Object.when implementation in ES5

object-when.js

/*!

Object-when implementation on ES5 (made by afonsomatos)

Released under the MIT license (2014-11-10)

*/
 
// object.when
Object.defineProperty(Object.prototype, "when", {
	enumerable: false,
	configurable: true,
	writable: true,
 
	value: function(prop, expect, handler) {
 		
		var expecting = expect instanceof Array ? expect : [expect]; // Don't do Array(expect) because Array(5) != [5]
 
		var setter    = function(val) {
			for (var i = 0; i < expecting.length; i++) {
				
				var possible = expecting[i] instanceof Function ? expecting[i].call(this, val) : expecting[i];
				
				if( possible === val) {
 
					handler.call(this, val);
 
					// Remove the `when` listener
					var temp = this[prop];
					delete this[prop];
					this[prop] = temp;
				}
 
			};
		};
 
		Object.defineProperty(this, prop, { set: setter });
	}
 
});
 
// object.unwhen
Object.defineProperty(Object.prototype, "unwhen", {
	enumerable: false,
	configurable: true,
	writable: true,
 
	value: function(prop) {
 
		// Remove the `when`listener
		var temp = this[prop];
		delete this[prop];
		this[prop] = temp;
 
	}
 
});