The coffee-script gem (https://github.com/josh/ruby-coffee-script https://github.com/josh/ruby-coffee-script) uses ExecJS (https://github.com/sstephenson/execjs

The coffee-script gem (https://github.com/josh/ruby-coffee-script https://github.com/josh/ruby-coffee-script) uses ExecJS (https://github.com/sstephenson/execjs https://github.com/sstephenson/execjs) to invoke the CS compiler. Rails 3.1's asset pipelining system will use these gems by way of Sprockets 2 (https://github.com/sstephenson/sprockets https://github.com/sstephenson/sprockets) to compile and serve CoffeeScript to the browser. If you're on Windows or Mac OS, ExecJS will use the JS runtime already installed on your system (Windows Script Host or Apple JavaScriptCore). You can also install a gem like therubyracer (V8 for MRI, available on Heroku) or therubyrhino (Rhino for JRuby) for faster compilation. And ExecJS can use Node.js or Spidermonkey if they're installed too.