Extend something, experimental alternative to SomeClass.prototype = AnotherClass() then extend further.

extend.js

function extend( proto, template ){
		var prop;
		for(prop in template){
		try{
			if( typeof(template[prop]) === 'function' ){
				// relay arguments, using predefined functions where possible
				proto[ prop ] = proto[ prop ] || (function(prop){
					return function(){ return this._xhr[ prop ].apply(this._xhr, arguments); };
				})(prop);
				continue;
			};
			Object.defineProperty(proto, prop, (function(prop){
				return {
					get: function(){ return this._xhr[prop]; }
					,set: function(val){ return this._xhr[prop] = val; }
				};
			})(prop));
		}catch(err){};
		};
		return template;
	};
	function FakeXMLHttpRequest(){ this._xhr = new XMLHttpRequest; };
	FakeXMLHttpRequest.prototype = {eg: function(){ return Math.random(); }};
	extend(FakeXMLHttpRequest.prototype,
		extend( FakeXMLHttpRequest.prototype, new XMLHttpRequest).constructor.prototype.constructor
	);