Proscenium - Integrated Frontend Development for Rails

🗣️ prow · see · nee · uhm

noun: proscenium

• the part of a theatre stage in front of the curtain.

Proscenium treats your frontend and client-side code as first class citizens of your Rails app, and assumes a "fast by default" internet. It bundles and minifies JavaScript (+ JSX), TypeScript (+TSX) and CSS in real time, on demand, and with zero configuration.

The highlights:

• Fast, real-time bundling, tree-shaking, code-splitting and minification of Javascript (.js,.jsx), Typescript (.ts,.tsx) and CSS (.css).
• NO JavaScript runtime needed (eg. Node) - just the browser!
• NO build step or pre-compilation.
• NO additional process or server - Just run rails s!
• Transforms newer JavaScript and CSS syntax to older syntax for older browsers.
• Deep integration with Rails.
• Automatically side-load your layouts, views, and partials.
• Import from NPM, URL's, and locally.
• Server-side import map support.
• CSS Modules & mixins.
• Source maps.

Table of Contents

• Getting Started
• Installation
• Client-Side Code Anywhere
• Side Loading
• Importing
  • Local Imports
• Import Maps
• Source Maps
• SVG
• Environment Variables
• i18n
• JavaScript
  • Tree Shaking
  • Code Splitting
  • JavaScript Caveats
• CSS
  • Importing CSS from JavaScript
  • CSS Modules
  • CSS Mixins
  • CSS Caveats
• Typescript
  • Typescript Caveats
• JSX
• JSON
• Phlex Support
• ViewComponent Support
• Cache Busting
• rjs is back!
• Resolution
  • Assets from Rails Engines
• Thanks
• Development

Getting Started

Getting started obviously depends on whether you are adding Proscenium to an existing Rails app, or creating a new one. So choose the appropriate guide below:

• Getting Started with a new Rails app
• Getting Started with an existing Rails app
  • Migrate from Sprockets
  • Migrate from Propshaft [Coming soon]
  • Migrate from Webpacker [Coming soon]
• Render a React component with Proscenium

Installation

Add this line to your Rails application's Gemfile, and you're good to go:

gem 'proscenium'

Please note that Proscenium is designed solely for use with Rails.

Now if you start your Rails app, you can open any front end code (JS, CSS, etc.). For example, a file at app/assets/stylesheets/application.css can be accessed at https://localhost:3000/app/assets/stylesheets/application.css, which will be bundled, transformed, and minified [in production] in real time.

Client-Side Code Anywhere

Proscenium believes that your frontend code is just as important as your backend code, and is not an afterthought - they should be first class citizens of your Rails app. So instead of having to throw all your JS and CSS into a "app/assets" directory, and then requiring a separate process to compile or bundle, just put them wherever you want within your app, and just run Rails!

For example, if you have some JS that is required by your app/views/users/index.html.erb view, just create a JS file alongside it at app/views/users/index.js. Or if you have some CSS that is used by your entire application, put it in app/views/layouts/application.css and load it alongside your layout. Maybe you have a few JS utility functions, so put them in lib/utils.js.

Simply put your JS(X) and CSS anywhere you want, and they will be served by your Rails app from the location where you placed them.

Using the examples above...

• app/views/users/index.js => https://localhost:3000/app/views/users/index.js
• app/views/layouts/application.css => https://localhost:3000/app/views/layouts/application.css
• lib/utils.js => https://localhost:3000/lib/utils.js
• app/components/menu_component.jsx => https://localhost:3000/app/components/menu_component.jsx
• config/properties.css => https://localhost:3000/config/properties.css

Side Loading

Proscenium is best experienced when your assets are automtically side loaded.

The Problem

With Rails you would typically declaratively load your JavaScript and CSS assets using the javascript_include_tag and stylesheet_link_tag helpers.

For example, you may have top-level "application" CSS located in a file at /app/assets/stylesheets/application.css. Likewise, you may have some global JavaScript located in a file at /app/javascript/application.js.

