Check for Session Timeout in Javascript and redirect to login page

  <script type="text/javascript">
        function CheckSession() {
            var session = '<%=Session["username"] != null%>';
            //session = '<%=Session["username"]%>';
            alert(session);
            if (session == false) {
                alert("Your Session has expired");
                window.location = "login.aspx";
            }
            else {
                alert(session);
            }
        }
    </script>