-
Notifications
You must be signed in to change notification settings - Fork 22.5k
/
index.md
170 lines (121 loc) · 5.1 KB
/
index.md
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
---
title: set
slug: Web/JavaScript/Reference/Functions/set
page-type: javascript-language-feature
browser-compat: javascript.functions.set
---
{{jsSidebar("Functions")}}
The **`set`** syntax binds an object property to a function to be called when there is an attempt to set that property. It can also be used in [classes](/en-US/docs/Web/JavaScript/Reference/Classes).
{{EmbedInteractiveExample("pages/js/functions-setter.html")}}
## Syntax
```js-nolint
{ set prop(val) { /* … */ } }
{ set [expression](val) { /* … */ } }
```
There are some additional syntax restrictions:
- A setter must have exactly one parameter.
### Parameters
- `prop`
- : The name of the property to bind to the given function. In the same way as other properties in [object initializers](/en-US/docs/Web/JavaScript/Reference/Operators/Object_initializer), it can be a string literal, a number literal, or an identifier.
- `val`
- : An alias for the variable that holds the value attempted to be assigned to
`prop`.
- `expression`
- : You can also use expressions for a computed property name to bind to the given function.
## Description
In JavaScript, a setter can be used to execute a function whenever an attempt is made to change a property's value. Setters are most often used in conjunction with getters.
An object property is either a data property or an accessor property, but it cannot simultaneously be both. Read {{jsxref("Object.defineProperty()")}} for more information. The setter syntax allows you to specify the setter function in an object initializer.
```js
const obj = {
set prop() {
// setter, the code executed when setting obj.prop
},
}
```
Properties defined using this syntax are own properties of the created object, and they are configurable and enumerable.
## Examples
### Defining a setter on new objects in object initializers
The following example defines a pseudo-property `current` of object
`language`. When `current` is assigned a value, it updates
`log` with that value:
```js
const language = {
set current(name) {
this.log.push(name);
},
log: [],
};
language.current = "EN";
console.log(language.log); // ['EN']
language.current = "FA";
console.log(language.log); // ['EN', 'FA']
```
Note that `current` is not defined, and any attempts to access it will
result in `undefined`.
### Using setters in classes
You can use the exact same syntax to define public instance setters that are available on class instances. In classes, you don't need the comma separator between methods.
```js
class ClassWithGetSet {
#msg = "hello world";
get msg() {
return this.#msg;
}
set msg(x) {
this.#msg = `hello ${x}`;
}
}
const instance = new ClassWithGetSet();
console.log(instance.msg); // "hello world"
instance.msg = "cake";
console.log(instance.msg); // "hello cake"
```
Setter properties are defined on the `prototype` property of the class and are thus shared by all instances of the class. Unlike setter properties in object literals, setter properties in classes are not enumerable.
Static setters and private setters use similar syntaxes, which are described in the [`static`](/en-US/docs/Web/JavaScript/Reference/Classes/static) and [private properties](/en-US/docs/Web/JavaScript/Reference/Classes/Private_properties) pages.
### Removing a setter with the `delete` operator
If you want to remove the setter, you can just {{jsxref("Operators/delete", "delete")}}
it:
```js
delete language.current;
```
### Defining a setter on existing objects using `defineProperty`
To append a setter to an _existing_ object, use
{{jsxref("Object.defineProperty()")}}.
```js
const o = { a: 0 };
Object.defineProperty(o, "b", {
set(x) {
this.a = x / 2;
},
});
o.b = 10;
// Runs the setter, which assigns 10 / 2 (5) to the 'a' property
console.log(o.a); // 5
```
### Using a computed property name
```js
const expr = "foo";
const obj = {
baz: "bar",
set [expr](v) {
this.baz = v;
},
};
console.log(obj.baz); // "bar"
obj.foo = "baz";
// Run the setter
console.log(obj.baz); // "baz"
```
## Specifications
{{Specifications}}
## Browser compatibility
{{Compat}}
## See also
- [Working with objects](/en-US/docs/Web/JavaScript/Guide/Working_with_objects) guide
- [Functions](/en-US/docs/Web/JavaScript/Reference/Functions)
- [`get`](/en-US/docs/Web/JavaScript/Reference/Functions/get)
- {{jsxref("Object.defineProperty()")}}
- [Object initializer](/en-US/docs/Web/JavaScript/Reference/Operators/Object_initializer)
- {{jsxref("Statements/class", "class")}}
- [Property accessors](/en-US/docs/Web/JavaScript/Reference/Operators/Property_accessors)
- [Incompatible ES5 change: literal getter and setter functions must now have exactly zero or one arguments](https://whereswalden.com/2010/08/22/incompatible-es5-change-literal-getter-and-setter-functions-must-now-have-exactly-zero-or-one-arguments/) by Jeff Walden (2010)
- [More SpiderMonkey changes: ancient, esoteric, very rarely used syntax for creating getters and setters is being removed](https://whereswalden.com/2010/04/16/more-spidermonkey-changes-ancient-esoteric-very-rarely-used-syntax-for-creating-getters-and-setters-is-being-removed/) by Jeff Walden (2010)