Simple Cross-browser XHR request with Cors support. Supported by any moder browser and IE>=8.

ajax.js

function ajax(type, url, data, callback) {
		 	
	var xhr = new(this.XMLHttpRequest || ActiveXObject)('MSXML2.XMLHTTP.3.0');
	var req = [];
	
	for(var item in data) {
		req.push(encodeURIComponent(item) + "=" + encodeURIComponent(data[item]));
	}
	
	data = req.join("&");

	xhr.open(data ? 'POST' : 'GET', url, 1);
	xhr.setRequestHeader('X-Requested-With', 'XMLHttpRequest');
	xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');

	xhr.open(type, url, true);

	xhr.onload = function() {

		if (this.status == 200) {
			callback(JSON.parse(this.responseText), this);
			return;
		};

	};

	xhr.send(data);

}