最初的ajax案例

----------------------原始的ajax:
 function guo() {
            var xhr
            if (XMLHttpRequest) {
                xhr = new XMLHttpRequest();
            }
            else {
                xhr = ActiveXObject("XMLHttpRequest");
            }
            //ie5 和ie6是activeobject;
            //请求方式，地址，是否异步；
            xhr.open("get", "../Handler/test.ashx", true);
            //发送请求
            xhr.send();
             //readystate:0未初始化；1，已打开，2，已发送，3正在接受，4，已加载
            xhr.onreadystatechange = function (data) {
                if (xhr.readyState == 4) {
                    alert(xhr.responseText);
                }
            };
        }
--------post方式
 function guo() {
            var xhr
            if (XMLHttpRequest) {
                xhr = new XMLHttpRequest();
            }
            else {
                xhr = ActiveXObject("XMLHttpRequest");
            }
            //ie5 和ie6是activeobject;
            //请求方式，地址，是否异步；
            //xhr.open("get", "../Handler/test.ashx", true);
            xhr.open("post", "../Handler/test.ashx", true);
            //发送请求
            //设置请求的头部；
            xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
            //xhr.send();
            xhr.send("a=3&b=4");
            xhr.onreadystatechange = function (data) {
                if (xhr.readyState == 4) {
                    alert(xhr.responseText);
                }
            };
        }