Usage Guide
There are quite a few tools in the Babel toolchain that try to make it easy for you to use Babel whether you're an "end-user" or building an integration of Babel itself. This will be a quick introduction to those tools and you can read more about them in the "Usage" section of the docs.
If you're using a framework, the work of configuring Babel might be different or actually already handled for you. Check out our interactive setup guide instead.
Overview
This guide will show you how to compile your JavaScript application code that uses ES2015 syntax into code that works in current browsers. That will involve both transforming new syntax and polyfilling missing features.
The entire process to set this up involves:
-
Running these commands to install the packages:
- npm
- Yarn
- pnpm
npm install --save-dev @babel/core @babel/cli @babel/preset-env
yarn add --dev @babel/core @babel/cli @babel/preset-env
pnpm add --save-dev @babel/core @babel/cli @babel/preset-env
-
Creating a config file named
babel.config.json
(requiresv7.8.0
and above) in the root of your project with this content:babel.config.json{
"presets": [
[
"@babel/preset-env",
{
"targets": {
"edge": "17",
"firefox": "60",
"chrome": "67",
"safari": "11.1"
},
"useBuiltIns": "usage",
"corejs": "3.6.5"
}
]
]
}The browsers list above is just an arbitrary example. You will have to adapt it for the browsers you want to support. See here for more
@babel/preset-env
options.
Or babel.config.js
if you are using an older Babel version
const presets = [
[
"@babel/preset-env",
{
targets: {
edge: "17",
firefox: "60",
chrome: "67",
safari: "11.1",
},
useBuiltIns: "usage",
corejs: "3.6.4",
},
],
];
module.exports = { presets };
-
And running this command to compile all your code from the
src
directory tolib
:Shell./node_modules/.bin/babel src --out-dir lib
You can use the npm package runner that comes with [email protected] to shorten that command by replacing
./node_modules/.bin/babel
withnpx babel
Read on for a step-by-step explanation of how this works and an introduction to each of the tools used.
Basic usage with CLI
All the Babel modules you'll need are published as separate npm packages scoped under @babel
(since version 7). This modular design allows for various tools each designed for a specific use case. Here we'll look at @babel/core
and @babel/cli
.
Core Library
The core functionality of Babel resides at the @babel/core module. After installing it:
- npm
- Yarn
- pnpm
npm install --save-dev @babel/core
yarn add --dev @babel/core
pnpm add --save-dev @babel/core
you can require
it directly in your JavaScript program and use it like this:
const babel = require("@babel/core");
babel.transformSync("code", optionsObject);
As an end-user though, you'll probably want to install other tools that serve as an interface to @babel/core
and integrate well with your development process. Even so, you might still want to check its documentation page to learn about the options, most of which can be set from the other tools as well.
CLI tool
@babel/cli is a tool that allows you to use babel from the terminal. Here's the installation command and a basic usage example:
- npm
- Yarn
- pnpm
npm install --save-dev @babel/core @babel/cli
./node_modules/.bin/babel src --out-dir lib
yarn add --dev @babel/core @babel/cli
./node_modules/.bin/babel src --out-dir lib
pnpm add --save-dev @babel/core @babel/cli
./node_modules/.bin/babel src --out-dir lib
This will parse all the JavaScript files in the src
directory, apply any transformations we have told it to, and output each file to the lib
directory. Since we haven't told it to apply any transformations yet, the output code will be identical to the input (exact code styling is not preserved). We can specify what transformations we want by passing them as options.
We used the --out-dir
option above. You can view the rest of the options accepted by the cli tool by running it with --help
. But the most important to us right now are --plugins
and --presets
.
Plugins & Presets
Transformations come in the form of plugins, which are small JavaScript programs that instruct Babel on how to carry out transformations to the code. You can even write your own plugins to apply any transformations you want to your code. To transform ES2015 syntax into ES5 we can rely on official plugins like @babel/plugin-transform-arrow-functions
:
- npm
- Yarn
- pnpm
npm install --save-dev @babel/plugin-transform-arrow-functions
./node_modules/.bin/babel src --out-dir lib --plugins=@babel/plugin-transform-arrow-functions
yarn add --dev @babel/plugin-transform-arrow-functions
./node_modules/.bin/babel src --out-dir lib --plugins=@babel/plugin-transform-arrow-functions
pnpm add --save-dev @babel/plugin-transform-arrow-functions
./node_modules/.bin/babel src --out-dir lib --plugins=@babel/plugin-transform-arrow-functions
Now any arrow functions in our code will be transformed into ES5 compatible function expressions:
const fn = () => 1;
// converted to
var fn = function fn() {
return 1;
};
That's a good start! But we also have other ES2015 features in our code that we want transformed. Instead of adding all the plugins we want one by one, we can use a "preset" which is just a pre-determined set of plugins.
