerator prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

27.4.3.3 AsyncGeneratorFunction.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "AsyncGeneratorFunction".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

27.4.4 AsyncGeneratorFunction Instances

Every AsyncGeneratorFunction instance is an ECMAScript function object and has the internal slots listed in Table 30. The value of the [[IsClassConstructor]] internal slot for all such instances is false.

Each AsyncGeneratorFunction instance has the following own properties:

27.4.4.1 length

The value of the "length" property is an integral Number that indicates the typical number of arguments expected by the AsyncGeneratorFunction. However, the language permits the function to be invoked with some other number of arguments. The behaviour of an AsyncGeneratorFunction when invoked on a number of arguments other than the number specified by its "length" property depends on the function.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

27.4.4.2 name

The specification for the "name" property of Function instances given in 20.2.4.2 also applies to AsyncGeneratorFunction instances.

27.4.4.3 prototype

Whenever an AsyncGeneratorFunction instance is created, another ordinary object is also created and is the initial value of the async generator function's "prototype" property. The value of the prototype property is used to initialize the [[Prototype]] internal slot of a newly created AsyncGenerator when the generator function object is invoked using [[Call]].

This property has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.

Note

Unlike function instances, the object that is the value of an AsyncGeneratorFunction's "prototype" property does not have a "constructor" property whose value is the AsyncGeneratorFunction instance.

27.5 Generator Objects

A Generator is an instance of a generator function and conforms to both the Iterator and Iterable interfaces.

Generator instances directly inherit properties from the object that is the initial value of the "prototype" property of the Generator function that created the instance. Generator instances indirectly inherit properties from the Generator Prototype intrinsic, %GeneratorFunction.prototype.prototype%.

27.5.1 Properties of the Generator Prototype Object

The Generator prototype object:

  • is %GeneratorFunction.prototype.prototype%.
  • is an ordinary object.
  • is not a Generator instance and does not have a [[GeneratorState]] internal slot.
  • has a [[Prototype]] internal slot whose value is %IteratorPrototype%.
  • has properties that are indirectly inherited by all Generator instances.

27.5.1.1 Generator.prototype.constructor

The initial value of Generator.prototype.constructor is %GeneratorFunction.prototype%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

27.5.1.2 Generator.prototype.next ( value )

  1. Return ? GeneratorResume(this value, value, empty).

27.5.1.3 Generator.prototype.return ( value )

This method performs the following steps when called:

  1. Let g be the this value.
  2. Let C be Completion Record { [[Type]]: return, [[Value]]: value, [[Target]]: empty }.
  3. Return ? GeneratorResumeAbrupt(g, C, empty).

27.5.1.4 Generator.prototype.throw ( exception )

This method performs the following steps when called:

  1. Let g be the this value.
  2. Let C be ThrowCompletion(exception).
  3. Return ? GeneratorResumeAbrupt(g, C, empty).

27.5.1.5 Generator.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "Generator".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

27.5.2 Properties of Generator Instances

Generator instances are initially created with the internal slots described in Table 82.

Table 82: Internal Slots of Generator Instances
Internal Slot Type Description
[[GeneratorState]] undefined, suspendedStart, suspendedYield, executing, or completed The current execution state of the generator.
[[GeneratorContext]] an execution context The execution context that is used when executing the code of this generator.
[[GeneratorBrand]] a String or empty A brand used to distinguish different kinds of generators. The [[GeneratorBrand]] of generators declared by ECMAScript source text is always empty.

27.5.3 Generator Abstract Operations

27.5.3.1 GeneratorStart ( generator, generatorBody )

The abstract operation GeneratorStart takes arguments generator (a Generator) and generatorBody (a FunctionBody Parse Node or an Abstract Closure with no parameters) and returns unused. It performs the following steps when called:

  1. Assert: The value of generator.[[GeneratorState]] is undefined.
  2. Let genContext be the running execution context.
  3. Set the Generator component of genContext to generator.
  4. Let closure be a new Abstract Closure with no parameters that captures generatorBody and performs the following steps when called:
    1. Let acGenContext be the running execution context.
    2. Let acGenerator be the Generator component of acGenContext.
    3. If generatorBody is a Parse Node, then
      1. Let result be Completion(Evaluation of generatorBody).
    4. Else,
      1. Assert: generatorBody is an Abstract Closure with no parameters.
      2. Let result be generatorBody().
    5. Assert: If we return here, the generator either threw an exception or performed either an implicit or explicit return.
    6. Remove acGenContext from the execution context stack and restore the execution context that is at the top of the execution context stack as the running execution context.
    7. Set acGenerator.[[GeneratorState]] to completed.
    8. NOTE: Once a generator enters the completed state it never leaves it and its associated execution context is never resumed. Any execution state associated with acGenerator can be discarded at this point.
    9. If result.[[Type]] is normal, let resultValue be undefined.
    10. Else if result.[[Type]] is return, let resultValue be result.[[Value]].
    11. Else,
      1. Assert: result.[[Type]] is throw.
      2. Return ? result.
    12. Return CreateIterResultObject(resultValue, true).
  5. Set the code evaluation state of genContext such that when evaluation is resumed for that execution context, closure will be called with no arguments.
  6. Set generator.[[GeneratorContext]] to genContext.
  7. Set generator.[[GeneratorState]] to suspendedStart.
  8. Return unused.

27.5.3.2 GeneratorValidate ( generator, generatorBrand )

The abstract operation GeneratorValidate takes arguments generator (an ECMAScript language value) and generatorBrand (a String or empty) and returns either a normal completion containing one of suspendedStart, suspendedYield, or completed, or a throw completion. It performs the following steps when called:

  1. Perform ? RequireInternalSlot(generator, [[GeneratorState]]).
  2. Perform ? RequireInternalSlot(generator, [[GeneratorBrand]]).
  3. If generator.[[GeneratorBrand]] is not generatorBrand, throw a TypeError exception.
  4. Assert: generator also has a [[GeneratorContext]] internal slot.
  5. Let state be generator.[[GeneratorState]].
  6. If state is executing, throw a TypeError exception.
  7. Return state.

27.5.3.3 GeneratorResume ( generator, value, generatorBrand )

The abstract operation GeneratorResume takes arguments generator (an ECMAScript language value), value (an ECMAScript language value or empty), and generatorBrand (a String or empty) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

  1. Let state be ? GeneratorValidate(generator, generatorBrand).
  2. If state is completed, return CreateIterResultObject(undefined, true).
  3. Assert: state is either suspendedStart or suspendedYield.
  4. Let genContext be generator.[[GeneratorContext]].
  5. Let methodContext be the running execution context.
  6. Suspend methodContext.
  7. Set generator.[[GeneratorState]] to executing.
  8. Push genContext onto the execution context stack; genContext is now the running execution context.
  9. Resume the suspended evaluation of genContext using NormalCompletion(value) as the result of the operation that suspended it. Let result be the value returned by the resumed computation.
  10. Assert: When we return here, genContext has already been removed from the execution context stack and methodContext is the currently running execution context.
  11. Return ? result.

27.5.3.4 GeneratorResumeAbrupt ( generator, abruptCompletion, generatorBrand )

The abstract operation GeneratorResumeAbrupt takes arguments generator (an ECMAScript language value), abruptCompletion (a return completion or a throw completion), and generatorBrand (a String or empty) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

  1. Let state be ? GeneratorValidate(generator, generatorBrand).
  2. If state is suspendedStart, then
    1. Set generator.[[GeneratorState]] to completed.
    2. NOTE: Once a generator enters the completed state it never leaves it and its associated execution context is never resumed. Any execution state associated with generator can be discarded at this point.
    3. Set state to completed.
  3. If state is completed, then
    1. If abruptCompletion.[[Type]] is return, then
      1. Return CreateIterResultObject(abruptCompletion.[[Value]], true).
    2. Return ? abruptCompletion.
  4. Assert: state is suspendedYield.
  5. Let genContext be generator.[[GeneratorContext]].
  6. Let methodContext be the running execution context.
  7. Suspend methodContext.
  8. Set generator.[[GeneratorState]] to executing.
  9. Push genContext onto the execution context stack; genContext is now the running execution context.
  10. Resume the suspended evaluation of genContext using abruptCompletion as the result of the operation that suspended it. Let result be the Completion Record returned by the resumed computation.
  11. Assert: When we return here, genContext has already been removed from the execution context stack and methodContext is the currently running execution context.
  12. Return ? result.

27.5.3.5 GetGeneratorKind ( )

The abstract operation GetGeneratorKind takes no arguments and returns non-generator, sync, or async. It performs the following steps when called:

  1. Let genContext be the running execution context.
  2. If genContext does not have a Generator component, return non-generator.
  3. Let generator be the Generator component of genContext.
  4. If generator has an [[AsyncGeneratorState]] internal slot, return async.
  5. Else, return sync.

27.5.3.6 GeneratorYield ( iterNextObj )

The abstract operation GeneratorYield takes argument iterNextObj (an Object that conforms to the IteratorResult interface) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It performs the following steps when called:

  1. Let genContext be the running execution context.
  2. Assert: genContext is the execution context of a generator.
  3. Let generator be the value of the Generator component of genContext.
  4. Assert: GetGeneratorKind() is sync.
  5. Set generator.[[GeneratorState]] to suspendedYield.
  6. Remove genContext from the execution context stack and restore the execution context that is at the top of the execution context stack as the running execution context.
  7. Let callerContext be the running execution context.
  8. Resume callerContext passing NormalCompletion(iterNextObj). If genContext is ever resumed again, let resumptionValue be the Completion Record with which it is resumed.
  9. Assert: If control reaches here, then genContext is the running execution context again.
  10. Return resumptionValue.

27.5.3.7 Yield ( value )

The abstract operation Yield takes argument value (an ECMAScript language value) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It performs the following steps when called:

  1. Let generatorKind be GetGeneratorKind().
  2. If generatorKind is async, return ? AsyncGeneratorYield(? Await(value)).
  3. Otherwise, return ? GeneratorYield(CreateIterResultObject(value, false)).

27.5.3.8 CreateIteratorFromClosure ( closure, generatorBrand, generatorPrototype )

The abstract operation CreateIteratorFromClosure takes arguments closure (an Abstract Closure with no parameters), generatorBrand (a String or empty), and generatorPrototype (an Object) and returns a Generator. It performs the following steps when called:

  1. NOTE: closure can contain uses of the Yield operation to yield an IteratorResult object.
  2. Let internalSlotsList be « [[GeneratorState]], [[GeneratorContext]], [[GeneratorBrand]] ».
  3. Let generator be OrdinaryObjectCreate(generatorPrototype, internalSlotsList).
  4. Set generator.[[GeneratorBrand]] to generatorBrand.
  5. Set generator.[[GeneratorState]] to undefined.
  6. Let callerContext be the running execution context.
  7. Let calleeContext be a new execution context.
  8. Set the Function of calleeContext to null.
  9. Set the Realm of calleeContext to the current Realm Record.
  10. Set the ScriptOrModule of calleeContext to callerContext's ScriptOrModule.
  11. If callerContext is not already suspended, suspend callerContext.
  12. Push calleeContext onto the execution context stack; calleeContext is now the running execution context.
  13. Perform GeneratorStart(generator, closure).
  14. Remove calleeContext from the execution context stack and restore callerContext as the running execution context.
  15. Return generator.

27.6 AsyncGenerator Objects

An AsyncGenerator is an instance of an async generator function and conforms to both the AsyncIterator and AsyncIterable interfaces.

AsyncGenerator instances directly inherit properties from the object that is the initial value of the "prototype" property of the AsyncGenerator function that created the instance. AsyncGenerator instances indirectly inherit properties from the AsyncGenerator Prototype intrinsic, %AsyncGeneratorFunction.prototype.prototype%.

27.6.1 Properties of the AsyncGenerator Prototype Object

The AsyncGenerator prototype object:

  • is %AsyncGeneratorFunction.prototype.prototype%.
  • is an ordinary object.
  • is not an AsyncGenerator instance and does not have an [[AsyncGeneratorState]] internal slot.
  • has a [[Prototype]] internal slot whose value is %AsyncIteratorPrototype%.
  • has properties that are indirectly inherited by all AsyncGenerator instances.

27.6.1.1 AsyncGenerator.prototype.constructor

The initial value of AsyncGenerator.prototype.constructor is %AsyncGeneratorFunction.prototype%.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

27.6.1.2 AsyncGenerator.prototype.next ( value )

  1. Let generator be the this value.
  2. Let promiseCapability be ! NewPromiseCapability(%Promise%).
  3. Let result be Completion(AsyncGeneratorValidate(generator, empty)).
  4. IfAbruptRejectPromise(result, promiseCapability).
  5. Let state be generator.[[AsyncGeneratorState]].
  6. If state is completed, then
    1. Let iteratorResult be CreateIterResultObject(undefined, true).
    2. Perform ! Call(promiseCapability.[[Resolve]], undefined, « iteratorResult »).
    3. Return promiseCapability.[[Promise]].
  7. Let completion be NormalCompletion(value).
  8. Perform AsyncGeneratorEnqueue(generator, completion, promiseCapability).
  9. If state is either suspendedStart or suspendedYield, then
    1. Perform AsyncGeneratorResume(generator, completion).
  10. Else,
    1. Assert: state is either executing or awaiting-return.
  11. Return promiseCapability.[[Promise]].

27.6.1.3 AsyncGenerator.prototype.return ( value )

  1. Let generator be the this value.
  2. Let promiseCapability be ! NewPromiseCapability(%Promise%).
  3. Let result be Completion(AsyncGeneratorValidate(generator, empty)).
  4. IfAbruptRejectPromise(result, promiseCapability).
  5. Let completion be Completion Record { [[Type]]: return, [[Value]]: value, [[Target]]: empty }.
  6. Perform AsyncGeneratorEnqueue(generator, completion, promiseCapability).
  7. Let state be generator.[[AsyncGeneratorState]].
  8. If state is either suspendedStart or completed, then
    1. Set generator.[[AsyncGeneratorState]] to awaiting-return.
    2. Perform ! AsyncGeneratorAwaitReturn(generator).
  9. Else if state is suspendedYield, then
    1. Perform AsyncGeneratorResume(generator, completion).
  10. Else,
    1. Assert: state is either executing or awaiting-return.
  11. Return promiseCapability.[[Promise]].

27.6.1.4 AsyncGenerator.prototype.throw ( exception )

  1. Let generator be the this value.
  2. Let promiseCapability be ! NewPromiseCapability(%Promise%).
  3. Let result be Completion(AsyncGeneratorValidate(generator, empty)).
  4. IfAbruptRejectPromise(result, promiseCapability).
  5. Let state be generator.[[AsyncGeneratorState]].
  6. If state is suspendedStart, then
    1. Set generator.[[AsyncGeneratorState]] to completed.
    2. Set state to completed.
  7. If state is completed, then
    1. Perform ! Call(promiseCapability.[[Reject]], undefined, « exception »).
    2. Return promiseCapability.[[Promise]].
  8. Let completion be ThrowCompletion(exception).
  9. Perform AsyncGeneratorEnqueue(generator, completion, promiseCapability).
  10. If state is suspendedYield, then
    1. Perform AsyncGeneratorResume(generator, completion).
  11. Else,
    1. Assert: state is either executing or awaiting-return.
  12. Return promiseCapability.[[Promise]].

27.6.1.5 AsyncGenerator.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "AsyncGenerator".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

27.6.2 Properties of AsyncGenerator Instances

AsyncGenerator instances are initially created with the internal slots described below:

Table 83: Internal Slots of AsyncGenerator Instances
Internal Slot Type Description
[[AsyncGeneratorState]] undefined, suspendedStart, suspendedYield, executing, awaiting-return, or completed The current execution state of the async generator.
[[AsyncGeneratorContext]] an execution context The execution context that is used when executing the code of this async generator.
[[AsyncGeneratorQueue]] a List of AsyncGeneratorRequest Records Records which represent requests to resume the async generator. Except during state transitions, it is non-empty if and only if [[AsyncGeneratorState]] is either executing or awaiting-return.
[[GeneratorBrand]] a String or empty A brand used to distinguish different kinds of async generators. The [[GeneratorBrand]] of async generators declared by ECMAScript source text is always empty.

27.6.3 AsyncGenerator Abstract Operations

27.6.3.1 AsyncGeneratorRequest Records

An AsyncGeneratorRequest is a Record value used to store information about how an async generator should be resumed and contains capabilities for fulfilling or rejecting the corresponding promise.

They have the following fields:

Table 84: AsyncGeneratorRequest Record Fields
Field Name Value Meaning
[[Completion]] a Completion Record The Completion Record which should be used to resume the async generator.
[[Capability]] a PromiseCapability Record The promise capabilities associated with this request.

27.6.3.2 AsyncGeneratorStart ( generator, generatorBody )

The abstract operation AsyncGeneratorStart takes arguments generator (an AsyncGenerator) and generatorBody (a FunctionBody Parse Node or an Abstract Closure with no parameters) and returns unused. It performs the following steps when called:

  1. Assert: generator.[[AsyncGeneratorState]] is undefined.
  2. Let genContext be the running execution context.
  3. Set the Generator component of genContext to generator.
  4. Let closure be a new Abstract Closure with no parameters that captures generatorBody and performs the following steps when called:
    1. Let acGenContext be the running execution context.
    2. Let acGenerator be the Generator component of acGenContext.
    3. If generatorBody is a Parse Node, then
      1. Let result be Completion(Evaluation of generatorBody).
    4. Else,
      1. Assert: generatorBody is an Abstract Closure with no parameters.
      2. Let result be Completion(generatorBody()).
    5. Assert: If we return here, the async generator either threw an exception or performed either an implicit or explicit return.
    6. Remove acGenContext from the execution context stack and restore the execution context that is at the top of the execution context stack as the running execution context.
    7. Set acGenerator.[[AsyncGeneratorState]] to completed.
    8. If result.[[Type]] is normal, set result to NormalCompletion(undefined).
    9. If result.[[Type]] is return, set result to NormalCompletion(result.[[Value]]).
    10. Perform AsyncGeneratorCompleteStep(acGenerator, result, true).
    11. Perform AsyncGeneratorDrainQueue(acGenerator).
    12. Return undefined.
  5. Set the code evaluation state of genContext such that when evaluation is resumed for that execution context, closure will be called with no arguments.
  6. Set generator.[[AsyncGeneratorContext]] to genContext.
  7. Set generator.[[AsyncGeneratorState]] to suspendedStart.
  8. Set generator.[[AsyncGeneratorQueue]] to a new empty List.
  9. Return unused.

27.6.3.3 AsyncGeneratorValidate ( generator, generatorBrand )

The abstract operation AsyncGeneratorValidate takes arguments generator (an ECMAScript language value) and generatorBrand (a String or empty) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

  1. Perform ? RequireInternalSlot(generator, [[AsyncGeneratorContext]]).
  2. Perform ? RequireInternalSlot(generator, [[AsyncGeneratorState]]).
  3. Perform ? RequireInternalSlot(generator, [[AsyncGeneratorQueue]]).
  4. If generator.[[GeneratorBrand]] is not generatorBrand, throw a TypeError exception.
  5. Return unused.

27.6.3.4 AsyncGeneratorEnqueue ( generator, completion, promiseCapability )

The abstract operation AsyncGeneratorEnqueue takes arguments generator (an AsyncGenerator), completion (a Completion Record), and promiseCapability (a PromiseCapability Record) and returns unused. It performs the following steps when called:

  1. Let request be AsyncGeneratorRequest { [[Completion]]: completion, [[Capability]]: promiseCapability }.
  2. Append request to generator.[[AsyncGeneratorQueue]].
  3. Return unused.

27.6.3.5 AsyncGeneratorCompleteStep ( generator, completion, done [ , realm ] )

The abstract operation AsyncGeneratorCompleteStep takes arguments generator (an AsyncGenerator), completion (a Completion Record), and done (a Boolean) and optional argument realm (a Realm Record) and returns unused. It performs the following steps when called:

  1. Assert: generator.[[AsyncGeneratorQueue]] is not empty.
  2. Let next be the first element of generator.[[AsyncGeneratorQueue]].
  3. Remove the first element from generator.[[AsyncGeneratorQueue]].
  4. Let promiseCapability be next.[[Capability]].
  5. Let value be completion.[[Value]].
  6. If completion.[[Type]] is throw, then
    1. Perform ! Call(promiseCapability.[[Reject]], undefined, « value »).
  7. Else,
    1. Assert: completion.[[Type]] is normal.
    2. If realm is present, then
      1. Let oldRealm be the running execution context's Realm.
      2. Set the running execution context's Realm to realm.
      3. Let iteratorResult be CreateIterResultObject(value, done).
      4. Set the running execution context's Realm to oldRealm.
    3. Else,
      1. Let iteratorResult be CreateIterResultObject(value, done).
    4. Perform ! Call(promiseCapability.[[Resolve]], undefined, « iteratorResult »).
  8. Return unused.

27.6.3.6 AsyncGeneratorResume ( generator, completion )

The abstract operation AsyncGeneratorResume takes arguments generator (an AsyncGenerator) and completion (a Completion Record) and returns unused. It performs the following steps when called:

  1. Assert: generator.[[AsyncGeneratorState]] is either suspendedStart or suspendedYield.
  2. Let genContext be generator.[[AsyncGeneratorContext]].
  3. Let callerContext be the running execution context.
  4. Suspend callerContext.
  5. Set generator.[[AsyncGeneratorState]] to executing.
  6. Push genContext onto the execution context stack; genContext is now the running execution context.
  7. Resume the suspended evaluation of genContext using completion as the result of the operation that suspended it. Let result be the Completion Record returned by the resumed computation.
  8. Assert: result is never an abrupt completion.
  9. Assert: When we return here, genContext has already been removed from the execution context stack and callerContext is the currently running execution context.
  10. Return unused.

27.6.3.7 AsyncGeneratorUnwrapYieldResumption ( resumptionValue )

The abstract operation AsyncGeneratorUnwrapYieldResumption takes argument resumptionValue (a Completion Record) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It performs the following steps when called:

  1. If resumptionValue.[[Type]] is not return, return ? resumptionValue.
  2. Let awaited be Completion(Await(resumptionValue.[[Value]])).
  3. If awaited.[[Type]] is throw, return ? awaited.
  4. Assert: awaited.[[Type]] is normal.
  5. Return Completion Record { [[Type]]: return, [[Value]]: awaited.[[Value]], [[Target]]: empty }.

