Youtube in React: incorporating the commentThread response into our global state

comments.js

function reduceCommentThread(response, videoId, prevState) {
  if (!response) {
    return prevState;
  }
  const newComments = response.items.reduce((acc, item) => {
    acc[item.id] = item;
    return acc;
  }, {});

  // if we have already fetched some comments for a particular video
  // we just append the ids for the new comments
  const prevCommentIds = prevState.byVideo[videoId] ? prevState.byVideo[videoId].ids : [];
  const commentIds = [...prevCommentIds, ...Object.keys(newComments)];

  const byVideoComment = {
    nextPageToken: response.nextPageToken,
    ids: commentIds,
  };

  return {
    ...prevState,
    byId: {
      ...prevState.byId,
      ...newComments,
    },
    byVideo: {
      ...prevState.byVideo,
      [videoId]: byVideoComment,
    }
  };
}