Jammit is an industrial-strength asset packaging library for Rails, providing both the CSS and JavaScript concatenation and compression that you'd expect, as well as YUI Compressor, Closure Compiler, and UglifyJS compatibility, ahead-of-time gzipping, built-in JavaScript template support, and optional Data-URI / MHTML image and font embedding.

Current Version: 0.7.0

You can report bugs and discuss features on the issues page, on Freenode in the #documentcloud channel, or send tweets to @documentcloud.

Jammit is an open-source component of DocumentCloud.

Table of Contents

Installation | Configuration | Usage | YUI, Closure & UglifyJS | Precaching Assets
Expires Headers | Embedding Assets | JavaScript Templates | Changes


Source Code | Internal Docs | Image Embedding Example


  1. Grab the gem:
    gem install jammit
  2. Add the gem to Rails' environment.rb inside of the initializer:
    config.gem "jammit"
  3. If you're using Rails 3 or higher, you're done! Jammit routes (under /assets by default) will be loaded automatically into your app.

    If you're using Rails 2, edit config/routes.rb to give Jammit routes for dynamic asset packaging and caching.

    ActionController::Routing::Routes.draw do |map|

Note: If you don't already have the ruby-yui-compressor, closure-compiler or uglifier gems installed, downloading may take a minute — the jar files together weigh in at 5 megabytes.


Jammit uses the config/assets.yml YAML configuration file to define packages and to set extra options. A package is an ordered set of directory glob rules that will be expanded into a unique list of files. An example of a complete assets.yml follows:

embed_assets: on

    - public/javascripts/vendor/jquery.js
    - public/javascripts/lib/*.js
    - public/javascripts/views/**/*.js
    - app/views/workspace/*.jst

    - public/stylesheets/reset.css
    - public/stylesheets/widgets/*.css
    - public/stylesheets/pages/workspace.css
    - public/stylesheets/pages/empty.css

There are a number of extra configuration options that you may add to the assets.yml configuration file to customize the way Jammit behaves. Here's an example configuration file using all of the possible options. The meaning of the options and their default values are listed below. Don't be bewildered by the length of the list — none of the options are required, they just give you fine-grained control over how your packages are generated.

package_assets on | off | always Defaults to on, packaging and caching assets in every environment but development and test. Never packages when off, always packages when always.
embed_assets on | off | datauri Defaults to off. When on, packages and caches Data-URI and MTHML variants of your stylesheets, with whitelisted images embedded inline. Using datauri serves embedded images only to browsers that support Data-URIs, and serves unmodified stylesheets to Internet Explorer 7 or lower.
compress_assets on | off Defaults to on. When off, JavaScript and CSS packages will be left uncompressed (by YUI, Closure, or UglifyJS). Disabling compression is only recommended if you're packaging assets in development.
rewrite_relative_paths on | off Defaults to on. When off, relative paths with CSS will be left unchanged.
gzip_assets on | off Defaults to on. When off, only the plain version of the asset will be written out, not the .gz alternative. Disable this if you don't plan on configuring your webserver to serve the static gzip alternatives.
javascript_compressor yui | closure | uglifier Defaults to yui. The Jammit gem can use either the YUI Compressor, the Google Closure Compiler, or UglifyJS to compress your JavaScript.
allow_debugging on | off Defaults to on. Leaving it on allows you to pass debug_assets=true as a query parameter, in order to load the page with uncompressed, unpackaged assets in production.
template_function on | off | ... The JavaScript function that compiles your JavaScript templates (JST). Defaults to on, which uses a bundled variant of Micro-Templating. Set it to _.template if you use Underscore.js, or new Template for Prototype. Turn it off to pass through the template strings unaltered.
template_namespace ... By default, all of your compiled templates will be added to a top-level window.JST object. If you'd like to add them instead to your own JavaScript namespace, change it to the object of your choice.
template_extension ... By default, Jammit treats files with a jst extension as JavaScript templates. If you'd prefer to use a different extension, like html.mustache, set the template_extension option.
package_path ... The URL at which packaged assets are cached and made available. Defaults to assets, but if you already have an existing AssetsController with a different purpose, you could change it to, say, packages. (Single directory names only, please.)
compressor_options ... Pass an options hash directly to the underlying JavaScript compressor to configure it. See the ruby-yui-compressor, closure-compiler, or uglifier gem documentation for the full list of available options.
css_compressor_options ... Pass an options hash directly to the YUI CSS Compressor. Available options are charset, and line_break, which can be used to write out each CSS rule on a separate line.