27.6.3.8 AsyncGeneratorYield ( value )

The abstract operation AsyncGeneratorYield takes argument value (an ECMAScript language value) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It performs the following steps when called:

  1. Let genContext be the running execution context.
  2. Assert: genContext is the execution context of a generator.
  3. Let generator be the value of the Generator component of genContext.
  4. Assert: GetGeneratorKind() is async.
  5. Let completion be NormalCompletion(value).
  6. Assert: The execution context stack has at least two elements.
  7. Let previousContext be the second to top element of the execution context stack.
  8. Let previousRealm be previousContext's Realm.
  9. Perform AsyncGeneratorCompleteStep(generator, completion, false, previousRealm).
  10. Let queue be generator.[[AsyncGeneratorQueue]].
  11. If queue is not empty, then
    1. NOTE: Execution continues without suspending the generator.
    2. Let toYield be the first element of queue.
    3. Let resumptionValue be Completion(toYield.[[Completion]]).
    4. Return ? AsyncGeneratorUnwrapYieldResumption(resumptionValue).
  12. Else,
    1. Set generator.[[AsyncGeneratorState]] to suspendedYield.
    2. Remove genContext from the execution context stack and restore the execution context that is at the top of the execution context stack as the running execution context.
    3. Let callerContext be the running execution context.
    4. Resume callerContext passing undefined. If genContext is ever resumed again, let resumptionValue be the Completion Record with which it is resumed.
    5. Assert: If control reaches here, then genContext is the running execution context again.
    6. Return ? AsyncGeneratorUnwrapYieldResumption(resumptionValue).

27.6.3.9 AsyncGeneratorAwaitReturn ( generator )

The abstract operation AsyncGeneratorAwaitReturn takes argument generator (an AsyncGenerator) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

  1. Let queue be generator.[[AsyncGeneratorQueue]].
  2. Assert: queue is not empty.
  3. Let next be the first element of queue.
  4. Let completion be Completion(next.[[Completion]]).
  5. Assert: completion.[[Type]] is return.
  6. Let promise be ? PromiseResolve(%Promise%, completion.[[Value]]).
  7. Let fulfilledClosure be a new Abstract Closure with parameters (value) that captures generator and performs the following steps when called:
    1. Set generator.[[AsyncGeneratorState]] to completed.
    2. Let result be NormalCompletion(value).
    3. Perform AsyncGeneratorCompleteStep(generator, result, true).
    4. Perform AsyncGeneratorDrainQueue(generator).
    5. Return undefined.
  8. Let onFulfilled be CreateBuiltinFunction(fulfilledClosure, 1, "", « »).
  9. Let rejectedClosure be a new Abstract Closure with parameters (reason) that captures generator and performs the following steps when called:
    1. Set generator.[[AsyncGeneratorState]] to completed.
    2. Let result be ThrowCompletion(reason).
    3. Perform AsyncGeneratorCompleteStep(generator, result, true).
    4. Perform AsyncGeneratorDrainQueue(generator).
    5. Return undefined.
  10. Let onRejected be CreateBuiltinFunction(rejectedClosure, 1, "", « »).
  11. Perform PerformPromiseThen(promise, onFulfilled, onRejected).
  12. Return unused.

27.6.3.10 AsyncGeneratorDrainQueue ( generator )

The abstract operation AsyncGeneratorDrainQueue takes argument generator (an AsyncGenerator) and returns unused. It drains the generator's AsyncGeneratorQueue until it encounters an AsyncGeneratorRequest which holds a return completion. It performs the following steps when called:

  1. Assert: generator.[[AsyncGeneratorState]] is completed.
  2. Let queue be generator.[[AsyncGeneratorQueue]].
  3. If queue is empty, return unused.
  4. Let done be false.
  5. Repeat, while done is false,
    1. Let next be the first element of queue.
    2. Let completion be Completion(next.[[Completion]]).
    3. If completion.[[Type]] is return, then
      1. Set generator.[[AsyncGeneratorState]] to awaiting-return.
      2. Perform ! AsyncGeneratorAwaitReturn(generator).
      3. Set done to true.
    4. Else,
      1. If completion.[[Type]] is normal, then
        1. Set completion to NormalCompletion(undefined).
      2. Perform AsyncGeneratorCompleteStep(generator, completion, true).
      3. If queue is empty, set done to true.
  6. Return unused.

27.6.3.11 CreateAsyncIteratorFromClosure ( closure, generatorBrand, generatorPrototype )

The abstract operation CreateAsyncIteratorFromClosure takes arguments closure (an Abstract Closure with no parameters), generatorBrand (a String or empty), and generatorPrototype (an Object) and returns an AsyncGenerator. It performs the following steps when called:

  1. NOTE: closure can contain uses of the Await operation and uses of the Yield operation to yield an IteratorResult object.
  2. Let internalSlotsList be « [[AsyncGeneratorState]], [[AsyncGeneratorContext]], [[AsyncGeneratorQueue]], [[GeneratorBrand]] ».
  3. Let generator be OrdinaryObjectCreate(generatorPrototype, internalSlotsList).
  4. Set generator.[[GeneratorBrand]] to generatorBrand.
  5. Set generator.[[AsyncGeneratorState]] to undefined.
  6. Let callerContext be the running execution context.
  7. Let calleeContext be a new execution context.
  8. Set the Function of calleeContext to null.
  9. Set the Realm of calleeContext to the current Realm Record.
  10. Set the ScriptOrModule of calleeContext to callerContext's ScriptOrModule.
  11. If callerContext is not already suspended, suspend callerContext.
  12. Push calleeContext onto the execution context stack; calleeContext is now the running execution context.
  13. Perform AsyncGeneratorStart(generator, closure).
  14. Remove calleeContext from the execution context stack and restore callerContext as the running execution context.
  15. Return generator.

27.7 AsyncFunction Objects

AsyncFunctions are functions that are usually created by evaluating AsyncFunctionDeclarations, AsyncFunctionExpressions, AsyncMethods, and AsyncArrowFunctions. They may also be created by calling the %AsyncFunction% intrinsic.

27.7.1 The AsyncFunction Constructor

The AsyncFunction constructor:

  • is %AsyncFunction%.
  • is a subclass of Function.
  • creates and initializes a new AsyncFunction when called as a function rather than as a constructor. Thus the function call AsyncFunction(…) is equivalent to the object creation expression new AsyncFunction(…) with the same arguments.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified AsyncFunction behaviour must include a super call to the AsyncFunction constructor to create and initialize a subclass instance with the internal slots necessary for built-in async function behaviour. All ECMAScript syntactic forms for defining async function objects create direct instances of AsyncFunction. There is no syntactic means to create instances of AsyncFunction subclasses.

27.7.1.1 AsyncFunction ( ...parameterArgs, bodyArg )

The last argument (if any) specifies the body (executable code) of an async function. Any preceding arguments specify formal parameters.

This function performs the following steps when called:

  1. Let C be the active function object.
  2. If bodyArg is not present, set bodyArg to the empty String.
  3. Return ? CreateDynamicFunction(C, NewTarget, async, parameterArgs, bodyArg).
 Note
See NOTE for 20.2.1.1.

27.7.2 Properties of the AsyncFunction Constructor

The AsyncFunction constructor:

  • is a standard built-in function object that inherits from the Function constructor.
  • has a [[Prototype]] internal slot whose value is %Function%.
  • has a "name" property whose value is "AsyncFunction".
  • has the following properties:

27.7.2.1 AsyncFunction.length

This is a data property with a value of 1. This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

27.7.2.2 AsyncFunction.prototype

The initial value of AsyncFunction.prototype is the AsyncFunction prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

27.7.3 Properties of the AsyncFunction Prototype Object

The AsyncFunction prototype object:

27.7.3.1 AsyncFunction.prototype.constructor

The initial value of AsyncFunction.prototype.constructor is %AsyncFunction%

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

27.7.3.2 AsyncFunction.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "AsyncFunction".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

27.7.4 AsyncFunction Instances

Every AsyncFunction instance is an ECMAScript function object and has the internal slots listed in Table 30. The value of the [[IsClassConstructor]] internal slot for all such instances is false. AsyncFunction instances are not constructors and do not have a [[Construct]] internal method. AsyncFunction instances do not have a prototype property as they are not constructible.

Each AsyncFunction instance has the following own properties:

27.7.4.1 length

The specification for the "length" property of Function instances given in 20.2.4.1 also applies to AsyncFunction instances.

27.7.4.2 name

The specification for the "name" property of Function instances given in 20.2.4.2 also applies to AsyncFunction instances.

27.7.5 Async Functions Abstract Operations

27.7.5.1 AsyncFunctionStart ( promiseCapability, asyncFunctionBody )

The abstract operation AsyncFunctionStart takes arguments promiseCapability (a PromiseCapability Record) and asyncFunctionBody (a FunctionBody Parse Node or an ExpressionBody Parse Node) and returns unused. It performs the following steps when called:

  1. Let runningContext be the running execution context.
  2. Let asyncContext be a copy of runningContext.
  3. NOTE: Copying the execution state is required for AsyncBlockStart to resume its execution. It is ill-defined to resume a currently executing context.
  4. Perform AsyncBlockStart(promiseCapability, asyncFunctionBody, asyncContext).
  5. Return unused.

27.7.5.2 AsyncBlockStart ( promiseCapability, asyncBody, asyncContext )

The abstract operation AsyncBlockStart takes arguments promiseCapability (a PromiseCapability Record), asyncBody (a Parse Node), and asyncContext (an execution context) and returns unused. It performs the following steps when called:

  1. Assert: promiseCapability is a PromiseCapability Record.
  2. Let runningContext be the running execution context.
  3. Let closure be a new Abstract Closure with no parameters that captures promiseCapability and asyncBody and performs the following steps when called:
    1. Let acAsyncContext be the running execution context.
    2. Let result be Completion(Evaluation of asyncBody).
    3. Assert: If we return here, the async function either threw an exception or performed an implicit or explicit return; all awaiting is done.
    4. Remove acAsyncContext from the execution context stack and restore the execution context that is at the top of the execution context stack as the running execution context.
    5. If result.[[Type]] is normal, then
      1. Perform ! Call(promiseCapability.[[Resolve]], undefined, « undefined »).
    6. Else if result.[[Type]] is return, then
      1. Perform ! Call(promiseCapability.[[Resolve]], undefined, « result.[[Value]] »).
    7. Else,
      1. Assert: result.[[Type]] is throw.
      2. Perform ! Call(promiseCapability.[[Reject]], undefined, « result.[[Value]] »).
    8. Return unused.
  4. Set the code evaluation state of asyncContext such that when evaluation is resumed for that execution context, closure will be called with no arguments.
  5. Push asyncContext onto the execution context stack; asyncContext is now the running execution context.
  6. Resume the suspended evaluation of asyncContext. Let result be the value returned by the resumed computation.
  7. Assert: When we return here, asyncContext has already been removed from the execution context stack and runningContext is the currently running execution context.
  8. Assert: result is a normal completion with a value of unused. The possible sources of this value are Await or, if the async function doesn't await anything, step 3.h above.
  9. Return unused.

27.7.5.3 Await ( value )

The abstract operation Await takes argument value (an ECMAScript language value) and returns either a normal completion containing either an ECMAScript language value or empty, or a throw completion. It performs the following steps when called:

  1. Let asyncContext be the running execution context.
  2. Let promise be ? PromiseResolve(%Promise%, value).
  3. Let fulfilledClosure be a new Abstract Closure with parameters (v) that captures asyncContext and performs the following steps when called:
    1. Let prevContext be the running execution context.
    2. Suspend prevContext.
    3. Push asyncContext onto the execution context stack; asyncContext is now the running execution context.
    4. Resume the suspended evaluation of asyncContext using NormalCompletion(v) as the result of the operation that suspended it.
    5. Assert: When we reach this step, asyncContext has already been removed from the execution context stack and prevContext is the currently running execution context.
    6. Return undefined.
  4. Let onFulfilled be CreateBuiltinFunction(fulfilledClosure, 1, "", « »).
  5. Let rejectedClosure be a new Abstract Closure with parameters (reason) that captures asyncContext and performs the following steps when called:
    1. Let prevContext be the running execution context.
    2. Suspend prevContext.
    3. Push asyncContext onto the execution context stack; asyncContext is now the running execution context.
    4. Resume the suspended evaluation of asyncContext using ThrowCompletion(reason) as the result of the operation that suspended it.
    5. Assert: When we reach this step, asyncContext has already been removed from the execution context stack and prevContext is the currently running execution context.
    6. Return undefined.
  6. Let onRejected be CreateBuiltinFunction(rejectedClosure, 1, "", « »).
  7. Perform PerformPromiseThen(promise, onFulfilled, onRejected).
  8. Remove asyncContext from the execution context stack and restore the execution context that is at the top of the execution context stack as the running execution context.
  9. Let callerContext be the running execution context.
  10. Resume callerContext passing empty. If asyncContext is ever resumed again, let completion be the Completion Record with which it is resumed.
  11. Assert: If control reaches here, then asyncContext is the running execution context again.
  12. Return completion.

28 Reflection

28.1 The Reflect Object

The Reflect object:

  • is %Reflect%.
  • is the initial value of the "Reflect" property of the global object.
  • is an ordinary object.
  • has a [[Prototype]] internal slot whose value is %Object.prototype%.
  • is not a function object.
  • does not have a [[Construct]] internal method; it cannot be used as a constructor with the new operator.
  • does not have a [[Call]] internal method; it cannot be invoked as a function.

28.1.1 Reflect.apply ( target, thisArgument, argumentsList )

This function performs the following steps when called:

  1. If IsCallable(target) is false, throw a TypeError exception.
  2. Let args be ? CreateListFromArrayLike(argumentsList).
  3. Perform PrepareForTailCall().
  4. Return ? Call(target, thisArgument, args).

28.1.2 Reflect.construct ( target, argumentsList [ , newTarget ] )

This function performs the following steps when called:

  1. If IsConstructor(target) is false, throw a TypeError exception.
  2. If newTarget is not present, set newTarget to target.
  3. Else if IsConstructor(newTarget) is false, throw a TypeError exception.
  4. Let args be ? CreateListFromArrayLike(argumentsList).
  5. Return ? Construct(target, args, newTarget).

28.1.3 Reflect.defineProperty ( target, propertyKey, attributes )

This function performs the following steps when called:

  1. If target is not an Object, throw a TypeError exception.
  2. Let key be ? ToPropertyKey(propertyKey).
  3. Let desc be ? ToPropertyDescriptor(attributes).
  4. Return ? target.[[DefineOwnProperty]](key, desc).

28.1.4 Reflect.deleteProperty ( target, propertyKey )

This function performs the following steps when called:

  1. If target is not an Object, throw a TypeError exception.
  2. Let key be ? ToPropertyKey(propertyKey).
  3. Return ? target.[[Delete]](key).

28.1.5 Reflect.get ( target, propertyKey [ , receiver ] )

This function performs the following steps when called:

  1. If target is not an Object, throw a TypeError exception.
  2. Let key be ? ToPropertyKey(propertyKey).
  3. If receiver is not present, then
    1. Set receiver to target.
  4. Return ? target.[[Get]](key, receiver).

28.1.6 Reflect.getOwnPropertyDescriptor ( target, propertyKey )

This function performs the following steps when called:

  1. If target is not an Object, throw a TypeError exception.
  2. Let key be ? ToPropertyKey(propertyKey).
  3. Let desc be ? target.[[GetOwnProperty]](key).
  4. Return FromPropertyDescriptor(desc).

28.1.7 Reflect.getPrototypeOf ( target )

This function performs the following steps when called:

  1. If target is not an Object, throw a TypeError exception.
  2. Return ? target.[[GetPrototypeOf]]().

28.1.8 Reflect.has ( target, propertyKey )

This function performs the following steps when called:

  1. If target is not an Object, throw a TypeError exception.
  2. Let key be ? ToPropertyKey(propertyKey).
  3. Return ? target.[[HasProperty]](key).

28.1.9 Reflect.isExtensible ( target )

This function performs the following steps when called:

  1. If target is not an Object, throw a TypeError exception.
  2. Return ? target.[[IsExtensible]]().

28.1.10 Reflect.ownKeys ( target )

This function performs the following steps when called:

  1. If target is not an Object, throw a TypeError exception.
  2. Let keys be ? target.[[OwnPropertyKeys]]().
  3. Return CreateArrayFromList(keys).

28.1.11 Reflect.preventExtensions ( target )

This function performs the following steps when called:

  1. If target is not an Object, throw a TypeError exception.
  2. Return ? target.[[PreventExtensions]]().

28.1.12 Reflect.set ( target, propertyKey, V [ , receiver ] )

This function performs the following steps when called:

  1. If target is not an Object, throw a TypeError exception.
  2. Let key be ? ToPropertyKey(propertyKey).
  3. If receiver is not present, then
    1. Set receiver to target.
  4. Return ? target.[[Set]](key, V, receiver).

28.1.13 Reflect.setPrototypeOf ( target, proto )

This function performs the following steps when called:

  1. If target is not an Object, throw a TypeError exception.
  2. If proto is not an Object and proto is not null, throw a TypeError exception.
  3. Return ? target.[[SetPrototypeOf]](proto).

28.1.14 Reflect [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "Reflect".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

28.2 Proxy Objects

28.2.1 The Proxy Constructor

The Proxy constructor:

  • is %Proxy%.
  • is the initial value of the "Proxy" property of the global object.
  • creates and initializes a new Proxy object when called as a constructor.
  • is not intended to be called as a function and will throw an exception when called in that manner.

28.2.1.1 Proxy ( target, handler )

This function performs the following steps when called:

  1. If NewTarget is undefined, throw a TypeError exception.
  2. Return ? ProxyCreate(target, handler).

28.2.2 Properties of the Proxy Constructor

The Proxy constructor:

  • has a [[Prototype]] internal slot whose value is %Function.prototype%.
  • does not have a "prototype" property because Proxy objects do not have a [[Prototype]] internal slot that requires initialization.
  • has the following properties:

28.2.2.1 Proxy.revocable ( target, handler )

This function creates a revocable Proxy object.

It performs the following steps when called:

  1. Let proxy be ? ProxyCreate(target, handler).
  2. Let revokerClosure be a new Abstract Closure with no parameters that captures nothing and performs the following steps when called:
    1. Let F be the active function object.
    2. Let p be F.[[RevocableProxy]].
    3. If p is null, return undefined.
    4. Set F.[[RevocableProxy]] to null.
    5. Assert: p is a Proxy object.
    6. Set p.[[ProxyTarget]] to null.
    7. Set p.[[ProxyHandler]] to null.
    8. Return undefined.
  3. Let revoker be CreateBuiltinFunction(revokerClosure, 0, "", « [[RevocableProxy]] »).
  4. Set revoker.[[RevocableProxy]] to proxy.
  5. Let result be OrdinaryObjectCreate(%Object.prototype%).
  6. Perform ! CreateDataPropertyOrThrow(result, "proxy", proxy).
  7. Perform ! CreateDataPropertyOrThrow(result, "revoke", revoker).
  8. Return result.

28.3 Module Namespace Objects

A Module Namespace Object is a module namespace exotic object that provides runtime property-based access to a module's exported bindings. There is no constructor function for Module Namespace Objects. Instead, such an object is created for each module that is imported by an ImportDeclaration that contains a NameSpaceImport.

In addition to the properties specified in 10.4.6 each Module Namespace Object has the following own property:

28.3.1 @@toStringTag

The initial value of the @@toStringTag property is the String value "Module".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

29 Memory Model

The memory consistency model, or memory model, specifies the possible orderings of Shared Data Block events, arising via accessing TypedArray instances backed by a SharedArrayBuffer and via methods on the Atomics object. When the program has no data races (defined below), the ordering of events appears as sequentially consistent, i.e., as an interleaving of actions from each agent. When the program has data races, shared memory operations may appear sequentially inconsistent. For example, programs may exhibit causality-violating behaviour and other astonishments. These astonishments arise from compiler transforms and the design of CPUs (e.g., out-of-order execution and speculation). The memory model defines both the precise conditions under which a program exhibits sequentially consistent behaviour as well as the possible values read from data races. To wit, there is no undefined behaviour.

The memory model is defined as relational constraints on events introduced by abstract operations on SharedArrayBuffer or by methods on the Atomics object during an evaluation.

Note

This section provides an axiomatic model on events introduced by the abstract operations on SharedArrayBuffers. It bears stressing that the model is not expressible algorithmically, unlike the rest of this specification. The nondeterministic introduction of events by abstract operations is the interface between the operational semantics of ECMAScript evaluation and the axiomatic semantics of the memory model. The semantics of these events is defined by considering graphs of all events in an evaluation. These are neither Static Semantics nor Runtime Semantics. There is no demonstrated algorithmic implementation, but instead a set of constraints that determine if a particular event graph is allowed or disallowed.

29.1 Memory Model Fundamentals

Shared memory accesses (reads and writes) are divided into two groups, atomic accesses and data accesses, defined below. Atomic accesses are sequentially consistent, i.e., there is a strict total ordering of events agreed upon by all agents in an agent cluster. Non-atomic accesses do not have a strict total ordering agreed upon by all agents, i.e., unordered.

Note 1

No orderings weaker than sequentially consistent and stronger than unordered, such as release-acquire, are supported.

A Shared Data Block event is either a ReadSharedMemory, WriteSharedMemory, or ReadModifyWriteSharedMemory Record.

Table 85: ReadSharedMemory Event Fields
Field Name Value Meaning
[[Order]] SeqCst or Unordered The weakest ordering guaranteed by the memory model for the event.
[[NoTear]] a Boolean Whether this event is allowed to read from multiple write events with equal range as this event.
[[Block]] a Shared Data Block The block the event operates on.
[[ByteIndex]] a non-negative integer The byte address of the read in [[Block]].
[[ElementSize]] a non-negative integer The size of the read.
Table 86: WriteSharedMemory Event Fields
Field Name Value Meaning
[[Order]] SeqCst, Unordered, or Init The weakest ordering guaranteed by the memory model for the event.
[[NoTear]] a Boolean Whether this event is allowed to be read from multiple read events with equal range as this event.
[[Block]] a Shared Data Block The block the event operates on.
[[ByteIndex]] a non-negative integer The byte address of the write in [[Block]].
[[ElementSize]] a non-negative integer The size of the write.
[[Payload]] a List of byte values The List of byte values to be read by other events.
Table 87: ReadModifyWriteSharedMemory Event Fields
Field Name Value Meaning
[[Order]] SeqCst Read-modify-write events are always sequentially consistent.
[[NoTear]] true Read-modify-write events cannot tear.
[[Block]] a Shared Data Block The block the event operates on.
[[ByteIndex]] a non-negative integer The byte address of the read-modify-write in [[Block]].
[[ElementSize]] a non-negative integer The size of the read-modify-write.
[[Payload]] a List of byte values The List of byte values to be passed to [[ModifyOp]].
[[ModifyOp]] a read-modify-write modification function An abstract closure that returns a modified List of byte values from a read List of byte values and [[Payload]].

