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Bundle and transpile JavaScript in Rails with bun, esbuild, rollup.js, or Webpack.


>= 6.0.0
 Project Readme

JavaScript Bundling for Rails

Use Bun, esbuild, rollup.js, or Webpack to bundle your JavaScript, then deliver it via the asset pipeline in Rails. This gem provides installers to get you going with the bundler of your choice in a new Rails application, and a convention to use app/assets/builds to hold your bundled output as artifacts that are not checked into source control (the installer adds this directory to .gitignore by default).

You develop using this approach by running the bundler in watch mode in a terminal with yarn build --watch (and your Rails server in another, if you're not using something like puma-dev). You can also use ./bin/dev, which will start both the Rails server and the JS build watcher (along with a CSS build watcher, if you're also using cssbundling-rails).

Whenever the bundler detects changes to any of the JavaScript files in your project, it'll bundle app/javascript/application.js into app/assets/builds/application.js (and all other entry points configured). You can refer to the build output in your layout using the standard asset pipeline approach with <%= javascript_include_tag "application", defer: true %>.

When you deploy your application to production, the javascript:build task attaches to the assets:precompile task to ensure that all your package dependencies from package.json have been installed via your javascript package manager (bun, npm, pnpm, or yarn), and then runs the build script defined in package.json to process all the entry points, as it would in development. The latter files are then picked up by the asset pipeline, digested, and copied into public/assets, as any other asset pipeline file.

This also happens in testing where the bundler attaches to the test:prepare task to ensure the JavaScript has been bundled before testing commences. If your testing library of choice does not call the test:prepare Rake task, ensure that your test suite runs javascript:build to bundle JavaScript before testing commences.

That's it!

You can configure your bundler options in the build script in package.json or via the installer-generated bun.config.js for Bun, rollup.config.js for rollup.js or webpack.config.json for Webpack (esbuild does not have a default configuration format, and we don't intend to use esbuild as an API in order to hack around it).

If you're already using webpacker and you're wondering if you should migrate to jsbundling-rails, have a look at the high-level comparison. If you're looking to migrate from webpacker, see the migration guide.

If you want to use webpack features like code splitting and hot module reloading, consider using the official fork of webpacker, shakapacker.


If you are installing esbuild, rollup, or webpack, you must already have node installed on your system. You will also need npx version 7.1.0 or later.

If you are using Bun, then you must have the Bun runtime already installed on your system.

To get started run:

./bin/bundle add jsbundling-rails
./bin/rails javascript:install:[bun|esbuild|rollup|webpack]

Or, in Rails 7+, you can preconfigure your new application to use a specific bundler with rails new myapp -j [bun|esbuild|rollup|webpack].


Is there a work-around for lack of glob syntax on Windows?

The default build script for esbuild relies on the app/javascript/*.* glob pattern to compile multiple entrypoints automatically. This glob pattern is not available by default on Windows, so you need to change the build script in package.json to manually list the entrypoints you wish to compile.

Why does bun/esbuild overwrite my application.css?

If you import CSS in your application.js while using esbuild or Bun, you'll be creating both an app/assets/builds/application.js and app/assets/builds/application.css file when bundling. The latter can conflict with the app/assets/builds/application.css produced by cssbundling-rails. The solution is to either change the output file for bun/esbuild (and the references for that) or for cssbundling. Both are specified in package.json.

How can I reference static assets in JavaScript code?

Suppose you have an image app/javascript/images/example.png that you need to reference in frontend code built with esbuild.

  1. Create the image at app/javascript/images/example.png.
  2. In package.json, under "scripts" and "build", add the additional arguments:
    • --loader:.png=file This instructs esbuild to copy png files to the build directory.
    • --asset-names=[name]-[hash].digested This tells esbuild to append .digested to the file name so that sprockets or propshaft will not append an additional digest hash to the file.
  3. When esbuild runs, it will copy the png file to something like app/assets/builds/example-5SRKKTLZ.digested.png.
  4. In frontend code, the image is available for import by its original name: import Example from "../images/example.png".
  5. The image itself can now be referenced by its imported name, e.g. in React, <img src={Example} />.
  6. The path of the image resolves to /assets/example-5SRKKTLZ.digested.png, which is served by the asset pipeline.


JavaScript Bundling for Rails is released under the MIT License.