Youtube reducing fetchMostPopularVideos request

video.js

function reduceFetchMostPopularVideos(response, prevState) {
  const videoMap = response.items.reduce((accumulator, video) => {
    accumulator[video.id] = video;
    return accumulator;
  }, {});

  let items = Object.keys(videoMap);
  if (response.hasOwnProperty('prevPageToken') && prevState.mostPopular) {
    items = [...prevState.mostPopular.items, ...items];
  }

  const mostPopular = {
    totalResults: response.pageInfo.totalResults,
    nextPageToken: response.nextPageToken,
    items,
  };

  return {
    ...prevState,
    mostPopular,
    byId: {...prevState.byId, ...videoMap},
  };
}