//1.00创建异步对象             var xhr = new XMLHttpRequest();             //2.0             xhr.open("post", url,params, true);             //3.0将参数使用Formdata属性传递             xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");             //4.0设置回调函数             xhr.onreadystatechange = function () {                 if (xhr.readyState == 4 && xhr.status == 200) {                     alert(xhr.responseText);                 }             }             //5.0传递参数             xhr.send(params);

 xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");

var ajaxHelp = {     CreatXHR: function () {         //创建异步对象         var xhr = new XMLHttpRequest();         return xhr;     },     //ajax的get请求     AjaxGet: function (url, callBack) {         this.AJaxCommon("get", url, null, callBack);     },     //ajax的post请求     AjaxPost: function (url, params, callBack) {         this.AJaxCommon("post", url, params, callBack);     },     AJaxCommon: function (method, url, params, callBack) {         //1.0         var xhr = this.CreatXHR();         //2.0         xhr.open(method, url, true);         //3.0         if (method == "get") {             xhr.setRequestHeader("If-Modified-Since", "0");         } else {             xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");         }         //4.0         xhr.onreadystatechange = function () {             if (xhr.readyState == 4 && xhr.status == 200) {                 var datas = JSON.parse(xhr.responseText);                 //执行回调函数                 callBack(datas);             }         }         //5.0         xhr.send(params);     } };