vanilla js Ajax

<script>
function loadDoc() {
var data = new FormData();
        data.append('id', id);
        
   var xhttp = new XMLHttpRequest();
   xhttp.onreadystatechange = function() {
      if (this.readyState == 4 && this.status == 200) {
        document.getElementById("demo").innerHTML = this.responseText;
       }
     };

   xhttp.open("POST", "fileName.php", true);
   xhttp.send();
}
</script>