index.html中有以下内容：
<div width="680">
<iframe src=top.html width="680" height="800" frameborder="0"name="event" id="event" style="overflow:hidden"></iframe>
</div>想在页面加载的时候，根据iframe的src的内容不同改变iframe的高度，写了以下函数。
function setOnloadFrameHeight(){
alert('JJJJJJJJ');
if(document.getElementById("event").src== "top.html")
{alert('FFFFFFFFFF'); document.getElementById("event").height= "200";}
else
{alert('GGGGGGG'); document.getElementById("event").height= "50";}
}注：调用的时候写window.onload = setOnloadFrameHeight()；运行的时候，alert('JJJJJJJJ')可以抛出来，alert('FFFFFFFFFF')和alert('GGGGGGG')都抛出不来。不知道是什么地方不对。
请问：应该怎么写（setOnloadFrameHeight）？
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
<style>
*{margin:0; padding:0;}</style>
</head><body>
<div width="680">
<iframe src=top.html width="680" height="800" frameborder="0"name="event" id="event" style="overflow:hidden"></iframe>
</div>
<script>
function setOnloadFrameHeight(){
alert('JJJJJJJJ');
if(document.getElementById("event").src== "top.html")
{alert('FFFFFFFFFF'); document.getElementById("event").height= "200";}
else
{alert('GGGGGGG'); document.getElementById("event").height= "50";}
}
window.onload = setOnloadFrameHeight; </script>
</body>
</html>