纯javascript 实现 Ajax 验证用户名是否被占用

var xmlHttpRequest;function createXMLHttpRequest() {    var xmlHttp;    if (window.ActiveXobject) {        xmlHttp = new ActiveXobject("MSXML2.XMLHTTP.3.0");    }    else if (window.XMLHttpRequest) {        xmlHttp = new XMLHttpRequest();    }    return xmlHttp;}function validateUserName() {    var username = document.getElementById("tbUserName").value;    var data = "username=" + escape(username);    xmlHttpRequest = createXMLHttpRequest();    xmlHttpRequest.onreadystatechange = callback;    xmlHttpRequest.open("Post", "AjaxPages/CheckUserIsRegister.aspx?num=" + Math.random(), true);    //使用post时需    xmlHttpRequest.setRequestHeader("Content-Length", urlData.length);    xmlHttpRequest.setRequestHeader("CONTENT-TYPE", "application/x-www-form-urlencoded");    xmlHttpRequest.send(data);}function callback() {    if (xmlHttpRequest.readyState == 4) {        if (xmlHttpRequest.status == 200) {            var result = xmlHttpRequest.responseText;            if (result == "0") {                document.getElementById("spCheck").innerHTML = "恭喜你，该用户名可以注册";            }            else if (result == "1") {                document.getElementById("spCheck").innerHTML = "对不起，该用户名已使用";            }            else {                document.getElementById("spCheck").innerHTML = result;            }        }    }}