XMLHttpRequest函数封装：

function ajax(Url,sccuessFn,failureFn) {	//1.创建XMLHttpRequest对象	var xhr = null;	if(window.XMLHttpRequest) {		xhr = new XMLHttpRequest();	}else {		xhr = new ActiveXObject(‘Microsoft.XMLHTTP‘);	}	//2.打开与服务器的链接	xhr = open(‘get‘,Url,true);	//3.发送给服务器	xhr = send(null);	//4.响应就绪	xhr.onreadystatechange = function() {		if(xhr.readyState == 4) {			if(xhr.status == 200) {				successFn(xhr.responseText);			}else {				if(failureFn) {					failureFn();				}else {					alert(xhr.status);				}			}		}	}}