Ayman Hourieh covers some interesting manipulation of Arrays that accommodates typical data structures. He also covers how to bind methods to objects, sorting with custom functions. He shows a custom AssertException, and covers using Firebug for debugging. He covers some JavaScript quirks: static local variables, null, undefined and delete.
Joe Hewitt, in this Dr Dobbs article talks about using the Firebug extensions to inspect and debug Ajax applications (or any browser-based JavaScript application). It also has CSS inspecting capabilities, editing JavaScript on the fly, inspecting HTTP request and responses, logging, breakpoint debugging and profiling. A useful howto.