检测浏览器的关闭事件

<script type="text/javascript">   
<!--
        window.onbeforeunload = onbeforeunload_handler;//监听浏览器关闭事件

        function onbeforeunload_handler() {
            var rstr = "确定退出";
            alert("-----------");
            return rstr;
        }

// -->   
</script>