S - Single Responsibility

❌ Vi phạm SRP - OrderProcessor làm hết

class OrderProcessor {
  checkout(order: Order) {
    // (1) Xu ly thanh toan
    const ok = chargeCard(order.card, order.total);
    if (!ok) throw new Error("Thanh toan that bai");

    // (2) Xu ly van chuyen
    const label = createShippingLabel(order.address);
    bookCourier(label);

    // (3) Gui email xac nhan
    sendEmail(order.email, "Don hang da xac nhan");
  }
}

✅ Tuân thủ SRP - tách trách nhiệm

class PaymentProcessor {
  pay(order: Order): void {
    const ok = chargeCard(order.card, order.total);
    if (!ok) throw new Error("Thanh toan that bai");
  }
}

class ShippingProcessor {
  ship(order: Order): void {
    const label = createShippingLabel(order.address);
    bookCourier(label);
  }
}

class OrderNotifier {
  confirm(order: Order): void {
    sendEmail(order.email, "Don hang da xac nhan");
  }
}

// Lop dieu phoi: chi GHEP cac buoc, khong tu lam chi tiet
class OrderProcessor {
  constructor(
    private payment: PaymentProcessor,
    private shipping: ShippingProcessor,
    private notifier: OrderNotifier,
  ) {}

  checkout(order: Order): void {
    this.payment.pay(order);
    this.shipping.ship(order);
    this.notifier.confirm(order);
  }
}

payment.test.ts - test độc lập

import { describe, it, expect, vi } from "vitest";
import { chargeCard } from "./payment-gateway";

vi.mock("./payment-gateway");   // mock module truoc, vi.mocked moi co tac dung

describe("PaymentProcessor", () => {
  it("nem loi khi the bi tu choi", () => {
    vi.mocked(chargeCard).mockReturnValue(false);
    const order = { id: 1, total: 499000 };
    expect(() => new PaymentProcessor().pay(order))
      .toThrow("Thanh toan that bai");
  });
});