[yblu]

Debugger is your friend :)

Start by adding breakpoints for some key actions of the app. Then step through the flows with the debugger. First pass you can just step over functions to get the high-level idea. In subsequent passes, you can step into functions that seem important. Rinse and repeat until you understand those actions well. Then move on to other areas in the app.

This works because you can see the actual end-to-end execution flow (not always clear from reading code), inspect runtime data (impossible by reading code) and even change the runtime data (variables, DOM) to validate assumptions about how the code works.