漳州175医院体检:怎么编一个获取URL的函数?

代码如下:
<script language="JavaScript">
<!--
function SelText(obj)
{
if (obj.value == "请输入要查询的关键词")
{
obj.value = "";
}
}
//-->
</script>
<script language=javascript>

function search(){
if(document.searchform.tn1.value!="webpage")
{document.searchform.action="http://www.baidu.com/s?tn="+searchform.tn1.value";searchform.submit();}
else if(document.searchform.tn1.value=="news")
{document.searchform.action="+http://www.news.baidu.com/s?tn="+searchform.tn1.value";searchform.submit();}
else if(document.searchform.tn1.value=="mp3")
{document.searchform.action ="http://www.mp3.baidu.com/s?tn="+searchform.tn1.value";searchform.submit();}
else if(document.searchform.tn1.value=="bbs")
{document.searchform.action="http://www.bbs.baidu.com/s?tn="+searchform.tn1.value";searchform.submit();}
else if(document.searchform.tn1.value=="pic")
document.searchform.action="http://www.pic.baidu.com/s?tn="+searchform.tn1.value";searchform.submit();}
else if(document.searchform.tn1.value=="software")
{document.searchform.action="http://www.software.baidu.com/s?tn="+searchform.tn1.value";
searchform.submit();}
}
</script>
</td>
<td align="left" valign="top">
<table width="770" border="0" cellspacing="0" cellpadding="0" bgcolor="#8CAAE7" height="55" style="margin-top:5px">
<form name="searchform"target=_blank>
<td width="275" rowspan="2" align="right">
<input type="text" name="word" value="请输入要查询的问题" onClick="SelText(this);" size="25" class="inputsearch" onMouseOver=this.focus() onFocus=this.select()>
<select name="tn1" class="inputsearch">
<option value=webpage selected>网页</option>
<option value=news>新闻</option>
<option value=pic>图片</option>
<option value=mp3>音乐</option>
<option value=bbs>论坛</option>
<option value=tomnews>站内</option>
<option value=software>软件</option>
</select> <input name=tn type=hidden value="1234"></td>
<td width="100" rowspan="2" align="center"><input type=submit value="搜索" onclick="javascript:window.external.search()" name="submit2" class="inputs"></td>
</form></td></tr>怎么才能根据不同的选择选择不同的URL
JAVASCRIPT编程