Twostroke is a Javascript implementation written in pure Ruby. It currently consists of:

• A hand-written parser
• A TSASM compiler (Twostroke's internal bytecode format)
• A VM that runs TSASM bytecode
• A minimal Javascript standard library implementation
• A test suite containing:
  • Some tests written for Twostroke
  • Underscore.js's complete test suite (minus the cases relying on the DOM)
  • Parts of mootools' test suite

yep, that's unmodified underscore.js: