reselect.js

import React from 'react';
import { useSelector } from 'react-redux';
import { createSelector } from 'reselect';

const selectUsers = state => state.users;

const selectNumOfUsers = createSelector(
  [selectUsers],
  users => users.length
);

export const UsersCounter = () => {
  const NumOfUsers = useSelector(selectNumOfUsers)
  return <div>{NumOfUsers}</div>
};

export const App = () => {
  return (
    <>
      <span>Number of users:</span>
      <UsersCounter />
    </>
  )
};