Break on a change of variable value

You don't even need an IDE - you can use "Object.watch()":

Object.Watch Tutorial

If you use any one debugger, I'd strongly recommend Firebug. For all your Javascript, HTML and CSS needs :-):

http://getfirebug.com/javascript

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Update for 2019:

• Object.Watch is Ancient History. Uncoincidentally, it's unavailable in most contemporary browsers.

• My personal favorite JS debugging tool these days is Chrome Developer Tools.

• My personal favorite JS IDE (for Angular, .Net Core, etc) is Microsoft Visual Studio Code (MSVC).

• You can do just about any "expected" debugging operation - including set watches - with the Chrome debugger (just as you could with FF Firebug).

• Chrome debugger is well integrated with the MSVC IDE.

• Both are "free" (at least "free as in beer"); both run well on Windows, Mac and Linux.

I'm having success with this library in Chrome and it looks to support all major browsers.

https://gist.github.com/eligrey/384583

Just include the .js file, then call:

yourObject.watch('someProperty', function() { 
    doWhatYouWant(); 
    debugger; 
    console.write('this too'); 
    alert('Object Changed'); //etc 
});