Shopping Cart Pricing with a useReducer State Machine and a Pure Discount Engine

import { CartItem } from './pricing';

export interface CartState {
  items: CartItem[];
  appliedCoupons: string[];
}

export type CartAction =
  | { type: 'ADD_ITEM'; item: CartItem }
  | { type: 'SET_QTY'; sku: string; qty: number }
  | { type: 'REMOVE_ITEM'; sku: string }
  | { type: 'APPLY_COUPON'; code: string }
  | { type: 'CLEAR' };

export const initialCart: CartState = { items: [], appliedCoupons: [] };

export function cartReducer(state: CartState, action: CartAction): CartState {
  switch (action.type) {
    case 'ADD_ITEM': {
      const existing = state.items.find((i) => i.sku === action.item.sku);
      const items = existing
        ? state.items.map((i) =>
            i.sku === action.item.sku ? { ...i, qty: i.qty + action.item.qty } : i
          )
        : [...state.items, action.item];
      return { ...state, items };
    }
    case 'SET_QTY': {
      if (action.qty <= 0) {
        return { ...state, items: state.items.filter((i) => i.sku !== action.sku) };
      }
      return {
        ...state,
        items: state.items.map((i) =>
          i.sku === action.sku ? { ...i, qty: action.qty } : i
        ),
      };
    }
    case 'REMOVE_ITEM':
      return { ...state, items: state.items.filter((i) => i.sku !== action.sku) };
    case 'APPLY_COUPON':
      if (state.appliedCoupons.includes(action.code)) return state;
      return { ...state, appliedCoupons: [...state.appliedCoupons, action.code] };
    case 'CLEAR':
      return initialCart;
    default:
      return state;
  }
}

import { useCallback, useMemo, useReducer } from 'react';
import { cartReducer, initialCart } from './cartReducer';
import { CartItem, DiscountRule, bogo, percentOff, priceCart } from './pricing';

const rules: DiscountRule[] = [
  percentOff('SAVE10', 10),
  bogo('SKU-COFFEE'),
];

export function useCart() {
  const [state, dispatch] = useReducer(cartReducer, initialCart);

  const totals = useMemo(
    () => priceCart(state.items, new Set(state.appliedCoupons), rules),
    [state]
  );

  const addItem = useCallback((item: CartItem) => dispatch({ type: 'ADD_ITEM', item }), []);
  const setQty = useCallback(
    (sku: string, qty: number) => dispatch({ type: 'SET_QTY', sku, qty }),
    []
  );
  const removeItem = useCallback((sku: string) => dispatch({ type: 'REMOVE_ITEM', sku }), []);
  const applyCoupon = useCallback(
    (code: string) => dispatch({ type: 'APPLY_COUPON', code }),
    []
  );
  const clear = useCallback(() => dispatch({ type: 'CLEAR' }), []);

  return {
    items: state.items,
    coupons: state.appliedCoupons,
    totals,
    addItem,
    setQty,
    removeItem,
    applyCoupon,
    clear,
  };
}