What wrong when I use function parser.parsefromstring(xml.responsetext, “text/XML”)?

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XML

I try to read xml files but I have a problem ... my code is :

<script >
    function myFunction(xml) {
          var i;
          var parser, xmlDoc;
          parser = new DOMParser();
          xmlDoc = parser.parseFromString(xml.responseText,"text/xml");
          alert(xmlDoc);
           var x = xmlDoc.getElementsByTagName("book");
           alert(x.length);
           alert(x.parentNode.nodeName);  
        }
 function loadDoc() {
      var xhttp = new XMLHttpRequest();
      xhttp.onreadystatechange = function() {
          myFunction(this);
      };
      xhttp.open("GET", "books.xml", true);
      xmlHttp.onreadystatechange = handleServerResponse;
      xhttp.send();
    }

</script>

my problem is when I run then

alert(xmlDoc); // print [object xmldoucument]
alert(x.length); // print 0
why alert(x.length); print zero ? what wrong ? I want to obtain the name of books ... but x.length is zero ... my xml file :

<bookstore>
   <book category="cooking">
       <title lang="en">Everyday Italian</title>
       <author>Giada De Laurentiis</author>
       <year>2005</year>
       <price>30.00</price> </book>
   <book category="children">
       <title lang="en">Harry Potter</title>
       <author>J K. Rowling</author>
       <year>2005</year>
       <price>29.99</price> </book>
   <book category="web">
       <title lang="en">XQuery Kick Start</title>
       <author>James McGovern</author>
       <author>Per Bothner</author>
       <author>Kurt Cagle</author>
       <author>James Linn</author>
       <author>Vaidyanathan Nagarajan</author>
       <year>2003</year>
       <price>49.99</price> </book>
   <book category="web" cover="paperback">
       <title lang="en">Learning XML</title>
       <author>Erik T. Ray</author>
       <year>2003</year>
       <price>39.95</price> </book>
</bookstore>

and I put my xml file in my project here here i put the file under WEB_INF folder

so any help plz ?

What I have tried:

my code is above so any help plz ?