docs/v4-upgrade

To update a Webpacker v3.5 app to v4, follow these steps:

  1. Update the webpacker gem and the @rails/webpacker package to v4. This will upgrade webpack itself from 3.x to 4.x, make sure you’re aware of any deprecations which might effect you. Also make sure any other packages you depend on support webpack 4 and don’t require any changes, e.g. if you explicitly include webpack you need to upgrade it to 4.x, and if you use webpack-dev-server you need to upgrade it to 3.x.
  2. Browser support definitions have been moved to .browserslistrc to /.
  3. Merge any differences between config/webpacker.yml and your config/webpacker.yml.
  4. Webpacker v4 upgrades Babel to v7, see also the release blog post. Many packages were moved to the @babel/ namespace, any babel plugins you have will need to be updated. It may be worth checking out babel-upgrade if you have problems. (#1564)
  5. .babelrc should be replaced with babel.config.js and .postcssrc.yml should be replaced with postcss.config.js (#1822). If you never changed these files from their defaults, the versions of babel.config.js and postcss.config.js in the webpacker repository should be usable.
  6. Due to the change in #1625, you’ll want to make sure that extract_css is set to true for the default environment in webpacker.yml if you want to have Webpacker supply your CSS.

Add SplitChunks

If you used the CommonsChunkPlugin you’ll need to upgrade to using the new splitChunks.

Originally, chunks (and modules imported inside them) were connected by a parent-child relationship in the internal webpack graph. The CommonsChunkPlugin was used to avoid duplicated dependencies across them, but further optimizations were not possible.

In webpack v4, CommonsChunkPlugin was removed in favor of optimization.splitChunks.

For the full configuration options of splitChunks, see the Webpack documentation.

// config/webpack/environment.js
const WebpackAssetsManifest = require('webpack-assets-manifest');

// Enable the default config
environment.splitChunks()

// or using custom config
environment.splitChunks((config) => Object.assign({}, config, { optimization: { splitChunks: false }}))

Then use, javascript_packs_with_chunks_tag helper to include all the transpiled
packs with the chunks in your view, which creates html tags for all the chunks.

<%= javascript_packs_with_chunks_tag 'calendar', 'map', 'data-turbolinks-track': 'reload' %>

Important: Pass all your pack names when using this helper otherwise you will
get duplicated chunks on the page.

<%# DO %>
<%= javascript_packs_with_chunks_tag 'calendar', 'map' %>

<%# DON'T %>
<%= javascript_packs_with_chunks_tag 'calendar' %>
<%= javascript_packs_with_chunks_tag 'map' %>

Package-specific notes:

  • If you’re using React, you need to add "@babel/preset-react", to the list of presets in your babel config.
  • If you’re using Vue Loader, you’ll need to upgrade to v15 for webpack 4.
  • To see what webpacker generates for a given framework with v4, you may want to re-run bundle exec rake webpacker:install:FRAMEWORK and let it override the files for your given JavaScript framework, and then compare them to see what changes you’ll need to make.

Excluding node_modules from being transpiled by babel-loader

One change to take into consideration, is that Webpacker 4 transpiles the
node_modules folder with the babel-loader. This folder used to be ignored by
webpacker 3. The new behavior helps in case some library contains ES6 code, but in
some cases it can lead to issues. To avoid running babel-loader in the
node_modules folder, replicating the same behavior as Webpacker 3, the
following code can be added to config/webpack/environment.js:

environment.loaders.delete('nodeModules')

Alternatively, in order to skip only a specific library in the node_modules
folder, this code can be added:

const nodeModulesLoader = environment.loaders.get('nodeModules')
if (!Array.isArray(nodeModulesLoader.exclude)) {
  nodeModulesLoader.exclude = (nodeModulesLoader.exclude == null)
    ? []
    : [nodeModulesLoader.exclude]
}
nodeModulesLoader.exclude.push(/some-library/) // replace `some-library` with
                                               // the actual path to exclude

Source Maps are enabled by default

Source maps are now enabled in production to make debugging in production easier. Enabling source maps doesn’t have drawbacks for most of the applications since maps are compressed by default and aren’t loaded by browsers unless Dev Tools are opened.

If you want to keep the old behavior source maps can be disabled in any environment configuration, e.g:

// config/webpack/production.js

const environment = require('./environment')
environment.config.merge({ devtool: 'none' })

module.exports = environment.toWebpackConfig()

Example upgrades

This is what an upgrade to Webpacker 4 looked like for existing Rails apps (please contribute yours!):