_showsReducer.js

export const initialState = {
  currentShowId: '74',
  show: null,
  episodes: [],
  actors: [],
};

export const showsReducer = baseReducer(initialState, {
  [ShowsAction.REQUEST_SHOW_FINISHED](state, action) {
    return {
      ...state,
      show: action.payload,
    };
  },

  [ShowsAction.REQUEST_EPISODES_FINISHED](state, action) {
    return {
      ...state,
      episodes: action.payload,
    };
  },

  [ShowsAction.REQUEST_CAST_FINISHED](state, action) {
    return {
      ...state,
      actors: action.payload,
    };
  },
});

baseReducer.js

export default function baseReducer(initialState, reducerDictionary) {
  // returns a redux reducing function
  return (state = initialState, action) => {
    // if the action type is used for a reducer name then this be a reference to it.
    const reducer = reducerDictionary[action.type];

    // if the action type "reducer" const is undefined or the action is an error
    // return the state.
    if (!reducer || action.error) {
      return state;
    }

    // if there is a valid reducer call it with the state and action objects.
    return reducer(state, action);
  };
}