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Mustang development is proudly sponsored by Cubox, Agile Rails Devshop

Mustang - V8 engine in Ruby's body

Mustang is ruby proxy library for awesome Google V8 JavaScript engine.


JavaScript integration testing suite which contains more than 4000 steps and running time more that 2 hours... I think that's enough. There is lot of different ways to handle headless testing, but they all have lot of issues as well. They are super slow (Selenium, Watir), or they are not working with ruby 1.9 (Johsnon), or they requires tricky external stuff (Culerity/Celerity, Zombie). Our goal is to create fast javascript engine working fine on all popular ruby implementations (1.8.x, 1.9.x, ree, rubinius), and make it core element of headless, virtual browser for testing purposes (of course integrated with capybara and cucubmer).

TheRubyRacer and PyV8 influence...

Yeap, TheRubyRacer from Charles Lowell and PyV8 made big influence on our Mustang code, so let say now big thanks to the authors...

You can of course ask why we wrote library which seems to be very similar to TheRubyRacer... yeah it seems, but there are a lot of differences. Here is short comparison of TheRubyRacer and Mustang:


  • Both are using similar integration with ruby's garbage collector. Actually, Mustang's code which handles that references is just improved version of TheRubyRacer's code (check the ext/v8/v8_ref.cpp).
  • Both are reflecting JS objects to ruby and vice-versa.


  • Mustang reflects all JavaScript objects and values properly. Properly, means all values from within V8 are reflected to Mustang::V8::* objects, regarding V8 inheritance tree which you can find here.
  • Once reflected ruby object (and vice-versa) is always the same object in ruby world.
  • All reflected V8 objects acts exacltly the same as ruby natives. For example you can deal with Mustang::V8::Array exactly the same as with native Ruby arrays (eg. they are comparable with ruby objects, etc).
  • Functions are reflected more accurately. Ruby objects' methods are represented properly.
  • Classes are converted to V8 function templates, so you can deal with them as with prototypes.
  • Different approach to deal with contexts, compiling JS scripts and exceptions handling.
  • Support for regexp reflections.
  • Way more friendly ruby api.
  • A very thorough test suite (over 250 tests passing on all ruby versions)
  • Clean codebase, less magic, fully documented.
  • Lower memory usage.

And the most important, like i said before, Mustang is used as core element of our bigger idea, so it have to exactly fit to our needs.


Before you install mustang gem make sure you have scons installed.

$ sudo apt-get install scons # on debian
$ sudo pacman -S scons       # on archlinux
$ brew install scons         # on mac...

Using rubygems:

$ gem install mustang


$ git clone git://
$ cd mustang
$ rake compile
$ rake install


For now it's only proof of concept. It implements very simple evaluation of javascript code:

require 'mustang'

cxt =
cxt.eval("'Hello' + ' World!';") # => 'Hello World!'
cxt.eval("var a=1;")
cxt.eval("a+5;") # => 6

cxt[:puts] = method(:puts)
cxt.eval("puts(a)") # displays "1" on the screen
cxt[:a] # => 1


TODO: coming soon...

Note on Patches/Pull Requests

  • Fork the project.
  • Make your feature addition or bug fix.
  • Add tests for it. This is important so I don't break it in a future version unintentionally.
  • Commit, do not mess with rakefile, version, or history. (if you want to have your own version, that is fine but bump version in a commit by itself I can ignore when I pull)
  • Send me a pull request. Bonus points for topic branches.


Copyright (c) 2010 Kriss 'nu7hatch' Kowalik. See LICENSE for details.


(proof-of-concept) Awesome V8 JavaScript engine embedded into Ruby's shiny body.



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