Next to the documentation it's also useful to look at the non-minified .js files that come with APEX.
You find them in ~your APEX directory~
In there you find many interesting .js files, based on the filename you have an idea which area they are in. For example if you want to work with cookies you can find that in storage.js. The navigation.js for example contains an interesting function to create popups and so much more...
Not everything is in the online documentation yet, but the documentation is updated frequently now, so definitely good to check it out again.