docs: function reviewsById(state = {}, action) {
    switch (action.type) {
      case RECEIVE_PRODUCT_REVIEWS:
      case RECEIVE_REVIEWS: {
        const nextReviews = action.reviews || [];
        return nextReviews.reduce((currentReviews, review) => ({
          ...currentReviews,
          [review.id]: review,
        }), state);
      }
      case RECEIVE_SUBMIT_REVIEW:
      case RECEIVE_USER_REVIEW:
        return {
          ...state,
          [action.review.id]: action.review,
        };
      default:
        return state;
    }
  }