I've studied Javascript in classes and on my own with other resources, and this book is definitely one of the best, clearest resources for understanding the language. It covers all the core elements clearly, and gradually adds more layers to what's being taught, which is especially important since so many people I know are overwhelmed with how complicated the language can be at times.

The only downside, as I thought, is that there's a LOT to cover. One already needs a basic understanding of HTML and CSS, and then these are two more closely-connected languages that need to be covered. So it takes commitment and persistence to really get the language down. If you have that, though, this is one of the best ways to learn the language outside of a classroom.