document.getElementById("parentControl") 根据ID来获取一个控件

有一点一定要指出，

<form id="form1" runat="server">

<div id="parentControl" >
<a href="#">aaaaaaaaaaa</a>
</div>
</form>

<script type="text/javascript" language="javascript">

var parentControl=document.getElementById("parentControl");

var e = document.createElement("input");
e.type = "button";
e.value = "bb";
parentControl.appendChild(e);

</script>

<div id="parentControl" >一定要在document.getElementById("parentControl");之前出现，否则 var parentControl=document.getElementById("parentControl");将会是NULL