js ajax代码

function getProductItem(){  var mypid = document.getElementById("pid").value;var url = "index.php?/product/getProductItemJson/"+ mypid;//实例化Ajax//var ajax = InitAjax();var ajax = false;//开始初始化XMLHttpRequest对象if(window.XMLHttpRequest) { //Mozilla 浏览器        ajax = new XMLHttpRequest();        if (ajax.overrideMimeType) {//设置MiME类别        ajax.overrideMimeType("text/xml");}}else if (window.ActiveXObject) { // IE浏览器    try {        ajax = new ActiveXObject("Msxml2.XMLHTTP");        } catch (e) {try {            ajax = new ActiveXObject("Microsoft.XMLHTTP");            } catch (e) {}        }}    if (!ajax) { // 异常，创建对象实例失败        window.alert("不能创建XMLHttpRequest对象实例.");        return false;    }               //通过Post方式打开连接ajax.open("GET", url, true);//发送GET数据,已经在URL中赋了值所以send那里只要加个空参.ajax.send(null);//获取执行状态ajax.onreadystatechange = function() {    //如果执行状态成功，那么就把返回信息写到指定的层里   if (ajax.readyState == 4 && ajax.status == 200) { var obj  = eval("("+ajax.responseText+")");var skuid = document.getElementById("skuid");skuid.options.length = 0;//清空select for(i=0;i<obj.length;i++)skuid.options.add(new Option(obj[i].sku,obj[i].skuid)); //添加selct    } }       }