瀏覽器關閉 彈出提示

<script type="text/javascript">
    window.a = 0;
    var rv = "肯定要關閉嗎？";
    window.onbeforeunload = function () {
        var isIE = document.all ? true : false;
        if (isIE) {
            var n = window.event.screenX - window.screenLeft;
            var b = n > document.documentElement.scrollWidth - 20;
            if (b && window.event.clientY < 0 || window.event.altKey) {
                //是關閉而非刷新
                window.event.returnValue = rv;
            }
        } else {
            if (document.documentElement.scrollWidth != 0) {
                if (window.a != 1) {
                    window.a = 0;
                    return rv;
                } else {
                    window.a = 0;
                }
            }
            else {
                window.a = 0;
                return rv;
            }
        }
    }
</script>