Guides.rubyonrails.org

Chapters

1. What Is Webpacker?
   • What is Webpack?
   • How is Webpacker Different from Sprockets?
2. Installing Webpacker
3. Usage
   • Using Webpacker for JavaScript
   • Using Webpacker for CSS
   • Using Webpacker for Static Assets
   • Webpacker in Rails Engines
   • Hot Module Replacement (HMR)
4. Webpacker in Different Environments
5. Running Webpacker in Development
   • Deploying Webpacker
6. Additional Documentation

1 What Is Webpacker?

Webpacker is a Rails wrapper around the webpack build system that provides a standard webpack configuration and reasonable defaults.

1.1 What is Webpack?

The goal of webpack, or any front-end build system, is to allow you to write your front-end code in a way that is convenient for developers and then package that code in a way that is convenient for browsers. With webpack, you can manage JavaScript, CSS, and static assets like images or fonts. Webpack will allow you to write your code, reference other code in your application, transform your code, and combine your code into easily downloadable packs.

See the webpack documentation for information.

1.2 How is Webpacker Different from Sprockets?

Rails also ships with Sprockets, an asset-packaging tool whose features overlap with Webpacker. Both tools will compile your JavaScript into browser-friendly files and also minify and fingerprint them in production. In a development environment, Sprockets and Webpacker allow you to incrementally change files.

Sprockets, which was designed to be used with Rails, is somewhat simpler to integrate. In particular, code can be added to Sprockets via a Ruby gem. However, webpack is better at integrating with more current JavaScript tools and NPM packages and allows for a wider range of integration. New Rails apps are configured to use webpack for JavaScript and Sprockets for CSS, although you can do CSS in webpack.

You should choose Webpacker over Sprockets on a new project if you want to use NPM packages and/or want access to the most current JavaScript features and tools. You should choose Sprockets over Webpacker for legacy applications where migration might be costly, if you want to integrate using Gems, or if you have a very small amount of code to package.

If you are familiar with Sprockets, the following guide might give you some idea of how to translate. Please note that each tool has a slightly different structure, and the concepts don't directly map onto each other.

2 Installing Webpacker

To use Webpacker, you must install the Yarn package manager, version 1.x or up, and you must have Node.js installed, version 10.13.0 and up.

To include Webpacker in a new project, add --webpack to the rails new command. To add Webpacker to an existing project, add the webpacker gem to the project's Gemfile, run bundle install, and then run bin/rails webpacker:install.

Installing Webpacker creates the following local files:

The installation also calls the yarn package manager, creates a package.json file with a basic set of packages listed, and uses Yarn to install these dependencies.

3 Usage

3.1 Using Webpacker for JavaScript

With Webpacker installed, any JavaScript file in the app/javascript/packs directory will get compiled to its own pack file by default.

So if you have a file called app/javascript/packs/application.js, Webpacker will create a pack called application, and you can add it to your Rails application with the code <%= javascript_pack_tag "application" %>. With that in place, in development, Rails will recompile the application.js file every time it changes, and you load a page that uses that pack. Typically, the file in the actual packs directory will be a manifest that mostly loads other files, but it can also have arbitrary JavaScript code.

The default pack created for you by Webpacker will link to Rails' default JavaScript packages if they have been included in the project:

import Rails from "@rails/ujs"
import Turbolinks from "turbolinks"
import * as ActiveStorage from "@rails/activestorage"
import "channels"

Rails.start()
Turbolinks.start()
ActiveStorage.start()

You'll need to include a pack that requires these packages to use them in your Rails application.

It is important to note that only webpack entry files should be placed in the app/javascript/packs directory; Webpack will create a separate dependency graph for each entry point, so a large number of packs will increase compilation overhead. The rest of your asset source code should live outside this directory though Webpacker does not place any restrictions or make any suggestions on how to structure your source code. Here is an example:

app/javascript:
  ├── packs:
  │   # only webpack entry files here
  │   └── application.js
  │   └── application.css
  └── src:
  │   └── my_component.js
  └── stylesheets:
  │   └── my_styles.css
  └── images:
      └── logo.svg

Typically, the pack file itself is largely a manifest that uses import or require to load the necessary files and may also do some initialization.

If you want to change these directories, you can adjust the source_path (default app/javascript) and source_entry_path (default packs) in the config/webpacker.yml file.

Within source files, import statements are resolved relative to the file doing the import, so import Bar from "./foo" finds a foo.js file in the same directory as the current file, while import Bar from "../src/foo" finds a file in a sibling directory named src.

3.2 Using Webpacker for CSS

Out of the box, Webpacker supports CSS and SCSS using the PostCSS processor.

