I've heard this from many people who've picked up CoffeeScript, especially those who are most fluent in Ruby. Dr. Nic of Engine Yard, an early proponent of CoffeeScript, gave me this blurb for my book:

> JavaScript the runtime language is fast, dynamic, and can run in places other languages can't. But JavaScript the development language is kind of icky. That's why CoffeeScript may turn out to be one of the great innovations in web application development; since I first discovered it, I've never had to write a line of pure JavaScript.

He could have build Prototype.js in CoffeeScript. :)