// BEFORE: Vuex module
const cartModule = {
  state: () => ({ items: [], total: 0 }),
  getters: {
    itemCount: (state) => state.items.length
  },
  mutations: {
    ADD_ITEM(state, item) {
      state.items.push(item)
      state.total += item.price
    }
  },
  actions: {
    async checkout({ state, commit }) {
      await api.checkout(state.items)
    }
  }
}

// AFTER: Pinia store
export const useCartStore = defineStore('cart', () => {
  const items = ref<CartItem[]>([])
  const total = computed(() => items.value.reduce((sum, i) => sum + i.price, 0))
  const itemCount = computed(() => items.value.length)

  function addItem(item: CartItem) {
    items.value.push(item)
  }

  async function checkout() {
    await api.checkout(items.value)
  }

  return { items, total, itemCount, addItem, checkout }
})

Key changes: No mutations, total is computed (not manual state), no commit/dispatch needed.