Raw JS Ajax handler with response time in success

ajax-request.js

fireAJAXRequest = function(method, url, async, callback) {
  var httpRequest,
	 	start= new Date().getTime(),
	 	end;

	if (window.XMLHttpRequest) {
		httpRequest = new XMLHttpRequest();
	} else if (window.ActiveXObject) {
		// Internet Explorer is stupid
		httpRequest = new ActiveXObject("Microsoft.XMLHTTP");
 	}

	httpRequest.onreadystatechange = function() {

		if (httpRequest.readyState === 4 && httpRequest.status === 200) {
			end = new Date().getTime()-start;
			httpRequest.responseTime = end;
			callback(httpRequest);
		}
		// TODO write additional response handlers

	};

	httpRequest.open(method, url, async);
	httpRequest.setRequestHeader("X-Requested-With", "XMLHttpRequest");
	httpRequest.send();
};