意外关闭浏览器时弹出提示

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)
  {
    window.event.returnValue = ""; 
  }
}