AJAX发送异步请求

<script type="text/javascript">

var xhr;

    function regist(){

         //1、创建XMLHttpRequest对象

         //判断当前浏览器是否支持XMLHttpRequest对象

         if(window.XMLHttpRequest){

             xhr=new XMLHttpRequest();

         }else if(window.ActiveXObject){

             xhr=new ActiveXObject("Microsoft.XMLHttp");

         }

        //获取表单数据

        var username=document.getElementByIdx("username").value;

        //2、创建与服务器的连接，调用open()函数

        //GET:连接方式为get请求，POSR：连接方式为post请求

        //UserAuthServlet:连接路径

        //true为异步请求，false为同步，一般写true

        xhr.open("GET","UserAuthServlet?username="+username,true);

        //3、注册回调函数

        xhr.onreadystatechange=callback;

        //4、发送请求

        //get请求写null,post请求写参数,例:xhr.send("username="+username);

        xhr.send(null);

    }

        //回调函数

        function callback(){

            //判断xhr状态

            if(xhr.readyState==4){

                 if(xhr.status==200){

                      var result=document.getElementByIdx("result");

                      result.innerHTML=xhr.responseText;

                  }

            }

        }

</script>