JS元素意外点击元素消失

<!DOCTYPE html>

<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style type="text/css">
*{margin: 0;padding: 0}
a{ display: inline-block; width: 50px;height: 50px;background: #f40;margin: 50px 0 0 50px;}
div{margin: 60px 0 0 50px; padding: 20px;outline: none;border:1px solid #eee;border-radius: 5px;width: 200px;}
</style>
</head>
<body>
<a id="search">1111</a>
<div id="searchBox"></div>
<script type="text/javascript">
window.onload = function(){
var obj=document.getElementById("searchBox");
      var oBtn=document.getElementById("search");
      searchBox.style.display = "none";
      oBtn.onclick=function(){
       obj.style.display="block";
      }
      document.onclick=function(event){
       //
       var e=event || window.event;//兼容ie和非ie的event
       var aim=e.srcElement || e.target; //兼容ie和非ie的事件源
       //
       if(e.srcElement){
        var aim=e.srcElement;
         if(aim!=oBtn && aim!=obj){
           obj.style.display="none";
         }
       }else{
         var aim=e.target;
         if(aim!=oBtn && aim!=obj){
           obj.style.display="none";
         }
       }
      }
}
</script>
</body>
</html>