ajax套路

var req;function UserNameCheck(){     var username = document.getElementById('username').value;     var url = "pre?user_name=" + escape(username);     if (window.XMLHttpRequest) {          req = new XMLHttpRequest();     }else if (window.ActiveXObject) {          req = new ActiveXObject("Microsoft.XMLHTTP");     }     if(req){          req.open("GET", url, true);          req.onreadystatechange = callback;          req.send(null);             }}function callback() {    if (req.readyState == 4) {        if (req.status == 200) {             parseMessage();                 // update the HTML DOM based on whether or not message is valid        }else{            alert ("Not able to retrieve description" + req.statusText);        }           }    else    {    document.getElementById("check_username").innerHTML = "正在验证用户名....";    }}function parseMessage() {    var xmlDoc = req.responseXML.documentElement;     var node = xmlDoc.getElementsByTagName('info');  document.getElementById('check_username').innerHTMLnode[0].firstChild.nodeValue;}