Java's documentation is pretty good, and it's usually the first place I look if I need to see how something in the Java world works.
Sencha's Ext-JS has interesting documentation. It seems comprehensive, but I've run into undocumented properties and undocumented interactions between documented properties before, and that makes me very wary of what I read there. Further, they only offer documentation for the most current version of their libraries. Unless you are always using the cutting edge, you're not guaranteed that the documentation will adequately describe what you need to do.