javascript 的事件--阻止冒泡

<!DOCTYPE html>
<html lang="en">
<head>
   <meta charset="UTF-8">
   <title>测试</title>
</head>
<body>
   <input type="button" id="btn1" value="点击">
   <div id="div1" style=" 200px; height: 200px; display: none;"></div>
   <script>
       window.onload=function(){
           var oBtn=document.getElementById('btn1');
           var oDiv=document.getElementById('div1');
            oBtn.onclick=function(eve){
               var oEvent=eve||event;    //处理ie和火狐的兼容性问题
                oDiv.style.display='block';
               alert('按钮点击了');
               oEvent.cancelBubble=true;    //阻止冒泡
            }
            document.onclick=function(){
               alert('冒泡');
            }
       }
   </script>
</body>
</html>