# JavaScript Bundling for Rails

Use [esbuild](https://esbuild.github.io), [rollup.js](https://rollupjs.org), or [Webpack](https://webpack.js.org) 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](https://github.com/puma/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 yarn, and then runs `yarn build` 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. (Note that this currently only applies to rails `test:*` tasks (like `test:all` or `test:controllers`), not "rails test", as that doesn't load `test:prepare`).

If your testing library of choice does not define a `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 `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`](https://github.com/rails/webpacker) and you're wondering if you should migrate to `jsbundling-rails`, have a look at [the high-level comparison](./docs/comparison_with_webpacker.md). If you're looking to migrate from webpacker, see the [migration guide](https://github.com/rails/jsbundling-rails/blob/main/docs/switch_from_webpacker.md).

If you want to use webpack features like [code splitting](https://webpack.js.org/guides/code-splitting/) and [hot module reloading](https://webpack.js.org/concepts/hot-module-replacement/), consider using the official fork of `webpacker`, [`shakapacker`](https://github.com/shakacode/shakapacker).

## Installation

You must already have node and yarn installed on your system. You will also need npx version 7.1.0 or later. Then:

1. Run `./bin/bundle add jsbundling-rails`
2. Run `./bin/rails javascript:install:[esbuild|rollup|webpack]`

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


## FAQ

### 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 esbuild overwrite my application.css?

If you [import CSS](https://esbuild.github.io/content-types/#css-from-js) in your application.js while using esbuild, 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](https://github.com/rails/cssbundling-rails). The solution is to either change the output file for 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`.
1. In `package.json`, under `"scripts"` and `"build"`, add the option `--loader:.png=file` to the esbuild script, which instructs esbuild to copy png files to the build directory.
1. When esbuild runs, it will copy the png file to something like `app/assets/builds/example-5SRKKTLZ.png`.
1. In frontend code, the image is available for import by its original name: `import Example from "../images/example.png"`.
1. The image itself can now be referenced by its imported name, e.g. in React, `<img src={Example} />`.
1. The path of the image resolves to `/assets/example-5SRKKTLZ.png`, which is served by the asset pipeline.

## License

JavaScript Bundling for Rails is released under the [MIT License](https://opensource.org/licenses/MIT).