object-watch.js

/*
 * object.watch polyfill
 *
 * 2012-04-03
 * Last Updated: 2015-03-24 by Travis Haynes
 *
 * By Eli Grey, http://eligrey.com
 * Public Domain.
 * 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, callback) {
      var getter, newval, oldval, setter;
      this.callbacks || (this.callbacks = []);
      if (!(this.callbacks.indexOf(callback) > -1)) {
        this.callbacks.push(callback);
      }
      oldval = this[prop];
      newval = oldval;
      getter = function() {
        return newval;
      };
      setter = function(val) {
        var index, result;
        oldval = newval;
        for (index in this.callbacks) {
          callback = this.callbacks[index];
          if (typeof callback === 'function') {
            result = callback.call(this, prop, oldval, val);
            if (result !== 'undefined') {
              newval = result;
            }
          }
        }
        return newval = val;
      };
      if (delete this[prop]) {
        // can't watch constants
        Object.defineProperty(this, prop, {
          get: getter,
          set: setter,
          enumerable: true,
          configurable: true
        });
      }
    }
  });
}

// object.unwatch
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;
		}
	});
}

Added multiple callback support, and the ability to override assigned values.