redux - full example

deriveState.js

import { compose } from "redux";

const usersSummary = function() {
  return this.users.map(user => ({
    ...user,
    booksCount: user.books.length,
    commentsCount: this.comments.filter(c => c.user === user.id).length
  }));
};

const booksSummary = function() {
  return this.books.map(book => ({
    ...book,
    readersCount: this.users.filter(u => u.books.indexOf(book.isbn) > -1)
      .length,
    commentsCount: this.comments.filter(c => c.book === book.isbn).length
  }));
};

const addUsersSummary = state =>
  Object.defineProperty(state, "usersSummary", {
    get: usersSummary
  });

const addBooksSummary = state =>
  Object.defineProperty(state, "booksSummary", {
    get: booksSummary
  });

export const deriveState = compose(addBooksSummary, addUsersSummary);