Fork of object.watch polyfill in ES5. Modified to not react when the value set is the same as previous.

object-watch.js

/*
 * object.watch polyfill
 *
 * 2012-04-03
 *
 * By Eli Grey, http://eligrey.com
 * Public Domain.
 *
 * Modified by Nenad Damnjanović
 * Nov 9, 2014
 *
 * NO WARRANTY EXPRESSED OR IMPLIED. USE AT YOUR OWN RISK.
 */
 
// object.watch
if (!Object.prototype.watch) {
	Object.defineProperty(Object.prototype, "watch", {
		  enumerable: false
		, configurable: true
		, writable: false
		, value: function (prop, handler) {
			var
			  oldval = this[prop]
			, getter = function () {
				return oldval;
			}
			, setter = function (newval) {
				if (oldval !== newval) {
					handler.call(this, prop, oldval, newval);
					oldval = newval;
				}
				else { return false }
			}
			;
			
			if (delete this[prop]) { // can't watch constants
				Object.defineProperty(this, prop, {
					  get: getter
					, set: setter
					, enumerable: true
					, configurable: true
				});
			}
		}
	});
}

if (!Object.prototype.unwatch) {
	Object.defineProperty(Object.prototype, "unwatch", {
		  enumerable: false
		, configurable: true
		, writable: false
		, value: function (prop) {
			var val = this[prop];
			delete this[prop]; // remove accessors
			this[prop] = val;
		}
	});
}

/**
 * 
 * Example usage:
 * 
 * 
	var o = {p: 1};

	o.watch("p", function (id, oldval, newval) {
	    console.log( "o." + id + " changed from " + oldval + " to " + newval );
	    return newval;
	});
	
	o.p = 2; // should log the change
	o.p = 2; // should do nothing
 * 
 * 
 */

Thank you!

p.s.: I wonder if by standard it should trigger if a variable is set to the same value again.