Listen a property for changes in JavaScript

There are two very interesting functions of Object in JavaScript - watch and unwatch. These two methods could be used to track changes of some specific properties of any Object. watch() watches for a property of an Object, if a value is assigned to that property, it will run registered function.



Example,
            Here i have an Object obj1 which has property myprop with value 1. Later watch method is used to register myprop property, if any changes/assignment is made to that property appropriate function is called. In this case whenever myprop of obj1 is assigned a value function is executed and alerts a msg.

obj1 = {myprop:1};

//  start watch on obj1.myprop from here
obj1.watch("myprop",
   function (prop_name, oldval, newval) {
      alert("obj1." + prop_name + " changed from " + oldval + " to " + newval)

      return newval;
   });

// assign new values
obj1.myprop = 2;
obj1.myprop = 3;
obj1.myprop = 4;

// unwatch on obj1.myprop from here

obj1.unwatch("myprop");

// assign new values

obj1.myprop = 5;

obj1.myprop = 6;

obj1.myprop = 7;