Just make a note！

var xmlHttpReq;function createXmlHttpRequest() {    //创建XMLHttpRequest对象    if(window.XMLHttpRequest) {        xmlHttpReq = new XMLHttpRequest();        if(xmlHttpReq.overrideMimeType) {            xmlHttpReq.overrideMimeType('text/xml');        }    } else if(window.ActiveXObject) {        try {            xmlHttpReq = new ActiveXObject("Msxml2.XMLHTTP");        } catch (e) {            try {                xmlHttpReq = new ActiveXObject("Microsoft.XMLHTTP");            } catch (e) {            }        }    }    if(!xmlHttpReq) {        alert('Can not create XMLHTTP instance!');        return false;    }}//入口，调用Ajax请求function callAjax() {    createXmlHttpRequest();    var url = "xxxxx";    var query = "methodName=run?a=2";//传递请求的参数    xmlHttpReq.open("POST", url, true);    xmlHttpReq.onreadystatechange = callBack;//指定回调函数    xmlHttpReq.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");    xmlHttpReq.send(query);//发送请求的参数}//回调函数function callBack() {    if(xmlHttpReq.readyState == 4) {        if(xmlHttpReq.status == 200) {            var returnStr = xmlHttp.responseXML; // xmlHttp.responseXML 即为执行后台操作后，返回的值                // do what you want with responseXML        }    }}