Personally I learned by reading examples and references. First msdn.microsoft.com has a great reference under dynamic html for all your javascript references. So if you have a peice of code that makes no sense and you want to understand it just pull up the examples. That is the best way to learn simply because you are becoming more interactive with the code and understanding it. They way your not only learning about that example you can brainstorm other ideas from the information you learn. Hope this helps.
P.S: MSDN also shows what version of browser it is compatible with.