js如何取得框架内页面的url？

框架集代码
<FRAMESET cols="200,*" onResize="if (navigator.family == 'nn4') window.location.reload()">
  <FRAME src="Left.Htm" name="treeframe" >
  <frameset rows="59,*" cols="*" >
<frame src="contrl.htm" name="topFrame" >
<FRAME SRC="main.htm" name="basefrm">
</frameset>
</FRAMESET>
var url=parent.window.frames["basefrm"].location.href