reducer.js

// This is a reducer, same as Redux's
const reducer = (state, action) => {
  switch(action.type) {
    case 'FETCHING_TWEETS':
      return {...state, fetchingTweets: true};
      break;
    case 'TWEETS_RECEIVED':
      return { ...state,
        fetchingTweets: false,
        tweets: action.tweets
      };
    default:
      return state;
  }
}