捕捉浏览器的刷新与关闭

<script language="javascript">
<!--
var s="close";
function window.onunload()
{
if(s=="fresh") 
    if(window.screenLeft>10000)
    {alert('关闭');}
    else{alert('刷新');}
else 
    alert('关闭');
}
function window.onbeforeunload()
{ s="fresh"; }
-->
</script>