Breakpoint on property change

Edit 2016.03: Object.observe is deprecated and removed in Chrome 50

Chrome 36 ships with native Object.observe implementation that can be leveraged here:

myObj = {a: 1, b: 2};
Object.observe(myObj, function (changes){
    console.log("Changes:");
    console.log(changes);
    debugger;
})
myObj.a = 42;

If you want it only temporarily, you should store callback in a variable and call Object.unobserve when done:

myObj = {a: 1, b: 2};
func = function() {debugger;}
Object.observe(myObj, func);
myObj.a = 42;
Object.unobserve(myObj, func);
myObj.a = 84;

Note that when using Object.observe, you'll not be notified when the assignment didn't change anything, e.g. if you've written myObj.a = 1.

To see the call stack, you need to enable "async call stack" option in Dev Tools:

Original answer (2012.07):

A console.watch sketch as suggested by @katspaugh:

var console = console || {}; // just in case
console.watch = function(oObj, sProp) {
   var sPrivateProp = "$_"+sProp+"_$"; // to minimize the name clash risk
   oObj[sPrivateProp] = oObj[sProp];

   // overwrite with accessor
   Object.defineProperty(oObj, sProp, {
       get: function () {
           return oObj[sPrivateProp];
       },

       set: function (value) {
           //console.log("setting " + sProp + " to " + value); 
           debugger; // sets breakpoint
           oObj[sPrivateProp] = value;
       }
   });
}

Invocation:

console.watch(obj, "someProp");

Compatibility:

• In Chrome 20, you can paste it directly in Dev Tools at runtime!
• For completeness: in Firebug 1.10 (Firefox 14), you have to inject it in your website (e.g. via Fiddler if you can't edit the source manually); sadly, functions defined from Firebug don't seem to break on debugger (or is it a matter of configuration? please correct me then), but console.log works.

>>> var obj = { foo: 42 }
>>> obj.watch('foo', function() { console.log('changed') })
>>> obj.foo = 69
changed
69

Edit: Object.watch was removed in Firefox 57.

If you don't mind messing around with the source, you could redefine the property with an accessor.

// original object
var obj = {
    someProp: 10
};

// save in another property
obj._someProp = obj.someProp;

// overwrite with accessor
Object.defineProperty(obj, 'someProp', {
    get: function () {
        return obj._someProp;
    },

    set: function (value) {
        debugger; // sets breakpoint
        obj._someProp = value;
    }
});