One little word: “debugger”.
It is the evidence of my background in DOS and Win32 applications and not ASP or java. The only ways I knew to break into debug mode when testing out javascript was either to put a line of intentional garbage into the javascript or to use I.E.'s “break on next statement” setting. There are probably add-ons that enable this as well.
But recently I just happened to come across the debugger statement for Javascript when looking for something else.
By inserting the word debugger in your code, the debugger will break at that line and voila you are in debug mode.
As in
function SomeCodetoTest( )
{
debugger
dosomething( );
dosomethingelse( );
}