babel-loader

What is babel-loader?

The babel-loader npm package is a Webpack plugin that integrates Babel with Webpack. Babel is a JavaScript compiler that allows you to use next-generation JavaScript, today. It compiles ES6 and beyond into ES5 code that can run in current browser environments. babel-loader enables Webpack to process and bundle JavaScript files using Babel.

What are babel-loader's main functionalities?

Transpiling ES6+ to ES5

This feature allows developers to write modern JavaScript code without worrying about compatibility issues with older browsers. The code sample shows a Webpack configuration that uses babel-loader to process files ending in .js, excluding node_modules, and transpiles them using the preset '@babel/preset-env'.

module.exports = {
  module: {
    rules: [
      {
        test: /\.js$/,
        exclude: /node_modules/,
        use: {
          loader: 'babel-loader',
          options: {
            presets: ['@babel/preset-env']
          }
        }
      }
    ]
  }
};

Using Babel plugins

babel-loader can be configured to use specific Babel plugins to enable experimental features or custom transformations. The code sample demonstrates how to include the '@babel/plugin-proposal-class-properties' plugin to allow the use of class properties syntax in JavaScript.

module.exports = {
  module: {
    rules: [
      {
        test: /\.js$/,
        exclude: /node_modules/,
        use: {
          loader: 'babel-loader',
          options: {
            plugins: ['@babel/plugin-proposal-class-properties']
          }
        }
      }
    ]
  }
};

Source Maps support

babel-loader supports source maps, which help in debugging transpiled code by mapping the transformed code back to the original source code. The code sample shows how to enable source maps in the babel-loader options.

module.exports = {
  module: {
    rules: [
      {
        test: /\.js$/,
        use: {
          loader: 'babel-loader',
          options: {
            sourceMaps: true
          }
        }
      }
    ]
  }
};

Other packages similar to babel-loader

ts-loader

ts-loader is a Webpack loader for TypeScript, similar to how babel-loader works for JavaScript. It compiles TypeScript code to JavaScript, allowing developers to use TypeScript's static typing features. While babel-loader focuses on JavaScript and its newer syntax, ts-loader is specifically designed for TypeScript.

awesome-typescript-loader

awesome-typescript-loader is another Webpack loader for TypeScript, providing similar functionality to ts-loader. It also compiles TypeScript to JavaScript and offers features like Babel integration and type checking. It is known for being fast and providing a smoother integration with Babel compared to ts-loader.

esbuild-loader

esbuild-loader is a Webpack loader that uses the esbuild bundler. It is designed for extremely fast bundling and minifying of JavaScript and TypeScript code. Unlike babel-loader, which focuses on compatibility and syntax transformations, esbuild-loader emphasizes performance and speed.

README

This README is for babel-loader v8 + 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

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.

• metadataSubscribers: Default []. Takes an array of context function names. E.g. if you passed ['myMetadataPlugin'], you'd assign a subscriber function to context.myMetadataPlugin within your webpack plugin's hooks & that function will be called with metadata.

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: {
      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: /\.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):

  {
    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:

  {
    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"
    ]
  }
}

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 => {
  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