if `fetch` ain't available, use this simple XHR wrapper (or use a polyfill)

xhrWrapper.js

function get(url, data, format) {
  return new Promise(function (resolve, reject) {
    var req = new XMLHttpRequest();
    req.open('GET', url);
    req.responseType = format || 'json';
    req.onload = function() {
      // It could be a successful response but not an OK one (e.g., 3xx, 4xx).
      if (req.status === 200) {
        resolve(req.response);
      } else {
        reject(Error(req.statusText));
      }
    };
    req.onerror = function() {
      reject(Error('Network Error'));
    };
    req.send(data);
  });
}