Sprockets: Rack-based asset packaging

Sprockets is a Ruby library for compiling and serving web assets. It features declarative dependency management for JavaScript and CSS assets, as well as a powerful preprocessor pipeline that allows you to write assets in languages like CoffeeScript, Sass, SCSS and LESS.


Install Sprockets from RubyGems:

$ gem install sprockets

Or include it in your project's Gemfile with Bundler:

gem 'sprockets', '~> 2.0'

Understanding the Sprockets Environment

You'll need an instance of the Sprockets::Environment class to access and serve assets from your application. Under Rails 3.1 and later, YourApp::Application.assets is a preconfigured Sprockets::Environment instance. For Rack-based applications, create an instance in config.ru.

The Sprockets Environment has methods for retrieving and serving assets, manipulating the load path, and registering processors. It is also a Rack application that can be mounted at a URL to serve assets over HTTP.

The Load Path

The load path is an ordered list of directories that Sprockets uses to search for assets.

In the simplest case, a Sprockets environment's load path will consist of a single directory containing your application's asset source files. When mounted, the environment will serve assets from this directory as if they were static files in your public root.

The power of the load path is that it lets you organize your source files into multiple directories -- even directories that live outside your application -- and combine those directories into a single virtual filesystem. That means you can easily bundle JavaScript, CSS and images into a Ruby library and import them into your application.

Manipulating the Load Path

To add a directory to your environment's load path, use the append_path and prepend_path methods. Directories at the beginning of the load path have precedence over subsequent directories.

environment = Sprockets::Environment.new
environment.append_path 'app/assets/javascripts'
environment.append_path 'lib/assets/javascripts'
environment.append_path 'vendor/assets/jquery'

In general, you should append to the path by default and reserve prepending for cases where you need to override existing assets.

Accessing Assets

Once you've set up your environment's load path, you can mount the environment as a Rack server and request assets via HTTP. You can also access assets programmatically from within your application.

Logical Paths

Assets in Sprockets are always referenced by their logical path.

The logical path is the path of the asset source file relative to its containing directory in the load path. For example, if your load path contains the directory app/assets/javascripts:

Asset source file Logical path
app/assets/javascripts/application.js application.js
app/assets/javascripts/models/project.js models/project.js

In this way, all directories in the load path are merged to create a virtual filesystem whose entries are logical paths.

Serving Assets Over HTTP

When you mount an environment, all of its assets are accessible as logical paths underneath the mount point. For example, if you mount your environment at /assets and request the URL /assets/application.js, Sprockets will search your load path for the file named application.js and serve it.

Under Rails 3.1 and later, your Sprockets environment is automatically mounted at /assets. If you are using Sprockets with a Rack application, you will need to mount the environment yourself. A good way to do this is with the map method in config.ru:

require 'sprockets'
map '/assets' do
  environment = Sprockets::Environment.new
  environment.append_path 'app/assets/javascripts'
  environment.append_path 'app/assets/stylesheets'
  run environment

Accessing Assets Programmatically

You can use the find_asset method (aliased as []) to retrieve an asset from a Sprockets environment. Pass it a logical path and you'll get a Sprockets::BundledAsset instance back:

# => #<Sprockets::BundledAsset ...>

Call to_s on the resulting asset to access its contents, length to get its length in bytes, mtime to query its last-modified time, and pathname to get its full path on the filesystem.

Using Engines

Asset source files can be written in another language, like SCSS or CoffeeScript, and automatically compiled to CSS or JavaScript by Sprockets. Compilers for these languages are called engines.

Engines are specified by additional extensions on the asset source filename. For example, a CSS file written in SCSS might have the name layout.css.scss, while a JavaScript file written in CoffeeScript might have the name dialog.js.coffee.

Styling with Sass and SCSS

Sass is a language that compiles to CSS and adds features like nested rules, variables, mixins and selector inheritance.

If the sass gem is available to your application, you can use Sass to write CSS assets in Sprockets.

Sprockets supports both Sass syntaxes. For the original whitespace-sensitive syntax, use the extension .css.sass. For the new SCSS syntax, use the extension .css.scss.

Styling with LESS

LESS extends CSS with dynamic behavior such as variables, mixins, operations and functions.

If the less gem is available to your application, you can use LESS to write CSS assets in Sprockets. Note that the LESS compiler is written in JavaScript, and at the time of this writing, the less gem depends on therubyracer which embeds the V8 JavaScript runtime in Ruby.

To write CSS assets with LESS, use the extension .css.less.

Scripting with CoffeeScript

CoffeeScript is a language that compiles to the "good parts" of JavaScript, featuring a cleaner syntax with array comprehensions, classes, and function binding.

If the coffee-script gem is available to your application, you can use CoffeeScript to write JavaScript assets in Sprockets. Note that the CoffeeScript compiler is written in JavaScript, and you will need an ExecJS-supported runtime on your system to invoke it.

