QUnit

QUnit is a powerful, easy-to-use, JavaScript test suite. It is primarily used to test jQuery code and plug-ins, but is capable of testing any generic JavaScript code (and even capable of testing JavaScript code on the server-side).

QUnit is especially useful for regression testing. Whenever a bug is reported, you can write a test that asserts the existence of that particular bug. Then fix it and commit both. Every time you work on the code again, run the tests. If the bug comes up again - a regression - you'll spot it immediately and know how to fix it, because you know what code you just changed.

Having good unit test coverage makes safe refactoring easy and cheap. You can run the tests after each small refactoring step and be able to always identify which change caused a specific problem.