These events are introduced by abstract operations or by methods on the Atomics object.

Some operations may also introduce Synchronize events. A Synchronize event has no fields, and exists purely to directly constrain the permitted orderings of other events.

In addition to Shared Data Block and Synchronize events, there are host-specific events.

Let the range of a ReadSharedMemory, WriteSharedMemory, or ReadModifyWriteSharedMemory event be the Set of contiguous integers from its [[ByteIndex]] to [[ByteIndex]] [[ElementSize]] - 1. Two events' ranges are equal when the events have the same [[Block]], and the ranges are element-wise equal. Two events' ranges are overlapping when the events have the same [[Block]], the ranges are not equal and their intersection is non-empty. Two events' ranges are disjoint when the events do not have the same [[Block]] or their ranges are neither equal nor overlapping.

Note 2

Examples of host-specific synchronizing events that should be accounted for are: sending a SharedArrayBuffer from one agent to another (e.g., by postMessage in a browser), starting and stopping agents, and communicating within the agent cluster via channels other than shared memory. It is assumed those events are appended to agent-order during evaluation like the other SharedArrayBuffer events.

Events are ordered within candidate executions by the relations defined below.

29.2 Agent Events Records

An Agent Events Record is a Record with the following fields.

Table 88: Agent Events Record Fields
Field Name Value Meaning
[[AgentSignifier]] an agent signifier The agent whose evaluation resulted in this ordering.
[[EventList]] a List of events Events are appended to the list during evaluation.
[[AgentSynchronizesWith]] a List of pairs of Synchronize events Synchronize relationships introduced by the operational semantics.

29.3 Chosen Value Records

A Chosen Value Record is a Record with the following fields.

Table 89: Chosen Value Record Fields
Field Name Value Meaning
[[Event]] a Shared Data Block event The ReadSharedMemory or ReadModifyWriteSharedMemory event that was introduced for this chosen value.
[[ChosenValue]] a List of byte values The bytes that were nondeterministically chosen during evaluation.

29.4 Candidate Executions

A candidate execution of the evaluation of an agent cluster is a Record with the following fields.

Table 90: Candidate Execution Record Fields
Field Name Value Meaning
[[EventsRecords]] a List of Agent Events Records Maps an agent to Lists of events appended during the evaluation.
[[ChosenValues]] a List of Chosen Value Records Maps ReadSharedMemory or ReadModifyWriteSharedMemory events to the List of byte values chosen during the evaluation.
[[AgentOrder]] an agent-order Relation Defined below.
[[ReadsBytesFrom]] a reads-bytes-from mathematical function Defined below.
[[ReadsFrom]] a reads-from Relation Defined below.
[[HostSynchronizesWith]] a host-synchronizes-with Relation Defined below.
[[SynchronizesWith]] a synchronizes-with Relation Defined below.
[[HappensBefore]] a happens-before Relation Defined below.

An empty candidate execution is a candidate execution Record whose fields are empty Lists and Relations.

29.5 Abstract Operations for the Memory Model

29.5.1 EventSet ( execution )

The abstract operation EventSet takes argument execution (a candidate execution) and returns a Set of events. It performs the following steps when called:

  1. Let events be an empty Set.
  2. For each Agent Events Record aer of execution.[[EventsRecords]], do
    1. For each event E of aer.[[EventList]], do
      1. Add E to events.
  3. Return events.

29.5.2 SharedDataBlockEventSet ( execution )

The abstract operation SharedDataBlockEventSet takes argument execution (a candidate execution) and returns a Set of events. It performs the following steps when called:

  1. Let events be an empty Set.
  2. For each event E of EventSet(execution), do
    1. If E is a ReadSharedMemory, WriteSharedMemory, or ReadModifyWriteSharedMemory event, add E to events.
  3. Return events.

29.5.3 HostEventSet ( execution )

The abstract operation HostEventSet takes argument execution (a candidate execution) and returns a Set of events. It performs the following steps when called:

  1. Let events be an empty Set.
  2. For each event E of EventSet(execution), do
    1. If E is not in SharedDataBlockEventSet(execution), add E to events.
  3. Return events.

29.5.4 ComposeWriteEventBytes ( execution, byteIndex, Ws )

The abstract operation ComposeWriteEventBytes takes arguments execution (a candidate execution), byteIndex (a non-negative integer), and Ws (a List of either WriteSharedMemory or ReadModifyWriteSharedMemory events) and returns a List of byte values. It performs the following steps when called:

  1. Let byteLocation be byteIndex.
  2. Let bytesRead be a new empty List.
  3. For each element W of Ws, do
    1. Assert: W has byteLocation in its range.
    2. Let payloadIndex be byteLocation - W.[[ByteIndex]].
    3. If W is a WriteSharedMemory event, then
      1. Let byte be W.[[Payload]][payloadIndex].
    4. Else,
      1. Assert: W is a ReadModifyWriteSharedMemory event.
      2. Let bytes be ValueOfReadEvent(execution, W).
      3. Let bytesModified be W.[[ModifyOp]](bytes, W.[[Payload]]).
      4. Let byte be bytesModified[payloadIndex].
    5. Append byte to bytesRead.
    6. Set byteLocation to byteLocation 1.
  4. Return bytesRead.
 Note 1

The read-modify-write modification [[ModifyOp]] is given by the function properties on the Atomics object that introduce ReadModifyWriteSharedMemory events.

 Note 2

This abstract operation composes a List of write events into a List of byte values. It is used in the event semantics of ReadSharedMemory and ReadModifyWriteSharedMemory events.

29.5.5 ValueOfReadEvent ( execution, R )

The abstract operation ValueOfReadEvent takes arguments execution (a candidate execution) and R (a ReadSharedMemory or ReadModifyWriteSharedMemory event) and returns a List of byte values. It performs the following steps when called:

  1. Let Ws be execution.[[ReadsBytesFrom]](R).
  2. Assert: Ws is a List of WriteSharedMemory or ReadModifyWriteSharedMemory events with length equal to R.[[ElementSize]].
  3. Return ComposeWriteEventBytes(execution, R.[[ByteIndex]], Ws).

29.6 Relations of Candidate Executions

29.6.1 agent-order

For a candidate execution execution, execution.[[AgentOrder]] is a Relation on events that satisfies the following.

  • For each pair (E, D) in EventSet(execution), execution.[[AgentOrder]] contains (E, D) if there is some Agent Events Record aer in execution.[[EventsRecords]] such that E and D are in aer.[[EventList]] and E is before D in List order of aer.[[EventList]].
Note

Each agent introduces events in a per-agent strict total order during the evaluation. This is the union of those strict total orders.

29.6.2 reads-bytes-from

For a candidate execution execution, execution.[[ReadsBytesFrom]] is a mathematical function mapping events in SharedDataBlockEventSet(execution) to Lists of events in SharedDataBlockEventSet(execution) that satisfies the following conditions.

29.6.3 reads-from

For a candidate execution execution, execution.[[ReadsFrom]] is the least Relation on events that satisfies the following.

  • For each pair (R, W) in SharedDataBlockEventSet(execution), execution.[[ReadsFrom]] contains (R, W) if execution.[[ReadsBytesFrom]](R) contains W.

29.6.4 host-synchronizes-with

For a candidate execution execution, execution.[[HostSynchronizesWith]] is a host-provided strict partial order on host-specific events that satisfies at least the following.

  • If execution.[[HostSynchronizesWith]] contains (E, D), E and D are in HostEventSet(execution).
  • There is no cycle in the union of execution.[[HostSynchronizesWith]] and execution.[[AgentOrder]].
Note 1

For two host-specific events E and D, E host-synchronizes-with D implies E happens-before D.

 Note 2

The host-synchronizes-with relation allows the host to provide additional synchronization mechanisms, such as postMessage between HTML workers.

29.6.5 synchronizes-with

For a candidate execution execution, execution.[[SynchronizesWith]] is the least Relation on events that satisfies the following.

  • For each pair (R, W) in execution.[[ReadsFrom]], execution.[[SynchronizesWith]] contains (W, R) if R.[[Order]] is SeqCst, W.[[Order]] is SeqCst, and R and W have equal ranges.
  • For each element eventsRecord of execution.[[EventsRecords]], the following is true.
    • For each pair (S, Sw) in eventsRecord.[[AgentSynchronizesWith]], execution.[[SynchronizesWith]] contains (S, Sw).
  • For each pair (E, D) in execution.[[HostSynchronizesWith]], execution.[[SynchronizesWith]] contains (E, D).
Note 1

Owing to convention, write events synchronizes-with read events, instead of read events synchronizes-with write events.

 Note 2

Init events do not participate in synchronizes-with, and are instead constrained directly by happens-before.

 Note 3

Not all SeqCst events related by reads-from are related by synchronizes-with. Only events that also have equal ranges are related by synchronizes-with.

 Note 4

For Shared Data Block events R and W such that W synchronizes-with R, R may reads-from other writes than W.

29.6.6 happens-before

For a candidate execution execution, execution.[[HappensBefore]] is the least Relation on events that satisfies the following.

  • For each pair (E, D) in execution.[[AgentOrder]], execution.[[HappensBefore]] contains (E, D).
  • For each pair (E, D) in execution.[[SynchronizesWith]], execution.[[HappensBefore]] contains (E, D).
  • For each pair (E, D) in SharedDataBlockEventSet(execution), execution.[[HappensBefore]] contains (E, D) if E.[[Order]] is Init and E and D have overlapping ranges.
  • For each pair (E, D) in EventSet(execution), execution.[[HappensBefore]] contains (E, D) if there is an event F such that the pairs (E, F) and (F, D) are in execution.[[HappensBefore]].
Note

Because happens-before is a superset of agent-order, candidate executions are consistent with the single-thread evaluation semantics of ECMAScript.

29.7 Properties of Valid Executions

29.7.1 Valid Chosen Reads

A candidate execution execution has valid chosen reads if the following algorithm returns true.

  1. For each ReadSharedMemory or ReadModifyWriteSharedMemory event R of SharedDataBlockEventSet(execution), do
    1. Let chosenValueRecord be the element of execution.[[ChosenValues]] whose [[Event]] field is R.
    2. Let chosenValue be chosenValueRecord.[[ChosenValue]].
    3. Let readValue be ValueOfReadEvent(execution, R).
    4. Let chosenLen be the number of elements in chosenValue.
    5. Let readLen be the number of elements in readValue.
    6. If chosenLen ≠ readLen, then
      1. Return false.
    7. If chosenValue[i] ≠ readValue[i] for some integer i in the interval from 0 (inclusive) to chosenLen (exclusive), then
      1. Return false.
  2. Return true.

29.7.2 Coherent Reads

A candidate execution execution has coherent reads if the following algorithm returns true.

  1. For each ReadSharedMemory or ReadModifyWriteSharedMemory event R of SharedDataBlockEventSet(execution), do
    1. Let Ws be execution.[[ReadsBytesFrom]](R).
    2. Let byteLocation be R.[[ByteIndex]].
    3. For each element W of Ws, do
      1. If execution.[[HappensBefore]] contains (R, W), then
        1. Return false.
      2. If there exists a WriteSharedMemory or ReadModifyWriteSharedMemory event V that has byteLocation in its range such that the pairs (W, V) and (V, R) are in execution.[[HappensBefore]], then
        1. Return false.
      3. Set byteLocation to byteLocation 1.
  2. Return true.

29.7.3 Tear Free Reads

A candidate execution execution has tear free reads if the following algorithm returns true.

  1. For each ReadSharedMemory or ReadModifyWriteSharedMemory event R of SharedDataBlockEventSet(execution), do
    1. If R.[[NoTear]] is true, then
      1. Assert: The remainder of dividing R.[[ByteIndex]] by R.[[ElementSize]] is 0.
      2. For each event W such that execution.[[ReadsFrom]] contains (R, W) and W.[[NoTear]] is true, do
        1. If R and W have equal ranges and there exists an event V such that V and W have equal ranges, V.[[NoTear]] is true, W is not V, and execution.[[ReadsFrom]] contains (R, V), then
          1. Return false.
  2. Return true.
 Note

An event's [[NoTear]] field is true when that event was introduced via accessing an integer TypedArray, and false when introduced via accessing a floating point TypedArray or DataView.

Intuitively, this requirement says when a memory range is accessed in an aligned fashion via an integer TypedArray, a single write event on that range must "win" when in a data race with other write events with equal ranges. More precisely, this requirement says an aligned read event cannot read a value composed of bytes from multiple, different write events all with equal ranges. It is possible, however, for an aligned read event to read from multiple write events with overlapping ranges.

29.7.4 Sequentially Consistent Atomics

For a candidate execution execution, memory-order is a strict total order of all events in EventSet(execution) that satisfies the following.

  • For each pair (E, D) in execution.[[HappensBefore]], (E, D) is in memory-order.

  • For each pair (R, W) in execution.[[ReadsFrom]], there is no WriteSharedMemory or ReadModifyWriteSharedMemory event V in SharedDataBlockEventSet(execution) such that V.[[Order]] is SeqCst, the pairs (W, V) and (V, R) are in memory-order, and any of the following conditions are true.

    • execution.[[SynchronizesWith]] contains the pair (W, R), and V and R have equal ranges.
    • The pairs (W, R) and (V, R) are in execution.[[HappensBefore]], W.[[Order]] is SeqCst, and W and V have equal ranges.
    • The pairs (W, R) and (W, V) are in execution.[[HappensBefore]], R.[[Order]] is SeqCst, and V and R have equal ranges.
    Note 1

    This clause additionally constrains SeqCst events on equal ranges.

  • For each WriteSharedMemory or ReadModifyWriteSharedMemory event W in SharedDataBlockEventSet(execution), if W.[[Order]] is SeqCst, then it is not the case that there is an infinite number of ReadSharedMemory or ReadModifyWriteSharedMemory events in SharedDataBlockEventSet(execution) with equal range that is memory-order before W.

    Note 2

    This clause together with the forward progress guarantee on agents ensure the liveness condition that SeqCst writes become visible to SeqCst reads with equal range in finite time.

A candidate execution has sequentially consistent atomics if a memory-order exists.

Note 3

While memory-order includes all events in EventSet(execution), those that are not constrained by happens-before or synchronizes-with are allowed to occur anywhere in the order.

29.7.5 Valid Executions

A candidate execution execution is a valid execution (or simply an execution) if all of the following are true.

  • The host provides a host-synchronizes-with Relation for execution.[[HostSynchronizesWith]].
  • execution.[[HappensBefore]] is a strict partial order.
  • execution has valid chosen reads.
  • execution has coherent reads.
  • execution has tear free reads.
  • execution has sequentially consistent atomics.

All programs have at least one valid execution.

29.8 Races

For an execution execution, two events E and D in SharedDataBlockEventSet(execution) are in a race if the following algorithm returns true.

  1. If E is not D, then
    1. If the pairs (E, D) and (D, E) are not in execution.[[HappensBefore]], then
      1. If E and D are both WriteSharedMemory or ReadModifyWriteSharedMemory events and E and D do not have disjoint ranges, then
        1. Return true.
      2. If execution.[[ReadsFrom]] contains either (E, D) or (D, E), then
        1. Return true.
  2. Return false.

29.9 Data Races

For an execution execution, two events E and D in SharedDataBlockEventSet(execution) are in a data race if the following algorithm returns true.

  1. If E and D are in a race in execution, then
    1. If E.[[Order]] is not SeqCst or D.[[Order]] is not SeqCst, then
      1. Return true.
    2. If E and D have overlapping ranges, then
      1. Return true.
  2. Return false.

29.10 Data Race Freedom

An execution execution is data race free if there are no two events in SharedDataBlockEventSet(execution) that are in a data race.

A program is data race free if all its executions are data race free.

The memory model guarantees sequential consistency of all events for data race free programs.

29.11 Shared Memory Guidelines

Note 1

The following are guidelines for ECMAScript programmers working with shared memory.

We recommend programs be kept data race free, i.e., make it so that it is impossible for there to be concurrent non-atomic operations on the same memory location. Data race free programs have interleaving semantics where each step in the evaluation semantics of each agent are interleaved with each other. For data race free programs, it is not necessary to understand the details of the memory model. The details are unlikely to build intuition that will help one to better write ECMAScript.

More generally, even if a program is not data race free it may have predictable behaviour, so long as atomic operations are not involved in any data races and the operations that race all have the same access size. The simplest way to arrange for atomics not to be involved in races is to ensure that different memory cells are used by atomic and non-atomic operations and that atomic accesses of different sizes are not used to access the same cells at the same time. Effectively, the program should treat shared memory as strongly typed as much as possible. One still cannot depend on the ordering and timing of non-atomic accesses that race, but if memory is treated as strongly typed the racing accesses will not "tear" (bits of their values will not be mixed).

 Note 2

The following are guidelines for ECMAScript implementers writing compiler transformations for programs using shared memory.

It is desirable to allow most program transformations that are valid in a single-agent setting in a multi-agent setting, to ensure that the performance of each agent in a multi-agent program is as good as it would be in a single-agent setting. Frequently these transformations are hard to judge. We outline some rules about program transformations that are intended to be taken as normative (in that they are implied by the memory model or stronger than what the memory model implies) but which are likely not exhaustive. These rules are intended to apply to program transformations that precede the introductions of the events that make up the agent-order.

Let an agent-order slice be the subset of the agent-order pertaining to a single agent.

Let possible read values of a read event be the set of all values of ValueOfReadEvent for that event across all valid executions.

Any transformation of an agent-order slice that is valid in the absence of shared memory is valid in the presence of shared memory, with the following exceptions.

  • Atomics are carved in stone: Program transformations must not cause the SeqCst events in an agent-order slice to be reordered with its Unordered operations, nor its SeqCst operations to be reordered with each other, nor may a program transformation remove a SeqCst operation from the agent-order.

    (In practice, the prohibition on reorderings forces a compiler to assume that every SeqCst operation is a synchronization and included in the final memory-order, which it would usually have to assume anyway in the absence of inter-agent program analysis. It also forces the compiler to assume that every call where the callee's effects on the memory-order are unknown may contain SeqCst operations.)

  • Reads must be stable: Any given shared memory read must only observe a single value in an execution.

    (For example, if what is semantically a single read in the program is executed multiple times then the program is subsequently allowed to observe only one of the values read. A transformation known as rematerialization can violate this rule.)

  • Writes must be stable: All observable writes to shared memory must follow from program semantics in an execution.

    (For example, a transformation may not introduce certain observable writes, such as by using read-modify-write operations on a larger location to write a smaller datum, writing a value to memory that the program could not have written, or writing a just-read value back to the location it was read from, if that location could have been overwritten by another agent after the read.)

  • Possible read values must be non-empty: Program transformations cannot cause the possible read values of a shared memory read to become empty.

    (Counterintuitively, this rule in effect restricts transformations on writes, because writes have force in memory model insofar as to be read by read events. For example, writes may be moved and coalesced and sometimes reordered between two SeqCst operations, but the transformation may not remove every write that updates a location; some write must be preserved.)

Examples of transformations that remain valid are: merging multiple non-atomic reads from the same location, reordering non-atomic reads, introducing speculative non-atomic reads, merging multiple non-atomic writes to the same location, reordering non-atomic writes to different locations, and hoisting non-atomic reads out of loops even if that affects termination. Note in general that aliased TypedArrays make it hard to prove that locations are different.

 Note 3

The following are guidelines for ECMAScript implementers generating machine code for shared memory accesses.

For architectures with memory models no weaker than those of ARM or Power, non-atomic stores and loads may be compiled to bare stores and loads on the target architecture. Atomic stores and loads may be compiled down to instructions that guarantee sequential consistency. If no such instructions exist, memory barriers are to be employed, such as placing barriers on both sides of a bare store or load. Read-modify-write operations may be compiled to read-modify-write instructions on the target architecture, such as LOCK-prefixed instructions on x86, load-exclusive/store-exclusive instructions on ARM, and load-link/store-conditional instructions on Power.

Specifically, the memory model is intended to allow code generation as follows.

  • Every atomic operation in the program is assumed to be necessary.
  • Atomic operations are never rearranged with each other or with non-atomic operations.
  • Functions are always assumed to perform atomic operations.
  • Atomic operations are never implemented as read-modify-write operations on larger data, but as non-lock-free atomics if the platform does not have atomic operations of the appropriate size. (We already assume that every platform has normal memory access operations of every interesting size.)

Naive code generation uses these patterns:

  • Regular loads and stores compile to single load and store instructions.
  • Lock-free atomic loads and stores compile to a full (sequentially consistent) fence, a regular load or store, and a full fence.
  • Lock-free atomic read-modify-write accesses compile to a full fence, an atomic read-modify-write instruction sequence, and a full fence.
  • Non-lock-free atomics compile to a spinlock acquire, a full fence, a series of non-atomic load and store instructions, a full fence, and a spinlock release.

That mapping is correct so long as an atomic operation on an address range does not race with a non-atomic write or with an atomic operation of different size. However, that is all we need: the memory model effectively demotes the atomic operations involved in a race to non-atomic status. On the other hand, the naive mapping is quite strong: it allows atomic operations to be used as sequentially consistent fences, which the memory model does not actually guarantee.

Local improvements to those basic patterns are also allowed, subject to the constraints of the memory model. For example:

  • There are obvious platform-dependent improvements that remove redundant fences. For example, on x86 the fences around lock-free atomic loads and stores can always be omitted except for the fence following a store, and no fence is needed for lock-free read-modify-write instructions, as these all use LOCK-prefixed instructions. On many platforms there are fences of several strengths, and weaker fences can be used in certain contexts without destroying sequential consistency.
  • Most modern platforms support lock-free atomics for all the data sizes required by ECMAScript atomics. Should non-lock-free atomics be needed, the fences surrounding the body of the atomic operation can usually be folded into the lock and unlock steps. The simplest solution for non-lock-free atomics is to have a single lock word per SharedArrayBuffer.
  • There are also more complicated platform-dependent local improvements, requiring some code analysis. For example, two back-to-back fences often have the same effect as a single fence, so if code is generated for two atomic operations in sequence, only a single fence need separate them. On x86, even a single fence separating atomic stores can be omitted, as the fence following a store is only needed to separate the store from a subsequent load.

