xml get tag names

var xhttp = new XMLHttpRequest();
xhttp.onreadystatechange = function() {

   if (this.readyState == 4 && this.status == 200) {

       myFunction(this);

   }

};
xhttp.open("GET", "books.xml", true);
xhttp.send();


function myFunction(xml) {
    var xmlDoc = xml.responseXML;
    var x = 
xmlDoc.getElementsByTagName("title")[0];
    
document.getElementById("demo").innerHTML =
    x.tagName;

}