判断浏览器是关闭还是刷新

//判断浏览器是刷新还是关闭
window.onbeforeunload = function()
{
       var n = window.event.screenX - window.screenLeft;
       var b = n > document.documentElement.scrollWidth-20;
       if(b && window.event.clientY < 0 || window.event.altKey)
       { 
           alert("是关闭")             
       }
}