A Grammar Summary

A.1 Lexical Grammar

SourceCharacter :: any Unicode code point InputElementDiv :: WhiteSpace LineTerminator Comment CommonToken DivPunctuator RightBracePunctuator InputElementRegExp :: WhiteSpace LineTerminator Comment CommonToken RightBracePunctuator RegularExpressionLiteral InputElementRegExpOrTemplateTail :: WhiteSpace LineTerminator Comment CommonToken RegularExpressionLiteral TemplateSubstitutionTail InputElementTemplateTail :: WhiteSpace LineTerminator Comment CommonToken DivPunctuator TemplateSubstitutionTail InputElementHashbangOrRegExp :: WhiteSpace LineTerminator Comment CommonToken HashbangComment RegularExpressionLiteral WhiteSpace :: <TAB> <VT> <FF> <ZWNBSP> <USP> LineTerminator :: <LF> <CR> <LS> <PS> LineTerminatorSequence :: <LF> <CR> [lookahead ≠ <LF>] <LS> <PS> <CR> <LF> Comment :: MultiLineComment SingleLineComment MultiLineComment :: /* MultiLineCommentCharsopt */ MultiLineCommentChars :: MultiLineNotAsteriskChar MultiLineCommentCharsopt * PostAsteriskCommentCharsopt PostAsteriskCommentChars :: MultiLineNotForwardSlashOrAsteriskChar MultiLineCommentCharsopt * PostAsteriskCommentCharsopt MultiLineNotAsteriskChar :: SourceCharacter but not * MultiLineNotForwardSlashOrAsteriskChar :: SourceCharacter but not one of / or * SingleLineComment :: // SingleLineCommentCharsopt SingleLineCommentChars :: SingleLineCommentChar SingleLineCommentCharsopt SingleLineCommentChar :: SourceCharacter but not LineTerminator HashbangComment :: #! SingleLineCommentCharsopt CommonToken :: IdentifierName PrivateIdentifier Punctuator NumericLiteral StringLiteral Template PrivateIdentifier :: # IdentifierName IdentifierName :: IdentifierStart IdentifierName IdentifierPart IdentifierStart :: IdentifierStartChar \ UnicodeEscapeSequence IdentifierPart :: IdentifierPartChar \ UnicodeEscapeSequence IdentifierStartChar :: UnicodeIDStart $ _ IdentifierPartChar :: UnicodeIDContinue $ <ZWNJ> <ZWJ> AsciiLetter :: one of a b c d e f g h i j k l m n o p q r s t u v w x y z A B C D E F G H I J K L M N O P Q R S T U V W X Y Z UnicodeIDStart :: any Unicode code point with the Unicode property “ID_Start” UnicodeIDContinue :: any Unicode code point with the Unicode property “ID_Continue” ReservedWord :: one of await break case catch class const continue debugger default delete do else enum export extends false finally for function if import in instanceof new null return super switch this throw true try typeof var void while with yield Punctuator :: OptionalChainingPunctuator OtherPunctuator OptionalChainingPunctuator :: ?. [lookahead ∉ DecimalDigit] OtherPunctuator :: one of { ( ) [ ] . ... ; , < > <= >= == != === !== - * % ** -- << >> >>> & | ^ ! ~ && || ?? ? : = = -= *= %= **= <<= >>= >>>= &= |= ^= &&= ||= ??= => DivPunctuator :: / /= RightBracePunctuator :: } NullLiteral :: null BooleanLiteral :: true false NumericLiteralSeparator :: _ NumericLiteral :: DecimalLiteral DecimalBigIntegerLiteral NonDecimalIntegerLiteral[ Sep] NonDecimalIntegerLiteral[ Sep] BigIntLiteralSuffix LegacyOctalIntegerLiteral DecimalBigIntegerLiteral :: 0 BigIntLiteralSuffix NonZeroDigit DecimalDigits[ Sep]opt BigIntLiteralSuffix NonZeroDigit NumericLiteralSeparator DecimalDigits[ Sep] BigIntLiteralSuffix NonDecimalIntegerLiteral[Sep] :: BinaryIntegerLiteral[?Sep] OctalIntegerLiteral[?Sep] HexIntegerLiteral[?Sep] BigIntLiteralSuffix :: n DecimalLiteral :: DecimalIntegerLiteral . DecimalDigits[ Sep]opt ExponentPart[ Sep]opt . DecimalDigits[ Sep] ExponentPart[ Sep]opt DecimalIntegerLiteral ExponentPart[ Sep]opt DecimalIntegerLiteral :: 0 NonZeroDigit NonZeroDigit NumericLiteralSeparatoropt DecimalDigits[ Sep] NonOctalDecimalIntegerLiteral DecimalDigits[Sep] :: DecimalDigit DecimalDigits[?Sep] DecimalDigit [ Sep] DecimalDigits[ Sep] NumericLiteralSeparator DecimalDigit DecimalDigit :: one of 0 1 2 3 4 5 6 7 8 9 NonZeroDigit :: one of 1 2 3 4 5 6 7 8 9 ExponentPart[Sep] :: ExponentIndicator SignedInteger[?Sep] ExponentIndicator :: one of e E SignedInteger[Sep] :: DecimalDigits[?Sep] DecimalDigits[?Sep] - DecimalDigits[?Sep] BinaryIntegerLiteral[Sep] :: 0b BinaryDigits[?Sep] 0B BinaryDigits[?Sep] BinaryDigits[Sep] :: BinaryDigit BinaryDigits[?Sep] BinaryDigit [ Sep] BinaryDigits[ Sep] NumericLiteralSeparator BinaryDigit BinaryDigit :: one of 0 1 OctalIntegerLiteral[Sep] :: 0o OctalDigits[?Sep] 0O OctalDigits[?Sep] OctalDigits[Sep] :: OctalDigit OctalDigits[?Sep] OctalDigit [ Sep] OctalDigits[ Sep] NumericLiteralSeparator OctalDigit LegacyOctalIntegerLiteral :: 0 OctalDigit LegacyOctalIntegerLiteral OctalDigit NonOctalDecimalIntegerLiteral :: 0 NonOctalDigit LegacyOctalLikeDecimalIntegerLiteral NonOctalDigit NonOctalDecimalIntegerLiteral DecimalDigit LegacyOctalLikeDecimalIntegerLiteral :: 0 OctalDigit LegacyOctalLikeDecimalIntegerLiteral OctalDigit OctalDigit :: one of 0 1 2 3 4 5 6 7 NonOctalDigit :: one of 8 9 HexIntegerLiteral[Sep] :: 0x HexDigits[?Sep] 0X HexDigits[?Sep] HexDigits[Sep] :: HexDigit HexDigits[?Sep] HexDigit [ Sep] HexDigits[ Sep] NumericLiteralSeparator HexDigit HexDigit :: one of 0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F StringLiteral :: " DoubleStringCharactersopt " ' SingleStringCharactersopt ' DoubleStringCharacters :: DoubleStringCharacter DoubleStringCharactersopt SingleStringCharacters :: SingleStringCharacter SingleStringCharactersopt DoubleStringCharacter :: SourceCharacter but not one of " or \ or LineTerminator <LS> <PS> \ EscapeSequence LineContinuation SingleStringCharacter :: SourceCharacter but not one of ' or \ or LineTerminator <LS> <PS> \ EscapeSequence LineContinuation LineContinuation :: \ LineTerminatorSequence EscapeSequence :: CharacterEscapeSequence 0 [lookahead ∉ DecimalDigit] LegacyOctalEscapeSequence NonOctalDecimalEscapeSequence HexEscapeSequence UnicodeEscapeSequence CharacterEscapeSequence :: SingleEscapeCharacter NonEscapeCharacter SingleEscapeCharacter :: one of ' " \ b f n r t v NonEscapeCharacter :: SourceCharacter but not one of EscapeCharacter or LineTerminator EscapeCharacter :: SingleEscapeCharacter DecimalDigit x u LegacyOctalEscapeSequence :: 0 [lookahead ∈ { 8, 9 }] NonZeroOctalDigit [lookahead ∉ OctalDigit] ZeroToThree OctalDigit [lookahead ∉ OctalDigit] FourToSeven OctalDigit ZeroToThree OctalDigit OctalDigit NonZeroOctalDigit :: OctalDigit but not 0 ZeroToThree :: one of 0 1 2 3 FourToSeven :: one of 4 5 6 7 NonOctalDecimalEscapeSequence :: one of 8 9 HexEscapeSequence :: x HexDigit HexDigit UnicodeEscapeSequence :: u Hex4Digits u{ CodePoint } Hex4Digits :: HexDigit HexDigit HexDigit HexDigit RegularExpressionLiteral :: / RegularExpressionBody / RegularExpressionFlags RegularExpressionBody :: RegularExpressionFirstChar RegularExpressionChars RegularExpressionChars :: [empty] RegularExpressionChars RegularExpressionChar RegularExpressionFirstChar :: RegularExpressionNonTerminator but not one of * or \ or / or [ RegularExpressionBackslashSequence RegularExpressionClass RegularExpressionChar :: RegularExpressionNonTerminator but not one of \ or / or [ RegularExpressionBackslashSequence RegularExpressionClass RegularExpressionBackslashSequence :: \ RegularExpressionNonTerminator RegularExpressionNonTerminator :: SourceCharacter but not LineTerminator RegularExpressionClass :: [ RegularExpressionClassChars ] RegularExpressionClassChars :: [empty] RegularExpressionClassChars RegularExpressionClassChar RegularExpressionClassChar :: RegularExpressionNonTerminator but not one of ] or \ RegularExpressionBackslashSequence RegularExpressionFlags :: [empty] RegularExpressionFlags IdentifierPartChar Template :: NoSubstitutionTemplate TemplateHead NoSubstitutionTemplate :: ` TemplateCharactersopt ` TemplateHead :: ` TemplateCharactersopt ${ TemplateSubstitutionTail :: TemplateMiddle TemplateTail TemplateMiddle :: } TemplateCharactersopt ${ TemplateTail :: } TemplateCharactersopt ` TemplateCharacters :: TemplateCharacter TemplateCharactersopt TemplateCharacter :: $ [lookahead ≠ {] \ TemplateEscapeSequence \ NotEscapeSequence LineContinuation LineTerminatorSequence SourceCharacter but not one of ` or \ or $ or LineTerminator TemplateEscapeSequence :: CharacterEscapeSequence 0 [lookahead ∉ DecimalDigit] HexEscapeSequence UnicodeEscapeSequence NotEscapeSequence :: 0 DecimalDigit DecimalDigit but not 0 x [lookahead ∉ HexDigit] x HexDigit [lookahead ∉ HexDigit] u [lookahead ∉ HexDigit] [lookahead ≠ {] u HexDigit [lookahead ∉ HexDigit] u HexDigit HexDigit [lookahead ∉ HexDigit] u HexDigit HexDigit HexDigit [lookahead ∉ HexDigit] u { [lookahead ∉ HexDigit] u { NotCodePoint [lookahead ∉ HexDigit] u { CodePoint [lookahead ∉ HexDigit] [lookahead ≠ }] NotCodePoint :: HexDigits[~Sep] but only if MV of HexDigits > 0x10FFFF CodePoint :: HexDigits[~Sep] but only if MV of HexDigits ≤ 0x10FFFF

A.2 Expressions

IdentifierReference[Yield, Await] : Identifier [~Yield] yield [~Await] await BindingIdentifier[Yield, Await] : Identifier yield await LabelIdentifier[Yield, Await] : Identifier [~Yield] yield [~Await] await Identifier : IdentifierName but not ReservedWord PrimaryExpression[Yield, Await] : this IdentifierReference[?Yield, ?Await] Literal ArrayLiteral[?Yield, ?Await] ObjectLiteral[?Yield, ?Await] FunctionExpression ClassExpression[?Yield, ?Await] GeneratorExpression AsyncFunctionExpression AsyncGeneratorExpression RegularExpressionLiteral TemplateLiteral[?Yield, ?Await, ~Tagged] CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await] CoverParenthesizedExpressionAndArrowParameterList[Yield, Await] : ( Expression[ In, ?Yield, ?Await] ) ( Expression[ In, ?Yield, ?Await] , ) ( ) ( ... BindingIdentifier[?Yield, ?Await] ) ( ... BindingPattern[?Yield, ?Await] ) ( Expression[ In, ?Yield, ?Await] , ... BindingIdentifier[?Yield, ?Await] ) ( Expression[ In, ?Yield, ?Await] , ... BindingPattern[?Yield, ?Await] )

When processing an instance of the production
PrimaryExpression[Yield, Await] : CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await]
the interpretation of CoverParenthesizedExpressionAndArrowParameterList is refined using the following grammar:

ParenthesizedExpression[Yield, Await] : ( Expression[ In, ?Yield, ?Await] )

 

Literal : NullLiteral BooleanLiteral NumericLiteral StringLiteral ArrayLiteral[Yield, Await] : [ Elisionopt ] [ ElementList[?Yield, ?Await] ] [ ElementList[?Yield, ?Await] , Elisionopt ] ElementList[Yield, Await] : Elisionopt AssignmentExpression[ In, ?Yield, ?Await] Elisionopt SpreadElement[?Yield, ?Await] ElementList[?Yield, ?Await] , Elisionopt AssignmentExpression[ In, ?Yield, ?Await] ElementList[?Yield, ?Await] , Elisionopt SpreadElement[?Yield, ?Await] Elision : , Elision , SpreadElement[Yield, Await] : ... AssignmentExpression[ In, ?Yield, ?Await] ObjectLiteral[Yield, Await] : { } { PropertyDefinitionList[?Yield, ?Await] } { PropertyDefinitionList[?Yield, ?Await] , } PropertyDefinitionList[Yield, Await] : PropertyDefinition[?Yield, ?Await] PropertyDefinitionList[?Yield, ?Await] , PropertyDefinition[?Yield, ?Await] PropertyDefinition[Yield, Await] : IdentifierReference[?Yield, ?Await] CoverInitializedName[?Yield, ?Await] PropertyName[?Yield, ?Await] : AssignmentExpression[ In, ?Yield, ?Await] MethodDefinition[?Yield, ?Await] ... AssignmentExpression[ In, ?Yield, ?Await] PropertyName[Yield, Await] : LiteralPropertyName ComputedPropertyName[?Yield, ?Await] LiteralPropertyName : IdentifierName StringLiteral NumericLiteral ComputedPropertyName[Yield, Await] : [ AssignmentExpression[ In, ?Yield, ?Await] ] CoverInitializedName[Yield, Await] : IdentifierReference[?Yield, ?Await] Initializer[ In, ?Yield, ?Await] Initializer[In, Yield, Await] : = AssignmentExpression[?In, ?Yield, ?Await] TemplateLiteral[Yield, Await, Tagged] : NoSubstitutionTemplate SubstitutionTemplate[?Yield, ?Await, ?Tagged] SubstitutionTemplate[Yield, Await, Tagged] : TemplateHead Expression[ In, ?Yield, ?Await] TemplateSpans[?Yield, ?Await, ?Tagged] TemplateSpans[Yield, Await, Tagged] : TemplateTail TemplateMiddleList[?Yield, ?Await, ?Tagged] TemplateTail TemplateMiddleList[Yield, Await, Tagged] : TemplateMiddle Expression[ In, ?Yield, ?Await] TemplateMiddleList[?Yield, ?Await, ?Tagged] TemplateMiddle Expression[ In, ?Yield, ?Await] MemberExpression[Yield, Await] : PrimaryExpression[?Yield, ?Await] MemberExpression[?Yield, ?Await] [ Expression[ In, ?Yield, ?Await] ] MemberExpression[?Yield, ?Await] . IdentifierName MemberExpression[?Yield, ?Await] TemplateLiteral[?Yield, ?Await, Tagged] SuperProperty[?Yield, ?Await] MetaProperty new MemberExpression[?Yield, ?Await] Arguments[?Yield, ?Await] MemberExpression[?Yield, ?Await] . PrivateIdentifier SuperProperty[Yield, Await] : super [ Expression[ In, ?Yield, ?Await] ] super . IdentifierName MetaProperty : NewTarget ImportMeta NewTarget : new . target ImportMeta : import . meta NewExpression[Yield, Await] : MemberExpression[?Yield, ?Await] new NewExpression[?Yield, ?Await] CallExpression[Yield, Await] : CoverCallExpressionAndAsyncArrowHead[?Yield, ?Await] SuperCall[?Yield, ?Await] ImportCall[?Yield, ?Await] CallExpression[?Yield, ?Await] Arguments[?Yield, ?Await] CallExpression[?Yield, ?Await] [ Expression[ In, ?Yield, ?Await] ] CallExpression[?Yield, ?Await] . IdentifierName CallExpression[?Yield, ?Await] TemplateLiteral[?Yield, ?Await, Tagged] CallExpression[?Yield, ?Await] . PrivateIdentifier

When processing an instance of the production
CallExpression[Yield, Await] : CoverCallExpressionAndAsyncArrowHead[?Yield, ?Await]
the interpretation of CoverCallExpressionAndAsyncArrowHead is refined using the following grammar:

CallMemberExpression[Yield, Await] : MemberExpression[?Yield, ?Await] Arguments[?Yield, ?Await]

 

SuperCall[Yield, Await] : super Arguments[?Yield, ?Await] ImportCall[Yield, Await] : import ( AssignmentExpression[ In, ?Yield, ?Await] ) Arguments[Yield, Await] : ( ) ( ArgumentList[?Yield, ?Await] ) ( ArgumentList[?Yield, ?Await] , ) ArgumentList[Yield, Await] : AssignmentExpression[ In, ?Yield, ?Await] ... AssignmentExpression[ In, ?Yield, ?Await] ArgumentList[?Yield, ?Await] , AssignmentExpression[ In, ?Yield, ?Await] ArgumentList[?Yield, ?Await] , ... AssignmentExpression[ In, ?Yield, ?Await] OptionalExpression[Yield, Await] : MemberExpression[?Yield, ?Await] OptionalChain[?Yield, ?Await] CallExpression[?Yield, ?Await] OptionalChain[?Yield, ?Await] OptionalExpression[?Yield, ?Await] OptionalChain[?Yield, ?Await] OptionalChain[Yield, Await] : ?. Arguments[?Yield, ?Await] ?. [ Expression[ In, ?Yield, ?Await] ] ?. IdentifierName ?. TemplateLiteral[?Yield, ?Await, Tagged] ?. PrivateIdentifier OptionalChain[?Yield, ?Await] Arguments[?Yield, ?Await] OptionalChain[?Yield, ?Await] [ Expression[ In, ?Yield, ?Await] ] OptionalChain[?Yield, ?Await] . IdentifierName OptionalChain[?Yield, ?Await] TemplateLiteral[?Yield, ?Await, Tagged] OptionalChain[?Yield, ?Await] . PrivateIdentifier LeftHandSideExpression[Yield, Await] : NewExpression[?Yield, ?Await] CallExpression[?Yield, ?Await] OptionalExpression[?Yield, ?Await] UpdateExpression[Yield, Await] : LeftHandSideExpression[?Yield, ?Await] LeftHandSideExpression[?Yield, ?Await] [no LineTerminator here] LeftHandSideExpression[?Yield, ?Await] [no LineTerminator here] -- UnaryExpression[?Yield, ?Await] -- UnaryExpression[?Yield, ?Await] UnaryExpression[Yield, Await] : UpdateExpression[?Yield, ?Await] delete UnaryExpression[?Yield, ?Await] void UnaryExpression[?Yield, ?Await] typeof UnaryExpression[?Yield, ?Await] UnaryExpression[?Yield, ?Await] - UnaryExpression[?Yield, ?Await] ~ UnaryExpression[?Yield, ?Await] ! UnaryExpression[?Yield, ?Await] [ Await] AwaitExpression[?Yield] ExponentiationExpression[Yield, Await] : UnaryExpression[?Yield, ?Await] UpdateExpression[?Yield, ?Await] ** ExponentiationExpression[?Yield, ?Await] MultiplicativeExpression[Yield, Await] : ExponentiationExpression[?Yield, ?Await] MultiplicativeExpression[?Yield, ?Await] MultiplicativeOperator ExponentiationExpression[?Yield, ?Await] MultiplicativeOperator : one of * / % AdditiveExpression[Yield, Await] : MultiplicativeExpression[?Yield, ?Await] AdditiveExpression[?Yield, ?Await] MultiplicativeExpression[?Yield, ?Await] AdditiveExpression[?Yield, ?Await] - MultiplicativeExpression[?Yield, ?Await] ShiftExpression[Yield, Await] : AdditiveExpression[?Yield, ?Await] ShiftExpression[?Yield, ?Await] << AdditiveExpression[?Yield, ?Await] ShiftExpression[?Yield, ?Await] >> AdditiveExpression[?Yield, ?Await] ShiftExpression[?Yield, ?Await] >>> AdditiveExpression[?Yield, ?Await] RelationalExpression[In, Yield, Await] : ShiftExpression[?Yield, ?Await] RelationalExpression[?In, ?Yield, ?Await] < ShiftExpression[?Yield, ?Await] RelationalExpression[?In, ?Yield, ?Await] > ShiftExpression[?Yield, ?Await] RelationalExpression[?In, ?Yield, ?Await] <= ShiftExpression[?Yield, ?Await] RelationalExpression[?In, ?Yield, ?Await] >= ShiftExpression[?Yield, ?Await] RelationalExpression[?In, ?Yield, ?Await] instanceof ShiftExpression[?Yield, ?Await] [ In] RelationalExpression[ In, ?Yield, ?Await] in ShiftExpression[?Yield, ?Await] [ In] PrivateIdentifier in ShiftExpression[?Yield, ?Await] EqualityExpression[In, Yield, Await] : RelationalExpression[?In, ?Yield, ?Await] EqualityExpression[?In, ?Yield, ?Await] == RelationalExpression[?In, ?Yield, ?Await] EqualityExpression[?In, ?Yield, ?Await] != RelationalExpression[?In, ?Yield, ?Await] EqualityExpression[?In, ?Yield, ?Await] === RelationalExpression[?In, ?Yield, ?Await] EqualityExpression[?In, ?Yield, ?Await] !== RelationalExpression[?In, ?Yield, ?Await] BitwiseANDExpression[In, Yield, Await] : EqualityExpression[?In, ?Yield, ?Await] BitwiseANDExpression[?In, ?Yield, ?Await] & EqualityExpression[?In, ?Yield, ?Await] BitwiseXORExpression[In, Yield, Await] : BitwiseANDExpression[?In, ?Yield, ?Await] BitwiseXORExpression[?In, ?Yield, ?Await] ^ BitwiseANDExpression[?In, ?Yield, ?Await] BitwiseORExpression[In, Yield, Await] : BitwiseXORExpression[?In, ?Yield, ?Await] BitwiseORExpression[?In, ?Yield, ?Await] | BitwiseXORExpression[?In, ?Yield, ?Await] LogicalANDExpression[In, Yield, Await] : BitwiseORExpression[?In, ?Yield, ?Await] LogicalANDExpression[?In, ?Yield, ?Await] && BitwiseORExpression[?In, ?Yield, ?Await] LogicalORExpression[In, Yield, Await] : LogicalANDExpression[?In, ?Yield, ?Await] LogicalORExpression[?In, ?Yield, ?Await] || LogicalANDExpression[?In, ?Yield, ?Await] CoalesceExpression[In, Yield, Await] : CoalesceExpressionHead[?In, ?Yield, ?Await] ?? BitwiseORExpression[?In, ?Yield, ?Await] CoalesceExpressionHead[In, Yield, Await] : CoalesceExpression[?In, ?Yield, ?Await] BitwiseORExpression[?In, ?Yield, ?Await] ShortCircuitExpression[In, Yield, Await] : LogicalORExpression[?In, ?Yield, ?Await] CoalesceExpression[?In, ?Yield, ?Await] ConditionalExpression[In, Yield, Await] : ShortCircuitExpression[?In, ?Yield, ?Await] ShortCircuitExpression[?In, ?Yield, ?Await] ? AssignmentExpression[ In, ?Yield, ?Await] : AssignmentExpression[?In, ?Yield, ?Await] AssignmentExpression[In, Yield, Await] : ConditionalExpression[?In, ?Yield, ?Await] [ Yield] YieldExpression[?In, ?Await] ArrowFunction[?In, ?Yield, ?Await] AsyncArrowFunction[?In, ?Yield, ?Await] LeftHandSideExpression[?Yield, ?Await] = AssignmentExpression[?In, ?Yield, ?Await] LeftHandSideExpression[?Yield, ?Await] AssignmentOperator AssignmentExpression[?In, ?Yield, ?Await] LeftHandSideExpression[?Yield, ?Await] &&= AssignmentExpression[?In, ?Yield, ?Await] LeftHandSideExpression[?Yield, ?Await] ||= AssignmentExpression[?In, ?Yield, ?Await] LeftHandSideExpression[?Yield, ?Await] ??= AssignmentExpression[?In, ?Yield, ?Await] AssignmentOperator : one of *= /= %= = -= <<= >>= >>>= &= ^= |= **=

