javascript如何判断页面是刷新还是跳转还是关闭?

Question

<script   language="javascript">   
  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("关闭");   
          window.event.returnValue="";
      }   
     else
     {
          alert("刷新");
     }
  }   
  </script>