Youtube in React: handling VIDEO_DETAILS_SUCCESS action

videos.js

function reduceVideoDetails(responses, prevState) {
  const videoResponses = responses.filter(response => response.result.kind === VIDEO_LIST_RESPONSE);
  const parsedVideos = videoResponses.reduce((videoMap, response) => {
      // we're explicitly asking for a video with a particular id
      // so the response set must either contain 0 items (if a video with the id does not exist)
      // or at most one item (i.e. the channel we've been asking for)
      const video = response.result.items ? response.result.items[0] : null;
      if (!video) {
        return videoMap;
      }
      videoMap[video.id] = video;
      return videoMap;
    }, {});

  return {
    ...prevState,
    byId: {...prevState.byId, ...parsedVideos},
  };
}