In certain circumstances when processing an instance of the production
AssignmentExpression[In, Yield, Await] : LeftHandSideExpression[?Yield, ?Await] = AssignmentExpression[?In, ?Yield, ?Await]
the interpretation of LeftHandSideExpression is refined using the following grammar:

AssignmentPattern[Yield, Await] : ObjectAssignmentPattern[?Yield, ?Await] ArrayAssignmentPattern[?Yield, ?Await] ObjectAssignmentPattern[Yield, Await] : { } { AssignmentRestProperty[?Yield, ?Await] } { AssignmentPropertyList[?Yield, ?Await] } { AssignmentPropertyList[?Yield, ?Await] , AssignmentRestProperty[?Yield, ?Await]opt } ArrayAssignmentPattern[Yield, Await] : [ Elisionopt AssignmentRestElement[?Yield, ?Await]opt ] [ AssignmentElementList[?Yield, ?Await] ] [ AssignmentElementList[?Yield, ?Await] , Elisionopt AssignmentRestElement[?Yield, ?Await]opt ] AssignmentRestProperty[Yield, Await] : ... DestructuringAssignmentTarget[?Yield, ?Await] AssignmentPropertyList[Yield, Await] : AssignmentProperty[?Yield, ?Await] AssignmentPropertyList[?Yield, ?Await] , AssignmentProperty[?Yield, ?Await] AssignmentElementList[Yield, Await] : AssignmentElisionElement[?Yield, ?Await] AssignmentElementList[?Yield, ?Await] , AssignmentElisionElement[?Yield, ?Await] AssignmentElisionElement[Yield, Await] : Elisionopt AssignmentElement[?Yield, ?Await] AssignmentProperty[Yield, Await] : IdentifierReference[?Yield, ?Await] Initializer[ In, ?Yield, ?Await]opt PropertyName[?Yield, ?Await] : AssignmentElement[?Yield, ?Await] AssignmentElement[Yield, Await] : DestructuringAssignmentTarget[?Yield, ?Await] Initializer[ In, ?Yield, ?Await]opt AssignmentRestElement[Yield, Await] : ... DestructuringAssignmentTarget[?Yield, ?Await] DestructuringAssignmentTarget[Yield, Await] : LeftHandSideExpression[?Yield, ?Await]

 

Expression[In, Yield, Await] : AssignmentExpression[?In, ?Yield, ?Await] Expression[?In, ?Yield, ?Await] , AssignmentExpression[?In, ?Yield, ?Await]

A.3 Statements

Statement[Yield, Await, Return] : BlockStatement[?Yield, ?Await, ?Return] VariableStatement[?Yield, ?Await] EmptyStatement ExpressionStatement[?Yield, ?Await] IfStatement[?Yield, ?Await, ?Return] BreakableStatement[?Yield, ?Await, ?Return] ContinueStatement[?Yield, ?Await] BreakStatement[?Yield, ?Await] [ Return] ReturnStatement[?Yield, ?Await] WithStatement[?Yield, ?Await, ?Return] LabelledStatement[?Yield, ?Await, ?Return] ThrowStatement[?Yield, ?Await] TryStatement[?Yield, ?Await, ?Return] DebuggerStatement Declaration[Yield, Await] : HoistableDeclaration[?Yield, ?Await, ~Default] ClassDeclaration[?Yield, ?Await, ~Default] LexicalDeclaration[ In, ?Yield, ?Await] HoistableDeclaration[Yield, Await, Default] : FunctionDeclaration[?Yield, ?Await, ?Default] GeneratorDeclaration[?Yield, ?Await, ?Default] AsyncFunctionDeclaration[?Yield, ?Await, ?Default] AsyncGeneratorDeclaration[?Yield, ?Await, ?Default] BreakableStatement[Yield, Await, Return] : IterationStatement[?Yield, ?Await, ?Return] SwitchStatement[?Yield, ?Await, ?Return] BlockStatement[Yield, Await, Return] : Block[?Yield, ?Await, ?Return] Block[Yield, Await, Return] : { StatementList[?Yield, ?Await, ?Return]opt } StatementList[Yield, Await, Return] : StatementListItem[?Yield, ?Await, ?Return] StatementList[?Yield, ?Await, ?Return] StatementListItem[?Yield, ?Await, ?Return] StatementListItem[Yield, Await, Return] : Statement[?Yield, ?Await, ?Return] Declaration[?Yield, ?Await] LexicalDeclaration[In, Yield, Await] : LetOrConst BindingList[?In, ?Yield, ?Await] ; LetOrConst : let const BindingList[In, Yield, Await] : LexicalBinding[?In, ?Yield, ?Await] BindingList[?In, ?Yield, ?Await] , LexicalBinding[?In, ?Yield, ?Await] LexicalBinding[In, Yield, Await] : BindingIdentifier[?Yield, ?Await] Initializer[?In, ?Yield, ?Await]opt BindingPattern[?Yield, ?Await] Initializer[?In, ?Yield, ?Await] VariableStatement[Yield, Await] : var VariableDeclarationList[ In, ?Yield, ?Await] ; VariableDeclarationList[In, Yield, Await] : VariableDeclaration[?In, ?Yield, ?Await] VariableDeclarationList[?In, ?Yield, ?Await] , VariableDeclaration[?In, ?Yield, ?Await] VariableDeclaration[In, Yield, Await] : BindingIdentifier[?Yield, ?Await] Initializer[?In, ?Yield, ?Await]opt BindingPattern[?Yield, ?Await] Initializer[?In, ?Yield, ?Await] BindingPattern[Yield, Await] : ObjectBindingPattern[?Yield, ?Await] ArrayBindingPattern[?Yield, ?Await] ObjectBindingPattern[Yield, Await] : { } { BindingRestProperty[?Yield, ?Await] } { BindingPropertyList[?Yield, ?Await] } { BindingPropertyList[?Yield, ?Await] , BindingRestProperty[?Yield, ?Await]opt } ArrayBindingPattern[Yield, Await] : [ Elisionopt BindingRestElement[?Yield, ?Await]opt ] [ BindingElementList[?Yield, ?Await] ] [ BindingElementList[?Yield, ?Await] , Elisionopt BindingRestElement[?Yield, ?Await]opt ] BindingRestProperty[Yield, Await] : ... BindingIdentifier[?Yield, ?Await] BindingPropertyList[Yield, Await] : BindingProperty[?Yield, ?Await] BindingPropertyList[?Yield, ?Await] , BindingProperty[?Yield, ?Await] BindingElementList[Yield, Await] : BindingElisionElement[?Yield, ?Await] BindingElementList[?Yield, ?Await] , BindingElisionElement[?Yield, ?Await] BindingElisionElement[Yield, Await] : Elisionopt BindingElement[?Yield, ?Await] BindingProperty[Yield, Await] : SingleNameBinding[?Yield, ?Await] PropertyName[?Yield, ?Await] : BindingElement[?Yield, ?Await] BindingElement[Yield, Await] : SingleNameBinding[?Yield, ?Await] BindingPattern[?Yield, ?Await] Initializer[ In, ?Yield, ?Await]opt SingleNameBinding[Yield, Await] : BindingIdentifier[?Yield, ?Await] Initializer[ In, ?Yield, ?Await]opt BindingRestElement[Yield, Await] : ... BindingIdentifier[?Yield, ?Await] ... BindingPattern[?Yield, ?Await] EmptyStatement : ; ExpressionStatement[Yield, Await] : [lookahead ∉ { {, function, async [no LineTerminator here] function, class, let [ }] Expression[ In, ?Yield, ?Await] ; IfStatement[Yield, Await, Return] : if ( Expression[ In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] else Statement[?Yield, ?Await, ?Return] if ( Expression[ In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] [lookahead ≠ else] IterationStatement[Yield, Await, Return] : DoWhileStatement[?Yield, ?Await, ?Return] WhileStatement[?Yield, ?Await, ?Return] ForStatement[?Yield, ?Await, ?Return] ForInOfStatement[?Yield, ?Await, ?Return] DoWhileStatement[Yield, Await, Return] : do Statement[?Yield, ?Await, ?Return] while ( Expression[ In, ?Yield, ?Await] ) ; WhileStatement[Yield, Await, Return] : while ( Expression[ In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] ForStatement[Yield, Await, Return] : for ( [lookahead ≠ let [] Expression[~In, ?Yield, ?Await]opt ; Expression[ In, ?Yield, ?Await]opt ; Expression[ In, ?Yield, ?Await]opt ) Statement[?Yield, ?Await, ?Return] for ( var VariableDeclarationList[~In, ?Yield, ?Await] ; Expression[ In, ?Yield, ?Await]opt ; Expression[ In, ?Yield, ?Await]opt ) Statement[?Yield, ?Await, ?Return] for ( LexicalDeclaration[~In, ?Yield, ?Await] Expression[ In, ?Yield, ?Await]opt ; Expression[ In, ?Yield, ?Await]opt ) Statement[?Yield, ?Await, ?Return] ForInOfStatement[Yield, Await, Return] : for ( [lookahead ≠ let [] LeftHandSideExpression[?Yield, ?Await] in Expression[ In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] for ( var ForBinding[?Yield, ?Await] in Expression[ In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] for ( ForDeclaration[?Yield, ?Await] in Expression[ In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] for ( [lookahead ∉ { let, async of }] LeftHandSideExpression[?Yield, ?Await] of AssignmentExpression[ In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] for ( var ForBinding[?Yield, ?Await] of AssignmentExpression[ In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] for ( ForDeclaration[?Yield, ?Await] of AssignmentExpression[ In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] [ Await] for await ( [lookahead ≠ let] LeftHandSideExpression[?Yield, ?Await] of AssignmentExpression[ In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] [ Await] for await ( var ForBinding[?Yield, ?Await] of AssignmentExpression[ In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] [ Await] for await ( ForDeclaration[?Yield, ?Await] of AssignmentExpression[ In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] ForDeclaration[Yield, Await] : LetOrConst ForBinding[?Yield, ?Await] ForBinding[Yield, Await] : BindingIdentifier[?Yield, ?Await] BindingPattern[?Yield, ?Await] ContinueStatement[Yield, Await] : continue ; continue [no LineTerminator here] LabelIdentifier[?Yield, ?Await] ; BreakStatement[Yield, Await] : break ; break [no LineTerminator here] LabelIdentifier[?Yield, ?Await] ; ReturnStatement[Yield, Await] : return ; return [no LineTerminator here] Expression[ In, ?Yield, ?Await] ; WithStatement[Yield, Await, Return] : with ( Expression[ In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] SwitchStatement[Yield, Await, Return] : switch ( Expression[ In, ?Yield, ?Await] ) CaseBlock[?Yield, ?Await, ?Return] CaseBlock[Yield, Await, Return] : { CaseClauses[?Yield, ?Await, ?Return]opt } { CaseClauses[?Yield, ?Await, ?Return]opt DefaultClause[?Yield, ?Await, ?Return] CaseClauses[?Yield, ?Await, ?Return]opt } CaseClauses[Yield, Await, Return] : CaseClause[?Yield, ?Await, ?Return] CaseClauses[?Yield, ?Await, ?Return] CaseClause[?Yield, ?Await, ?Return] CaseClause[Yield, Await, Return] : case Expression[ In, ?Yield, ?Await] : StatementList[?Yield, ?Await, ?Return]opt DefaultClause[Yield, Await, Return] : default : StatementList[?Yield, ?Await, ?Return]opt LabelledStatement[Yield, Await, Return] : LabelIdentifier[?Yield, ?Await] : LabelledItem[?Yield, ?Await, ?Return] LabelledItem[Yield, Await, Return] : Statement[?Yield, ?Await, ?Return] FunctionDeclaration[?Yield, ?Await, ~Default] ThrowStatement[Yield, Await] : throw [no LineTerminator here] Expression[ In, ?Yield, ?Await] ; TryStatement[Yield, Await, Return] : try Block[?Yield, ?Await, ?Return] Catch[?Yield, ?Await, ?Return] try Block[?Yield, ?Await, ?Return] Finally[?Yield, ?Await, ?Return] try Block[?Yield, ?Await, ?Return] Catch[?Yield, ?Await, ?Return] Finally[?Yield, ?Await, ?Return] Catch[Yield, Await, Return] : catch ( CatchParameter[?Yield, ?Await] ) Block[?Yield, ?Await, ?Return] catch Block[?Yield, ?Await, ?Return] Finally[Yield, Await, Return] : finally Block[?Yield, ?Await, ?Return] CatchParameter[Yield, Await] : BindingIdentifier[?Yield, ?Await] BindingPattern[?Yield, ?Await] DebuggerStatement : debugger ;

A.4 Functions and Classes

UniqueFormalParameters[Yield, Await] : FormalParameters[?Yield, ?Await] FormalParameters[Yield, Await] : [empty] FunctionRestParameter[?Yield, ?Await] FormalParameterList[?Yield, ?Await] FormalParameterList[?Yield, ?Await] , FormalParameterList[?Yield, ?Await] , FunctionRestParameter[?Yield, ?Await] FormalParameterList[Yield, Await] : FormalParameter[?Yield, ?Await] FormalParameterList[?Yield, ?Await] , FormalParameter[?Yield, ?Await] FunctionRestParameter[Yield, Await] : BindingRestElement[?Yield, ?Await] FormalParameter[Yield, Await] : BindingElement[?Yield, ?Await] FunctionDeclaration[Yield, Await, Default] : function BindingIdentifier[?Yield, ?Await] ( FormalParameters[~Yield, ~Await] ) { FunctionBody[~Yield, ~Await] } [ Default] function ( FormalParameters[~Yield, ~Await] ) { FunctionBody[~Yield, ~Await] } FunctionExpression : function BindingIdentifier[~Yield, ~Await]opt ( FormalParameters[~Yield, ~Await] ) { FunctionBody[~Yield, ~Await] } FunctionBody[Yield, Await] : FunctionStatementList[?Yield, ?Await] FunctionStatementList[Yield, Await] : StatementList[?Yield, ?Await, Return]opt ArrowFunction[In, Yield, Await] : ArrowParameters[?Yield, ?Await] [no LineTerminator here] => ConciseBody[?In] ArrowParameters[Yield, Await] : BindingIdentifier[?Yield, ?Await] CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await] ConciseBody[In] : [lookahead ≠ {] ExpressionBody[?In, ~Await] { FunctionBody[~Yield, ~Await] } ExpressionBody[In, Await] : AssignmentExpression[?In, ~Yield, ?Await]

When processing an instance of the production
ArrowParameters[Yield, Await] : CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await]
the interpretation of CoverParenthesizedExpressionAndArrowParameterList is refined using the following grammar:

ArrowFormalParameters[Yield, Await] : ( UniqueFormalParameters[?Yield, ?Await] )

 

AsyncArrowFunction[In, Yield, Await] : async [no LineTerminator here] AsyncArrowBindingIdentifier[?Yield] [no LineTerminator here] => AsyncConciseBody[?In] CoverCallExpressionAndAsyncArrowHead[?Yield, ?Await] [no LineTerminator here] => AsyncConciseBody[?In] AsyncConciseBody[In] : [lookahead ≠ {] ExpressionBody[?In, Await] { AsyncFunctionBody } AsyncArrowBindingIdentifier[Yield] : BindingIdentifier[?Yield, Await] CoverCallExpressionAndAsyncArrowHead[Yield, Await] : MemberExpression[?Yield, ?Await] Arguments[?Yield, ?Await]

When processing an instance of the production
AsyncArrowFunction[In, Yield, Await] : CoverCallExpressionAndAsyncArrowHead[?Yield, ?Await] [no LineTerminator here] => AsyncConciseBody[?In]
the interpretation of CoverCallExpressionAndAsyncArrowHead is refined using the following grammar:

AsyncArrowHead : async [no LineTerminator here] ArrowFormalParameters[~Yield, Await]

 

MethodDefinition[Yield, Await] : ClassElementName[?Yield, ?Await] ( UniqueFormalParameters[~Yield, ~Await] ) { FunctionBody[~Yield, ~Await] } GeneratorMethod[?Yield, ?Await] AsyncMethod[?Yield, ?Await] AsyncGeneratorMethod[?Yield, ?Await] get ClassElementName[?Yield, ?Await] ( ) { FunctionBody[~Yield, ~Await] } set ClassElementName[?Yield, ?Await] ( PropertySetParameterList ) { FunctionBody[~Yield, ~Await] } PropertySetParameterList : FormalParameter[~Yield, ~Await] GeneratorDeclaration[Yield, Await, Default] : function * BindingIdentifier[?Yield, ?Await] ( FormalParameters[ Yield, ~Await] ) { GeneratorBody } [ Default] function * ( FormalParameters[ Yield, ~Await] ) { GeneratorBody } GeneratorExpression : function * BindingIdentifier[ Yield, ~Await]opt ( FormalParameters[ Yield, ~Await] ) { GeneratorBody } GeneratorMethod[Yield, Await] : * ClassElementName[?Yield, ?Await] ( UniqueFormalParameters[ Yield, ~Await] ) { GeneratorBody } GeneratorBody : FunctionBody[ Yield, ~Await] YieldExpression[In, Await] : yield yield [no LineTerminator here] AssignmentExpression[?In, Yield, ?Await] yield [no LineTerminator here] * AssignmentExpression[?In, Yield, ?Await] AsyncGeneratorDeclaration[Yield, Await, Default] : async [no LineTerminator here] function * BindingIdentifier[?Yield, ?Await] ( FormalParameters[ Yield, Await] ) { AsyncGeneratorBody } [ Default] async [no LineTerminator here] function * ( FormalParameters[ Yield, Await] ) { AsyncGeneratorBody } AsyncGeneratorExpression : async [no LineTerminator here] function * BindingIdentifier[ Yield, Await]opt ( FormalParameters[ Yield, Await] ) { AsyncGeneratorBody } AsyncGeneratorMethod[Yield, Await] : async [no LineTerminator here] * ClassElementName[?Yield, ?Await] ( UniqueFormalParameters[ Yield, Await] ) { AsyncGeneratorBody } AsyncGeneratorBody : FunctionBody[ Yield, Await] AsyncFunctionDeclaration[Yield, Await, Default] : async [no LineTerminator here] function BindingIdentifier[?Yield, ?Await] ( FormalParameters[~Yield, Await] ) { AsyncFunctionBody } [ Default] async [no LineTerminator here] function ( FormalParameters[~Yield, Await] ) { AsyncFunctionBody } AsyncFunctionExpression : async [no LineTerminator here] function BindingIdentifier[~Yield, Await]opt ( FormalParameters[~Yield, Await] ) { AsyncFunctionBody } AsyncMethod[Yield, Await] : async [no LineTerminator here] ClassElementName[?Yield, ?Await] ( UniqueFormalParameters[~Yield, Await] ) { AsyncFunctionBody } AsyncFunctionBody : FunctionBody[~Yield, Await] AwaitExpression[Yield] : await UnaryExpression[?Yield, Await] ClassDeclaration[Yield, Await, Default] : class BindingIdentifier[?Yield, ?Await] ClassTail[?Yield, ?Await] [ Default] class ClassTail[?Yield, ?Await] ClassExpression[Yield, Await] : class BindingIdentifier[?Yield, ?Await]opt ClassTail[?Yield, ?Await] ClassTail[Yield, Await] : ClassHeritage[?Yield, ?Await]opt { ClassBody[?Yield, ?Await]opt } ClassHeritage[Yield, Await] : extends LeftHandSideExpression[?Yield, ?Await] ClassBody[Yield, Await] : ClassElementList[?Yield, ?Await] ClassElementList[Yield, Await] : ClassElement[?Yield, ?Await] ClassElementList[?Yield, ?Await] ClassElement[?Yield, ?Await] ClassElement[Yield, Await] : MethodDefinition[?Yield, ?Await] static MethodDefinition[?Yield, ?Await] FieldDefinition[?Yield, ?Await] ; static FieldDefinition[?Yield, ?Await] ; ClassStaticBlock ; FieldDefinition[Yield, Await] : ClassElementName[?Yield, ?Await] Initializer[ In, ?Yield, ?Await]opt ClassElementName[Yield, Await] : PropertyName[?Yield, ?Await] PrivateIdentifier ClassStaticBlock : static { ClassStaticBlockBody } ClassStaticBlockBody : ClassStaticBlockStatementList ClassStaticBlockStatementList : StatementList[~Yield, Await, ~Return]opt

A.5 Scripts and Modules

Script : ScriptBodyopt ScriptBody : StatementList[~Yield, ~Await, ~Return] Module : ModuleBodyopt ModuleBody : ModuleItemList ModuleItemList : ModuleItem ModuleItemList ModuleItem ModuleItem : ImportDeclaration ExportDeclaration StatementListItem[~Yield, Await, ~Return] ModuleExportName : IdentifierName StringLiteral ImportDeclaration : import ImportClause FromClause ; import ModuleSpecifier ; ImportClause : ImportedDefaultBinding NameSpaceImport NamedImports ImportedDefaultBinding , NameSpaceImport ImportedDefaultBinding , NamedImports ImportedDefaultBinding : ImportedBinding NameSpaceImport : * as ImportedBinding NamedImports : { } { ImportsList } { ImportsList , } FromClause : from ModuleSpecifier ImportsList : ImportSpecifier ImportsList , ImportSpecifier ImportSpecifier : ImportedBinding ModuleExportName as ImportedBinding ModuleSpecifier : StringLiteral ImportedBinding : BindingIdentifier[~Yield, Await] ExportDeclaration : export ExportFromClause FromClause ; export NamedExports ; export VariableStatement[~Yield, Await] export Declaration[~Yield, Await] export default HoistableDeclaration[~Yield, Await, Default] export default ClassDeclaration[~Yield, Await, Default] export default [lookahead ∉ { function, async [no LineTerminator here] function, class }] AssignmentExpression[ In, ~Yield, Await] ; ExportFromClause : * * as ModuleExportName NamedExports NamedExports : { } { ExportsList } { ExportsList , } ExportsList : ExportSpecifier ExportsList , ExportSpecifier ExportSpecifier : ModuleExportName ModuleExportName as ModuleExportName

A.6 Number Conversions

StringNumericLiteral ::: StrWhiteSpaceopt StrWhiteSpaceopt StrNumericLiteral StrWhiteSpaceopt StrWhiteSpace ::: StrWhiteSpaceChar StrWhiteSpaceopt StrWhiteSpaceChar ::: WhiteSpace LineTerminator StrNumericLiteral ::: StrDecimalLiteral NonDecimalIntegerLiteral[~Sep] StrDecimalLiteral ::: StrUnsignedDecimalLiteral StrUnsignedDecimalLiteral - StrUnsignedDecimalLiteral StrUnsignedDecimalLiteral ::: Infinity DecimalDigits[~Sep] . DecimalDigits[~Sep]opt ExponentPart[~Sep]opt . DecimalDigits[~Sep] ExponentPart[~Sep]opt DecimalDigits[~Sep] ExponentPart[~Sep]opt

All grammar symbols not explicitly defined by the StringNumericLiteral grammar have the definitions used in the Lexical Grammar for numeric literals.

