So how do I trace what Javascript is doing on my machine?
Generally mitmproxy gives a feeling what sites the browser talks too. And strace gives often a good feeling what a Linux binary does. But the browser is too big and complicated to read strace output in most cases.
Can anybody recommend a tool to look what Javascript code loaded by a certain page is doing?
Almost all modern browsers have debug panels that will list all of the requests made by a page, assets cached, cookies and local db, and of course it's trivial to just view the source of a site and read the javascript if it isn't compressed.
Haven't really used the Javascript debugger, but my guess would be completely infeasible to follow everything a random "modern" Web site might do. And as you say some Javascript might be compressed or obfuscated. What I really would want is a somewhat higher level / more filtered approach: Like strace lets me just trace file operations for example.