hiddenfield - unable to set ConfirmBox value in Hidden field using javascript -

when click on confirm box button , try set value in hidden fields showing error:

unable set confirm box value in hidden field using javascript

function confirm() {   var confirm_value = document.getelementbyid('<%=hdnconfirm.clientid%>');   var answer = confirm("record saved successfully!want scan document?");   if (answer) {     confirm_value.value = "yes";     return true;   } else {     confirm_value.value = "no";     return false;   } }