javascript: vanilla example of ajax request. Taken from https://www.sitepoint.com/guide-vanilla-ajax-without-jquery/

ajaxReq.js

var xhr = new XMLHttpRequest();
xhr.open('GET', 'send-ajax-data.php');
xhr.send(null);

xhr.onreadystatechange = function () {
  var DONE = 4; // readyState 4 means the request is done.
  var OK = 200; // status 200 is a successful return.
  if (xhr.readyState === DONE) {
    if (xhr.status === OK) 
      console.log(xhr.responseText); // 'This is the returned text.'
    } else {
      console.log('Error: ' + xhr.status); // An error occurred during the request.
    }
  }
};