Automatic Asset Minimization and Packaging with Rails 2.0.x

With the recent release of Rails 2.0, many of us are reviewing our approaches to common problems. Many new features have been added to Rails, and some old tricks are either no longer necessary or no longer work.

I am developing a project with Rails 2.0 and am getting close to putting it into production. A recurring issue for today’s web developers is that of asset packaging, or the combination of multiple site assets into a single file. Specifically, we’re talking about Javascript and CSS.

A given “Web 2.0” (a term I wish had recently been found dead in a cramped apartment in Brooklyn) site might have a half dozen Javascript or CSS files to deliver to a user, and web browsers are not all that efficient at retrieving them. Each one requires a separate TCP connection to the server, and many browsers are only capable of getting two of these files concurrently. This means delays for your users.

In Rails 2.0 (and previously in Edge Rails), it’s possible to combine multiple Javascript and CSS files using the javascript_include_tag and stylesheet_link_tag functions in your html.erb files; simply add :cache => true to the parameters like this:

<%= javascript_include_tag 'prototype', 'effects', :cache => true %><%= stylesheet_link_tag 'main', 'shop', 'form', :cache => true %>

With :cache => true and when running in your production environment, Rails will automatically combine your Javascript and CSS assets into single files (all.js and all.css, respectively) and significantly reduce your site’s load time.

However, this really only solves part of the problem. A common technique used to further improve site performance is to compress Javascript and CSS by removing unnecessary whitespace and comments. I am not sure why this wasn’t included as part of Rails’ built-in caching features, but it seemed to me it should be easy to add.

Turns out I was mostly right. Google “javascript minimization” (or minification) and you’ll see it’s a pretty hot topic. The Asset Packager plugin from Scott Becker does this, as well as CSS compression, but is targeted at Rails 1.x and doesn’t really make sense in the face of Rails 2.0.

So I set out to solve this problem in an elegant way for Rails 2.0. Asset Packager uses a Ruby script called jsmin.rb by Uladzislau Latynski which is based on jsmin.c by Douglas Crockford. The thing is, jsmin.rb is not a class or library, but rather a standalone executable that operates on stdin and stdout. Asset Pacakger actually forks a ruby shell process to do its Javascript minimization, and this seemed like folly if it could be done internal to Rails.

Accordingly, I modified jsmin.rb to operate as a singleton class and with a class method you could pass Javascript data to. Then it was simply a matter of monkey patching this function into ActionView::Helpers::AssetTagHelper, home of javascript_include_tag and stylesheet_link_tag.

I also wanted to add in CSS compression, which turned out to be easy. The javascript_include_tag and stylesheet_link_tag functions both use the same underlying functions to package their assets, so it was a simple case of replacing them with equivalents that do compression appropriately, based on whether we are dealing with CSS or JS.


module ActionView  module Helpers    module AssetTagHelper      require 'jsminlib'

      def compress_css(source)        source.gsub!(/\s+/, " ")           # collapse space        source.gsub!(/\/\*(.*?)\*\/ /, "") # remove comments        source.gsub!(/\} /, "}\n")         # add line breaks        source.gsub!(/\n$/, "")            # remove last break        source.gsub!(/ \{ /, " {")         # trim inside brackets        source.gsub!(/; \}/, "}")          # trim inside brackets      end

      def get_file_contents(filename)        contents =        if filename =~ /\.js$/          JSMin.minimize(contents)        elsif filename =~ /\.css$/          compress_css(contents)        end      end

      def join_asset_file_contents(paths)        paths.collect { |path|          get_file_contents(File.join(ASSETS_DIR, path.split("?").first)) }.join("\n\n")      end

    end  endend

By simply modifying join_asset_file_contents to use our new function get_file_contents instead of, we quickly get to the heart of the matter. CSS files get compress_css run on them, while Javascript files get JSMin.minimize run on them. Your :cache => true Javascript and CSS assets will now be gloriously combined and compressed!

Note that the above monkey patch requires jsminlib.rb, which you can download here. It is just a modified version of the original jsmin.rb, and you will want to put it into your Rails lib directory.

A good next step would be to further enhance get_file_contents to do Javascript obfuscation, which allows for the replacement of variable names and thus even further compression; it also tends to make Javascript code nearly incomprehensible and thus harder to steal, which may be desirable for some developers. I haven’t found any native Ruby ways to do this yet, but it seems to me that this would be a good place for a C extension (or similar), and that this should all be put into a tiny and lightweight plugin.

I’m always amazed at how easy it is to bend Rails (and Ruby) to one’s will, and in this case it’s really quite elegant and straightforward. I’d love to hear your ideas about how to take this idea forward, potentially even including it in Rails itself.

Download the files here:


#1 Erik on 01.13.08 at 7:06 pm

Thanks for writing this up. I had some trouble with an “undefined method ‘options’ for []:Array” in mongrel.rb. It turns out the downloadable version of your javascript_minimization.rb requires jsmin instead of jsminlib. Once I changed that it worked fine.

#2 Dave Troy on 01.14.08 at 8:49 am

Erik — you’re right. I will fix this now. Thanks for the heads up!

#3 Ryan on 07.28.08 at 1:51 pm

Looking forward to using this great little tweak, but your download links appear to be broken