javascript - Displaying data from XML file using AJAX

Question

I need to make a communicator using AJAX and XML file. I've already done function to download the data from the form and send it via AJAX to the server(XML file). The code below is in JavaScript.

function send(){
    var xhttp = new XMLHttpRequest();
    xhttp.onreadystatechange = function() {
        if (this.readyState == 4 && this.status == 20) {
        download(this);
    }
    xhttp.open("GET", "file.xml", true);
    xhttp.send();
}

But I don't know how to make function that is sending an AJAX request to the server to download new messages and displaying them in the communicator.

function download(xml){
    var xmlDoc = xml.responseXML;
    var x = xmlDoc.getElementsByTagName("author");
}

I am downloading the data from the input in HTML and this is the way I save it to the XML file.

<?php

$author = $_GET["author"];
$message = $_GET["message"];
$file = 'file.xml';

if (!empty($author) && !empty($message)) {
    $xml = simplexml_load_file($file);
    $entry = $xml->addChild("entry");
    
    $sxe = simplexml_import_dom($xml);
    $sxe ->addChild("author", $author);
    $sxe ->addChild("message", $message);
    
    $xml->asXml($file);
}
header('location: index.html');
?>

My XML file looks like this after the send function.

<?xml version="1.0"?>
<entry>
  <author>gnow</author>
  <message>BAKDA</message>
<entry/>

Answer 1

somthing like that...

function downloadXML()
  {
  let xmlhttp = new XMLHttpRequest() 
  
  xmlhttp.onreadystatechange = function()
    {
    if (this.readyState===4)  /* DONE */
      {  
      if (this.status!=200)  /* not LOADING */
        {
        console.warn("Error %d on load %s !", this.status, 'fileName.xml' )
        }
      else
        {
        let xmlDoc  = this.responseXML
          , author  = xmlDoc.querySelector('entry author').textContent
          , message = xmlDoc.querySelector('entry message').textContent
        }
      }
    }
  }