D - Dependency Inversion

❌ Vi phạm DIP - tự new chi tiết cụ thể

class SMSSender {
  send(message: string): void {
    console.log("Gui SMS:", message);
  }
}

class NotificationService {
  private sender = new SMSSender();   // bam chat vao SMS cu the
  notify(message: string): void {
    this.sender.send(message);
  }
}

✅ Tuân thủ DIP - đảo chiều phụ thuộc

// Abstraction do tang cao dinh nghia
interface Sender {
  send(message: string): void;
}

// Cac chi tiet cap thap "tuan theo" abstraction
class SmsSender implements Sender {
  send(message: string) { console.log("SMS:", message); }
}
class EmailSender implements Sender {
  send(message: string) { console.log("Email:", message); }
}

class NotificationService {
  // Chi biet Sender (abstraction), khong biet la SMS hay Email
  constructor(private sender: Sender) {}
  notify(message: string) { this.sender.send(message); }
}

// "Diem lap rap" (composition root): quyet dinh dung cai nao
const service = new NotificationService(new EmailSender());
service.notify("Don hang da xac nhan");

notification.test.ts - tiêm bản giả

import { describe, it, expect } from "vitest";

class FakeSender implements Sender {
  sent: string[] = [];
  send(message: string) { this.sent.push(message); }
}

describe("NotificationService", () => {
  it("goi sender voi dung noi dung", () => {
    const fake = new FakeSender();
    new NotificationService(fake).notify("xin chao");
    expect(fake.sent).toEqual(["xin chao"]);
  });
});