You would manually and declaratively include those two files in your application layout, something like this:

<%# /app/views/layouts/application.html.erb %>

<!DOCTYPE html>
<html>
  <head>
    <title>Hello World</title>
    <%= stylesheet_link_tag 'application' %> <!-- << Your app CSS -->
  </head>
  <body>
    <%= yield %>
    <%= javascript_include_tag 'application' %> <!-- << Your app JS -->
  </body>
</html>

Now, you may have some CSS and JavaScript that is only required by a specific view and partial, so you would load that in your view (or layout), something like this:

<%# /app/views/users/index.html.erb %>

<%= stylesheet_link_tag 'users' %>
<%= javascript_include_tag 'users' %>

<%# needed by the `users/_user.html.erb` partial %>
<%= javascript_include_tag '_user' %>

<% render @users %>

The main problem is that you have to keep track of all these assets, and make sure each is loaded by all the views that require them, but also avoid loading them when not needed. This can be a real pain, especially when you have a lot of views.

The Solution

When side loading your JavaScript, Typescript and CSS with Proscenium, they are automatically included alongside your views, partials, layouts, and components, and only when needed.

Side loading works by looking for a JS/TS/CSS file with the same name as your view, partial, layout or component. For example, if you have a view at app/views/users/index.html.erb, then Proscenium will look for a JS/TS/CSS file at app/views/users/index.js, app/views/users/index.ts or app/views/users/index.css. If it finds one, it will include it in the HTML for that view.

JSX is also supported for JavaScript and Typescript. Simply use the .jsx or .tsx extension instead of .js or .ts.

Usage

Simply create a JS and/or CSS file with the same name as any view, partial or layout.

Let's continue with our problem example above, where we have the following assets

• /app/assets/application.css
• /app/assets/application.js
• /app/assets/users.css
• /app/assets/users.js
• /app/assets/user.js

Your application layout is at /app/views/layouts/application.hml.erb, and the view that needs the users assets is at /app/views/users/index.html.erb, so move your assets JS and CSS alongside them:

• /app/views/layouts/application.css
• /app/views/layouts/application.js
• /app/views/users/index.css
• /app/views/users/index.js
• /app/views/users/_user.js (partial)

Now, in your layout and view, replace the javascript_include_tag and stylesheet_link_tag helpers with the include_asset helper from Proscenium. Something like this:

<!DOCTYPE html>
<html>
  <head>
    <title>Hello World</title>
    <%= include_assets # <-- %>
  </head>
  <body>
    <%= yield %>
  </body>
</html>

On each page request, Proscenium will check if any of your views, layouts and partials have a JS/TS/CSS file of the same name, and then include them wherever your placed the include_assets helper.

Now you never have to remember to include your assets again. Just create them alongside your views, partials and layouts, and Proscenium will take care of the rest.

Side loading is enabled by default, but you can disable it by setting config.proscenium.side_load to false in your /config/application.rb.

There are also include_stylesheets and include_javascripts helpers to allow you to control where the CSS and JS assets are included in the HTML. These helpers should be used instead of include_assets if you want to control exactly where the assets are included.

Bundling

To bundle a file means to inline any imported dependencies into the file itself. This process is recursive so dependencies of dependencies (and so on) will also be inlined.

Proscenium will bundle by default, and in real time. So there is no separate build step or pre-compilation.

Proscenium supports importing JS, JSX, TS, TSX, CSS and SVG from NPM, by URL, your local app, and even from other Ruby Gems.

Both static (import) and dynamic (import()) imports are supported for JavaScript and TypeScript, and can be used to import JS, TS, JSX, TSX, JSON, CSS and SVG files.

The @import CSS at-rule is supported for CSS.

Non-analyzable imports

Import paths are currently only bundled if they are a string literal or a glob pattern. Other forms of import paths are not bundled, and are instead preserved verbatim in the generated output. This is because bundling is a compile-time operation and Proscenium doesn't support all forms of run-time path resolution.

