怎么判断父窗口已关闭

如何判断父窗口已关闭

if(opener){     if(typeof(window.opener.document)==’unknown’||typeof(window.opener.document) == ’undefined’){         //父窗口已关闭     }else{         //父窗口没有关闭     } }else{        //父窗口已关闭 }