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babel
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Description

📦 Babel loader for webpack

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This README is for babel-loader v8 + Babel v7 If you are using legacy Babel v6, see the 7.x branch docs

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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

webpack

4.x || 5.x
| babel-loader 8.x | babel 7.x
npm install -D babel-loader @babel/core @babel/preset-env webpack

Usage

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: /\.m?js$/,
      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: /\.m?js$/,
      exclude: /node_modules/,
      use: {
        loader: 'babel-loader',
        options: {
          presets: [
            ['@babel/preset-env', { targets: "defaults" }]
          ],
          plugins: ['@babel/plugin-proposal-class-properties']
        }
      }
    }
  ]
}

This loader also supports the following loader-specific option:

  • cacheDirectory
    : Default
    false
    . 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 to
    true
    in options (
    {cacheDirectory: true}
    ), the loader will use the default cache directory in
    node_modules/.cache/babel-loader
    or fallback to the default OS temporary file directory if no
    node_modules
    folder could be found in any root directory.
  • cacheIdentifier
    : Default is a string composed by the
    @babel/core
    's version, the
    babel-loader
    's version, the contents of
    .babelrc
    file if it exists, and the value of the environment variable
    BABEL_ENV
    with a fallback to the
    NODE_ENV
    environment variable. This can be set to a custom value to force cache busting if the identifier changes.
  • cacheCompression
    : Default
    true
    . When set, each Babel transform output will be compressed with Gzip. If you want to opt-out of cache compression, set it to
    false
    -- your project may benefit from this if it transpiles thousands of files.
  • customize
    : Default
    null
    . The path of a module that exports a
    custom
    callback 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
    .custom
    to create a wrapper loader. Only use this if you must continue using
    babel-loader
    directly, but still want to customize.

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.

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: /\.m?js$/,
    exclude: {
      test: /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: /\.m?js$/,
    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):
javascript
  {
    test: /\.m?js$/,
    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:
javascript
  {
    test: /\.m?js$/,
    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"
    ]
  }
}

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 => {
  function myPlugin() {
    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) {
  if (cfg.hasFilesystemConfig()) {
    // Use the normal config
    return cfg.options;
  }

  return {
    ...cfg.options,
    plugins: [
      ...(cfg.options.plugins || []),

      // Include a custom plugin in the options.
      myPlugin,
    ],
  };
},

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): 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.

License

MIT

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