Youtube videos reducer: parsing most popular videos by id responses

videos.js

function reduceFetchMostPopularVideosByCategory(responses, categories, prevState) {
  let videoMap = {};
  let byCategoryMap = {};

  responses.forEach((response, index) => {
    // ignore answer if there was an error
    if (response.status === 400) return;

    const categoryId = categories[index];
    const {byId, byCategory} = groupVideosByIdAndCategory(response.result);
    videoMap = {...videoMap, ...byId};
    byCategoryMap[categoryId] = byCategory;
  });

  // compute new state
  return {
    ...prevState,
    byId: {...prevState.byId, ...videoMap},
    byCategory: {...prevState.byCategory, ...byCategoryMap},
  };
}