Javascript is a special language. You can learn it easily, you can write useful code easily, and more importantly, you can introduce bugs even more easily.
JSLint is a code quality checker that can catch some common bugs in your javascript:
http://www.jslint.com/
A fork of JSLint that can catch fewer bugs, but is more convinient to use if you already have a big buggy code-base:
http://www.jshint.com/
A great site explaining the reasoning behind the errors and warnings flagged by JSLint:
http://jslinterrors.com/
