XmlHttp利用

点击按钮后，不要调用服务器端事件，而是调用客户端事件， 
<script   type="text/javascript"   language="javascript" > 
                var   xmlHttp   =   null; 
                function   PostData() 
                { 
                        try 
                        { 
                                xmlHttp   =   new   ActiveXObject("Msxml2.XMLHTTP"); 
                        } 
                        catch(err1) 
                        { 
                                try 
                                { 
                                        xmlHttp   =   new   ActiveXObject("Microsoft.XMLHTTP"); 
                                } 
                                catch(err2) 
                                { 
                                        xmlHttp   =   new   XMLHttpRequest(); 
                                } 
                        } 
                        var   Text1   =   document.getElementById("Text1"); 
                        var   url   =   "addData.aspx?User="   +   Text1.value; 
                        xmlHttp.open("GET",url,true); 
                        xmlHttp.onreadystatechange   =   OnHandler; 
                        xmlHttp.send(null); 

                } 
                function   OnHandler() 
                { 
                        if(xmlHttp.readyState   ==   4) 
                        { 
                                var   isValid   =   xmlHttp.responseText; 
                                var   exists   =   document.getElementById("exsits"); 
                                exists.innerHTML   =   isValid.substring(0,4); 
                        } 
                } 
</script > 
然后addData.aspx是一个空页面，其后台代码为 
protected   void   Page_Load(object   sender,   EventArgs   e) 
        { 
                string   user   =   this.Request.QueryString["User"]; 
                //add   to   db 
                response.write("added"); 
        }