Functional Updates with Reselect 5.0

Demonstrate using Reselect 5.0 createSelector with functional updates to compute derived data when the state changes, showing performance improvements.

import { createSelector } from 'reselect';

Import the createSelector function from Reselect library.

// Assuming we have a state with users and posts
const selectUsers = state => state.users;
const selectPosts = state => state.posts;

Define simple selector functions for users and posts.

// Creates a memoized selector that calculates the number of posts per user
const selectPostsPerUser = createSelector(
  [selectUsers, selectPosts],
  (users, posts) => {
    return users.map(user => ({
      ...user,
      postCount: posts.filter(post => post.userId === user.id).length
    }));
  }
);

Create a memoized selector to compute the number of posts per user. It depends on users and posts selected from the state. It's a performance optimization to prevent unnecessary recalculations.

// This would be used within a component or with `mapStateToProps` in a connected component
// const mapStateToProps = state => ({
//   usersWithPostCount: selectPostsPerUser(state)
// });

Example usage of the selector in a React component or with `mapStateToProps` for a Redux connected component.