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Webpacker gives you a default set of configuration files for test, development and production environments in config/webpack/*.js. You can configure each individual environment in their respective files or configure them all in the base config/webpack/environment.js file.

By default, you don't need to make any changes to config/webpack/*.js files since it's all standard production-ready configuration. However, if you do need to customize or add a new loader, this is where you would go.

Here is how you can modify webpack configuration:

// config/webpack/custom.js
module.exports = {
  resolve: {
    alias: {
      jquery: 'jquery/src/jquery',
      vue: 'vue/dist/vue.js',
      React: 'react',
      ReactDOM: 'react-dom',
      vue_resource: 'vue-resource/dist/vue-resource',

// config/webpack/environment.js
const { environment } = require('@rails/webpacker')
const customConfig = require('./custom')

// Set nested object prop using path notation
environment.config.set('resolve.extensions', ['.foo', '.bar'])
environment.config.set('output.filename', '[name].js')

// Merge custom config

// Delete a property

module.exports = environment

If you need access to configs within Webpacker's configuration, you can import them like so:

const { config } = require('@rails/webpacker')



You can add additional loaders beyond the base set that Webpacker provides by adding it to your environment. We'll use json-loader as an example:

yarn add json-loader
// config/webpack/environment.js
const { environment } = require('@rails/webpacker')

environment.loaders.append('json', {
  test: /\.json$/,
  use: 'json-loader'

// Insert json loader at the top of list
environment.loaders.prepend('json', jsonLoader)

// Insert json loader after/before a given loader
environment.loaders.insert('json', jsonLoader, { after: 'style'} )
environment.loaders.insert('json', jsonLoader, { before: 'babel'} )

module.exports = environment

Finally add .json to the list of extensions in config/webpacker.yml. Now if you import() any .json files inside your JavaScript they will be processed using json-loader. Voila!

You can also modify the loaders that Webpacker pre-configures for you. We'll update the babel loader as an example:

// config/webpack/environment.js
const { environment } = require('@rails/webpacker')

const babelLoader = environment.loaders.get('babel')
babelLoader.options.cacheDirectory = false

module.exports = environment

Coffeescript 2

Out of the box webpacker supports coffeescript 1, but here is how you can use Coffeescript 2:

yarn add coffeescript@2.0.1
// config/webpack/environment.js
const { environment } = require('@rails/webpacker')

const babelLoader = environment.loaders.get('babel')

// Replace existing coffee loader with CS2 version
environment.loaders.insert('coffee', {
  test: /\.coffee(\.erb)?$/,
  use:  babelLoader.use.concat(['coffee-loader'])

module.exports = environment

React SVG loader

To use react svg loader, you should append svg loader before file loader:

const { environment } = require('@rails/webpacker')

const babelLoader = environment.loaders.get('babel')

environment.loaders.insert('svg', {
  test: /\.svg$/,
  use: babelLoader.use.concat([
      loader: 'react-svg-loader',
      options: {
        jsx: true // true outputs JSX tags
}, { after: 'file' })

const fileLoader = environment.loaders.get('file')
fileLoader.exclude = /\.(svg)$/i

Url Loader

// config/webpack/loaders/url.js

module.exports = {
  test: [/\.bmp$/, /\.gif$/, /\.jpe?g$/, /\.png$/],
  use: [{
    loader: 'url-loader',
    options: {
      limit: 10000,
      name: '[name]-[hash].[ext]'

// config/webpack/environment.js

const { environment } = require('@rails/webpacker')
const url = require('./loaders/url')

environment.loaders.prepend('url', url)

// avoid using both file and url loaders
environment.loaders.get('file').test = /\.(tiff|ico|svg|eot|otf|ttf|woff|woff2)$/i

Overriding Loader Options in webpack 3+ (for CSS Modules etc.)

In webpack 3+, if you'd like to specify additional or different options for a loader, edit config/webpack/environment.js and provide an options object to override. This is similar to the technique shown above, but the following example shows specifically how to apply CSS Modules, which is what you may be looking for:

const { environment } = require('@rails/webpacker')
const merge = require('webpack-merge')

const myCssLoaderOptions = {
  modules: true,
  sourceMap: true,
  localIdentName: '[name]__[local]___[hash:base64:5]'

const CSSLoader = environment.loaders.get('sass').use.find(el => el.loader === 'css-loader')

CSSLoader.options = merge(CSSLoader.options, myCssLoaderOptions)

module.exports = environment

See issue #756 for additional discussion of this.

For this to work, don't forget to use the stylesheet_pack_tag, for example:

<%= stylesheet_pack_tag 'YOUR_PACK_NAME_HERE' %>


The process for adding or modifying webpack plugins is the same as the process for loaders above:

// config/webpack/environment.js
const { environment } = require('@rails/webpacker')
const webpack = require('webpack')

// Get a pre-configured plugin
const manifestPlugin = environment.plugins.get('Manifest')
manifestPlugin.opts.writeToFileEmit = false

// Add an additional plugin of your choosing : ProvidePlugin
  new webpack.ProvidePlugin({
    $: 'jquery',
    jQuery: 'jquery',
    jquery: 'jquery',
    'window.Tether': 'tether',
    Popper: ['popper.js', 'default'],
    ActionCable: 'actioncable',
    Vue: 'vue',
    VueResource: 'vue-resource',

// Insert before a given plugin
  new webpack.optimize.CommonsChunkPlugin({
    name: 'vendor', // Vendor code
    minChunks: (module) => module.context && module.context.indexOf('node_modules') !== -1
, { before: 'manifest' })

module.exports = environment

Resolved modules

To add new paths to resolve.modules, the API is same as loaders and plugins:

const { environment } = require('@rails/webpacker')

// Resolved modules list API - prepend, append, insert
environment.resolvedModules.append('vendor', 'vendor')

Add common chunks

The CommonsChunkPlugin is an opt-in feature that creates a separate file (known as a chunk), consisting of common modules shared between multiple entry points. By separating common modules from bundles, the resulting chunked file can be loaded once initially, and stored in the cache for later use. This results in page speed optimizations as the browser can quickly serve the shared code from the cache, rather than being forced to load a larger bundle whenever a new page is visited.

Add the plugins in config/webpack/environment.js:

const webpack = require('webpack')

  new webpack.optimize.CommonsChunkPlugin({
    name: 'vendor',
    minChunks: (module) => {
      // this assumes your vendor imports exist in the node_modules directory
      return module.context && module.context.indexOf('node_modules') !== -1

  new webpack.optimize.CommonsChunkPlugin({
    name: 'manifest',
    minChunks: Infinity

Now, add these files to your layouts/application.html.erb:

<%# Head %>

<%= javascript_pack_tag "manifest" %>
<%= javascript_pack_tag "vendor" %>

<%# If importing any styles from node_modules in your JS app %>

<%= stylesheet_pack_tag "vendor" %>

More detailed guides available here: webpack guides