simple-selector.js

// selectors.js
export const getItems = state => state.items;

// Component.jsx
import React from 'react';
import { connect } from 'react-redux';
import { getItems } from './selectors';

function MyComponent({ items }) {
  return <div>{items.length}</div> 
}

const mapStateToProps = state => ({ items: getItems(state) });

export default connect(mapStateToProps)(MyComponent);