StringIntegerLiteral ::: StrWhiteSpaceopt StrWhiteSpaceopt StrIntegerLiteral StrWhiteSpaceopt StrIntegerLiteral ::: SignedInteger[~Sep] NonDecimalIntegerLiteral[~Sep]

A.7 Time Zone Offset String Format

UTCOffset ::: TemporalSign Hour TemporalSign Hour HourSubcomponents[ Extended] TemporalSign Hour HourSubcomponents[~Extended] TemporalSign ::: ASCIISign <MINUS> ASCIISign ::: one of - Hour ::: 0 DecimalDigit 1 DecimalDigit 20 21 22 23 HourSubcomponents[Extended] ::: TimeSeparator[?Extended] MinuteSecond TimeSeparator[?Extended] MinuteSecond TimeSeparator[?Extended] MinuteSecond TemporalDecimalFractionopt TimeSeparator[Extended] ::: [ Extended] : [~Extended] [empty] MinuteSecond ::: 0 DecimalDigit 1 DecimalDigit 2 DecimalDigit 3 DecimalDigit 4 DecimalDigit 5 DecimalDigit TemporalDecimalFraction ::: TemporalDecimalSeparator DecimalDigit TemporalDecimalSeparator DecimalDigit DecimalDigit TemporalDecimalSeparator DecimalDigit DecimalDigit DecimalDigit TemporalDecimalSeparator DecimalDigit DecimalDigit DecimalDigit DecimalDigit TemporalDecimalSeparator DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit TemporalDecimalSeparator DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit TemporalDecimalSeparator DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit TemporalDecimalSeparator DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit TemporalDecimalSeparator DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit TemporalDecimalSeparator ::: one of . ,

A.8 Regular Expressions

Pattern[UnicodeMode, N] :: Disjunction[?UnicodeMode, ?N] Disjunction[UnicodeMode, N] :: Alternative[?UnicodeMode, ?N] Alternative[?UnicodeMode, ?N] | Disjunction[?UnicodeMode, ?N] Alternative[UnicodeMode, N] :: [empty] Alternative[?UnicodeMode, ?N] Term[?UnicodeMode, ?N] Term[UnicodeMode, N] :: Assertion[?UnicodeMode, ?N] Atom[?UnicodeMode, ?N] Atom[?UnicodeMode, ?N] Quantifier Assertion[UnicodeMode, N] :: ^ $ \b \B (?= Disjunction[?UnicodeMode, ?N] ) (?! Disjunction[?UnicodeMode, ?N] ) (?<= Disjunction[?UnicodeMode, ?N] ) (?<! Disjunction[?UnicodeMode, ?N] ) Quantifier :: QuantifierPrefix QuantifierPrefix ? QuantifierPrefix :: * ? { DecimalDigits[~Sep] } { DecimalDigits[~Sep] ,} { DecimalDigits[~Sep] , DecimalDigits[~Sep] } Atom[UnicodeMode, N] :: PatternCharacter . \ AtomEscape[?UnicodeMode, ?N] CharacterClass[?UnicodeMode] ( GroupSpecifier[?UnicodeMode]opt Disjunction[?UnicodeMode, ?N] ) (?: Disjunction[?UnicodeMode, ?N] ) SyntaxCharacter :: one of ^ $ \ . * ? ( ) [ ] { } | PatternCharacter :: SourceCharacter but not SyntaxCharacter AtomEscape[UnicodeMode, N] :: DecimalEscape CharacterClassEscape[?UnicodeMode] CharacterEscape[?UnicodeMode] [ N] k GroupName[?UnicodeMode] CharacterEscape[UnicodeMode] :: ControlEscape c AsciiLetter 0 [lookahead ∉ DecimalDigit] HexEscapeSequence RegExpUnicodeEscapeSequence[?UnicodeMode] IdentityEscape[?UnicodeMode] ControlEscape :: one of f n r t v GroupSpecifier[UnicodeMode] :: ? GroupName[?UnicodeMode] GroupName[UnicodeMode] :: < RegExpIdentifierName[?UnicodeMode] > RegExpIdentifierName[UnicodeMode] :: RegExpIdentifierStart[?UnicodeMode] RegExpIdentifierName[?UnicodeMode] RegExpIdentifierPart[?UnicodeMode] RegExpIdentifierStart[UnicodeMode] :: IdentifierStartChar \ RegExpUnicodeEscapeSequence[ UnicodeMode] [~UnicodeMode] UnicodeLeadSurrogate UnicodeTrailSurrogate RegExpIdentifierPart[UnicodeMode] :: IdentifierPartChar \ RegExpUnicodeEscapeSequence[ UnicodeMode] [~UnicodeMode] UnicodeLeadSurrogate UnicodeTrailSurrogate RegExpUnicodeEscapeSequence[UnicodeMode] :: [ UnicodeMode] u HexLeadSurrogate \u HexTrailSurrogate [ UnicodeMode] u HexLeadSurrogate [ UnicodeMode] u HexTrailSurrogate [ UnicodeMode] u HexNonSurrogate [~UnicodeMode] u Hex4Digits [ UnicodeMode] u{ CodePoint } UnicodeLeadSurrogate :: any Unicode code point in the inclusive interval from U D800 to U DBFF UnicodeTrailSurrogate :: any Unicode code point in the inclusive interval from U DC00 to U DFFF

Each \u HexTrailSurrogate for which the choice of associated u HexLeadSurrogate is ambiguous shall be associated with the nearest possible u HexLeadSurrogate that would otherwise have no corresponding \u HexTrailSurrogate.

 

HexLeadSurrogate :: Hex4Digits but only if the MV of Hex4Digits is in the inclusive interval from 0xD800 to 0xDBFF HexTrailSurrogate :: Hex4Digits but only if the MV of Hex4Digits is in the inclusive interval from 0xDC00 to 0xDFFF HexNonSurrogate :: Hex4Digits but only if the MV of Hex4Digits is not in the inclusive interval from 0xD800 to 0xDFFF IdentityEscape[UnicodeMode] :: [ UnicodeMode] SyntaxCharacter [ UnicodeMode] / [~UnicodeMode] SourceCharacter but not UnicodeIDContinue DecimalEscape :: NonZeroDigit DecimalDigits[~Sep]opt [lookahead ∉ DecimalDigit] CharacterClassEscape[UnicodeMode] :: d D s S w W [ UnicodeMode] p{ UnicodePropertyValueExpression } [ UnicodeMode] P{ UnicodePropertyValueExpression } UnicodePropertyValueExpression :: UnicodePropertyName = UnicodePropertyValue LoneUnicodePropertyNameOrValue UnicodePropertyName :: UnicodePropertyNameCharacters UnicodePropertyNameCharacters :: UnicodePropertyNameCharacter UnicodePropertyNameCharactersopt UnicodePropertyValue :: UnicodePropertyValueCharacters LoneUnicodePropertyNameOrValue :: UnicodePropertyValueCharacters UnicodePropertyValueCharacters :: UnicodePropertyValueCharacter UnicodePropertyValueCharactersopt UnicodePropertyValueCharacter :: UnicodePropertyNameCharacter DecimalDigit UnicodePropertyNameCharacter :: AsciiLetter _ CharacterClass[UnicodeMode] :: [ [lookahead ≠ ^] ClassRanges[?UnicodeMode] ] [^ ClassRanges[?UnicodeMode] ] ClassRanges[UnicodeMode] :: [empty] NonemptyClassRanges[?UnicodeMode] NonemptyClassRanges[UnicodeMode] :: ClassAtom[?UnicodeMode] ClassAtom[?UnicodeMode] NonemptyClassRangesNoDash[?UnicodeMode] ClassAtom[?UnicodeMode] - ClassAtom[?UnicodeMode] ClassRanges[?UnicodeMode] NonemptyClassRangesNoDash[UnicodeMode] :: ClassAtom[?UnicodeMode] ClassAtomNoDash[?UnicodeMode] NonemptyClassRangesNoDash[?UnicodeMode] ClassAtomNoDash[?UnicodeMode] - ClassAtom[?UnicodeMode] ClassRanges[?UnicodeMode] ClassAtom[UnicodeMode] :: - ClassAtomNoDash[?UnicodeMode] ClassAtomNoDash[UnicodeMode] :: SourceCharacter but not one of \ or ] or - \ ClassEscape[?UnicodeMode] ClassEscape[UnicodeMode] :: b [ UnicodeMode] - CharacterClassEscape[?UnicodeMode] CharacterEscape[?UnicodeMode]

B Additional ECMAScript Features for Web Browsers

The ECMAScript language syntax and semantics defined in this annex are required when the ECMAScript host is a web browser. The content of this annex is normative but optional if the ECMAScript host is not a web browser.

Note

This annex describes various legacy features and other characteristics of web browser ECMAScript hosts. All of the language features and behaviours specified in this annex have one or more undesirable characteristics and in the absence of legacy usage would be removed from this specification. However, the usage of these features by large numbers of existing web pages means that web browsers must continue to support them. The specifications in this annex define the requirements for interoperable implementations of these legacy features.

These features are not considered part of the core ECMAScript language. Programmers should not use or assume the existence of these features and behaviours when writing new ECMAScript code. ECMAScript implementations are discouraged from implementing these features unless the implementation is part of a web browser or is required to run the same legacy ECMAScript code that web browsers encounter.

B.1 Additional Syntax

B.1.1 HTML-like Comments

The syntax and semantics of 12.4 is extended as follows except that this extension is not allowed when parsing source text using the goal symbol Module:

Syntax

Comment :: MultiLineComment SingleLineComment SingleLineHTMLOpenComment SingleLineHTMLCloseComment SingleLineDelimitedComment MultiLineComment :: /* FirstCommentLineopt LineTerminator MultiLineCommentCharsopt */ HTMLCloseCommentopt FirstCommentLine :: SingleLineDelimitedCommentChars SingleLineHTMLOpenComment :: <!-- SingleLineCommentCharsopt SingleLineHTMLCloseComment :: LineTerminatorSequence HTMLCloseComment SingleLineDelimitedComment :: /* SingleLineDelimitedCommentCharsopt */ HTMLCloseComment :: WhiteSpaceSequenceopt SingleLineDelimitedCommentSequenceopt --> SingleLineCommentCharsopt SingleLineDelimitedCommentChars :: SingleLineNotAsteriskChar SingleLineDelimitedCommentCharsopt * SingleLinePostAsteriskCommentCharsopt SingleLineNotAsteriskChar :: SourceCharacter but not one of * or LineTerminator SingleLinePostAsteriskCommentChars :: SingleLineNotForwardSlashOrAsteriskChar SingleLineDelimitedCommentCharsopt * SingleLinePostAsteriskCommentCharsopt SingleLineNotForwardSlashOrAsteriskChar :: SourceCharacter but not one of / or * or LineTerminator WhiteSpaceSequence :: WhiteSpace WhiteSpaceSequenceopt SingleLineDelimitedCommentSequence :: SingleLineDelimitedComment WhiteSpaceSequenceopt SingleLineDelimitedCommentSequenceopt

Similar to a MultiLineComment that contains a line terminator code point, a SingleLineHTMLCloseComment is considered to be a LineTerminator for purposes of parsing by the syntactic grammar.

B.1.2 Regular Expressions Patterns

The syntax of 22.2.1 is modified and extended as follows. These changes introduce ambiguities that are broken by the ordering of grammar productions and by contextual information. When parsing using the following grammar, each alternative is considered only if previous production alternatives do not match.

This alternative pattern grammar and semantics only changes the syntax and semantics of BMP patterns. The following grammar extensions include productions parameterized with the [UnicodeMode] parameter. However, none of these extensions change the syntax of Unicode patterns recognized when parsing with the [UnicodeMode] parameter present on the goal symbol.

Syntax

Term[UnicodeMode, N] :: [ UnicodeMode] Assertion[ UnicodeMode, ?N] [ UnicodeMode] Atom[ UnicodeMode, ?N] Quantifier [ UnicodeMode] Atom[ UnicodeMode, ?N] [~UnicodeMode] QuantifiableAssertion[?N] Quantifier [~UnicodeMode] Assertion[~UnicodeMode, ?N] [~UnicodeMode] ExtendedAtom[?N] Quantifier [~UnicodeMode] ExtendedAtom[?N] Assertion[UnicodeMode, N] :: ^ $ \b \B [ UnicodeMode] (?= Disjunction[ UnicodeMode, ?N] ) [ UnicodeMode] (?! Disjunction[ UnicodeMode, ?N] ) [~UnicodeMode] QuantifiableAssertion[?N] (?<= Disjunction[?UnicodeMode, ?N] ) (?<! Disjunction[?UnicodeMode, ?N] ) QuantifiableAssertion[N] :: (?= Disjunction[~UnicodeMode, ?N] ) (?! Disjunction[~UnicodeMode, ?N] ) ExtendedAtom[N] :: . \ AtomEscape[~UnicodeMode, ?N] \ [lookahead = c] CharacterClass[~UnicodeMode] ( GroupSpecifier[~UnicodeMode]opt Disjunction[~UnicodeMode, ?N] ) (?: Disjunction[~UnicodeMode, ?N] ) InvalidBracedQuantifier ExtendedPatternCharacter InvalidBracedQuantifier :: { DecimalDigits[~Sep] } { DecimalDigits[~Sep] ,} { DecimalDigits[~Sep] , DecimalDigits[~Sep] } ExtendedPatternCharacter :: SourceCharacter but not one of ^ $ \ . * ? ( ) [ | AtomEscape[UnicodeMode, N] :: [ UnicodeMode] DecimalEscape [~UnicodeMode] DecimalEscape but only if the CapturingGroupNumber of DecimalEscape is ≤ CountLeftCapturingParensWithin(the Pattern containing DecimalEscape) CharacterClassEscape[?UnicodeMode] CharacterEscape[?UnicodeMode, ?N] [ N] k GroupName[?UnicodeMode] CharacterEscape[UnicodeMode, N] :: ControlEscape c AsciiLetter 0 [lookahead ∉ DecimalDigit] HexEscapeSequence RegExpUnicodeEscapeSequence[?UnicodeMode] [~UnicodeMode] LegacyOctalEscapeSequence IdentityEscape[?UnicodeMode, ?N] IdentityEscape[UnicodeMode, N] :: [ UnicodeMode] SyntaxCharacter [ UnicodeMode] / [~UnicodeMode] SourceCharacterIdentityEscape[?N] SourceCharacterIdentityEscape[N] :: [~N] SourceCharacter but not c [ N] SourceCharacter but not one of c or k ClassAtomNoDash[UnicodeMode, N] :: SourceCharacter but not one of \ or ] or - \ ClassEscape[?UnicodeMode, ?N] \ [lookahead = c] ClassEscape[UnicodeMode, N] :: b [ UnicodeMode] - [~UnicodeMode] c ClassControlLetter CharacterClassEscape[?UnicodeMode] CharacterEscape[?UnicodeMode, ?N] ClassControlLetter :: DecimalDigit _ Note

When the same left-hand sides occurs with both [ UnicodeMode] and [~UnicodeMode] guards it is to control the disambiguation priority.

B.1.2.1 Static Semantics: Early Errors

The semantics of 22.2.1.1 is extended as follows:

ExtendedAtom :: InvalidBracedQuantifier
  • It is a Syntax Error if any source text is matched by this production.

Additionally, the rules for the following productions are modified with the addition of the highlighted text:

NonemptyClassRanges :: ClassAtom - ClassAtom ClassRanges NonemptyClassRangesNoDash :: ClassAtomNoDash - ClassAtom ClassRanges

B.1.2.2 Static Semantics: CountLeftCapturingParensWithin and CountLeftCapturingParensBefore

In the definitions of CountLeftCapturingParensWithin and CountLeftCapturingParensBefore, references to “ Atom :: ( GroupSpecifieropt Disjunction ) ” are to be interpreted as meaning “ Atom :: ( GroupSpecifieropt Disjunction ) ” or “ ExtendedAtom :: ( GroupSpecifieropt Disjunction ) ”.

B.1.2.3 Static Semantics: IsCharacterClass

The semantics of 22.2.1.5 is extended as follows:

ClassAtomNoDash :: \ [lookahead = c]
  1. Return false.

B.1.2.4 Static Semantics: CharacterValue

The semantics of 22.2.1.6 is extended as follows:

ClassAtomNoDash :: \ [lookahead = c]
  1. Return the numeric value of U 005C (REVERSE SOLIDUS).
 ClassEscape :: c ClassControlLetter
  1. Let ch be the code point matched by ClassControlLetter.
  2. Let i be the numeric value of ch.
  3. Return the remainder of dividing i by 32.
 CharacterEscape :: LegacyOctalEscapeSequence
  1. Return the MV of LegacyOctalEscapeSequence (see 12.9.4.3).

B.1.2.5 Runtime Semantics: CompileSubpattern

The semantics of CompileSubpattern is extended as follows:

The rule for Term :: QuantifiableAssertion Quantifier is the same as for Term :: Atom Quantifier but with QuantifiableAssertion substituted for Atom.

The rule for Term :: ExtendedAtom Quantifier is the same as for Term :: Atom Quantifier but with ExtendedAtom substituted for Atom.

The rule for Term :: ExtendedAtom is the same as for Term :: Atom but with ExtendedAtom substituted for Atom.

B.1.2.6 Runtime Semantics: CompileAssertion

CompileAssertion rules for the Assertion :: (?= Disjunction ) and Assertion :: (?! Disjunction ) productions are also used for the QuantifiableAssertion productions, but with QuantifiableAssertion substituted for Assertion.

B.1.2.7 Runtime Semantics: CompileAtom

CompileAtom rules for the Atom productions except for Atom :: PatternCharacter are also used for the ExtendedAtom productions, but with ExtendedAtom substituted for Atom. The following rules, with parameter direction, are also added:

ExtendedAtom :: \ [lookahead = c]
  1. Let A be the CharSet containing the single character \ U 005C (REVERSE SOLIDUS).
  2. Return CharacterSetMatcher(rer, A, false, direction).
 ExtendedAtom :: ExtendedPatternCharacter
  1. Let ch be the character represented by ExtendedPatternCharacter.
  2. Let A be a one-element CharSet containing the character ch.
  3. Return CharacterSetMatcher(rer, A, false, direction).

B.1.2.8 Runtime Semantics: CompileToCharSet

The semantics of 22.2.2.9 is extended as follows:

The following two rules replace the corresponding rules of CompileToCharSet.

NonemptyClassRanges :: ClassAtom - ClassAtom ClassRanges
  1. Let A be CompileToCharSet of the first ClassAtom with argument rer.
  2. Let B be CompileToCharSet of the second ClassAtom with argument rer.
  3. Let C be CompileToCharSet of ClassRanges with argument rer.
  4. Let D be CharacterRangeOrUnion(rer, A, B).
  5. Return the union of D and C.
 NonemptyClassRangesNoDash :: ClassAtomNoDash - ClassAtom ClassRanges
  1. Let A be CompileToCharSet of ClassAtomNoDash with argument rer.
  2. Let B be CompileToCharSet of ClassAtom with argument rer.
  3. Let C be CompileToCharSet of ClassRanges with argument rer.
  4. Let D be CharacterRangeOrUnion(rer, A, B).
  5. Return the union of D and C.

In addition, the following rules are added to CompileToCharSet.

