Static asset bundler for rails
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Tame your static assets without adding additional processing time on your server. Lash will

  • Bundle multiple JavaScript files into a single minified script.
  • Minify JavaScript files to be included on demand.
  • Build SASS style sheets using "production" settings.
  • Bundle loose PNG, GIF and JPEG files into a single CSS image sprite.
  • Optimize PNG files to reduce size by 5%-35%.
  • Generate static gzipped versions of static assets for use with nginx's gzip_static plugin.

Based on "Optimizing asset bundling and serving with Rails" at github.


# Gemfile
gem 'lash'

Bundling Assets

Lash includes several rake tasks to bundle loose, development versions of your static assets into minified and compressed versions. To bundle all assets simply run

rake lash:all                   # Runs all lash tasks including javascripts and style sheets
rake lash:deploy                # Called by capistrano to generate static assets on the server

Individual assets can be bundled on demand using their respective lash:asset_type tasks.


Lash expects to find JavaScript assets as subfolders of the public/javascripts folder like so

  • public/javascripts
    • cdn
    • demand
    • application
    • ie

For each folder that lash finds, it will bundle all the .js files found in that folder into a single bundle_#{folder}.js file in the public/javascripts folder. For example, the following structure...

  • public/javascripts/application
    • application.js
    • utility.js
    • rails.js

...will be bundled and minified into public/javascripts/bundle_application.js and a gzipped version bundle_application.js.gz in the same folder.

Special Folders

Lash recognizes two special folders: cdn and demand.

public/javascripts/cdn : Lash expects to find scripts that will normally be referenced via CDN (like the jquery via google's cdn) for local use during development mode and to support outages on the CDN.

public/javascripts/demand : Lash will not bundle scripts in the demand folder. Instead, each script is individually minified and gzipped in place. This is where you would put large scripts that you don't use on every page - like jquery.forms.

CDN JavaScripts and Developer Mode

Shared CDN hosts are great for optimizing your site's user experience but there are two issues that developers regularly have to deal with - developing offline and accounting for CDN unavailability. Lash handles both of these cases with ease.

During development, Lash will use the copies of the libraries found in your public/javascripts/cdn directory. This makes it easy to work offline.

In production, Lash will use the CDN versions of the scripts and if a runtime test is provided, fallback to the local version if the CDN is unavailable.

<%= javascript_cdn 'jquery', '//', 'jQuery' -%>

# => <script src="//" type="text/javascript"></script>
# => <script type="text/javascript">
# =>  if(typeof jQuery === "undefined" ) 
# =>    document.write( unescape("<script src=\"/javascripts/cdn/jquery.min.js?1297459967\" type=\"text/javascript\"><\/script>") );
# => </script>

Referencing JavaScript Bundles in Layouts

To include references to bundled scripts in a view simply call {Lash::BundleHelper#javascript_bundle}


<%= javascript_bundle 'application', 'ie' %>

When {Lash::BundleHelper#bundle_files?} is true (on by default in production), lash will include a reference to the single bundled script. When false, it will include each individual javascript that would have been bundled.

See {Lash::BundleHelper#javascript_bundle} for details.


When referring to assets in your javascripts you loose the convenience of the rails cache busting asset tags. During bundling, lash will generate an public/javascripts/application/application.bundleVersion.js which gets bundled into the application script. This script simply declares a global variable bundleVersion which you can append to asset urls in your scripts.


$('#waiting-div').append( $('<img src="/images/wainting.gif?' + bundleVersion + '" />' ) )

JavaScript bundling tasks

rake lash:js                    # Bundles and minifies javascripts
rake lash:js_bundle             # Bundles javascripts folders into single minified files
rake lash:js_gzip               # Compresses minified javascripts for nginx gzip_static support
rake lash:js_min                # Minifies all javascripts

CSS Sprites

CSS sprites is are an effective optimization technique, but can be very time consuming to produce. Lash makes it easy. Just drop the desired images into a folder, run a rake task and you've got a highly optimized sprite image and accompanying CSS file.

Lash looks for sprite images in public/sprites. Each sub folder will be bundled into a single image and css file. Given the following folder structure:

  • public/sprites/ui
    • accept.png
    • add.png

Lash will generate a sprite image public/images/ui-sprite.png and corresponding css file public/stylesheets/sass/_ui-sprite.scss.

To generate your apps sprites

rake lash:sprites


When referring to assets in your css scripts you loose the convenience of the rails cache busting asset tags. During bundling, lash will generate an public/stylesheets/sass/_version.scss. You can include this into any of your SASS scripts when you reference static assets like images.


@import 'version';
.smiley { background-image: url(/images/smiley.png?#{$bundle-version})}

CSS bundling tasks

rake lash:css        # Process CSS scripts
rake lash:css_gzip   # Compresses stylesheets for use with nginx gzip_static
rake lash:sass       # Pre-generate sass scripts
rake lash:sprites    # Generate CSS sprites from the public/sprites folders

Optimizing PNG Images

Most image editors will compress PNG files using a very basic compression algorithm. However the PNG format allows for much more aggressive optimization at the cost of speed. Since image resources are some of the heaviest assets downloaded from your site, optimizing them is often worth the effort.

See A guide to PNG optimization for a more detailed discussion.

To optimize your pngs

rake lash:png

Integrating With Capistrano

Lash was designed to work with capistrano to easily run static asset bundling during the deployment process. This makes sure that all assets are primed for static serving from your website without interfering with any existing requests that are currently being served.

To run bundling tasks during deployment

# in config/deploy.rb
require 'lash/capistrano'

If you use capistrano to publish your app (and really who isn't?) you'll want to add some additional filters to your .gitignore file

# lash asset helpers

# lash generated filed



Copyright (C) 2011 Apps In Your Pants.

Dual licensed under MIT and GPLv3

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