Show alert box before session expire

<script language="javascript" type="text/javascript">
       var sessionTimeoutWarning =
    "<%= System.Configuration.ConfigurationSettings.AppSettings
    ["SessionWarning"].ToString()%>";
        var sessionTimeout = "<%= Session.Timeout %>";

        var sTimeout = parseInt(sessionTimeoutWarning) * 60 * 1000;
        setTimeout('SessionWarning()', sTimeout);

        function SessionWarning() {
var message = "Your session will expire in another " +
    (parseInt(sessionTimeout) - parseInt(sessionTimeoutWarning)) +
    " mins! Please Save the data before the session expires";
alert(message);
        }
</script>