一般ajax写法

function test(url, username, password, jQuery,callback) {    url = url || "/test.jsp";   jQuery.ajax({type:Get,     url: url,     async: true,     password:password,     username:username,     complete:function(xmlhttp, status){     var ok = (200 <= xmlhttp.status && xmlhttp.status < 300) || xmlhttp.status == 1223; // status 204 -> 1223 in IE     if (ok) {       callback(true);     } else {       callback(false);     }   }});    var xmlhttp;    if (window.XMLHttpRequest) {        xmlhttp = new XMLHttpRequest()    } else {        xmlhttp = new ActiveXObject("MSXML2.XMLHTTP");    }    xmlhttp.onreadystatechange = function() {        if (xmlhttp.readyState == 4) {            var ok = (200 <= xmlhttp.status && xmlhttp.status < 300) || xmlhttp.status == 1223; // status 204 -> 1223 in IE            if (ok) {                callback(true);            } else {                callback(false);            }        }    };    xmlhttp.open("Get", url, true, username, password);    try {        xmlhttp.send(null);//error    } catch (e) {        callback(null);    }}