cache.js

if (_cacheValues.has(key)) {
  return Promise.resolve(_cacheValues.get(key));
} else if (_cachePromises.has(key)) {
        return _cachePromises.get(key);
} else {
  const promise = new Promise(function (resolve, reject) {
    return getData(key).then(data => {
      _cacheValues.set(key, data);
      _cachePromises.delete(key);
      const now = new Date().getTime();
      _cacheResolvedTime.set(key, now);
      resolve(data);
    });
  });
  _cachePromises.set(key, promise);
  return promise;
}