Here are some examples:

// Analyzable imports (will be bundled)
import "pkg";
import("pkg");
import(`./locale-${foo}.json`);

// Non-analyzable imports (will not be bundled)
import(`pkg/${foo}`);

The way to work around non-analyzable imports is to mark the package containing this problematic code as unbundled so that it's not included in the bundle. You will then need to ensure that a copy of the external package is available to your bundled code at run-time.

Import from NPM (node_modules)

Bare imports (imports not beginning with ./, /, https://, http://) are fully supported, and will use your package manager of choice (eg, NPM, Yarn, pnpm) via the package.json file located at the root of your Rails app.

Install the package you want to import using your package manager of choice...

npm install react

...and then import it as you would any other package.

import React from "react";

Local Imports

And of course you can import your own code, using relative or absolute paths (file extension is optional, and absolute paths use your Rails root as the base):

import utils from "/lib/utils";
import constants from "./constants";
import Header from "/app/components/header";

@import "/lib/reset";

Unbundling

Sometimes you don't want to bundle an import. For example, you want to ensure that only one instance of React is loaded. In this cases, you can use the unbundle prefix

import React from "unbundle:react";

This only works any bare and local imports.

You can also use the unbundle prefix in your import map, which ensures that all imports of a particular path is always unbundled:

{
  "imports": {
    "react": "unbundle:react"
  }
}

Then just import as normal:

import React from "react";

Import Maps

[WIP]

Import maps for both JS and CSS is supported out of the box, and works with no regard to the browser being used. This is because the import map is parsed and resolved by Proscenium on the server, instead of by the browser. This is faster, and also allows you to use import maps in browsers that do not support them yet.

If you are not familiar with import maps, think of them as a way to define aliases.

Just create config/import_map.json and specify the imports you want to use. For example:

{
  "imports": {
    "react": "https://esm.sh/react@18.2.0",
    "start": "/lib/start.js",
    "common": "/lib/common.css",
    "@radix-ui/colors/": "https://esm.sh/@radix-ui/colors@0.1.8/"
  }
}

Using the above import map, we can do...

import { useCallback } from "react";
import startHere from "start";
import styles from "common";

and for CSS...

@import "common";
@import "@radix-ui/colors/blue.css";

You can also write your import map in JavaScript instead of JSON. So instead of config/import_map.json, create config/import_map.js, and define an anonymous function. This function accepts a single environment argument.

(env) => ({
  imports: {
    react:
      env === "development"
        ? "https://esm.sh/react@18.2.0?dev"
        : "https://esm.sh/react@18.2.0",
  },
});

Source Maps

Source maps can make it easier to debug your code. They encode the information necessary to translate from a line/column offset in a generated output file back to a line/column offset in the corresponding original input file. This is useful if your generated code is sufficiently different from your original code (e.g. your original code is TypeScript or you enabled minification). This is also useful if you prefer looking at individual files in your browser's developer tools instead of one big bundled file.

Source map output is supported for both JavaScript and CSS. Each file is appended with the link to the source map. For example:

//# sourceMappingURL=/app/views/layouts/application.js.map

Your browsers dev tools should pick this up and automatically load the source map when and where needed.

SVG

You can import SVG from JS(X), which will bundle the SVG source code. Additionally, if importing from JSX or TSX, the SVG source code will be rendered as a JSX/TSX component.

Environment Variables

Available in >=0.10.0

You can define and access any environment variable from your JavaScript and Typescript under the proscenium.env namespace.

For performance and security reasons you must declare the environment variable names that you wish to expose in your config/application.rb file.

config.proscenium.env_vars = Set['API_KEY', 'SOME_SECRET_VARIABLE']
config.proscenium.env_vars << 'ANOTHER_API_KEY'

This assumes that the environment variable of the same name has already been defined. If not, you will need to define it yourself either in your code using Ruby's ENV object, or in your shell.

These declared environment variables will be replaced with constant expressions, allowing you to use this like this:

console.log(proscenium.env.RAILS_ENV); // console.log("development")
console.log(proscenium.env.RAILS_ENV === "development"); // console.log(true)

The RAILS_ENV and NODE_ENV environment variables will always automatically be declared for you.

