sense-action-creator-2.js

const getUser = (userId) => (dispatch, getState) => {
  // create a unique key for this request
  const key = `@@getUser:${userId}`;

  // if the action creator has already fired off a request, return it
  const fetching = getState().fetching[key];
  if (fetching) {
    return fetching;
  }

  // check if the request is cached
  const cacheItem = getState().cache[key];
  if (cacheItem && cacheItem.expireTime > Date.now()) {
    return;
  }

  const promise = api.get(`/user/${userId}`).then(user => {
    // populate the store with the received data
    dispatch(receiveUser(user));

    // set the request as cached
    dispatch(setCache(key));

    // let components know we finished fetching
    dispatch(stopFetching(key));
  }, () => {
    dispatch(stopFetching(key));
  });

  // let components know we are fetching this data
  dispatch(startFetching(key, promise));
  
  return promise;
};