JS判断标签下是否存在另一个标签

例如：判断div下是否存在a标签

 <div id="hello">
      <a href="#">AAA</a> 
      <a href="#">BBB</a> 
      <a href="#">CCC</a>
      <a href="#">DDD</a>
 </div>

<script>
window.onload=function(){
    var obj = document.getElementById("hello");
    var a = obj.getElementsByTagName("a");
    if(a.length>0) alert("有a");
    else alert("无a");
}
</script>