ngrx - この @ngrx 効果の例で、reduce 関数は何をしますか?

Question

この@ngrx エフェクトの例では、次の行の関数は何をしreduceますか?

const newBookEntities = newBooks.reduce((entities: { [id: string]: Book }, book: Book) => {
        return Object.assign(entities, {
          [book.id]: book
        });
      }, {});

newBooksの代わりに を使用することはできませんnewBookEntitiesか?

export function reducer(state = initialState, action: book.Actions | collection.Actions): State {
  switch (action.type) {
    case book.SEARCH_COMPLETE:
    case collection.LOAD_SUCCESS: {
      const books = action.payload;
      const newBooks = books.filter(book => !state.entities[book.id]);

      const newBookIds = newBooks.map(book => book.id);
      const newBookEntities = newBooks.reduce((entities: { [id: string]: Book }, book: Book) => {
        return Object.assign(entities, {
          [book.id]: book
        });
      }, {});

      return {
        ids: [ ...state.ids, ...newBookIds ],
        entities: Object.assign({}, state.entities, newBookEntities),
        selectedBookId: state.selectedBookId
      };
    }

    case book.LOAD: {
      const book = action.payload;

      if (state.ids.indexOf(book.id) > -1) {
        return state;
      }

      return {
        ids: [ ...state.ids, book.id ],
        entities: Object.assign({}, state.entities, {
          [book.id]: book
        }),
        selectedBookId: state.selectedBookId
      };
    }

    case book.SELECT: {
      return {
        ids: state.ids,
        entities: state.entities,
        selectedBookId: action.payload
      };
    }

    default: {
      return state;
    }
  }
}