It's a horrible solution. Just store them hashed on the server end, and make a secure connection. I would argue that more browsers support SSL than JavaScript...
1. The server is non-malicious and competently written. The passwords are correctly hashed with something like bcrypt or scrypt. (This is super-easy, so server programmers have no excuse for not doing this.) Browser-side hashing has no advantage.
2. The server is non-malicious, but incompetently written, e.g. they store plaintext passwords or some crap like that. Hashing passwords in javascript would help, but it's not as good as fixing the server, and almost certainly a lot harder.
3. The server is malicious. In which case it can serve up malicious javascript, so you're just as screwed.