In addition to this, Proscenium also provides a process.env.NODE_ENV variable, which is set to the same value as proscenium.env.RAILS_ENV. It is provided to support the community's existing tooling, which often relies on this variable.

Environment variables are particularly powerful in aiding tree shaking.

function start() {
  console.log("start");
}
function doSomethingDangerous() {
  console.log("resetDatabase");
}

proscenium.env.RAILS_ENV === "development" && doSomethingDangerous();

start();

In development the above code will be transformed into the following code, discarding the definition, and call todoSomethingDangerous().

function start() {
  console.log("start");
}
start();

Please note that for security reasons environment variables are not replaced in URL imports.

An undefined environment variable will be replaced with undefined.

console.log(proscenium.env.UNKNOWN); // console.log((void 0).UNKNOWN)

This means that code that relies on this will not be tree shaken. You can work around this by using the optional chaining operator:

if (typeof proscenium.env?.UNKNOWN !== "undefined") {
  // do something if UNKNOWN is defined
}

i18n

Basic support is provided for importing your Rails locale files from config/locales/*.yml, exporting them as JSON.

import translations from "@proscenium/i18n";
// translations.en.*

Javascript

By default, Proscenium's output will take advantage of all modern JS features from the ES2022 spec and earlier. For example, a !== void 0 && a !== null ? a : b will become a ?? b when minifying (enabled by default in production), which makes use of syntax from the ES2020 version of JavaScript. Any syntax feature that is not supported by ES2020 will be transformed into older JavaScript syntax that is more widely supported.

Tree Shaking

Tree shaking is the term the JavaScript community uses for dead code elimination, a common compiler optimization that automatically removes unreachable code. Tree shaking is enabled by default in Proscenium.

function one() {
  console.log("one");
}
function two() {
  console.log("two");
}
one();

The above code will be transformed to the following code, discarding two(), as it is never called.

function one() {
  console.log("one");
}
one();

Code Splitting

Available in >=0.10.0.

Side loaded assets are automatically code split. This means that if you have a file that is imported and used imported several times, and by different files, it will be split off into a separate file.

As an example:

// /lib/son.js
import father from "./father";

father() + " and Son";

// /lib/daughter.js
import father from "./father";

father() + " and Daughter";

// /lib/father.js
export default () => "Father";

Both son.js and daughter.js import father.js, so both son and daughter would usually include a copy of father, resulting in duplicated code and larger bundle sizes.

If these files are side loaded, then father.js will be split off into a separate file or chunk, and only downloaded once.

• Code shared between multiple entry points is split off into a separate shared file that both entry points import. That way if the user first browses to one page and then to another page, they don't have to download all of the JavaScript for the second page from scratch if the shared part has already been downloaded and cached by their browser.

• Code referenced through an asynchronous import() expression will be split off into a separate file and only loaded when that expression is evaluated. This allows you to improve the initial download time of your app by only downloading the code you need at startup, and then lazily downloading additional code if needed later.

• Without code splitting, an import() expression becomes Promise.resolve().then(() => require()) instead. This still preserves the asynchronous semantics of the expression but it means the imported code is included in the same bundle instead of being split off into a separate file.

Code splitting is enabled by default. You can disable it by setting the code_splitting configuration option to false in your application's /config/application.rb:

config.proscenium.code_splitting = false

JavaScript Caveats

There are a few important caveats as far as JavaScript is concerned. These are detailed on the esbuild site.

CSS

CSS is a first-class content type in Proscenium, which means it can bundle CSS files directly without needing to import your CSS from JavaScript code. You can @import other CSS files and reference image and font files with url() and Proscenium will bundle everything together.

Note that by default, Proscenium's output will take advantage of all modern CSS features. For example, color: rgba(255, 0, 0, 0.4) will become color: #f006 after minifying in production, which makes use of syntax from CSS Color Module Level 4.

