Essential Design Patterns in TypeScript: เขียนโค้ดให้ตายยาก

class Database {
  private static instance: Database;

  private constructor() {
    /* connect db */
  }

  public static getInstance(): Database {
    if (!Database.instance) {
      Database.instance = new Database();
    }
    return Database.instance;
  }
}

// ใช้กี่ครั้งก็ได้ Database ตัวเดิม
const db1 = Database.getInstance();
const db2 = Database.getInstance(); // db1 === db2

interface PaymentProcessor {
  pay(amount: number): void;
}

class Stripe implements PaymentProcessor { ... }
class PayPal implements PaymentProcessor { ... }

class PaymentFactory {
  static create(type: 'stripe' | 'paypal'): PaymentProcessor {
    if (type === 'stripe') return new Stripe();
    if (type === 'paypal') return new PayPal();
    throw new Error('Unknown payment method');
  }
}

// คนเรียกใช้ไม่ต้องรู้ว่าข้างในสร้างยังไง แค่ขอมาก็พอ
const payment = PaymentFactory.create('stripe');
payment.pay(100);

class NewsAgency {
  // Subject
  private subscribers: Function[] = [];

  subscribe(fn: Function) {
    this.subscribers.push(fn);
  }

  notify(news: string) {
    this.subscribers.forEach((sub) => sub(news));
  }
}

const cnn = new NewsAgency();
cnn.subscribe((news) => console.log("Reader 1 got:", news));
cnn.subscribe((news) => console.log("Reader 2 got:", news));

cnn.notify("Breaking News!"); // Reader 1 & 2 เด้งพร้อมกัน