Just like with plugins, you can create your own presets too to share any combination of plugins you need. For our use case here, there's an excellent preset named env
.
- npm
- Yarn
- pnpm
npm install --save-dev @babel/preset-env
./node_modules/.bin/babel src --out-dir lib --presets=@babel/env
yarn add --dev @babel/preset-env
./node_modules/.bin/babel src --out-dir lib --presets=@babel/env
pnpm add --save-dev @babel/preset-env
./node_modules/.bin/babel src --out-dir lib --presets=@babel/env
Without any configuration, this preset will include all plugins to support modern JavaScript (ES2015, ES2016, etc.). But presets can take options too. Rather than passing both cli and preset options from the terminal, let's look at another way of passing options: configuration files.
Configuration
There are a few different ways to use configuration files depending on your needs. Be sure to read our in-depth guide on how to configure Babel for more information.
For now, let's create a file called babel.config.json
(requires v7.8.0
and above) with the following content:
{
"presets": [
[
"@babel/preset-env",
{
"targets": {
"edge": "17",
"firefox": "60",
"chrome": "67",
"safari": "11.1"
}
}
]
]
}
Now the env
preset will only load transformation plugins for features that are not available in our target browsers. We're all set for syntax. Let's look at polyfills next.
Polyfill
🚨 As of Babel 7.4.0, this package has been deprecated in favor of directly including
core-js/stable
(to polyfill ECMAScript features):JavaScriptimport "core-js/stable";
If you are compiling generators or async function to ES5, and you are using a version of
@babel/core
or@babel/plugin-transform-regenerator
older than7.18.0
, you must also load theregenerator runtime
package. It is automatically loaded when using@babel/preset-env
'suseBuiltIns: "usage"
option or@babel/plugin-transform-runtime
.
The @babel/polyfill module includes core-js and a custom regenerator runtime to emulate a full ES2015 environment.
This means you can use new built-ins like Promise
or WeakMap
, static methods like Array.from
or Object.assign
, instance methods like Array.prototype.includes
, and generator functions (when used alongside the regenerator plugin). The polyfill adds to the global scope as well as native prototypes like String
in order to do this.
For library/tool authors this may be too much. If you don't need the instance methods like Array.prototype.includes
you can do without polluting the global scope altogether by using the transform runtime plugin instead of @babel/polyfill
.
To go one step further, if you know exactly what features you need polyfills for, you can require them directly from core-js.
Since we're building an application we can just install @babel/polyfill
:
- npm
- Yarn
- pnpm
npm install --save @babel/polyfill
yarn add @babel/polyfill
pnpm add @babel/polyfill
Note the
--save
option instead of--save-dev
as this is a polyfill that needs to run before your source code.
Now luckily for us, we're using the env
preset which has a "useBuiltIns"
option that when set to "usage"
will practically apply the last optimization mentioned above where you only include the polyfills you need. With this new option the configuration changes like this:
{
"presets": [
[
"@babel/preset-env",
{
"targets": {
"edge": "17",
"firefox": "60",
"chrome": "67",
"safari": "11.1"
},
"useBuiltIns": "usage"
}
]
]
}
Babel will now inspect all your code for features that are missing in your target environments and include only the required polyfills. For example this code:
Promise.resolve().finally();
would turn into this (because Edge 17 doesn't have Promise.prototype.finally
):
require("core-js/modules/es.promise.finally");
Promise.resolve().finally();
If we weren't using the env
preset with the "useBuiltIns"
option set to "usage"
(defaults to "false") we would've had to require the full polyfill only once in our entry point before any other code.
For example:
{
"presets": [
[
"@babel/preset-env",
{
"targets": {
"edge": "17",
"firefox": "60",
"chrome": "67",
"safari": "11.1"
},
"useBuiltIns": "entry"
}
]
]
}
Then import core-js (to polyfill ECMAScript features) first, in our entry file to emulate a full ES2015 environment since @babel/polyfill has been deprecated:
import "core-js/stable";
Summary
We used @babel/cli
to run Babel from the terminal, @babel/polyfill
to polyfill all the new JavaScript features, and the env
preset to only include the transformations and polyfills for the features that we use and that are missing in our target browsers.
For more information on setting up Babel with your build system, IDE, and more, check out our interactive setup guide.