redux - full example

deriveStateSelectors.js

import { compose } from "redux";
import { createSelector } from "reselect";

const getBooks = state => state.books;
const getUsers = state => state.users;
const getComments = state => state.comments;

const usersSummary = createSelector(
  [getUsers, getComments],
  (users, comments) =>
    users.map(user => ({
      ...user,
      booksCount: user.books.length,
      commentsCount: comments.filter(c => c.user === user.id).length
    }))
);

const booksSummary = createSelector(
  [getBooks, getUsers, getComments],
  (books, users, comments) =>
    books.map(book => ({
      ...book,
      readersCount: users.filter(u => u.books.indexOf(book.isbn) > -1).length,
      commentsCount: comments.filter(c => c.book === book.isbn).length
    }))
);

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

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


export const deriveState = compose(
  addBooksSummary,
  addUsersSummary
);