javascript如何获取iframe的src属性

以下是引用片段：
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<HEAD>
<TITLE> New Document </TITLE>
<META NAME="Generator" CONTENT="EditPlus">
<META NAME="Author" CONTENT="">
<META NAME="Keywords" CONTENT="">
<META NAME="Description" CONTENT="">
</HEAD>
<BODY>

   <script language="javascript">
   window.onload=haha;
   function haha()
   {

    for(var i=0;i<document.all.length;i++)
    {
      if(document.all[i].tagName=="IFRAME")
   {
     alert(document.all[i].src);
   }

    }
   }

  //  alert(document.getElementById("haha").src);
   </script>
</BODY>
</HTML>
<iframe  src="http://baidu.com" height="0" width="0" frameborder="0"></iframe>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML> <HEAD> <TITLE> New Document </TITLE> <META NAME="Generator" CONTENT="EditPlus"> <META NAME="Author" CONTENT=""> <META NAME="Keywords" CONTENT=""> <META NAME="Description" CONTENT=""> </HEAD> <BODY> <script language="javascript"> window.onload=haha; function haha() { for(var i=0;i<document.all.length;i++) { if(document.all[i].tagName=="IFRAME") { alert(document.all[i].src); } } } // alert(document.getElementById("haha").src); </script> </BODY> </HTML> <iframe src="http://baidu.com" height="0" width="0" frameborder="0"></iframe>
[提示：你可先修改部分代码，再按运行]

注意:这个例子的iframe在HTML最后,所以一定要利用window.onload事件来调用函数,这样可以让代码载入完毕再调用函数,以免出错.