The new CSS nesting syntax is supported, and transformed into non-nested CSS for older browsers.

Proscenium will also automatically insert vendor prefixes so that your CSS will work in older browsers.

Importing CSS from JavaScript

You can also import CSS from JavaScript. When you do this, Proscenium will automatically append each stylesheet to the document's head as a <link> element.

import "./button.css";

export let Button = ({ text }) => {
  return <div className="button">{text}</div>;
};

CSS Modules

Proscenium implements a subset of CSS Modules. It supports the :local and :global keywords, but not the composes property. (it is recommended that you use mixins instead of composes, as they will work everywhere, even in plain CSS files.)

Give any CSS file a .module.css extension, and Proscenium will treat it as a CSS Module, transforming all class names with a suffix unique to the file.

.title {
  font-size: 20em;
}

The above input produces:

.title-5564cdbb {
  font-size: 20em;
}

You now have a unique class name that you can use pretty much anywhere.

In your Views

You can reference CSS modules from your Rails views, partials, and layouts using the css_module helper, which accepts one or more class names, and will return the equivilent CSS module names - the class name with the unique suffix appended.

With side-loading setup, you can use the css_module helper as follows.

<div>
  <h1 class="<%= css_module :hello_title %>">Hello World</h1>
  <p class="<%= css_module :body, paragraph: %>">
    Lorem ipsum dolor sit amet, consectetur adipiscing elit.
  </p>
</div>

css_module accepts multiple class names, and will return a space-separated string of transformed CSS module names.

css_module :my_module_name
# => "my_module_name-ABCD1234"

You can even reference a class from any CSS file by passing the URL path to the file, as a prefix to the class name. Doing so will automatically side load the stylesheet.

css_module '/app/components/button.css@big_button'
# => "big_button"

It also supports NPM packages (already installed in /node_modules):

css_module 'mypackage/button@big_button'
# => "big_button"

css_module also accepts a path keyword argument, which allows you to specify the path to the CSS file. Note that this will use the given path for all class names passed to that instance of css_module.

css_module :my_module_name, path: Rails.root.join('app/components/button.css')

In your JavaScript

Importing a CSS module from JS will automatically append the stylesheet to the document's head. And the result of the import will be an object of CSS class to module names.

import styles from "./styles.module.css";
// styles == { header: 'header-5564cdbb' }

It is important to note that the exported object of CSS module names is actually a JavaScript Proxy object. So destructuring the object will not work. Instead, you must access the properties directly.

Also, importing a CSS module into another CSS module will result in the same digest string for all classes.

CSS Mixins

Proscenium provides functionality for including or "mixing in" onr or more CSS classes into another. This is similar to the composes property of CSS Modules, but works everywhere, and is not limited to CSS Modules.

CSS mixins are supported using the @define-mixin and @mixin at-rules.

A mixin is defined using the @define-mixin at-rule. Pass it a name, which should adhere to class name semantics, and declare your rules:

// /lib/mixins.css
@define-mixin bigText {
  font-size: 50px;
}

Use a mixin using the @mixin at-rule. Pass it the name of the mixin you want to use, and the url where the mixin is declared. The url is used to resolve the mixin, and can be relative, absolute, a URL, or even from an NPM packacge.

// /app/views/layouts/application.css
p {
  @mixin bigText from url("/lib/mixins.css");
  color: red;
}

The above produce this output:

p {
  font-size: 50px;
  color: red;
}

Mixins can be declared in any CSS file. They do not need to be declared in the same file as where they are used. however, if you declare and use a mixin in the same file, you don't need to specify the URL of where the mixin is declared.

@define-mixin bigText {
  font-size: 50px;
}

p {
  @mixin bigText;
  color: red;
}

CSS modules and Mixins works perfectly together. You can include a mixin in a CSS module.

CSS Caveats

There are a few important caveats as far as CSS is concerned. These are detailed on the esbuild site.

Typescript

Typescript and TSX is supported out of the box, and has built-in support for parsing TypeScript syntax and discarding the type annotations. Just rename your files to .ts or .tsx and you're good to go.

