The answer is simple: MDN. The Mozilla Developer Network has the best JavaScript reference and tutorial documents available for free. This site has a plethora of information on all web technologies:

Web APIs & DOM,
and much more…

You might recall that Mozilla was one of the first browsers ever invented. In the early days of the Internet everyone used Netscape (or Internet Explorer… ick.) Today, Mozilla published Firefox, which is one of the most popular and advanced browsers available.

I've been using MDN as reference for JavaScript for many, many years. It has never let me down. Give a it try, I'm sure you'll like it.