Getting the Value of Hidden Field

See more:

ASP.NET

Javascript

Hi,
I have a problem. I call a java script in onChange event of dropdownlist.In that java script i have set a value to a hidden field but when the page is post backthe value of hidden field becomes null(DropdownList has a SelectedindexChanged Event and its AutopostBack Property is True.This Property Must be true.).I want to check the value of Hidden Field on Server side code of dropdownlist which was set by java script.Any one can help me.
My javascript Code is-

function ChaekDdl(id)
    {
   
    var ddl=document.getElementById(id);
    var hiddenField = document.getElementById('HiddenField1');
    if(ddl.options[ddl.selectedIndex].value=="1")
     {   //var check=confirm('Are You Sure?');    
            if(confirm('Are You Sure?'))
             {   
                //document.getElementById('TextBox1').value='1';       
                hiddenField.value = '1';
                return true;
            }
            else
            {
                hiddenField.value = '0';
                return false;
            }

    }
  }

Thanks in advance.