Please note that Proscenium does not do any type checking so you will still need to run tsc -noEmit in parallel with Proscenium to check types.

Typescript Caveats

There are a few important caveats as far as Typescript is concerned. These are detailed on the esbuild site.

JSX

Using JSX syntax usually requires you to manually import the JSX library you are using. For example, if you are using React, by default you will need to import React into each JSX file like this:

import * as React from "react";
render(<div />);

This is because the JSX transform turns JSX syntax into a call to React.createElement but it does not itself import anything, so the React variable is not automatically present.

Proscenium generates these import statements for you. Keep in mind that this also completely changes how the JSX transform works, so it may break your code if you are using a JSX library that is not React.

In the [not too distant] future, you will be able to configure Proscenium to use a different JSX library, or to disable this auto-import completely.

JSON

Importing .json files parses the JSON file into a JavaScript object, and exports the object as the default export. Using it looks something like this:

import object from "./example.json";
console.log(object);

In addition to the default export, there are also named exports for each top-level property in the JSON object. Importing a named export directly means Proscenium can automatically remove unused parts of the JSON file from the bundle, leaving only the named exports that you actually used. For example, this code will only include the version field when bundled:

import { version } from "./package.json";
console.log(version);

Phlex Support

Phlex is a framework for building fast, reusable, testable views in pure Ruby. Proscenium works perfectly with Phlex, with support for side-loading, CSS modules, and more. Simply write your Phlex classes and inherit from Proscenium::Phlex.

class MyView < Proscenium::Phlex
  def view_template
    h1 { 'Hello World' }
  end
end

In your layouts, include Proscenium::Phlex::AssetInclusions, and call the include_assets helper.

class ApplicationLayout < Proscenium::Phlex
  include Proscenium::Phlex::AssetInclusions # <--

  def view_template(&)
    doctype
    html do
      head do
        title { 'My Awesome App' }
        include_assets # <--
      end
      body(&)
    end
  end
end

You can specifically include CCS and JS assets using the include_stylesheets and include_javascripts helpers, allowing you to control where they are included in the HTML.

Side-loading

Any Phlex class that inherits Proscenium::Phlex will automatically be side-loaded.

CSS Modules

CSS Modules are fully supported in Phlex classes, with access to the css_module helper if you need it. However, there is a better and more seemless way to reference CSS module classes in your Phlex classes.

Within your Phlex classes, any class names that begin with @ will be treated as a CSS module class.

# /app/views/users/show_view.rb
class Users::ShowView < Proscenium::Phlex
  def view_template
    h1 class: :@user_name do
      @user.name
    end
  end
end

/* /app/views/users/show_view.module.css */
.userName {
  color: red;
  font-size: 50px;
}

In the above Users::ShowView Phlex class, the @user_name class will be resolved to the userName class in the users/show_view.module.css file.

The view above will be rendered something like this:

<h1 class="user_name-ABCD1234"></h1>

You can of course continue to reference regular class names in your view, and they will be passed through as is. This will allow you to mix and match CSS modules and regular CSS classes in your views.

# /app/views/users/show_view.rb
class Users::ShowView < Proscenium::Phlex
  def view_template
    h1 class: :[@user_name, :title] do
      @user.name
    end
  end
end

<h1 class="user_name-ABCD1234 title">Joel Moss</h1>

ViewComponent Support

ViewComponent iA framework for creating reusable, testable & encapsulated view components, built to integrate seamlessly with Ruby on Rails. Proscenium works perfectly with ViewComponent, with support for side-loading, CSS modules, and more. Simply write your ViewComponent classes and inherit from Proscenium::ViewComponent.

class MyView < Proscenium::ViewComponent
  def call
    tag.h1 'Hello World'
  end
end

Side-loading

Any ViewComponent class that inherits Proscenium::ViewComponent will automatically be side-loaded.

CSS Modules

CSS Modules are fully supported in ViewComponent classes, with access to the css_module helper if you need it.

