javascript alertbox with countdown

<script type="text/javascript" language="javascript">
        var counter = 10; var timerId;

        timerId = setInterval("ShowAlert()", 1000);

        function ShowAlert() {
            if (counter != 0) {
                alert(counter);
                counter = counter - 1;
            }
            else {
                clearInterval(timerId);
            }
        }
    </script>