I was actually looking into that over the last few days and haven't found anything satisfying yet.
I have tried Aptana, WebStorm (from Jet brains), textmate, bbedit, jseclipse, visual studio, vim. But still haven"t found what I am hoping to find.
I find the eclipse based solutions (Aptana and jseclipse) are too slow for my taste and while they do provide things like completion, it is fairly limited to the obvious things. Aptana probably does what you are looking for, it has a debugger that integrate with your browser.
I tought webstorm seemed better but it is also a bit slow and brings a bit too much with it. Give it a try it may be what you are looking it also has debugger integration.
I couldn't find any special editing facilities other than text highlighting and indentation in bbedit and eventually gave up. TextMate does it ok but since I am not that familiar with javascript and the libraries I am using, I was hoping for something that would let me edit and introspect things at the same time. its really comparable to what you could achieve with vim or emacs.