movies-selectors.js

export const moviesSelector = ({ movies }) => movies.movies

movies-subscription-selectors.js

import { createSelector } from 'reselect'
import { moviesSelector } from './movies-selectors'
import { subscriptionsOfUserSelector } from './subscription-selectors'

export const getUserSubscriptions = createSelector(
  moviesSelector,
  subscriptionsOfUserSelector,
  (movies, subscriptions) =>
    subscriptions.map((s) => s.movieId) // create array of movie id's
      .filter((movieId) => !~subscriptions.indexOf(+movieId)) // get movies the user is not subscribed to
      .map((id) => movies[id]) // get the movies
)

subscription-selectors.js

import { createSelector } from 'reselect'

export const subscriptionsSelector = ({ subscriptions }) => subscriptions.subscriptions
export const subscriptionsOfUserSelector = createSelector(
  subscriptionsSelector,
  (_, { userId }) => +userId, // userId is passed as a prop to some component
  (subscriptions, userId) => ({
    subscriptions: Object.keys(subscriptions).map(key => subscriptions[key]).filter(s => s.userId === userId)
  })