How to put Grid Value to hidden Field

Dear Experts 
I am using obout gird control i want to put some grid fields valaue into hidden field can any one suggust me how can i do that . 
i am trying this code 
<asp:HiddenField ID="hdQuantityVal1" runat="server" />
then on Java Script i am doing this 
On BeforClientSide update i am telling that go to this java script function 
<ClientSideEvents OnBeforeClientUpdate= "getvalue"> </ClientSideEvents>

Here is javaScript Code 
<script type="text/javascript">

function getvalue(Record) {

alert( document.getElementById("hdQuantityVal").value)
confirm("After Script " + document.getElementById("hdQuantityVal").value);
}
</script>
...