What is NgRx?

import { createReducer, on, createAction, props, createSelector } from '@ngrx/store';

export const add = createAction('[Cart] Add', props<{ item: string }>());

export const cartReducer = createReducer(
  [] as string[],
  on(add, (state, { item }) => [...state, item]),   // pure, returns NEW state
);

// Memoized selector to read a slice of the store:
export const selectCart = (s: { cart: string[] }) => s.cart;
export const selectCount = createSelector(selectCart, (cart) => cart.length);

// In a component: this.store.dispatch(add({ item: 'book' }));
//                 count$ = this.store.select(selectCount);