babelThis README is for babel-loader v8/v9 with Babel v7 If you are using legacy Babel v6, see the 7.x branch docs
Babel Loader
This package allows transpiling JavaScript files using Babel and webpack.
Note: Issues with the output should be reported on the Babel Issues tracker.
Install
babel-loader supported webpack versions supported Babel versions supported Node.js versions 8.x 4.x or 5.x 7.x >= 8.9 9.x 5.x ^7.12.0 >= 14.15.0
npm install -D babel-loader @babel/core @babel/preset-env webpackUsage
webpack documentation: Loaders
Within your webpack configuration object, you'll need to add the babel-loader to the list of modules, like so:
module: {
rules: [
{
test: /\.(?:js|mjs|cjs)$/,
exclude: /node_modules/,
use: {
loader: 'babel-loader',
options: {
presets: [
['@babel/preset-env', { targets: "defaults" }]
]
}
}
}
]
}Options
See the babel options.
You can pass options to the loader by using the options property:
module: {
rules: [
{
test: /\.(?:js|mjs|cjs)$/,
exclude: /node_modules/,
use: {
loader: 'babel-loader',
options: {
presets: [
['@babel/preset-env', { targets: "defaults" }]
],
plugins: ['@babel/plugin-proposal-class-properties']
}
}
}
]
}The options passed here will be merged with Babel config files, e.g. babel.config.js or .babelrc.
This loader also supports the following loader-specific option:
cacheDirectory: Defaultfalse. When set, the given directory will be used to cache the results of the loader. Future webpack builds will attempt to read from the cache to avoid needing to run the potentially expensive Babel recompilation process on each run. If the value is set totruein options ({cacheDirectory: true}), the loader will use the default cache directory innode_modules/.cache/babel-loaderor fallback to the default OS temporary file directory if nonode_modulesfolder could be found in any root directory.cacheIdentifier: Default is a string composed by- the
@babel/core's version and thebabel-loader's version - the merged Babel config, including options passed to
babel-loaderand the contents ofbabel.config.jsor.babelrcfile if they exist - the value of the environment variable
BABEL_ENVwith a fallback to theNODE_ENVenvironment variable. This can be set to a custom value to force cache busting if the identifier changes.
- the
cacheCompression: Defaulttrue. When set, each Babel transform output will be compressed with Gzip. If you want to opt-out of cache compression, set it tofalse-- your project may benefit from this if it transpiles thousands of files.customize: Defaultnull. The path of a module that exports acustomcallback like the one that you'd pass to.custom(). Since you already have to make a new file to use this, it is recommended that you instead use.customto create a wrapper loader. Only use this if you must continue usingbabel-loaderdirectly, but still want to customize.metadataSubscribers: Default[]. Takes an array of context function names. E.g. if you passed ['myMetadataPlugin'], you'd assign a subscriber function tocontext.myMetadataPluginwithin your webpack plugin's hooks & that function will be called withmetadata.
Troubleshooting
babel-loader is slow!
Make sure you are transforming as few files as possible. Because you are probably matching /\.m?js$/, you might be transforming the node_modules folder or other unwanted source.
To exclude node_modules, see the exclude option in the loaders config as documented above.
You can also speed up babel-loader by as much as 2x by using the cacheDirectory option. This will cache transformations to the filesystem.
Some files in my node_modules are not transpiled for IE 11
Although we typically recommend not compiling node_modules, you may need to when using libraries that do not support IE 11 or any legacy targets.
For this, you can either use a combination of test and not, or pass a function to your exclude option. You can also use negative lookahead regex as suggested here.
{
test: /\.(?:js|mjs|cjs)$/,
exclude: {
and: [/node_modules/], // Exclude libraries in node_modules ...
not: [
// Except for a few of them that needs to be transpiled because they use modern syntax
/unfetch/,
/d3-array|d3-scale/,
/@hapi[\\/]joi-date/,
]
},
use: {
loader: 'babel-loader',
options: {
presets: [
['@babel/preset-env', { targets: "ie 11" }]
]
}
}
}Babel is injecting helpers into each file and bloating my code!
Babel uses very small helpers for common functions such as _extend. By default, this will be added to every file that requires it.
You can instead require the Babel runtime as a separate module to avoid the duplication.
The following configuration disables automatic per-file runtime injection in Babel, requiring @babel/plugin-transform-runtime instead and making all helper references use it.
See the docs for more information.
NOTE: You must run npm install -D @babel/plugin-transform-runtime to include this in your project and @babel/runtime itself as a dependency with npm install @babel/runtime.
rules: [
// the 'transform-runtime' plugin tells Babel to
// require the runtime instead of inlining it.
