XML DOM replaceData() Method

var xhttp = new XMLHttpRequest();
xhttp.onreadystatechange = function() {
if (xhttp.readyState == 4 && xhttp.status == 200) {
myFunction(xhttp);
}
};
xhttp.open("GET", "books_comment.xml", true);
xhttp.send();

function myFunction(xml) {
    var x, i, txt, xmlDoc;
    xmlDoc = xml.responseXML;
   txt = "";
    x = xmlDoc.getElementsByTagName("book")[0].childNodes;
    for (i = 0; i < x.length; i++) {
    // Process only comment nodes
        if (x[i].nodeType == 8) {
            x[i].replaceData(4, 6, "Easy");
            txt += x[i].data + "<br>";
        }
    }
    document.getElementById("demo").innerHTML = txt;
}

commentNode.replaceData(start,length,string)