Webpacker Upgrade Guide

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

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.
Browser support definitions have been moved from .browserslistrc to /.
Merge any differences between config/webpacker.yml and your config/webpacker.yml.
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)
.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.
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.

SplitChunks Configuration

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, the 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 the javascript_packs_with_chunks_tag and stylesheet_packs_with_chunks_tag helpers 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' %>

<!-- Creates the following: -->
<script src="/packs/vendor-16838bab065ae1e314.js" data-turbolinks-track="reload"></script>
<script src="/packs/calendar~runtime-16838bab065ae1e314.js" data-turbolinks-track="reload"></script>
<script src="/packs/calendar-1016838bab065ae1e314.js" data-turbolinks-track="reload"></script>
<script src="/packs/map~runtime-16838bab065ae1e314.js" data-turbolinks-track="reload"></script>
<script src="/packs/map-16838bab065ae1e314.js" data-turbolinks-track="reload"></script>

Important: Pass all your pack names to the 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 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: false })

module.exports = environment.toWebpackConfig()

Namespaces

The compiled packs in the public directory are now stored under namespaces:

JavaScripts are stored under js
Stylesheets are stored under css
Other resources are stored under media

# Before
"runtime~hello_react" => "/packs/runtime~hello_react-da2baf7fd07b0e8b6d17.js"

# After
"runtime~hello_react" => "/packs/js/runtime~hello_react-da2baf7fd07b0e8b6d17.js"

Upgrading projects with custom Webpack setups that use only the view helpers

The default value for extract_css is false in config/webpack.yml. Custom webpack builds that extract the CSS such as often used with React on Rails should set this value to true or else no CSS link tags are generated.

default: &default
   # other stuff
   extract_css: true
   # by default, extract and emit a css file. The default is false

Example Upgrades

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

connorshea/vglist#77

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