How to traverse all child nodes in javascript?

<?xml version="1.0"?>
<bookstore>
  <book>
     <author>name1</author>
     <bookname>C# Programming</bookname>
     <price>$50</price>
  </book>
  <book>
     <author>name2</author>
     <bookname>C++ Programming</bookname>
     <price>$60</price>
  </book>
  <book>
     <author>name3</author>
     <bookname>Java Programming</bookname>
     <price>$60</price>
  </book>
</bookstore>

<script type="text/javascript">
      function getXMLValue() {

          if (window.XMLHttpRequest) {
              xmlhttp = new XMLHttpRequest();
          }
          else {
              xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
          }

          xmlhttp.open("GET", "books.xml", false);
          xmlhttp.send(null);
          xmlDoc = xmlhttp.responseXML;

          var bNumber = xmlDoc.getElementsByTagName("book");
          for (var i = 0; i < bNumber.length; i++) {
              for (var j = 0; j < 3; j++) {
                  // Visit belong to all child nodes  of 'book' .
                 window.alert(xmlDoc.getElementsByTagName("book")[i].childNodes[j].childNodes[0].nodeValue);
              }
          }

      }

  </script>