What is the most effective way to debug JavaScript code that runs in a popup window? Specifically I need to trace what is happening as the page loads.

This is for SCORM 1.2 courses running in an LMS, which depends on other JavaScript objects in a parent window, so debugging the popup by itself won't work.

I could use a technique for other contexts though most of my time is debugging these courses.

I could use something like an option in the in-browser debugger that pauses on the first line of JavaScript that executes for a popup page, as if I put a breakpoint there. (I can't, or at least don't know how, to set breakpoints until well after the page has started)

Edit:

The debugger; statement works, but only for code I control. I sometimes need to trace JavaScript that runs as some popup window opens, and can't add breakpoints because the code has already run.