XMLHttpRequest wrapped in ES6 Promise

xhr.js

// https://github.com/mdn/js-examples/blob/master/promises-test/index.html
xhr = function(url, responseType) {
  return new Promise(function(resolve, reject) {
    var request = new XMLHttpRequest();
    request.open('GET', url, true);
    request.responseType = responseType;
    request.onload = function() {
      if (request.status === 200) {
        resolve(request.response);
      } else {
        reject(Error(`${url} failed to load; error code: ${request.statusText}`));
      }
    };
    request.onerror = function() {
      reject(Error(`${url} failed to load; there was a network error.`));
    };
    request.send();
  });
};