The Power Trifecta: Combining Factory, Strategy, and Adapter Patterns in TypeScript

// ==========================================
// 1. STRATEGY PATTERN
// Defines the common interface for all behaviors
// ==========================================

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

// Concrete Strategy A: Modern implementation
class CreditCardPayment implements PaymentStrategy {
  pay(amount: number): void {
    console.log(`✅ Paid $${amount} via Credit Card.`);
  }
}

// ==========================================
// 2. ADAPTER PATTERN
// Adapts an incompatible legacy class to the Strategy interface
// ==========================================

// The Incompatible Class (e.g., old banking system)
class LegacyBankSystem {
  // Note: Different method name and return type than PaymentStrategy
  makeTransfer(totalAmount: number): string {
    console.log(`...Connecting to legacy bank for transfer of $${totalAmount}`);
    return "TX_LEGACY_SUCCESS";
  }
}

// The Adapter
class BankTransferAdapter implements PaymentStrategy {
  private legacySystem: LegacyBankSystem;

  constructor(legacySystem: LegacyBankSystem) {
    this.legacySystem = legacySystem;
  }

  // Translates the 'pay' call into the legacy specific call
  pay(amount: number): void {
    const response = this.legacySystem.makeTransfer(amount);
    console.log(`✅ Bank Adapter finished: ${response}`);
  }
}

// ==========================================
// 3. FACTORY PATTERN
// Orchestrates creation logic (hiding instantiation details)
// ==========================================

class PaymentStrategyFactory {
  static getPaymentStrategy(method: 'creditcard' | 'banktransfer'): PaymentStrategy {
    switch (method) {
      case 'creditcard':
        return new CreditCardPayment();

      case 'banktransfer':
        // Complex creation logic (instantiating legacy system + adapter) 
        // is hidden from the client here.
        const legacySystem = new LegacyBankSystem();
        return new BankTransferAdapter(legacySystem);

      default:
        throw new Error('Unknown payment method');
    }
  }
}

// ==========================================
// CLIENT CODE
// Clean, decoupled, and unaware of the complexity
// ==========================================

function processUserCheckout(amount: number, method: 'creditcard' | 'banktransfer') {
  try {
    // 1. Ask Factory for the tool
    const strategy = PaymentStrategyFactory.getPaymentStrategy(method);

    // 2. Use the tool (Polymorphism)
    strategy.pay(amount);

  } catch (err) {
    console.error(err);
  }
}

// Usage
console.log("--- Transaction 1 ---");
processUserCheckout(100, 'creditcard');

console.log("\n--- Transaction 2 ---");
processUserCheckout(5000, 'banktransfer');