DOM Promises are read only

gistfile1.js

function ajax(url) {
  return new Promise(function (resolver) {
    var xhr = new XMLHttpRequest();
    xhr.onreadystatechange = function () {
      if (xhr.readyState === 4) {
        if (xhr.status === 200) {
          resolver.resolve(xhr.responseText);
        } else {
          resolver.reject(new Error('There was a problem with the request.'));
        }
      }
    };
    xhr.open('GET', url);
    xhr.send();
  });
}