derived-state.js

import { select } from 'easy-peasy'; // 👈 import the helper

const store = createStore({
  shoppingBasket: {
    products: [{ name: 'Shoes', price: 123 }, { name: 'Hat', price: 75 }],
    // 👇 define your derived state
    totalPrice: select(state =>
      state.products.reduce((acc, cur) => acc + cur.price, 0)
    )
  }
});