ClassEscape :: c ClassControlLetter
  1. Let cv be the CharacterValue of this ClassEscape.
  2. Let c be the character whose character value is cv.
  3. Return the CharSet containing the single character c.
 ClassAtomNoDash :: \ [lookahead = c]
  1. Return the CharSet containing the single character \ U 005C (REVERSE SOLIDUS).
 Note
This production can only be reached from the sequence \c within a character class where it is not followed by an acceptable control character.

B.1.2.8.1 CharacterRangeOrUnion ( rer, A, B )

The abstract operation CharacterRangeOrUnion takes arguments rer (a RegExp Record), A (a CharSet), and B (a CharSet) and returns a CharSet. It performs the following steps when called:

  1. If rer.[[Unicode]] is false, then
    1. If A does not contain exactly one character or B does not contain exactly one character, then
      1. Let C be the CharSet containing the single character - U 002D (HYPHEN-MINUS).
      2. Return the union of CharSets A, B and C.
  2. Return CharacterRange(A, B).

B.1.2.9 Static Semantics: ParsePattern ( patternText, u )

The semantics of 22.2.3.4 is extended as follows:

The abstract operation ParsePattern takes arguments patternText (a sequence of Unicode code points) and u (a Boolean). It performs the following steps when called:

  1. If u is true, then
    1. Let parseResult be ParseText(patternText, Pattern[ UnicodeMode, N]).
  2. Else,
    1. Let parseResult be ParseText(patternText, Pattern[~UnicodeMode, ~N]).
    2. If parseResult is a Parse Node and parseResult contains a GroupName, then
      1. Set parseResult to ParseText(patternText, Pattern[~UnicodeMode, N]).
  3. Return parseResult.

B.2 Additional Built-in Properties

When the ECMAScript host is a web browser the following additional properties of the standard built-in objects are defined.

B.2.1 Additional Properties of the Global Object

The entries in Table 91 are added to Table 6.

Table 91: Additional Well-known Intrinsic Objects
Intrinsic Name Global Name ECMAScript Language Association
%escape% escape The escape function (B.2.1.1)
%unescape% unescape The unescape function (B.2.1.2)

B.2.1.1 escape ( string )

This function is a property of the global object. It computes a new version of a String value in which certain code units have been replaced by a hexadecimal escape sequence.

When replacing a code unit of numeric value less than or equal to 0x00FF, a two-digit escape sequence of the form %xx is used. When replacing a code unit of numeric value strictly greater than 0x00FF, a four-digit escape sequence of the form %uxxxx is used.

It is the %escape% intrinsic object.

It performs the following steps when called:

  1. Set string to ? ToString(string).
  2. Let len be the length of string.
  3. Let R be the empty String.
  4. Let unescapedSet be the string-concatenation of the ASCII word characters and "@* -./".
  5. Let k be 0.
  6. Repeat, while k < len,
    1. Let C be the code unit at index k within string.
    2. If unescapedSet contains C, then
      1. Let S be C.
    3. Else,
      1. Let n be the numeric value of C.
      2. If n < 256, then
        1. Let hex be the String representation of n, formatted as an uppercase hexadecimal number.
        2. Let S be the string-concatenation of "%" and ! StringPad(hex, 2𝔽, "0", start).
      3. Else,
        1. Let hex be the String representation of n, formatted as an uppercase hexadecimal number.
        2. Let S be the string-concatenation of "%u" and ! StringPad(hex, 4𝔽, "0", start).
    4. Set R to the string-concatenation of R and S.
    5. Set k to k 1.
  7. Return R.
 Note

The encoding is partly based on the encoding described in RFC 1738, but the entire encoding specified in this standard is described above without regard to the contents of RFC 1738. This encoding does not reflect changes to RFC 1738 made by RFC 3986.

B.2.1.2 unescape ( string )

This function is a property of the global object. It computes a new version of a String value in which each escape sequence of the sort that might be introduced by the escape function is replaced with the code unit that it represents.

It is the %unescape% intrinsic object.

It performs the following steps when called:

  1. Set string to ? ToString(string).
  2. Let len be the length of string.
  3. Let R be the empty String.
  4. Let k be 0.
  5. Repeat, while k < len,
    1. Let C be the code unit at index k within string.
    2. If C is the code unit 0x0025 (PERCENT SIGN), then
      1. Let hexDigits be the empty String.
      2. Let optionalAdvance be 0.
      3. If k 5 < len and the code unit at index k 1 within string is the code unit 0x0075 (LATIN SMALL LETTER U), then
        1. Set hexDigits to the substring of string from k 2 to k 6.
        2. Set optionalAdvance to 5.
      4. Else if k 3 ≤ len, then
        1. Set hexDigits to the substring of string from k 1 to k 3.
        2. Set optionalAdvance to 2.
      5. Let parseResult be ParseText(StringToCodePoints(hexDigits), HexDigits[~Sep]).
      6. If parseResult is a Parse Node, then
        1. Let n be the MV of parseResult.
        2. Set C to the code unit whose numeric value is n.
        3. Set k to k optionalAdvance.
    3. Set R to the string-concatenation of R and C.
    4. Set k to k 1.
  6. Return R.

B.2.2 Additional Properties of the String.prototype Object

B.2.2.1 String.prototype.substr ( start, length )

This method returns a substring of the result of converting the this value to a String, starting from index start and running for length code units (or through the end of the String if length is undefined). If start is negative, it is treated as sourceLength start where sourceLength is the length of the String. The result is a String value, not a String object.

It performs the following steps when called:

  1. Let O be ? RequireObjectCoercible(this value).
  2. Let S be ? ToString(O).
  3. Let size be the length of S.
  4. Let intStart be ? ToIntegerOrInfinity(start).
  5. If intStart = -∞, set intStart to 0.
  6. Else if intStart < 0, set intStart to max(size intStart, 0).
  7. Else, set intStart to min(intStart, size).
  8. If length is undefined, let intLength be size; otherwise let intLength be ? ToIntegerOrInfinity(length).
  9. Set intLength to the result of clamping intLength between 0 and size.
  10. Let intEnd be min(intStart intLength, size).
  11. Return the substring of S from intStart to intEnd.
 Note

This method is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.

B.2.2.2 String.prototype.anchor ( name )

This method performs the following steps when called:

  1. Let S be the this value.
  2. Return ? CreateHTML(S, "a", "name", name).

B.2.2.2.1 CreateHTML ( string, tag, attribute, value )

The abstract operation CreateHTML takes arguments string (an ECMAScript language value), tag (a String), attribute (a String), and value (an ECMAScript language value) and returns either a normal completion containing a String or a throw completion. It performs the following steps when called:

  1. Let str be ? RequireObjectCoercible(string).
  2. Let S be ? ToString(str).
  3. Let p1 be the string-concatenation of "<" and tag.
  4. If attribute is not the empty String, then
    1. Let V be ? ToString(value).
    2. Let escapedV be the String value that is the same as V except that each occurrence of the code unit 0x0022 (QUOTATION MARK) in V has been replaced with the six code unit sequence "&quot;".
    3. Set p1 to the string-concatenation of:
      • p1
      • the code unit 0x0020 (SPACE)
      • attribute
      • the code unit 0x003D (EQUALS SIGN)
      • the code unit 0x0022 (QUOTATION MARK)
      • escapedV
      • the code unit 0x0022 (QUOTATION MARK)
  5. Let p2 be the string-concatenation of p1 and ">".
  6. Let p3 be the string-concatenation of p2 and S.
  7. Let p4 be the string-concatenation of p3, "</", tag, and ">".
  8. Return p4.

B.2.2.3 String.prototype.big ( )

This method performs the following steps when called:

  1. Let S be the this value.
  2. Return ? CreateHTML(S, "big", "", "").

B.2.2.4 String.prototype.blink ( )

This method performs the following steps when called:

  1. Let S be the this value.
  2. Return ? CreateHTML(S, "blink", "", "").

B.2.2.5 String.prototype.bold ( )

This method performs the following steps when called:

  1. Let S be the this value.
  2. Return ? CreateHTML(S, "b", "", "").

B.2.2.6 String.prototype.fixed ( )

This method performs the following steps when called:

  1. Let S be the this value.
  2. Return ? CreateHTML(S, "tt", "", "").

B.2.2.7 String.prototype.fontcolor ( color )

This method performs the following steps when called:

  1. Let S be the this value.
  2. Return ? CreateHTML(S, "font", "color", color).

B.2.2.8 String.prototype.fontsize ( size )

This method performs the following steps when called:

  1. Let S be the this value.
  2. Return ? CreateHTML(S, "font", "size", size).

B.2.2.9 String.prototype.italics ( )

This method performs the following steps when called:

  1. Let S be the this value.
  2. Return ? CreateHTML(S, "i", "", "").

B.2.2.10 String.prototype.link ( url )

This method performs the following steps when called:

  1. Let S be the this value.
  2. Return ? CreateHTML(S, "a", "href", url).

B.2.2.11 String.prototype.small ( )

This method performs the following steps when called:

  1. Let S be the this value.
  2. Return ? CreateHTML(S, "small", "", "").

B.2.2.12 String.prototype.strike ( )

This method performs the following steps when called:

  1. Let S be the this value.
  2. Return ? CreateHTML(S, "strike", "", "").

B.2.2.13 String.prototype.sub ( )

This method performs the following steps when called:

  1. Let S be the this value.
  2. Return ? CreateHTML(S, "sub", "", "").

B.2.2.14 String.prototype.sup ( )

This method performs the following steps when called:

  1. Let S be the this value.
  2. Return ? CreateHTML(S, "sup", "", "").

B.2.2.15 String.prototype.trimLeft ( )

Note

The property "trimStart" is preferred. The "trimLeft" property is provided principally for compatibility with old code. It is recommended that the "trimStart" property be used in new ECMAScript code.

The initial value of the "trimLeft" property is %String.prototype.trimStart%, defined in 22.1.3.32.

B.2.2.16 String.prototype.trimRight ( )

Note

The property "trimEnd" is preferred. The "trimRight" property is provided principally for compatibility with old code. It is recommended that the "trimEnd" property be used in new ECMAScript code.

The initial value of the "trimRight" property is %String.prototype.trimEnd%, defined in 22.1.3.31.

B.2.3 Additional Properties of the Date.prototype Object

B.2.3.1 Date.prototype.getYear ( )

Note

The getFullYear method is preferred for nearly all purposes, because it avoids the “year 2000 problem.”

This method performs the following steps when called:

  1. Let t be ? thisTimeValue(this value).
  2. If t is NaN, return NaN.
  3. Return YearFromTime(LocalTime(t)) - 1900𝔽.

B.2.3.2 Date.prototype.setYear ( year )

Note

The setFullYear method is preferred for nearly all purposes, because it avoids the “year 2000 problem.”

This method performs the following steps when called:

  1. Let t be ? thisTimeValue(this value).
  2. Let y be ? ToNumber(year).
  3. If t is NaN, set t to 0𝔽; otherwise, set t to LocalTime(t).
  4. If y is NaN, then
    1. Set the [[DateValue]] internal slot of this Date object to NaN.
    2. Return NaN.
  5. Let yi be ! ToIntegerOrInfinity(y).
  6. If 0 ≤ yi ≤ 99, let yyyy be 1900𝔽 𝔽(yi).
  7. Else, let yyyy be y.
  8. Let d be MakeDay(yyyy, MonthFromTime(t), DateFromTime(t)).
  9. Let date be UTC(MakeDate(d, TimeWithinDay(t))).
  10. Set the [[DateValue]] internal slot of this Date object to TimeClip(date).
  11. Return the value of the [[DateValue]] internal slot of this Date object.

B.2.3.3 Date.prototype.toGMTString ( )

Note

The toUTCString method is preferred. This method is provided principally for compatibility with old code.

The initial value of the "toGMTString" property is �te.prototype.toUTCString%, defined in 21.4.4.43.

B.2.4 Additional Properties of the RegExp.prototype Object

B.2.4.1 RegExp.prototype.compile ( pattern, flags )

This method performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? RequireInternalSlot(O, [[RegExpMatcher]]).
  3. If pattern is an Object and pattern has a [[RegExpMatcher]] internal slot, then
    1. If flags is not undefined, throw a TypeError exception.
    2. Let P be pattern.[[OriginalSource]].
    3. Let F be pattern.[[OriginalFlags]].
  4. Else,
    1. Let P be pattern.
    2. Let F be flags.
  5. Return ? RegExpInitialize(O, P, F).
 Note

This method completely reinitializes the this value RegExp with a new pattern and flags. An implementation may interpret use of this method as an assertion that the resulting RegExp object will be used multiple times and hence is a candidate for extra optimization.

B.3 Other Additional Features

B.3.1 Labelled Function Declarations

Prior to ECMAScript 2015, the specification of LabelledStatement did not allow for the association of a statement label with a FunctionDeclaration. However, a labelled FunctionDeclaration was an allowable extension for non-strict code and most browser-hosted ECMAScript implementations supported that extension. In ECMAScript 2015 and later, the grammar production for LabelledStatement permits use of FunctionDeclaration as a LabelledItem but 14.13.1 includes an Early Error rule that produces a Syntax Error if that occurs. That rule is modified with the addition of the highlighted text:

LabelledItem : FunctionDeclaration
  • It is a Syntax Error if any source text that is strict mode code is matched by this production.
Note

The early error rules for WithStatement, IfStatement, and IterationStatement prevent these statements from containing a labelled FunctionDeclaration in non-strict code.

B.3.2 Block-Level Function Declarations Web Legacy Compatibility Semantics

Prior to ECMAScript 2015, the ECMAScript specification did not define the occurrence of a FunctionDeclaration as an element of a Block statement's StatementList. However, support for that form of FunctionDeclaration was an allowable extension and most browser-hosted ECMAScript implementations permitted them. Unfortunately, the semantics of such declarations differ among those implementations. Because of these semantic differences, existing web ECMAScript source text that uses Block level function declarations is only portable among browser implementations if the usage only depends upon the semantic intersection of all of the browser implementations for such declarations. The following are the use cases that fall within that intersection semantics:

  1. A function is declared and only referenced within a single block.

    • One or more FunctionDeclarations whose BindingIdentifier is the name f occur within the function code of an enclosing function g and that declaration is nested within a Block.
    • No other declaration of f that is not a var declaration occurs within the function code of g.
    • All occurrences of f as an IdentifierReference are within the StatementList of the Block containing the declaration of f.

  2. A function is declared and possibly used within a single Block but also referenced by an inner function definition that is not contained within that same Block.

    • One or more FunctionDeclarations whose BindingIdentifier is the name f occur within the function code of an enclosing function g and that declaration is nested within a Block.
    • No other declaration of f that is not a var declaration occurs within the function code of g.
    • There may be occurrences of f as an IdentifierReference within the StatementList of the Block containing the declaration of f.
    • There is at least one occurrence of f as an IdentifierReference within another function h that is nested within g and no other declaration of f shadows the references to f from within h.
    • All invocations of h occur after the declaration of f has been evaluated.

  3. A function is declared and possibly used within a single block but also referenced within subsequent blocks.

    • One or more FunctionDeclaration whose BindingIdentifier is the name f occur within the function code of an enclosing function g and that declaration is nested within a Block.
    • No other declaration of f that is not a var declaration occurs within the function code of g.
    • There may be occurrences of f as an IdentifierReference within the StatementList of the Block containing the declaration of f.
    • There is at least one occurrence of f as an IdentifierReference within the function code of g that lexically follows the Block containing the declaration of f.

The first use case is interoperable with the semantics of Block level function declarations provided by ECMAScript 2015. Any pre-existing ECMAScript source text that employs that use case will operate using the Block level function declarations semantics defined by clauses 10, 14, and 15.

ECMAScript 2015 interoperability for the second and third use cases requires the following extensions to the clause 10, clause 15, clause 19.2.1 and clause 16.1.7 semantics.

If an ECMAScript implementation has a mechanism for reporting diagnostic warning messages, a warning should be produced when code contains a FunctionDeclaration for which these compatibility semantics are applied and introduce observable differences from non-compatibility semantics. For example, if a var binding is not introduced because its introduction would create an early error, a warning message should not be produced.

B.3.2.1 Changes to FunctionDeclarationInstantiation

During FunctionDeclarationInstantiation the following steps are performed in place of step 29:

  1. If strict is false, then
    1. For each FunctionDeclaration f that is directly contained in the StatementList of a Block, CaseClause, or DefaultClause, do
      1. Let F be StringValue of the BindingIdentifier of f.
      2. If replacing the FunctionDeclaration f with a VariableStatement that has F as a BindingIdentifier would not produce any Early Errors for func and parameterNames does not contain F, then
        1. NOTE: A var binding for F is only instantiated here if it is neither a VarDeclaredName, the name of a formal parameter, or another FunctionDeclaration.
        2. If initializedBindings does not contain F and F is not "arguments", then
          1. Perform ! varEnv.CreateMutableBinding(F, false).
          2. Perform ! varEnv.InitializeBinding(F, undefined).
          3. Append F to instantiatedVarNames.
        3. When the FunctionDeclaration f is evaluated, perform the following steps in place of the FunctionDeclaration Evaluation algorithm provided in 15.2.6:
          1. Let fenv be the running execution context's VariableEnvironment.
          2. Let benv be the running execution context's LexicalEnvironment.
          3. Let fobj be ! benv.GetBindingValue(F, false).
          4. Perform ! fenv.SetMutableBinding(F, fobj, false).
          5. Return unused.

B.3.2.2 Changes to GlobalDeclarationInstantiation

During GlobalDeclarationInstantiation the following steps are performed in place of step 12:

  1. Perform the following steps:
    1. Let strict be IsStrict of script.
    2. If strict is false, then
      1. Let declaredFunctionOrVarNames be the list-concatenation of declaredFunctionNames and declaredVarNames.
      2. For each FunctionDeclaration f that is directly contained in the StatementList of a Block, CaseClause, or DefaultClause Contained within script, do
        1. Let F be StringValue of the BindingIdentifier of f.
        2. If replacing the FunctionDeclaration f with a VariableStatement that has F as a BindingIdentifier would not produce any Early Errors for script, then
          1. If env.HasLexicalDeclaration(F) is false, then
            1. Let fnDefinable be ? env.CanDeclareGlobalVar(F).
            2. If fnDefinable is true, then
              1. NOTE: A var binding for F is only instantiated here if it is neither a VarDeclaredName nor the name of another FunctionDeclaration.
              2. If declaredFunctionOrVarNames does not contain F, then
                1. Perform ? env.CreateGlobalVarBinding(F, false).
                2. Append F to declaredFunctionOrVarNames.
              3. When the FunctionDeclaration f is evaluated, perform the following steps in place of the FunctionDeclaration Evaluation algorithm provided in 15.2.6:
                1. Let genv be the running execution context's VariableEnvironment.
                2. Let benv be the running execution context's LexicalEnvironment.
                3. Let fobj be ! benv.GetBindingValue(F, false).
                4. Perform ? genv.SetMutableBinding(F, fobj, false).
                5. Return unused.

B.3.2.3 Changes to EvalDeclarationInstantiation

During EvalDeclarationInstantiation the following steps are performed in place of step 11:

  1. If strict is false, then
    1. Let declaredFunctionOrVarNames be the list-concatenation of declaredFunctionNames and declaredVarNames.
    2. For each FunctionDeclaration f that is directly contained in the StatementList of a Block, CaseClause, or DefaultClause Contained within body, do
      1. Let F be StringValue of the BindingIdentifier of f.
      2. If replacing the FunctionDeclaration f with a VariableStatement that has F as a BindingIdentifier would not produce any Early Errors for body, then
        1. Let bindingExists be false.
        2. Let thisEnv be lexEnv.
        3. Assert: The following loop will terminate.
        4. Repeat, while thisEnv is not varEnv,
          1. If thisEnv is not an Object Environment Record, then
            1. If ! thisEnv.HasBinding(F) is true, then
              1. Let bindingExists be true.
          2. Set thisEnv to thisEnv.[[OuterEnv]].
        5. If bindingExists is false and varEnv is a Global Environment Record, then
          1. If varEnv.HasLexicalDeclaration(F) is false, then
            1. Let fnDefinable be ? varEnv.CanDeclareGlobalVar(F).
          2. Else,
            1. Let fnDefinable be false.
        6. Else,
          1. Let fnDefinable be true.
        7. If bindingExists is false and fnDefinable is true, then
          1. If declaredFunctionOrVarNames does not contain F, then
            1. If varEnv is a Global Environment Record, then
              1. Perform ? varEnv.CreateGlobalVarBinding(F, true).
            2. Else,
              1. Let bindingExists be ! varEnv.HasBinding(F).
              2. If bindingExists is false, then
                1. Perform ! varEnv.CreateMutableBinding(F, true).
                2. Perform ! varEnv.InitializeBinding(F, undefined).
            3. Append F to declaredFunctionOrVarNames.
          2. When the FunctionDeclaration f is evaluated, perform the following steps in place of the FunctionDeclaration Evaluation algorithm provided in 15.2.6:
            1. Let genv be the running execution context's VariableEnvironment.
            2. Let benv be the running execution context's LexicalEnvironment.
            3. Let fobj be ! benv.GetBindingValue(F, false).
            4. Perform ? genv.SetMutableBinding(F, fobj, false).
            5. Return unused.

B.3.2.4 Changes to Block Static Semantics: Early Errors

The rules for the following production in 14.2.1 are modified with the addition of the highlighted text:

Block : { StatementList }

B.3.2.5 Changes to switch Statement Static Semantics: Early Errors

The rules for the following production in 14.12.1 are modified with the addition of the highlighted text:

SwitchStatement : switch ( Expression ) CaseBlock

B.3.2.6 Changes to BlockDeclarationInstantiation

During BlockDeclarationInstantiation the following steps are performed in place of step 3.a.ii.1:

  1. If ! env.HasBinding(dn) is false, then
    1. Perform ! env.CreateMutableBinding(dn, false).

During BlockDeclarationInstantiation the following steps are performed in place of step 3.b.iii:

  1. Perform the following steps:
    1. If the binding for fn in env is an uninitialized binding, then
      1. Perform ! env.InitializeBinding(fn, fo).
    2. Else,
      1. Assert: d is a FunctionDeclaration.
      2. Perform ! env.SetMutableBinding(fn, fo, false).

