I like to try to instrument every function call (or some higher level equivalent) that gets called, then perform some basic operation that I'm interested in with the code, and just start reading important-looking code from the start of the timeline to the end.

Chrome has a nice timeline view, and it now works with Node as a backend instead of the browser, so looking at a timeline chart would be a nice way to browse a new-to-me large JS codebase.