Vanilla JavaScript AJAX Function

ajax.js

var ajax = (function () {
	var ajax = {};

	ajax.get = function (url, callbackFunc) {
		var req = prepareRequest('GET', url, callbackFunc);
		req.send();
	}

	ajax.post = function (url, params, callbackFunc) {
		var req = prepareRequest('POST', url, callbackFunc);
		req.send(params); //e.g. "firstname=John&lastname=Doe"
	}

	function prepareRequest(method, url, callbackFunc) {
		var req = new XMLHttpRequest();
		req.open(method, url, true);
		req.onreadystatechange = function () {
			if (req.readyState != 4 || req.status != 200) return;
			callbackFunc(req);
		};
		if(method == 'POST') r.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
		return req;
	}

	return ajax;
}());