javascript needs built-in ability for stopping, pausing, resuming client code in progress through built-in or add-on debugger. right now, add-on debuggers are unable to halt an initially running process unless it is running wild, then the brouwser offers to halt the script, but then the debugger doesn't kick in like you want it to because at that point, the debugger is unresponsive.

I don't know if this requires that the debugger be in a separate thread or not. possibly. if so, that could introduce parallel-programming "synchronization issues". those who have done any multithreaded programming know what I am talking about. if you don't, get a book on multithreaded or parallel programming.

I don't know how easy and/or possible this is. just having a debugger; statement alone is not going to do all the debugging tasks for people who need a debugger.

we also need to be able especially to set the Program Counter (PC) to a specific line number in whichever file. a specific statement basically. we should be able to change a variable's value, and go back/forward 2 (whatever) lines and re-execute a function that somehow got messed up, until a person can figure out how to fix it right.