I use it this way.
<asp:TextBox ID="txtValue" runat="server" CssClass="txtValue" style="display: none;" />
Then I use jQuery to access the value.
<script>
var txtValue = $(".txtValue").val();
</script>
Or even on a control.
<asp:LinkButton ID="lnkPrint" runat="server" CausesValidation="true"
CommandName="Print" CommandArgument='<%# Bind("PrintData") %>'
Text="<img src='/images/print_on.png' onmouseover='cursor: pointer;'
class='PrintButton' />" ToolTip="Print" OnClientClick="if ($('.txtValue').val()
!= '1') { $('.txtValue').val('1'); alert('Warning: Please enable pop-ups for
this site!'); }" />
This method survives ajax and postbacks.
Cheers