JavaScript中判断页面关闭、页面刷新的实现代码

window.onunload = function(){      
        var a_n = window.event.screenX - window.screenLeft;      
        var a_b = a_n > document.documentElement.scrollWidth-20;      
        if(a_b && window.event.clientY< 0 || window.event.altKey){      
                  alert('关闭页面行为');
        }else{
                  alert('跳转或者刷新页面行为');    
            }
}