The debugger that comes built-in with Google Chrome is a powerful tool for learning JavaScript. Not only does it help you track down errors, but more importantly, it helps you visualize what the JavaScript engine is actually doing behind the scenes.
I often say that programming is an invisible art, in that the functions and loops all fire off very quickly inside the machine and we humans typically have no way of visualizing what is happening step by step. Well, the debugger changes all that. You'll use these skills to understand how your programs work on a deeper level, tackle tough algorithms, and find errors in your code. Overall, using the debugger will likely make you a better programmer by helping you think like the JavaScript engine.