The contents of assets.yml are passed through ERB before being loaded, in case you find the need to configure Jammit for different environments.

Warning: In Rails 3 and higher, static assets are not served by default. You either have to configure your webserver to serve static assets out of public (recommended), or set config.serve_static_assets = true


To access your packages in views, use the corresponding helper. The helper methods can include multiple packages at once:

<%= include_stylesheets :common, :workspace, :media => 'all' %>
<%= include_javascripts :workspace %>

In development, no packaging is performed, so you'll see a list of individual references to all of the JavaScript and CSS files. The assets.yml configuration file is reloaded on every development request, so you can change the contents of your packages without needing to restart Rails. In all other environments, or if package_assets is set to "always", you'll get tags for the merged packages.

If allow_debugging is left on, you'll be able to request the development version of the assets for any given page, by passing debug_assets=true as a query parameter.

You can easily use Jammit within your Rakefile, and other scripts:

require 'jammit'

YUI, Closure & UglifyJS

Jammit can be configured to use the YUI Compressor, the Google Closure Compiler, or UglifyJS to compress and optimize your JavaScript. (CSS is always run through the YUI Compressor.) Specify the javascript_compressor to choose either yui, closure, or uglifier backends. If left blank, Jammit defaults to yui.

You can configure the JavaScript compilation by adding compressor_options to your assets.yml. The compressor_options will be passed directly to the Gem backend of your chosen compressor. See the ruby-yui-compressor, closure-compiler, or uglifier gem documentation for all the available options. For example, to configure the Closure Compiler to use its advanced optimizations, you would add the compilation_level:

javascript_compressor: closure
  compilation_level: "ADVANCED_OPTIMIZATIONS"

Jammit always uses the YUI CSS Compressor to compress CSS files. You can configure it by specifying the css_compressor_options, in assets.yml.

Warning: Google's Closure Compiler has been known to choke on certain already pre-compressed JavaScripts, such as the production version of jQuery. Using the uncompressed development version is recommended.

Precaching Assets

Installing the Jammit gem provides the optional but handy jammit command-line utility, which can be hooked into your deployment process. The jammit command reads in your configuration file, generates all of the defined packages, and gzips them at the highest compression setting. In order to serve these static gzipped versions, configure your Nginx http_gzip_static_module, or your Apache Content Negotiation MultiViews. It's also a good idea to have gzip compression turned on for the remainder of your static assets, including any asset packages that aren't gzipped-in-advance. Adding Nginx's http_gzip_module or Apache's mod_deflate will do the trick.

The jammit command can be passed options to configure the path to assets.yml, and the output directory in which all packages are compiled. Run jammit --help to see all of the options. For speedy builds, jammit will check the modification times of your packages and source files: only the packages that have changed are rebuilt. If you'd like to force all packages to build, use jammit --force.

In production, you'll want to run Jammit during deployment, somewhere in between updating the source and symlinking to the new release. Whether you're using Capistrano, Vlad, or just good 'ol Rake, it shouldn't be too hard to add a step that calls the jammit command.

Expires Headers

To get the fastest page load times for your regular visitors, it's recommended to set the HTTP Expires header to a date far in the future. You don't need to worry about clearing the cached assets when you deploy a new release, because Rails will write out the asset's modification time as part of the URL, causing browsers to fetch a fresh copy of the asset.

If you're using an Nginx webserver, add the following snippet to the
server { ... } block for your application:

location ~ ^/assets/ {
  passenger_enabled on;
  expires max;

If you're using Apache, make sure that mod_expires is enabled, and then add the following snippet to the appropriate VirtualHost section of your configuration, filling in the path to your deployed application:

ExpiresActive On
<Directory "/path/to/public/assets">
  ExpiresDefault "access plus 1 year"

If for any reason (multiple physical servers, rsync-only deploys) you don't wish to rely on modification times for cache expiry, Jammit uses the standard Rails helpers to generate URLs, and will respect the RAILS_ASSET_ID environment variable.

Embedding Images ...

After you've finished concatenating and compressing your JavaScript and CSS files into streamlined downloads, the slowest part of your page load is probably the images. It's common to use image sprites to avoid the avalanche of HTTP requests that are needed to load a bunch of small images. Unfortunately, image sprites can be complicated to position (especially with horizontal and vertical tiling), and a real pain to create and maintain. With a little elbow grease from Jammit, your spriting woes can be a thing of the past.