# /app/components/user_component.rb
class UserComponent < Proscenium::ViewComponent
  def view_template
    div.h1 @user.name, class: css_module(:user_name)
  end
end

/* # /app/components/user_component.module.css */
.userName {
  color: red;
  font-size: 50px;
}

The view above will be rendered something like this:

<h1 class="user_name-ABCD1234">Joel Moss</h1>

Cache Busting

COMING SOON

By default, all assets are not cached by the browser. But if in production, you populate the REVISION env variable, all CSS and JS URL's will be appended with its value as a query string, and the Cache-Control response header will be set to public and a max-age of 30 days.

For example, if you set REVISION=v1, URL's will be appended with ?v1: /my/imported/file.js?v1.

It is assumed that the REVISION env var will be unique between deploys. If it isn't, then assets will continue to be cached as the same version between deploys. I recommend you assign a version number or to use the Git commit hash of the deploy. Just make sure it is unique for each deploy.

You can set the cache_query_string config option directly to define any query string you wish:

Rails.application.config.proscenium.cache_query_string = 'my-cache-busting-version-string'

The cache is set with a max-age of 30 days. You can customise this with the cache_max_age config option:

Rails.application.config.proscenium.cache_max_age = 12.months.to_i

rjs is back

Proscenium brings back RJS! Any path ending in .rjs will be served from your Rails app. This allows you to import server rendered javascript.

Resolution

Proscenium will serve files ending with any of these extension: js,mjs,ts,css,jsx,tsx from the following directories, and their sub-directories of your Rails application's root: /app, /lib, /config, /node_modules, /vendor.

So a file at /app/views/users/index.js will be served from https://localhost:3000/app/views/users/index.js.

You can continue to access any file in the /public directory as you normally would. Proscenium will not process files in the /public directory.

If requesting a file that exists in a root directory and the public directory, the file in the public directory will be served. For example, if you have a file at /lib/foo.js and /public/lib/foo.js, and you request /lib/foo.js, the file in the public directory (/public/lib/foo.js) will be served.

Assets from Rails Engines

Proscenium can serve assets from Rails Engines that are installed in your Rails app.

An engine that wants to expose its assets via Proscenium to the application must add Proscenium as a dependency, and add itself to the list of engines in the Proscenium config options Proscenium.config.engines.

For example, we have a gem called gem1 that has Proscenium as a dependency, and exposes a Rails engine. It has some assets that it wants to expose to the application. To do this, it adds itself to the list of engines in the Proscenium config engines option:

class Gem1::Engine < ::Rails::Engine
  config.proscenium.engines << self
end

When this gem is installed in any Rails application, its assets will be available at the URL /gem1/.... For example, if the gem has a file lib/styles.css, it can be requested at /gem1/lib/styles.css.

The same directories and file extensions are supported as for the application itself.

It is important to note that the application takes precedence over the gem. So if the application has a file at /public/gem1/lib/styles.css, and the gem also has a file at /lib/styles.css, then the file in the application will be served. This is because both files would be accessible at the same URL: /gem1/lib/styles.css.

Thanks

HUGE thanks 🙏 go to Evan Wallace and his amazing esbuild project. Proscenium would not be possible without it, and it is esbuild that makes this so fast and efficient.

Because Proscenium uses esbuild extensively, some of these docs are taken directly from the esbuild docs, with links back to the esbuild site where appropriate.

Development

Before doing anything else, you will need compile a local version of the Go binary. This is because the Go binary is not checked into the repo. To compile the binary, run:

bundle exec rake compile:local

Running tests

We have tests for both Ruby and Go. To run the Ruby tests:

bin/test

To run the Go tests:

go test ./test

Running Go benchmarks

go test ./internal/builder -bench=. -run="^$" -count=10 -benchmem

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/joelmoss/proscenium. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the code of conduct.

License

The gem is available as open source under the terms of the MIT License.

Code of Conduct

Everyone interacting in the Proscenium project's codebases, issue trackers, chat rooms and mailing lists is expected to follow the code of conduct.