c# - how to access and set the value of server control in javascript -

i in need access hidden field , set value something, can access hidden field value in c# code behind.

i used following:

var = document.getelementbyid(id).getattribute("value", true);  

but giving undefined or object null error.

any appreciated.

<asp:button id="backbtn" runat="server"  text="go back" onclientclick="callconfirmbox('<%= textbox2.clientid %>)';" onclick="btnback_click" /> <br/>  <script type="text/javascript">             function callconfirmbox(id) {                 var usrresponse;                 alert("in call confirm box called");                 if ('<%=this.test%>') {                      var usrresponse = window.confirm("do want go without saving data");                 }                 if (usrresponse) //userresponse true                 {                     alert(usrresponse);                    window.location = window.location.href;                 }                 else                {                   var e  =document.getelementbyid(id).getattribute("value",true);                     alert(e)                 }             }      </script> 

put iside

window.onload = function(){       function callconfirmbox(id) {                   //your code               } } 

your accessing element before dom finish loading.that's why getting such error.

edit :

suppose id of button backbtn given in html.

window.onload = function(){           var callconfirmbox = function (id) {                       //your code                   }          var backbtn = document.getelementbyid('backbtn');          backbtn.onclick = callconfirmbox;      }