I wrote a similar one for CoffeeScript: http://www.mrspeaker.net/2010/04/29/instant-coffeescript/ but it uses the HTML5 cache manifest stuff to let it work offline (kind of) like a native app. I wrote to the creator of the JS IDE saying "please do the same!" but actually, I really want it so I'm also doing it myself (and will post him the changes when I'm done).