A brief discussion on jQuery asynchronous object (XMLHttpRequest)_javascript skills

//1.00Create asynchronous object
          var xhr = new XMLHttpRequest();
                    //2.0
                xhr.open("post", url,params, true);
//3.0 uses the Formdata attribute to pass parameters
               xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
//4.0 Set callback function
                 xhr.onreadystatechange = function () {
If (xhr.readyState == 4 && xhr.status == 200) {
alert(xhr.responseText);
                }
            }
                                                                                                                                                                                         //5.0 passing parameters
                 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);
    }
};