With embed_assets turned on, Jammit will inline image files directly into your compiled CSS, using Data-URIs in supported browsers, and MHTML in Internet Explorer 7 and below. Instead of ten CSS files referencing 30 images, you can have a single, packaged, minified, gzipped CSS file, with the images coming in all at once instead of piecemeal, making just a single HTTP request.

Take a look at this example (especially on a slow connection or wifi):
Normal Image Loading vs. Jammit Image Embedding

Embedded images can be a little tricky, which is why using them is strictly on an opt-in basis. After enabling embed_assets in the configuration file, you'll need to whitelist the images that you'd like to make embeddable. When processing CSS, Jammit will only embed images that have .../embed/... somewhere in their path — the other images will be left untouched. You can make a single public/images/embed folder for your embedded images, or organize them into directories however you prefer. It's not recommended to embed all of your site's images, just the ones that conform to the following three rules:

  1. Images that are small. Large images will simply delay the rendering of your CSS. Jammit won't embed images larger than 32 kilobytes, because Internet Explorer 8 won't render them.
  2. Images that are immediately visible. It's better to leave the images that are hidden at page load time to download in the background.
  3. Images that are referenced by a single CSS rule. Referencing the same embedded image in multiple rules will cause that image's contents to be embedded more than once, defeating the purpose. Replace the duplicated rules with an image-specific HTML class, and you're good to go.

A final cautionary note. Internet Explorer's implementation of MHTML requires the use of absolute paths in image references. This means that the timestamp that Rails appends to the stylesheet URL (to allow far-future expires headers) needs to also be in the contents of the stylesheet. If a process on your webserver changes the modification time of the cached MHTML stylesheet, it will break the image references. To fix it, use the jammit command (with --base-url) to rebuild your assets, or simply delete the cached file, and let Jammit automatically rebuild the file on the next request.

If the MHTML stylesheets sound too fragile, or if you encounter any problems with them in Internet Explorer (such as mixed-mode warnings when serving MHTML through SSL), we recommend setting embed_assets to "datauri". Using "datauri" will cause Internet Explorer 7 and below to receive plain versions of the packaged stylesheets, while all other browsers get the Data-URI flavor.

... and Embedding Fonts

Embedded fonts work in largely the same way as images, TTF and OTF files that are whitelisted within an "embed" folder will be inlined as Data-URIs. There is no MHTML variant for fonts, because Internet Explorer only supports unembeddable EOT files. Embedding is especially important for fonts in general, because it helps avoid the flash of unstyled text that the browser would otherwise display while waiting for the font to download. If you're looking to get started with web fonts, FontSquirrel has a great tool that can generate the proper fonts and styles you'll need. Here's an example of a CSS rule that would activate the proper Jammit embedding:

@font-face {
  font-family: 'DroidSans';
  src: url(/fonts/DroidSans.eot);
  src: local('Droid Sans'), local('DroidSans'), url(/fonts/embed/DroidSans.ttf) format('truetype');

JavaScript Templates

If you're using enough JavaScript to want it compressed and concatenated, you're probably using it to generate at least a little of your HTML. Traditionally, this has been accomplished by joining together strings and inserting them into the DOM. A far more agreeable way to generate HTML on the client is to use JavaScript templates (referred to here as JST).

Jammit helps keep your JavaScript views organized alongside your Rails views, bundling them together into packages, and providing them pre-compiled for your client-side code to evaluate. If left unspecified, Jammit uses a variant of John Resig's Micro Templating to compile the templates, but you can choose your preferred JavaScript templating engine of your choosing by setting the template_function in assets.yml. Jammit will run all of your templates through the function, and assign each one by filename to a top-level JST object. For example, the following template, app/views/drivers/license.jst:

<div class="drivers-license">
  <h2>Name: <%= name %></h2>
  <em>Hometown: <%= birthplace %></em>
  <div class="biography">
    <%= bio %>

Including this template within a JavaScript asset package makes it available within the browser as the JST.license function.

JST.license({name : "Moe", birthplace : "Brooklyn", bio : "Moe was always..."});

Since 0.5.0, templates should be included in the appropriate javascripts package. Here's an example of an assets.yml that uses templates, and shows the location at which they'll be available on the client. When including templates from different directories, the common prefix is ignored, and the rest of the path becomes the name of the template:

    - app/views/accounts/badge.jst
    - app/views/common/dialog.jst
    - app/views/common/menu.jst

Then, from your JavaScript:


To use Underscore.js templates, set template_function to _.template.
To use Prototype templates, set it to new Template.
To use Mustache.js templates, you'll need a little extra setup.
If you'd rather leave your template strings as strings, and compile them later, set template_function to off.

