babel-loader

Babel is a compiler for writing next generation JavaScript.

This package allows transpiling JavaScript files using Babel and webpack.

Notes: Issues with the output should be reported on the babel issue tracker;

Installation

npm install babel-loader --save-dev

Note: npm will deprecate auto-installing of peerDependencies on the next major release, so required peer dependencies like babel-core and webpack will have to be listed explicitly in your package.json.

Usage

Documentation: Using loaders

Within your webpack configuration object, you'll need to add the babel-loader to the list of modules, like so:

module: {
loaders: [
  {
    test: /\.jsx?$/,
    exclude: /(node_modules|bower_components)/,
    loader: 'babel'
  }
]
}

Options

See the babel options.

You can pass options to the loader by writting them as a query string:

module: {
loaders: [
  {
    test: /\.jsx?$/,
    exclude: /(node_modules|bower_components)/,
    loader: 'babel?optional[]=runtime&stage=0'
  }
]
}

or by using the query property:

module: {
loaders: [
  {
    test: /\.jsx?$/,
    exclude: /(node_modules|bower_components)/,
    loader: 'babel',
    query: {
      optional: ['runtime'],
      stage: 0
    }
  }
]
}

This loader also supports the following loader-specific option:

• cacheDirectory: 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. The default value (loader: 'babel-loader?cacheDirectory') will cause the loader to use the default OS temporary file directory.

• cacheIdentifier: When set, it will add the given identifier to the cached files. This can be used to force cache busting if the identifier changes. By default the identifier is composed by the babel-core's version, the babel-loader's version and the .babelrc file if it exists.

Note: The sourceMap option is ignored, instead sourceMaps are automatically enabled when webpack is configured to use them (via the devtool config option).

Troubleshooting

babel-loader is slow!

Make sure you are transforming as few files as possible. Because you are probably matching /\.js$/, you might be transforming the node_modules folder or other unwanted source.

See the exclude option in the loaders config as documented above.

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, instead requiring babel-runtime and making all helper references use it.

See the docs for more information.

NOTE: You must run npm install babel-runtime --save to include this in your project.

loaders: [
  // the optional 'runtime' transformer tells babel to require the runtime
  // instead of inlining it.
  {
    test: /\.jsx?$/,
    exclude: /(node_modules|bower_components)/,
    loader: 'babel?optional[]=runtime'
  }
]

using cacheDirectory fails with ENOENT Error

If using cacheDirectory results in an error similar to the following:

ERROR in ./frontend/src/main.jsx
Module build failed: Error: ENOENT, open 'true/350c59cae6b7bce3bb58c8240147581bfdc9cccc.json.gzip'
 @ multi app

(notice the true/ in the filepath)

That means that most likely, you're not setting the options correctly, and you're doing something similar to:

loaders: [
  {
    test: /\.jsx?$/,
    exclude: /(node_modules|bower_components)/,
    loader: 'babel?cacheDirectory=true'
  }
]

That's not the correct way of setting boolean values. You should do instead:

loaders: [
  {
    test: /\.jsx?$/,
    exclude: /(node_modules|bower_components)/,
    loader: 'babel-loader?cacheDirectory'
  }
]

or use the query property:

loaders: [
  // the optional 'runtime' transformer tells babel to require the runtime
  // instead of inlining it.
  {
    test: /\.jsx?$/,
    exclude: /(node_modules|bower_components)/,
    loader: 'babel-loader',
    query: {
      cacheDirectory: true
    }
  }
]

custom polyfills (e.g. Promise library)

Since Babel 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');

License