To include CSS code in your packs, first include your CSS files in your top-level pack file as though it was a JavaScript file. So if your CSS top-level manifest is in app/javascript/styles/styles.scss, you can import it with import styles/styles. This tells webpack to include your CSS file in the download. To actually load it in the page, include <%= stylesheet_pack_tag "application" %> in the view, where the application is the same pack name that you were using.

If you are using a CSS framework, you can add it to Webpacker by following the instructions to load the framework as an NPM module using yarn, typically yarn add <framework>. The framework should have instructions on importing it into a CSS or SCSS file.

3.3 Using Webpacker for Static Assets

The default Webpacker configuration should work out of the box for static assets. The configuration includes several image and font file format extensions, allowing webpack to include them in the generated manifest.json file.

With webpack, static assets can be imported directly in JavaScript files. The imported value represents the URL to the asset. For example:

import myImageUrl from '../images/my-image.jpg'

// ...
let myImage = new Image();
myImage.src = myImageUrl;
myImage.alt = "I'm a Webpacker-bundled image";
document.body.appendChild(myImage);

If you need to reference Webpacker static assets from a Rails view, the assets need to be explicitly required from Webpacker-bundled JavaScript files. Unlike Sprockets, Webpacker does not import your static assets by default. The default app/javascript/packs/application.js file has a template for importing files from a given directory, which you can uncomment for every directory you want to have static files in. The directories are relative to app/javascript. The template uses the directory images, but you can use anything in app/javascript:

const images = require.context("../images", true)
const imagePath = name => images(name, true)

Static assets will be output into a directory under public/packs/media. For example, an image located and imported at app/javascript/images/my-image.jpg will be output at public/packs/media/images/my-image-abcd1234.jpg. To render an image tag for this image in a Rails view, use image_pack_tag 'media/images/my-image.jpg.

The Webpacker ActionView helpers for static assets correspond to asset pipeline helpers according to the following table:

Also, the generic helper asset_pack_path takes the local location of a file and returns its Webpacker location for use in Rails views.

You can also access the image by directly referencing the file from a CSS file in app/javascript.

3.4 Webpacker in Rails Engines

As of Webpacker version 6, Webpacker is not "engine-aware," which means Webpacker does not have feature-parity with Sprockets when it comes to using within Rails engines.

Gem authors of Rails engines who wish to support consumers using Webpacker are encouraged to distribute frontend assets as an NPM package in addition to the gem itself and provide instructions (or an installer) to demonstrate how host apps should integrate. A good example of this approach is Alchemy CMS.

3.5 Hot Module Replacement (HMR)

Webpacker out-of-the-box supports HMR with webpack-dev-server, and you can toggle it by setting dev_server/hmr option inside webpacker.yml.

Check out webpack's documentation on DevServer for more information.

To support HMR with React, you would need to add react-hot-loader. Check out React Hot Loader's Getting Started guide.

Don't forget to disable HMR if you are not running webpack-dev-server; otherwise, you will get a "not found error" for stylesheets.

4 Webpacker in Different Environments

Webpacker has three environments by default development, test, and production. You can add additional environment configurations in the webpacker.yml file and set different defaults for each environment. Webpacker will also load the file config/webpack/<environment>.js for additional environment setup.

5 Running Webpacker in Development

Webpacker ships with two binstub files to run in development: ./bin/webpack and ./bin/webpack-dev-server. Both are thin wrappers around the standard webpack.js and webpack-dev-server.js executables and ensure that the right configuration files and environmental variables are loaded based on your environment.

By default, Webpacker compiles automatically on demand in development when a Rails page loads. This means that you don't have to run any separate processes, and compilation errors will be logged to the standard Rails log. You can change this by changing to compile: false in the config/webpacker.yml file. Running bin/webpack will force the compilation of your packs.

If you want to use live code reloading or have enough JavaScript that on-demand compilation is too slow, you'll need to run ./bin/webpack-dev-server or ruby ./bin/webpack-dev-server. This process will watch for changes in the app/javascript/packs/*.js files and automatically recompile and reload the browser to match.

Windows users will need to run these commands in a terminal separate from bundle exec rails server.

Once you start this development server, Webpacker will automatically start proxying all webpack asset requests to this server. When you stop the server, it'll revert to on-demand compilation.

The Webpacker Documentation gives information on environment variables you can use to control webpack-dev-server. See additional notes in the rails/webpacker docs on the webpack-dev-server usage.

5.1 Deploying Webpacker

Webpacker adds a webpacker:compile task to the assets:precompile rake task, so any existing deploy pipeline that was using assets:precompile should work. The compile task will compile the packs and place them in public/packs.

6 Additional Documentation

For more information on advanced topics, such as using Webpacker with popular frameworks, consult the Webpacker Documentation.

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