Getting hidden variable value from server side in client side

hi,
        I am struck in problem. I have a hidden field in my form and i am setting some value in that.
         Now i wnt to use this value from client side but i m not able to retrieve it. All i am getting there is an object.
         I feel the hidden field is not able to hold that value.
         <input type="hidden" id="hdnVar" runat="server" />
         in code behind
         hdnVar.Value = "abc";
         client side
         alert('value '+document.getElementById('<%= hdnVar.ClientID %>'));
         alert only shows string value not the value of hidden field
...