Webpacker makes it easy to use the JavaScript preprocessor and bundler Webpack to manage application-like JavaScript in Rails. It coexists with the asset pipeline, as the purpose is only to use Webpack for app-like JavaScript, not images, css, or even JavaScript Sprinkles (that all continues to live in app/assets).

It's designed to work with Rails 5.1+ and makes use of the Yarn dependency management that's been made default from that version forward.

Webpacker is currently compatible with Rails 5.0 stable, but there's no guarantee it will still be in the future.

You can either make use of Webpacker during setup of a new application with --webpack or you can add the gem and run bin/rails webpacker:install in an existing application.

As the rubygems version isn't promised to be kept up to date until the release of Rails 5.1, you may want to include the gem directly from GitHub:

gem 'webpacker', github: 'rails/webpacker'

Webpacker ships with three binstubs: ./bin/webpack, ./bin/webpack-watcher and ./bin/webpack-dev-server. They're thin wrappers around the standard webpack.js executable, just to ensure that the right configuration file is loaded and the node_modules from vendor are used.

A binstub is also created to install your npm dependencies declared in vendor, and can be called via ./bin/yarn.

In development, you'll need to run ./bin/webpack-watcher in a separate terminal from ./bin/rails server to have your app/javascript/packs/*.js files compiled as you make changes. If you'd rather not have to run the two processes separately by hand, you can use Foreman.

Alternatively, you can run ./bin/webpack-dev-server. This will launch a Webpack Dev Server listening on http://localhost:8080/ serving your pack files. It will recompile your files as you make changes. You also need to set config.x.webpacker[:dev_server_host] in your config/environment/development.rb to tell Webpacker to load your packs from the Webpack Dev Server. This setup allows you to leverage advanced Webpack features, such as Hot Module Replacement.

Webpacker gives you a default set of configuration files for development and production. They all live together with the shared points in config/webpack/*.js. By default, you shouldn't have to make any changes for a basic setup out the box. But this is where you do go if you need something more advanced.

The configuration for what Webpack is supposed to compile by default rests on the convention that every file in app/javascript/packs/* should be turned into their own output files (or entry points, as Webpack calls it).

Let's say you're building a calendar. Your structure could look like this:

<%# app/views/layout/application.html.erb %>
<%= javascript_pack_tag 'calendar' %>

// app/javascript/packs/calendar.js
require('calendar')

app/javascript/calendar/index.js // gets loaded by require('calendar')
app/javascript/calendar/components/grid.jsx
app/javascript/calendar/models/month.js

But it could also look a million other ways. The only convention that Webpacker enforces is the one where entry points are automatically configured by the files in app/javascript/packs.

To compile all the packs during deployment, you can use the rails webpacker:compile command. This will invoke the production configuration, which includes digesting. The javascript_pack_tag helper method will automatically insert the correct digest when run in production mode. Just like the asset pipeline does it.

It's possible to link to assets that have been precompiled by sprockets. Add the .erb extension to your javascript file, then you can use Sprockets' asset helpers:

// app/javascript/my_pack/example.js.erb

<% helpers = ActionController::Base.helpers %>
var railsImagePath = "<%= helpers.image_path('rails.png') %>";

This is enabled by the rails-erb-loader loader rule in config/webpack/shared.js.

To use Webpacker with React, just create a new app with rails new myapp --webpack=react (or run rails webpacker:install:react on a Rails 5.1 app already setup with webpack), and all the relevant dependencies will be added via yarn and changes to the configuration files made. Now you can create JSX files and have them properly compiled automatically.

To use Webpacker with Angular, just create a new app with rails new myapp --webpack=angular (or run rails webpacker:install:angular on a Rails 5.1 app already setup with webpack). TypeScript support and the Angular core libraires will be added via yarn and changes to the configuration files made. An example component written in TypeScript is also added to your project in app/javascript so that you can experiment Angular right away.

• Improve process for linking to assets compiled by sprockets - shouldn't need to specify <% helpers = ActionController::Base.helpers %> at the beginning of each file
• Consider chunking setup
• Consider on-demand compiling with digests when digesting=true
• I'm sure a ton of other shit

Webpacker is released under the MIT License.