B.3.3 FunctionDeclarations in IfStatement Statement Clauses

The following augments the IfStatement production in 14.6:

IfStatement[Yield, Await, Return] : if ( Expression[ In, ?Yield, ?Await] ) FunctionDeclaration[?Yield, ?Await, ~Default] else Statement[?Yield, ?Await, ?Return] if ( Expression[ In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] else FunctionDeclaration[?Yield, ?Await, ~Default] if ( Expression[ In, ?Yield, ?Await] ) FunctionDeclaration[?Yield, ?Await, ~Default] else FunctionDeclaration[?Yield, ?Await, ~Default] if ( Expression[ In, ?Yield, ?Await] ) FunctionDeclaration[?Yield, ?Await, ~Default] [lookahead ≠ else]

This production only applies when parsing non-strict code. Source text matched by this production is processed as if each matching occurrence of FunctionDeclaration[?Yield, ?Await, ~Default] was the sole StatementListItem of a BlockStatement occupying that position in the source text. The semantics of such a synthetic BlockStatement includes the web legacy compatibility semantics specified in B.3.2.

B.3.4 VariableStatements in Catch Blocks

The content of subclause 14.15.1 is replaced with the following:

Catch : catch ( CatchParameter ) Block Note

The Block of a Catch clause may contain var declarations that bind a name that is also bound by the CatchParameter. At runtime, such bindings are instantiated in the VariableDeclarationEnvironment. They do not shadow the same-named bindings introduced by the CatchParameter and hence the Initializer for such var declarations will assign to the corresponding catch parameter rather than the var binding.

This modified behaviour also applies to var and function declarations introduced by direct eval calls contained within the Block of a Catch clause. This change is accomplished by modifying the algorithm of 19.2.1.3 as follows:

Step 3.d.i.2.a.i is replaced by:

  1. If thisEnv is not the Environment Record for a Catch clause, throw a SyntaxError exception.

Step 11.b.ii.4.a.i.i is replaced by:

  1. If thisEnv is not the Environment Record for a Catch clause, let bindingExists be true.

B.3.5 Initializers in ForIn Statement Heads

The following augments the ForInOfStatement production in 14.7.5:

ForInOfStatement[Yield, Await, Return] : for ( var BindingIdentifier[?Yield, ?Await] Initializer[~In, ?Yield, ?Await] in Expression[ In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return]

This production only applies when parsing non-strict code.

The static semantics of ContainsDuplicateLabels in 8.3.1 are augmented with the following:

ForInOfStatement : for ( var BindingIdentifier Initializer in Expression ) Statement
  1. Return ContainsDuplicateLabels of Statement with argument labelSet.

The static semantics of ContainsUndefinedBreakTarget in 8.3.2 are augmented with the following:

ForInOfStatement : for ( var BindingIdentifier Initializer in Expression ) Statement
  1. Return ContainsUndefinedBreakTarget of Statement with argument labelSet.

The static semantics of ContainsUndefinedContinueTarget in 8.3.3 are augmented with the following:

ForInOfStatement : for ( var BindingIdentifier Initializer in Expression ) Statement
  1. Return ContainsUndefinedContinueTarget of Statement with arguments iterationSet and « ».

The static semantics of IsDestructuring in 14.7.5.2 are augmented with the following:

BindingIdentifier : Identifier yield await
  1. Return false.

The static semantics of VarDeclaredNames in 8.2.6 are augmented with the following:

ForInOfStatement : for ( var BindingIdentifier Initializer in Expression ) Statement
  1. Let names1 be the BoundNames of BindingIdentifier.
  2. Let names2 be the VarDeclaredNames of Statement.
  3. Return the list-concatenation of names1 and names2.

The static semantics of VarScopedDeclarations in 8.2.7 are augmented with the following:

ForInOfStatement : for ( var BindingIdentifier Initializer in Expression ) Statement
  1. Let declarations1 be « BindingIdentifier ».
  2. Let declarations2 be the VarScopedDeclarations of Statement.
  3. Return the list-concatenation of declarations1 and declarations2.

The runtime semantics of ForInOfLoopEvaluation in 14.7.5.5 are augmented with the following:

ForInOfStatement : for ( var BindingIdentifier Initializer in Expression ) Statement
  1. Let bindingId be StringValue of BindingIdentifier.
  2. Let lhs be ? ResolveBinding(bindingId).
  3. If IsAnonymousFunctionDefinition(Initializer) is true, then
    1. Let value be ? NamedEvaluation of Initializer with argument bindingId.
  4. Else,
    1. Let rhs be ? Evaluation of Initializer.
    2. Let value be ? GetValue(rhs).
  5. Perform ? PutValue(lhs, value).
  6. Let keyResult be ? ForIn/OfHeadEvaluation(« », Expression, enumerate).
  7. Return ? ForIn/OfBodyEvaluation(BindingIdentifier, Statement, keyResult, enumerate, varBinding, labelSet).

B.3.6 The [[IsHTMLDDA]] Internal Slot

An [[IsHTMLDDA]] internal slot may exist on host-defined objects. Objects with an [[IsHTMLDDA]] internal slot behave like undefined in the ToBoolean and IsLooselyEqual abstract operations and when used as an operand for the typeof operator.

Note

Objects with an [[IsHTMLDDA]] internal slot are never created by this specification. However, the document.all object in web browsers is a host-defined exotic object with this slot that exists for web compatibility purposes. There are no other known examples of this type of object and implementations should not create any with the exception of document.all.

B.3.6.1 Changes to ToBoolean

The following step replaces step 3 of ToBoolean:

  1. If argument is an Object and argument has an [[IsHTMLDDA]] internal slot, return false.

B.3.6.2 Changes to IsLooselyEqual

The following steps replace step 4 of IsLooselyEqual:

  1. Perform the following steps:
    1. If x is an Object, x has an [[IsHTMLDDA]] internal slot, and y is either null or undefined, return true.
    2. If x is either null or undefined, y is an Object, and y has an [[IsHTMLDDA]] internal slot, return true.

B.3.6.3 Changes to the typeof Operator

The following step replaces step 12 of the evaluation semantics for typeof:

  1. If val has an [[IsHTMLDDA]] internal slot, return "undefined".

B.3.7 Non-default behaviour in HostMakeJobCallback

The HostMakeJobCallback abstract operation allows hosts which are web browsers to specify non-default behaviour.

B.3.8 Non-default behaviour in HostEnsureCanAddPrivateElement

The HostEnsureCanAddPrivateElement abstract operation allows hosts which are web browsers to specify non-default behaviour.

C The Strict Mode of ECMAScript

The strict mode restriction and exceptions

D Host Layering Points

See 4.2 for the definition of host.

D.1 Host Hooks

HostCallJobCallback(...)

HostEnqueueFinalizationRegistryCleanupJob(...)

HostEnqueuePromiseJob(...)

HostEnsureCanCompileStrings(...)

HostFinalizeImportMeta(...)

HostGetImportMetaProperties(...)

HostHasSourceTextAvailable(...)

HostLoadImportedModule(...)

HostMakeJobCallback(...)

HostPromiseRejectionTracker(...)

InitializeHostDefinedRealm(...)

D.2 Host-defined Fields

[[HostDefined]] on Realm Records: See Table 24.

[[HostDefined]] on Script Records: See Table 39.

[[HostDefined]] on Module Records: See Table 40.

[[HostDefined]] on JobCallback Records: See Table 28.

[[HostSynchronizesWith]] on Candidate Executions: See Table 90.

[[IsHTMLDDA]]: See B.3.6.

D.3 Host-defined Objects

The global object: See clause 19.

D.4 Running Jobs

Preparation steps before, and cleanup steps after, invocation of Job Abstract Closures. See 9.5.

D.5 Internal Methods of Exotic Objects

Any of the essential internal methods in Table 4 for any exotic object not specified within this specification.

D.6 Built-in Objects and Methods

Any built-in objects and methods not defined within this specification, except as restricted in 17.1.

E Corrections and Clarifications in ECMAScript 2015 with Possible Compatibility Impact

9.1.1.4.15-9.1.1.4.18 Edition 5 and 5.1 used a property existence test to determine whether a global object property corresponding to a new global declaration already existed. ECMAScript 2015 uses an own property existence test. This corresponds to what has been most commonly implemented by web browsers.

10.4.2.1: The 5th Edition moved the capture of the current array length prior to the integer conversion of the array index or new length value. However, the captured length value could become invalid if the conversion process has the side-effect of changing the array length. ECMAScript 2015 specifies that the current array length must be captured after the possible occurrence of such side-effects.

21.4.1.17: Previous editions permitted the TimeClip abstract operation to return either 0𝔽 or -0𝔽 as the representation of a 0 time value. ECMAScript 2015 specifies that 0𝔽 always returned. This means that for ECMAScript 2015 the time value of a Date is never observably -0𝔽 and methods that return time values never return -0𝔽.

21.4.1.18: If a UTC offset representation is not present, the local time zone is used. Edition 5.1 incorrectly stated that a missing time zone should be interpreted as "z".

21.4.4.36: If the year cannot be represented using the Date Time String Format specified in 21.4.1.18 a RangeError exception is thrown. Previous editions did not specify the behaviour for that case.

21.4.4.41: Previous editions did not specify the value returned by Date.prototype.toString when this time value is NaN. ECMAScript 2015 specifies the result to be the String value "Invalid Date".

22.2.4.1, 22.2.6.13.1: Any LineTerminator code points in the value of the "source" property of a RegExp instance must be expressed using an escape sequence. Edition 5.1 only required the escaping of /.

22.2.6.8, 22.2.6.11: In previous editions, the specifications for String.prototype.match and String.prototype.replace was incorrect for cases where the pattern argument was a RegExp value whose global flag is set. The previous specifications stated that for each attempt to match the pattern, if lastIndex did not change, it should be incremented by 1. The correct behaviour is that lastIndex should be incremented by 1 only if the pattern matched the empty String.

23.1.3.30: Previous editions did not specify how a NaN value returned by a comparefn was interpreted by Array.prototype.sort. ECMAScript 2015 specifies that such as value is treated as if 0𝔽 was returned from the comparefn. ECMAScript 2015 also specifies that ToNumber is applied to the result returned by a comparefn. In previous editions, the effect of a comparefn result that is not a Number value was implementation-defined. In practice, implementations call ToNumber.

F Additions and Changes That Introduce Incompatibilities with Prior Editions

6.2.5: In ECMAScript 2015, Function calls are not allowed to return a Reference Record.

7.1.4.1: In ECMAScript 2015, ToNumber applied to a String value now recognizes and converts BinaryIntegerLiteral and OctalIntegerLiteral numeric strings. In previous editions such strings were converted to NaN.

9.3: In ECMAScript 2018, Template objects are canonicalized based on Parse Node (source location), instead of across all occurrences of that template literal or tagged template in a Realm in previous editions.

12.2: In ECMAScript 2016, Unicode 8.0.0 or higher is mandated, as opposed to ECMAScript 2015 which mandated Unicode 5.1. In particular, this caused U 180E MONGOLIAN VOWEL SEPARATOR, which was in the Space_Separator (Zs) category and thus treated as whitespace in ECMAScript 2015, to be moved to the Format (Cf) category (as of Unicode 6.3.0). This causes whitespace-sensitive methods to behave differently. For example, "\u180E".trim().length was 0 in previous editions, but 1 in ECMAScript 2016 and later. Additionally, ECMAScript 2017 mandated always using the latest version of the Unicode Standard.

12.7: In ECMAScript 2015, the valid code points for an IdentifierName are specified in terms of the Unicode properties “ID_Start” and “ID_Continue”. In previous editions, the valid IdentifierName or Identifier code points were specified by enumerating various Unicode code point categories.

12.10.1: In ECMAScript 2015, Automatic Semicolon Insertion adds a semicolon at the end of a do-while statement if the semicolon is missing. This change aligns the specification with the actual behaviour of most existing implementations.

13.2.5.1: In ECMAScript 2015, it is no longer an early error to have duplicate property names in Object Initializers.

13.15.1: In ECMAScript 2015, strict mode code containing an assignment to an immutable binding such as the function name of a FunctionExpression does not produce an early error. Instead it produces a runtime error.

14.2: In ECMAScript 2015, a StatementList beginning with the token let followed by the input elements LineTerminator then Identifier is the start of a LexicalDeclaration. In previous editions, automatic semicolon insertion would always insert a semicolon before the Identifier input element.

14.5: In ECMAScript 2015, a StatementListItem beginning with the token let followed by the token [ is the start of a LexicalDeclaration. In previous editions such a sequence would be the start of an ExpressionStatement.

14.6.2: In ECMAScript 2015, the normal result of an IfStatement is never the value empty. If no Statement part is evaluated or if the evaluated Statement part produces a normal completion containing empty, the result of the IfStatement is undefined.

14.7: In ECMAScript 2015, if the ( token of a for statement is immediately followed by the token sequence let [ then the let is treated as the start of a LexicalDeclaration. In previous editions such a token sequence would be the start of an Expression.

14.7: In ECMAScript 2015, if the ( token of a for-in statement is immediately followed by the token sequence let [ then the let is treated as the start of a ForDeclaration. In previous editions such a token sequence would be the start of an LeftHandSideExpression.

14.7: Prior to ECMAScript 2015, an initialization expression could appear as part of the VariableDeclaration that precedes the in keyword. In ECMAScript 2015, the ForBinding in that same position does not allow the occurrence of such an initializer. In ECMAScript 2017, such an initializer is permitted only in non-strict code.

14.7: In ECMAScript 2015, the result of evaluating an IterationStatement is never a normal completion whose [[Value]] is empty. If the Statement part of an IterationStatement is not evaluated or if the final evaluation of the Statement part produces a normal completion whose [[Value]] is empty, the result of evaluating the IterationStatement is a normal completion whose [[Value]] is undefined.

14.11.2: In ECMAScript 2015, the result of evaluating a WithStatement is never a normal completion whose [[Value]] is empty. If evaluation of the Statement part of a WithStatement produces a normal completion whose [[Value]] is empty, the result of evaluating the WithStatement is a normal completion whose [[Value]] is undefined.

14.12.4: In ECMAScript 2015, the result of evaluating a SwitchStatement is never a normal completion whose [[Value]] is empty. If evaluation of the CaseBlock part of a SwitchStatement produces a normal completion whose [[Value]] is empty, the result of evaluating the SwitchStatement is a normal completion whose [[Value]] is undefined.

14.15: In ECMAScript 2015, it is an early error for a Catch clause to contain a var declaration for the same Identifier that appears as the Catch clause parameter. In previous editions, such a variable declaration would be instantiated in the enclosing variable environment but the declaration's Initializer value would be assigned to the Catch parameter.

14.15, 19.2.1.3: In ECMAScript 2015, a runtime SyntaxError is thrown if a Catch clause evaluates a non-strict direct eval whose eval code includes a var or FunctionDeclaration declaration that binds the same Identifier that appears as the Catch clause parameter.

14.15.3: In ECMAScript 2015, the result of a TryStatement is never the value empty. If the Block part of a TryStatement evaluates to a normal completion containing empty, the result of the TryStatement is undefined. If the Block part of a TryStatement evaluates to a throw completion and it has a Catch part that evaluates to a normal completion containing empty, the result of the TryStatement is undefined if there is no Finally clause or if its Finally clause evaluates to an empty normal completion.

15.4.5 In ECMAScript 2015, the function objects that are created as the values of the [[Get]] or [[Set]] attribute of accessor properties in an ObjectLiteral are not constructor functions and they do not have a "prototype" own property. In the previous edition, they were constructors and had a "prototype" property.

20.1.2.6: In ECMAScript 2015, if the argument to Object.freeze is not an object it is treated as if it was a non-extensible ordinary object with no own properties. In the previous edition, a non-object argument always causes a TypeError to be thrown.

20.1.2.8: In ECMAScript 2015, if the argument to Object.getOwnPropertyDescriptor is not an object an attempt is made to coerce the argument using ToObject. If the coercion is successful the result is used in place of the original argument value. In the previous edition, a non-object argument always causes a TypeError to be thrown.

20.1.2.10: In ECMAScript 2015, if the argument to Object.getOwnPropertyNames is not an object an attempt is made to coerce the argument using ToObject. If the coercion is successful the result is used in place of the original argument value. In the previous edition, a non-object argument always causes a TypeError to be thrown.

20.1.2.12: In ECMAScript 2015, if the argument to Object.getPrototypeOf is not an object an attempt is made to coerce the argument using ToObject. If the coercion is successful the result is used in place of the original argument value. In the previous edition, a non-object argument always causes a TypeError to be thrown.

20.1.2.15: In ECMAScript 2015, if the argument to Object.isExtensible is not an object it is treated as if it was a non-extensible ordinary object with no own properties. In the previous edition, a non-object argument always causes a TypeError to be thrown.

20.1.2.16: In ECMAScript 2015, if the argument to Object.isFrozen is not an object it is treated as if it was a non-extensible ordinary object with no own properties. In the previous edition, a non-object argument always causes a TypeError to be thrown.

20.1.2.17: In ECMAScript 2015, if the argument to Object.isSealed is not an object it is treated as if it was a non-extensible ordinary object with no own properties. In the previous edition, a non-object argument always causes a TypeError to be thrown.

20.1.2.18: In ECMAScript 2015, if the argument to Object.keys is not an object an attempt is made to coerce the argument using ToObject. If the coercion is successful the result is used in place of the original argument value. In the previous edition, a non-object argument always causes a TypeError to be thrown.

20.1.2.19: In ECMAScript 2015, if the argument to Object.preventExtensions is not an object it is treated as if it was a non-extensible ordinary object with no own properties. In the previous edition, a non-object argument always causes a TypeError to be thrown.

20.1.2.21: In ECMAScript 2015, if the argument to Object.seal is not an object it is treated as if it was a non-extensible ordinary object with no own properties. In the previous edition, a non-object argument always causes a TypeError to be thrown.

20.2.3.2: In ECMAScript 2015, the [[Prototype]] internal slot of a bound function is set to the [[GetPrototypeOf]] value of its target function. In the previous edition, [[Prototype]] was always set to %Function.prototype%.

20.2.4.1: In ECMAScript 2015, the "length" property of function instances is configurable. In previous editions it was non-configurable.

20.5.6.2: In ECMAScript 2015, the [[Prototype]] internal slot of a NativeError constructor is the Error constructor. In previous editions it was the Function prototype object.

21.4.4 In ECMAScript 2015, the Date prototype object is not a Date instance. In previous editions it was a Date instance whose TimeValue was NaN.

22.1.3.11 In ECMAScript 2015, the String.prototype.localeCompare function must treat Strings that are canonically equivalent according to the Unicode Standard as being identical. In previous editions implementations were permitted to ignore canonical equivalence and could instead use a bit-wise comparison.

22.1.3.27 and 22.1.3.29 In ECMAScript 2015, lowercase/upper conversion processing operates on code points. In previous editions such the conversion processing was only applied to individual code units. The only affected code points are those in the Deseret block of Unicode.

22.1.3.30 In ECMAScript 2015, the String.prototype.trim method is defined to recognize white space code points that may exist outside of the Unicode BMP. However, as of Unicode 7 no such code points are defined. In previous editions such code points would not have been recognized as white space.

22.2.4.1 In ECMAScript 2015, If the pattern argument is a RegExp instance and the flags argument is not undefined, a new RegExp instance is created just like pattern except that pattern's flags are replaced by the argument flags. In previous editions a TypeError exception was thrown when pattern was a RegExp instance and flags was not undefined.

22.2.6 In ECMAScript 2015, the RegExp prototype object is not a RegExp instance. In previous editions it was a RegExp instance whose pattern is the empty String.

22.2.6 In ECMAScript 2015, "source", "global", "ignoreCase", and "multiline" are accessor properties defined on the RegExp prototype object. In previous editions they were data properties defined on RegExp instances.

25.4.13: In ECMAScript 2019, Atomics.wake has been renamed to Atomics.notify to prevent confusion with Atomics.wait.

27.1.4.4, 27.6.3.6: In ECMAScript 2019, the number of Jobs enqueued by await was reduced, which could create an observable difference in resolution order between a then() call and an await expression.

G Colophon

This specification is authored on GitHub in a plaintext source format called Ecmarkup. Ecmarkup is an HTML and Markdown dialect that provides a framework and toolset for authoring ECMAScript specifications in plaintext and processing the specification into a full-featured HTML rendering that follows the editorial conventions for this document. Ecmarkup builds on and integrates a number of other formats and technologies including Grammarkdown for defining syntax and Ecmarkdown for authoring algorithm steps. PDF renderings of this specification are produced by printing the HTML rendering to a PDF.

Prior editions of this specification were authored using Word—the Ecmarkup source text that formed the basis of this edition was produced by converting the ECMAScript 2015 Word document to Ecmarkup using an automated conversion tool.

H Bibliography

  1. IEEE 754-2019: IEEE Standard for Floating-Point Arithmetic. Institute of Electrical and Electronic Engineers, New York (2019) Note

    There are no normative changes between IEEE 754-2008 and IEEE 754-2019 that affect the ECMA-262 specification.
  2. The Unicode Standard, available at <https://unicode.org/versions/latest>
  3. Unicode Technical Note #5: Canonical Equivalence in Applications, available at <https://unicode.org/notes/tn5/>
  4. Unicode Technical Standard #10: Unicode Collation Algorithm, available at <https://unicode.org/reports/tr10/>
  5. Unicode Standard Annex #15, Unicode Normalization Forms, available at <https://unicode.org/reports/tr15/>
  6. Unicode Standard Annex #18: Unicode Regular Expressions, available at <https://unicode.org/reports/tr18/>
  7. Unicode Standard Annex #24: Unicode Script Property, available at <https://unicode.org/reports/tr24/>
  8. Unicode Standard Annex #31, Unicode Identifiers and Pattern Syntax, available at <https://unicode.org/reports/tr31/>
  9. Unicode Standard Annex #44: Unicode Character Database, available at <https://unicode.org/reports/tr44/>
  10. Unicode Technical Standard #51: Unicode Emoji, available at <https://unicode.org/reports/tr51/>
  11. IANA Time Zone Database, available at <https://www.iana.org/time-zones>
  12. ISO 8601:2004(E) Data elements and interchange formats — Information interchange — Representation of dates and times
  13. RFC 1738 “Uniform Resource Locators (URL)”, available at <https://tools.ietf.org/html/rfc1738>
  14. RFC 2396 “Uniform Resource Identifiers (URI): Generic Syntax”, available at <https://tools.ietf.org/html/rfc2396>
  15. RFC 3629 “UTF-8, a transformation format of ISO 10646”, available at <https://tools.ietf.org/html/rfc3629>
  16. RFC 7231 “Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content”, available at <https://tools.ietf.org/html/rfc7231>

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