# Importmap for Rails [Import maps](https://github.com/WICG/import-maps) let you import JavaScript modules using logical names that map to versioned/digested files – directly from the browser. So you can [build modern JavaScript applications using JavaScript libraries made for ESM without the need for transpiling or bundling](https://world.hey.com/dhh/modern-web-apps-without-javascript-bundling-or-transpiling-a20f2755).This frees you from needing Webpack, Yarn, npm, or any other part of the JavaScript toolchain. All you need is the asset pipeline that's already included in Rails. With this approach you'll ship many small JavaScript files instead of one big JavaScript file. Thanks to HTTP2 that no longer carries a material performance penalty during the initial transport, and in fact offers substantial benefits over the long run due to better caching dynamics. Whereas before any change to any JavaScript file included in your big bundle would invalidate the cache for the the whole bundle, now only the cache for that single file is invalidated. There's [native support for import maps in Chrome/Edge 89+](https://caniuse.com/?search=importmap), and [a shim available](https://github.com/guybedford/es-module-shims) for any browser with basic ESM support. So your app will be able to work with all the evergreen browsers. ## Installation Importmap for Rails is automatically included in Rails 7+ for new applications, but you can also install it manually in existing applications: 1. Add `importmap-rails` to your Gemfile with `gem 'importmap-rails'` 2. Run `./bin/bundle install` 3. Run `./bin/rails importmap:install` Note: In order to use JavaScript from Rails frameworks like Action Cable, Action Text, and Active Storage, you must be running Rails 7.0+. This was the first version that shipped with ESM compatible builds of these libraries. ## Usage The import map is setup through `Rails.application.config.importmap` via the configuration in `config/importmap.rb`. This file is automatically reloaded in development upon changes, but note that you must restart the server if you remove pins and need them gone from the rendered importmap or list of preloads. This import map is inlined in the `` of your application layout using `<%= javascript_importmap_tags %>`, which will setup the JSON configuration inside a ``. That logical entrypoint, `application`, is mapped in the importmap script tag to the file `app/javascript/application.js`. It's in `app/javascript/application.js` you setup your application by importing any of the modules that have been defined in the import map. You can use the full ESM functionality of importing any particular export of the modules or everything. It makes sense to use logical names that match the package names used by npm, such that if you later want to start transpiling or bundling your code, you won't have to change any module imports. ## Using node modules via JavaScript CDNs Importmap for Rails is designed to be used with JavaScript CDNs for your node package dependencies. The CDNs provide pre-compiled distribution versions ready to use, and offer a fast, efficient way of serving them. You can use the `./bin/importmap` command that's added as part of the install to pin, unpin, or update node packages in your import map. This command uses an API from [JSPM.org](https://jspm.org) to resolve your package dependencies efficiently, and then add the pins to your `config/importmap.rb` file. It can resolve these dependencies from JSPM itself, but also from other CDNs, like [unpkg.com](https://unpkg.com) and [jsdelivr.com](https://www.jsdelivr.com). It works like so: ```bash /bin/importmap pin react react-dom Pinning "react" to https://ga.jspm.io/npm:react@17.0.2/index.js Pinning "react-dom" to https://ga.jspm.io/npm:react-dom@17.0.2/index.js Pinning "object-assign" to https://ga.jspm.io/npm:object-assign@4.1.1/index.js Pinning "scheduler" to https://ga.jspm.io/npm:scheduler@0.20.2/index.js ./bin/importmap json { "imports": { "application": "/application.js", "react": "https://ga.jspm.io/npm:react@17.0.2/index.js", "react-dom": "https://ga.jspm.io/npm:react-dom@17.0.2/index.js", "object-assign": "https://ga.jspm.io/npm:object-assign@4.1.1/index.js", "scheduler": "https://ga.jspm.io/npm:scheduler@0.20.2/index.js" } } ``` As you can see, the two packages react and react-dom resolve to a total of four dependencies, when resolved via the jspm default. Now you can use these in your application.js entrypoint like you would any other module: ```js import React from "react" import ReactDOM from "react-dom" ``` You can also designate a specific version to pin: ```bash ./bin/importmap pin react@17.0.1 Pinning "react" to https://ga.jspm.io/npm:react@17.0.1/index.js Pinning "object-assign" to https://ga.jspm.io/npm:object-assign@4.1.1/index.js ``` Or even remove pins: ```bash ./bin/importmap unpin react Unpinning "react" Unpinning "object-assign" ``` If you pin a package that has already been pinned, it'll be updated inline, along with its dependencies. You can control the environment of the package for packages with separate "production" (the default) and "development" builds: ```bash ./bin/importmap pin react --env development Pinning "react" to https://ga.jspm.io/npm:react@17.0.2/dev.index.js Pinning "object-assign" to https://ga.jspm.io/npm:object-assign@4.1.1/index.js ``` You can also pick an alternative, supported CDN provider when pinning, like `unpkg` or `jsdelivr` (`jspm` is the default): ```bash ./bin/importmap pin react --from jsdelivr Pinning "react" to https://cdn.jsdelivr.net/npm/react@17.0.2/index.js ``` Remember, though, that if you switch a pin from one provider to another, you may have to clean up dependencies added by the first provider that isn't used by the second provider. Run `./bin/importmap` to see all options. Note that this command is merely a convenience wrapper to resolving logical package names to CDN URLs. You can also just lookup the CDN URLs yourself, and then pin those. For example, if you wanted to use Skypack for React, you could just add the following to `config/importmap.rb`: ```ruby pin "react", to: "https://cdn.skypack.dev/react" ``` ## What if I don't like to use a JavaScript CDN? You always have the option to simply download the compiled JavaScript packages from the CDNs, and saving them locally in your application. You can put such files in app/javascript/vendor, and then reference them with local pins, like: ```ruby # config/importmap.rb pin "react", to: "vendor/react@17.0.2.js" ``` But using a JavaScript CDN is fast, secure, and easier to deal with. Start there. ## Preloading pinned modules To avoid the waterfall effect where the browser has to load one file after another before it can get to the deepest nested import, we use [modulepreload links](https://developers.google.com/web/updates/2017/12/modulepreload). Pinned modules are preloaded by default, but you can turn this off with `preload: false`. Example: ```ruby # config/importmap.rb pin "@github/hotkey", to: "https://ga.jspm.io/npm:@github/hotkey@1.4.4/dist/index.js" pin "md5", to: "https://cdn.jsdelivr.net/npm/md5@2.3.0/md5.js", preload: false # app/views/layouts/application.html.erb <%= javascript_importmap_tags %> # will include the following link before the importmap is setup: ... ``` ## Caching the import map and preload modules The import map should be cached in production, and is so by default via the `config.importmap.cached` option that will be set to the same value as `config.action_controller.perform_caching`, unless explicitly set differently. ## Expected errors from using the es-module-shim While import maps are native in Chrome and Edge, they need a shim in other browsers that'll produce a JavaScript console error like `TypeError: Module specifier, 'application' does not start with "/", "./", or "../".`. This error is normal and does not have any user-facing consequences. ## License Importmap for Rails is released under the [MIT License](https://opensource.org/licenses/MIT).