ESP send A0 to my web page

-------------- Script I found ------------------------
<script>
   function obtenirVariables()
   {
      var uniqueURL = "reqEtatVariables" + "&aleatoire=" + Math.trunc(Math.random() * 1000000);
      var request = new XMLHttpRequest(); // http://www.toutjavascript.com/reference/ref-xmlhttprequest.php
      // la fonction à appeler lors d'un changement d'avancement de la requête AJAX
      request.onreadystatechange = function()
      {
         if (this.readyState == 4) {
            // Indicateur de l'avancement de l'appel AJAX == 4 => Données complètement accessibles
            if (this.status == 200) {
               // Code retour du serveur après l'appel AJAX == 200 => OK, tout s'est bien passé
               if (this.responseXML != null) {
                  // si on a bien obtenu une réponse non nulle
                  // alors on va extraire du XML les éléments qui nous intéressent
                  /*document.getElementById("boutonID").innerHTML =
                  this.responseXML.getElementsByTagName('bouton')[0].childNodes[0].nodeValue;

                  document.getElementById("digital1ID").innerHTML =
                  this.responseXML.getElementsByTagName('digital1')[0].childNodes[0].nodeValue;*/

                  document.getElementById("analog1ID").innerHTML =
                  this.responseXML.getElementsByTagName('sendsensor1')[0].childNodes[0].nodeValue;
                  document.getElementById("analog2ID").innerHTML =
                  this.responseXML.getElementsByTagName('sendsensor2')[0].childNodes[0].nodeValue;
                  document.getElementById("analog3ID").innerHTML =
                  this.responseXML.getElementsByTagName('sendsensor3')[0].childNodes[0].nodeValue;
                  document.getElementById("analog4ID").innerHTML =
                  this.responseXML.getElementsByTagName('sendsensor4')[0].childNodes[0].nodeValue;
               }
            }
         }
      }
      request.open("GET", uniqueURL , true); // ici on envoie la requête GET sur l'URL /reqEtatVariables
      request.send(null);
      setTimeout("obtenirVariables()", 1000); // on rappelle obtenirVariables() dans 1s
   }
   </script>

<p>la pin D4 vaut <span id="digital1ID">...</span></p> -->
   <p>Plant Soil Humidity Sensor1 <span id="analog1ID">...</span></p>
   <p>Plant Soil Humidity Sensor2 <span id="analog2ID">...</span></p>
   <p>Plant Soil Humidity Sensor3 <span id="analog3ID">...</span></p>
   <p>Plant Soil Humidity Sensor4 <span id="analog4ID">...</span></p>

----------------------------------  ESP8266 CODE -----------------------------
#include <ESP8266WebServer.h>
#include <FS.h>
const char* ssid = "****";
const char* password = "****";
const String DISPLAY_DATA_HTML = "/html/display_data.html";

ESP8266WebServer server(80);
#define Sensor1 A0

**** function to display the page ****
void DisplayData()
{
 String form = "";
 File f = SPIFFS.open(DISPLAY_DATA_HTML, "r");
 if (!f){
  Serial.println("Can't open update html file");
  server.send(404, "text/html", "File not found");
  }
  else{
   char buf[1024];
   int siz = f.size();
   while(siz > 0) {
    size_t len = std::min((int)(sizeof(buf) - 1), siz);
    f.read((uint8_t *)buf, len);
    buf[len] = 0;
    form += buf;
    siz -= sizeof(buf) - 1;
   }
   f.close();
   server.send(200, "text/html", form);
  }
}

void setup(void){
 Serial.begin(115200);

 if (!SPIFFS.begin()) {
  Serial.println("Failed to mount file system");
  return;
  }

 WiFi.begin(ssid, password);
 Serial.println("");
 // Wait for connection
 while (WiFi.status() != WL_CONNECTED) {
  delay(500);
  Serial.print(".");
  }
 Serial.println("");
 Serial.print("Connected to ");
 Serial.println(ssid);
 Serial.print("IP address: ");
 Serial.println(WiFi.localIP());

 if (MDNS.begin("esp8266")) {
  Serial.println("MDNS responder started");
  }

 server.on("/display_data",DisplayData);
 server.on("/reqEtatVariables", [&](){
 server.send(200, "text/XML", String((int)Web_Soil_Sensor1)); // send to someones browser when asked
  });

server.serveStatic("/css", SPIFFS, "/html/css");
server.serveStatic("/display_data.html", SPIFFS, "/html/display_data.html");

server.begin();
  Serial.println("HTTP server started");
  if (!MDNS.begin(host)) {
    Serial.println("Error setting up MDNS responder!");
    while(1){
      delay(1000);
    }
  }
  Serial.println("mDNS responder started");
  // Add service to MDNS-SD
  MDNS.addService("http", "tcp", 80);
}

void loop(void){
 server.handleClient();
}