The default extension for template files is jst, but you can set template_extension in your assets.yml if you're using an engine with a preferred extension, such as html.mustache


0.7.0Oct 28, 2015
Added Rails 4 compatibility. (It's been compatible for a while, but only if you installed from GitHub.) Fixed Sass/SCSS compilation, but not yet compatible with the Rails development cycle since the SCSS won't be compiled to CSS in dev.

0.6.6Mar 20, 2013
Significant internal refactor. Added ability to customize CSS compressor. Defaulted to JSMin and CSSMin.

0.6.5Nov 30, 2011
Added the ability to pass HTML tag options to include_javascripts. Fixed a bug that would always cause JS to be recompiled, regardless of mtimes.

0.6.3May 26, 2011
Quick bugfix release for Rails 3.1 compatibility, which changes the method signatures of javascript_include_tag and stylesheet_link_tag in a backwards-incompatible fashion.

0.6.1May 25, 2011
Jammit now supports UglifyJS as a Java-free alternative to YUI and Closure, for JavaScript compression. You can now pass --packages core,embed,ui to the jammit command, in order to only build a subset of your asset packages. The built-in JS templating function has been upgraded to the equivalent of the latest _.template. Environment-specific settings are now supported in your assets.yml file.

You can now pass debug_assets=true as a query parameter to debug the un-packaged versions of your assets in production ... and disable this feature by specifying allow_debugging: off. Fixes for relative URL rewriting when Regex-unsafe characters exist in the base path. Fixes binary-mode file reading for some versions of Ruby 1.8 on Windows. It's now possible to embed .woff-formatted fonts in stylesheets, along with the rest.

Added a Jammit.package! helper function, for use in Rake tasks. Fixed a request_uri deprecation warning for Rails 3. Bugfix to allow template_extension to include periods.

You can now embed WOFF-formatted fonts. Jammit warnings avoid use of Rails.logger. Bugfixes for Ruby 1.9.2 with asset files containing Unicode characters.

Included missing rails/routes.rb file in the Gem manifest for Rails 3.

The Jammit gem is now compatible with Rails 3. JST template packages are no longer specified separately. This is a backwards-incompatible change, and you'll have to update your assets.yml file to simply include the JST in your existing JavaScript packages, as shown above. Alongside this change, template names are now better namespaced — see the section on JavaScript Templates for the details. Bugfix for IE8 and Data-URIs slightly under 32kb in size, as well as for MHTML in IE7-mode in Vista.

Jammit will now add the RAILS_ASSET_ID timestamp to image URLs within your CSS packages, if configured, for better cache-busting. Greg Hazel contributed a series of Jammit/Windows bug fixes.

Bugfix for building on Windows with drive-letter absolute paths.

Added a logged warning when you have rules in assets.yml that don't match any files.

Jammit is now able to embed @font-face web fonts in TTF and OTF formats. The embed_images option has been renamed to embed_assets for this change. The assets.yml file is now passed through ERB before being loaded, so you can configure environment-specific settings, just like you would with database.yml.

Added css_compressor_options to assets.yml, so you can set charset and line_break for the YUI CSS Compressor.

If Java isn't available on your machine, Jammit will now run in a graceful degraded mode, where assets are packaged but not compressed. You can now pass :embed_assets => false to the include_stylesheets helper to disable image embedding on a per-package basis.

Jammit now correctly rewrites relative image URLs within CSS, for easier integration with partial Rails apps deployed on sub-paths.

The jammit command has been enhanced to check the modification times of your packages — if no source files have changed, the package isn't rebuilt.

Jammit now raises an exception if Java isn't installed, or if the Java version is unsupported by your JavaScript compressor of choice.

When specifying your asset packages as directory globs, absolute globs are now absolute, and relative globs are relative to RAILS_ROOT.

Jammit now throws a ConfigurationNotFound error when attempting to load a nonexistent configuration file. Resolved an issue with asset caching from daemonized mongrels.

The include_stylesheets helper now takes the same options as the Rails stylesheet_link_tag helper (such as :media => 'print').

Jammit now supports the Google Closure Compiler as an alternative to the YUI compressor.

Fixed a bug that conflicted with other plugins trying to alter ApplicationController in development.

Added support for embedding images with stylesheet-relative paths. Shortened the MHTML identifiers.

Initial Jammit release.

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