ajax.js

// ES6
function ajax(method, url, responseType, onsuccess = () => {}, onfailure = () => {
    console.error(method + "request to " + url + " failed with status " + request.status);
  }
) {
  var request = new XMLHttpRequest();
  request.responseType = responseType;
  request.onreadystatechange = () => {
    if (request.readyState == XMLHttpRequest.DONE) {
      if (request.status == 200)
        onsuccess(request.response);
      else
        onfailure(request.response);
    }
  };
  request.open(method, url, true);
  request.send();
}

// Legacy
function ajax(method, url, responseType, onsuccess, onfailure) {
  onsuccess = onsuccess || function() {};
  onfailure = onfailure || function() {
    console.error(method + "request to " + url + " failed with status " + request.status);
  };
  var request = new XMLHttpRequest();
  request.responseType = responseType;
  request.onreadystatechange = function() {
    if (request.readyState == XMLHttpRequest.DONE) {
      if (request.status == 200) {
        onsuccess(request.response);
      } else {
        onfailure(request.response);
      }
    }
  };
  request.open(method, url, true);
  request.send();
}