Simple promise based ajax helper function

ajax.js

/**
 * Ajax helper, returns a promise. Assumes both request and response are json.
 */
function ajax(type, url, data) {
  return new Promise(function (resolve, reject) {
    var req = new XMLHttpRequest();
    req.onload = function () {
      if (req.status >= 200 && req.status < 400) {
        resolve(JSON.parse(req.responseText));
      } else {
        reject(JSON.parse(req.responseText));
      }
    };
    req.onerror = function () {
      reject({success: false, error: 'Network interuption'});
    };
    req.open(type, url);
    if (type === 'PUT' || type === 'POST') {
      req.setRequestHeader('Content-Type', 'application/json');
    }

    req.send(JSON.stringify(data));
  });
}