如何在js读取xml时进行判断

xml文件：  
<?xml version='1.0' encoding='utf-8' ?>
  <lastTf>
  <bh>201010</bh>
  <mc>NAMELESS</mc>
  <lastTime>3020-01-01 00:00:00</lastTime>
  </lastTf>

首先想要读取“mc”的值，但当“mc”的值为“NAMELESS”时，改读“bh”的值的后两位数字进行显示。求助哪位大侠帮帮忙。。。

JScript code

<script type="text/javascript">
if (window.XMLHttpRequest)
  {// code for IE7+, Firefox, Chrome, Opera, Safari
  xmlhttp=new XMLHttpRequest();
  }
else
  {// code for IE6, IE5
  xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
  }

// load xml from the remote  
//xmlhttp.open("GET","the xml uri",false);
//xmlhttp.send();
//xmlDoc=xmlhttp.responseXML;

// load xml just for test 
  
  
var txt="<lastTf><bh>201010</bh><mc>NAMELESS</mc>";
txt=txt+"<lastTime>3020-01-01 00:00:00</lastTime><lastTf>";

var xmlDoc;
if (window.DOMParser)
  {
  parser=new DOMParser();
  xmlDoc=parser.parseFromString(txt,"text/xml");
  }
else // Internet Explorer
  {
  xmlDoc=new ActiveXObject("Microsoft.XMLDOM");
  xmlDoc.async="false";
  xmlDoc.loadXML(txt);
  } 
var v = xmlDoc.getElementsByTagName("mc")[0].childNodes[0].nodeValue;
if( v=="NAMELESS" ){
  v = xmlDoc.getElementsByTagName("bh")[0].childNodes[0].nodeValue;
}
alert(v);

</script>