{
test: /\.(?:js|mjs|cjs)$/,
exclude: /node_modules/,
use: {
loader: 'babel-loader',
options: {
presets: [
['@babel/preset-env', { targets: "defaults" }]
],
plugins: ['@babel/plugin-transform-runtime']
}
}
}
]NOTE: transform-runtime & custom polyfills (e.g. Promise library)
Since @babel/plugin-transform-runtime includes a polyfill that includes a custom regenerator-runtime and core-js, the following usual shimming method using webpack.ProvidePlugin will not work:
// ...
new webpack.ProvidePlugin({
'Promise': 'bluebird'
}),
// ...The following approach will not work either:
require('@babel/runtime/core-js/promise').default = require('bluebird');
var promise = new Promise;which outputs to (using runtime):
'use strict';
var _Promise = require('@babel/runtime/core-js/promise')['default'];
require('@babel/runtime/core-js/promise')['default'] = require('bluebird');
var promise = new _Promise();The previous Promise library is referenced and used before it is overridden.
One approach is to have a "bootstrap" step in your application that would first override the default globals before your application:
// bootstrap.js
require('@babel/runtime/core-js/promise').default = require('bluebird');
// ...
require('./app');The Node.js API for babel has been moved to babel-core.
If you receive this message, it means that you have the npm package babel installed and are using the short notation of the loader in the webpack config (which is not valid anymore as of webpack 2.x):
{
test: /\.(?:js|mjs|cjs)$/,
loader: 'babel',
}webpack then tries to load the babel package instead of the babel-loader.
To fix this, you should uninstall the npm package babel, as it is deprecated in Babel v6. (Instead, install @babel/cli or @babel/core.)
In the case one of your dependencies is installing babel and you cannot uninstall it yourself, use the complete name of the loader in the webpack config:
{
test: /\.(?:js|mjs|cjs)$/,
loader: 'babel-loader',
}Exclude libraries that should not be transpiled
core-js and webpack/buildin will cause errors if they are transpiled by Babel.
You will need to exclude them form babel-loader.
{
"loader": "babel-loader",
"options": {
"exclude": [
// \\ for Windows, / for macOS and Linux
/node_modules[\\/]core-js/,
/node_modules[\\/]webpack[\\/]buildin/,
],
"presets": [
"@babel/preset-env"
]
}
}Top level function (IIFE) is still arrow (on Webpack 5)
That function is injected by Webpack itself after running babel-loader. By default Webpack asumes that your target environment supports some ES2015 features, but you can overwrite this behavior using the output.environment Webpack option (documentation).
To avoid the top-level arrow function, you can use output.environment.arrowFunction:
// webpack.config.js
module.exports = {
// ...
output: {
// ...
environment: {
// ...
arrowFunction: false, // <-- this line does the trick
},
},
};Customize config based on webpack target
Webpack supports bundling multiple targets. For cases where you may want different Babel configurations for each target (like web and node), this loader provides a target property via Babel's caller API.
For example, to change the environment targets passed to @babel/preset-env based on the webpack target:
// babel.config.js
module.exports = api => {
return {
plugins: [
"@babel/plugin-proposal-nullish-coalescing-operator",
"@babel/plugin-proposal-optional-chaining"
],
presets: [
[
"@babel/preset-env",
{
useBuiltIns: "entry",
// caller.target will be the same as the target option from webpack
targets: api.caller(caller => caller && caller.target === "node")
? { node: "current" }
: { chrome: "58", ie: "11" }
}
]
]
}
}Customized Loader
babel-loader exposes a loader-builder utility that allows users to add custom handling
of Babel's configuration for each file that it processes.
.custom accepts a callback that will be called with the loader's instance of
babel so that tooling can ensure that it using exactly the same @babel/core
instance as the loader itself.
In cases where you want to customize without actually having a file to call .custom, you
may also pass the customize option with a string pointing at a file that exports
your custom callback function.
Example
// Export from "./my-custom-loader.js" or whatever you want.
module.exports = require("babel-loader").custom(babel => {
// Extract the custom options in the custom plugin
function myPlugin(api, { opt1, opt2 }) {
return {
visitor: {},
};
}
return {
// Passed the loader options.
customOptions({ opt1, opt2, ...loader }) {
return {
// Pull out any custom options that the loader might have.
custom: { opt1, opt2 },
// Pass the options back with the two custom options removed.
loader,
};
},
// Passed Babel's 'PartialConfig' object.
config(cfg, { customOptions }) {
if (cfg.hasFilesystemConfig()) {
// Use the normal config
return cfg.options;
}
return {
...cfg.options,
plugins: [
...(cfg.options.plugins || []),
// Include a custom plugin in the options and passing it the customOptions object.
[myPlugin, customOptions],
],
};
},
result(result) {
return {
...result,
code: result.code + "\n// Generated by some custom loader",
};
},
};
});// And in your Webpack config
module.exports = {
// ..
module: {
rules: [{
// ...
loader: path.join(__dirname, 'my-custom-loader.js'),
// ...
}]
}
};customOptions(options: Object): { custom: Object, loader: Object }
Given the loader's options, split custom options out of babel-loader's
options.
config(cfg: PartialConfig, options: { source, customOptions }): Object
Given Babel's PartialConfig object, return the options object that should
be passed to babel.transform.
result(result: Result): Result
Given Babel's result object, allow loaders to make additional tweaks to it.
