How to findout session remining time in asp.net through javascript?

The Controls collection cannot be modified because the control contains code blocks (i.e. <% ... %>).

<script type="text/javascript" language="javascript">
        var sessionTimeout = "<%= Session.Timeout %>";
        function DisplaySessionTimeout() {
            alert(sessionTimeout);
            //if session is not less than 0
            if (sessionTimeout >= 0)
            //call the function again after 1 minute delay
                window.setTimeout("DisplaySessionTimeout()", 5000);
            else {
                //show message box
                alert("Your current Session is over.");
            }
        }
    </script>