xhr.onreadystatechange=f1;

                                     xhr.send(null);

                            b,发送post请求

                                     xhr.open('post');

                                     ...

                   step3 服务器端编写处理请求的代码

                   step4 在监听器当中，处理服务器返回的数据，

                   一般是通过XmlHttpRequest获得数据之后，更

                   新页面。

                                     function f1(){

                                               //只有状态为4时，才能正确地接收服务器

                                               //返回的数据

                                               if(xhr.readyState == 4){

                                                        //获得服务器返回的文本数据

                                                        var txt = xhr.responseText;

                                                        //通过dom，更新页面