封锁浏览器前出发事件

<a href="javascript:;">点我一下</a>
<a href="javascript:alert('test');">点我一下</a>
<a href="###" onclick="alert('test');">点我一下</a>
<script type="text/javascript">//<![CDATA[

window.onbeforeunload = function(e){
  return 'leave or stay';
}
window.onunload = function(){
  alert('onunload');
}
//]]></script>