To write JavaScript assets with CoffeeScript, use the extension .js.coffee.

JavaScript Templating with EJS and Eco

Sprockets supports JavaScript templates for client-side rendering of strings or markup. JavaScript templates have the special format extension .jst and are compiled to JavaScript functions.

When loaded, a JavaScript template function can be accessed by its logical path as a property on the global JST object. Invoke a template function to render the template as a string. The resulting string can then be inserted into the DOM.

<!-- templates/hello.jst.ejs -->
<div>Hello, <span><%= name %></span>!</div>

// application.js
//= require templates/hello
$("#hello").html(JST["templates/hello"]({ name: "Sam" }));

Sprockets supports two JavaScript template languages: EJS, for embedded JavaScript, and Eco, for embedded CoffeeScript. Both languages use the familiar <% … %> syntax for embedding logic in templates.

If the ejs gem is available to your application, you can use EJS templates in Sprockets. EJS templates have the extension .jst.ejs.

If the eco gem is available to your application, you can use Eco templates in Sprockets. Eco templates have the extension .jst.eco. Note that the eco gem depends on the CoffeeScript compiler, so the same caveats apply as outlined above for the CoffeeScript engine.

Invoking Ruby with ERB

Sprockets provides an ERB engine for preprocessing assets using embedded Ruby code. Append .erb to a CSS or JavaScript asset's filename to enable the ERB engine.

Note: Sprockets processes multiple engine extensions in order from right to left, so you can use multiple engines with a single asset. For example, to have a CoffeeScript asset that is first preprocessed with ERB, use the extension .js.coffee.erb.

Ruby code embedded in an asset is evaluated in the context of a Sprockets::Context instance for the given asset. Common uses for ERB include:

  • embedding another asset as a Base64-encoded data: URI with the asset_data_uri helper
  • inserting the URL to another asset, such as with the asset_path helper provided by the Sprockets Rails plugin
  • embedding other application resources, such as a localized string database, in a JavaScript asset via JSON
  • embedding version constants loaded from another file

See the Helper Methods section for more information about interacting with Sprockets::Context instances via ERB.

String Interpolation Syntax

If you need access to Ruby from an asset but cannot use ERB's <% … %> syntax, Sprockets also supports Ruby string interpolation syntax (#{ … }) with the .str engine extension.

Managing and Bundling Dependencies

You can create asset bundles -- ordered concatenations of asset source files -- by specifying dependencies in a special comment syntax at the top of each source file.

Sprockets reads these comments, called directives, and processes them to recursively build a dependency graph. When you request an asset with dependencies, the dependencies will be included in order at the top of the file.

The Directive Processor

Sprockets runs the directive processor on each CSS and JavaScript source file. The directive processor scans for comment lines beginning with = in comment blocks at the top of the file.

//= require jquery
//= require jquery-ui
//= require backbone
//= require_tree .

The first word immediately following = specifies the directive name. Any words following the directive name are treated as arguments. Arguments may be placed in single or double quotes if they contain spaces, similar to commands in the Unix shell.

Note: Non-directive comment lines will be preserved in the final asset, but directive comments are stripped after processing. Sprockets will not look for directives in comment blocks that occur after the first line of code.

Supported Comment Types

The directive processor understands comment blocks in three formats:

/* Multi-line comment blocks (CSS, SCSS, JavaScript)
 *= require foo

// Single-line comment blocks (SCSS, JavaScript)
//= require foo

# Single-line comment blocks (CoffeeScript)
#= require foo

Sprockets Directives

You can use the following directives to declare dependencies in asset source files.

For directives that take a path argument, you may specify either a logical path or a relative path. Relative paths begin with ./ and reference files relative to the location of the current file.

The require Directive

require path inserts the contents of the asset source file specified by path. If the file is required multiple times, it will appear in the bundle only once.

The include Directive

include path works like require, but inserts the contents of the specified source file even if it has already been included or required.

The require_directory Directive

require_directory path requires all source files of the same format in the directory specified by path. Files are required in alphabetical order.

The require_tree Directive

require_tree path works like require_directory, but operates recursively to require all files in all subdirectories of the directory specified by path.

The require_self Directive

require_self tells Sprockets to insert the body of the current source file before any subsequent require or include directives.

The depend_on Directive

depend_on path declares a dependency on the given path without including it in the bundle. This is useful when you need to expire an asset's cache in response to a change in another file.


The Sprockets source code is hosted on GitHub. You can check out a copy of the latest code using Git:

$ git clone https://github.com/sstephenson/sprockets.git

If you've found a bug or have a question, please open an issue on the Sprockets issue tracker. Or, clone the Sprockets repository, write a failing test case, fix the bug and submit a pull request.


Copyright © 2011 Sam Stephenson <[email protected]>

Copyright © 2011 Joshua Peek <[email protected]>

Sprockets is distributed under an MIT-style license. See LICENSE for details.