JSLint

Lint is a tool which finds common errors in Java code. It's very handy and is usually able to find *something* you ought to fix. Well, there's a JSLint for Javascript too!

http://www.jslint.com

Note, it will complain about globals (like alert). You can, at the top of your javascript, put a comment and define a comma delimited list of globals to ignore:

/*global alert, console, $, $$ */