Well since you can compile python, clojure, etc etc to JavaScript I guess the .js extension is meaningless. In my opinion it isn't at all OK, because a .js extension means that the code is in JavaScript, so that I can look at it, and maybe contribute. I'm not interested in CoffeeScript widgetmajigs that turn CoffeeScript into JavaScript. Use .coffee or maybe CoffeeScript just can't stand on its own. Maybe it's just riding JS's coattails and the moment it separates it becomes abandoned.