Youtube robust watch details reducer

video.js

function reduceWatchDetails(responses, prevState) {
  const videoDetailResponse = responses.find(r => r.result.kind === 'youtube#videoListResponse');
  // we know that items will only have one element
  // because we explicitly asked for a video with a specific id
  const video = videoDetailResponse.result.items[0];

  return {
    ...prevState,
    byId: {
      ...prevState.byId,
      [video.id]: video
    },
  };
}