在基于Gecko的浏览器中检测窗口关闭事件

Question

如何在用户按下Alt + F4或在基于Gecko的浏览器中使用javascript window.close（）时检测窗口关闭事件？

Answer 1

window.onbeforeunload =function(){return '';}

Answer 2

.......

window.onunload = function(){
   alert('I am getting closed !!');
};