Structural Patterns

// Adapter — make incompatible interfaces work together
// Old payment interface
class OldPayPal { makePayment(amount: number, currency: string) { console.log(\`PayPal: \${amount} \${currency}\`); }
}

// New interface your app expects
interface PaymentProcessor { charge(cents: number): void;
}

// Adapter bridges the gap
class PayPalAdapter implements PaymentProcessor { constructor(private paypal: OldPayPal) {} charge(cents: number) { this.paypal.makePayment(cents / 100, 'USD'); }
}

const processor: PaymentProcessor = new PayPalAdapter(new OldPayPal());
processor.charge(4999); // PayPal: 49.99 USD

// Decorator — add behavior without subclassing
interface Logger { log(message: string): void;
}

class ConsoleLogger implements Logger { log(message: string) { console.log(message); }
}

class TimestampLogger implements Logger { constructor(private wrapped: Logger) {} log(message: string) { this.wrapped.log(\`[\${new Date().toISOString()}] \${message}\`); }
}

class PrefixLogger implements Logger { constructor(private wrapped: Logger, private prefix: string) {} log(message: string) { this.wrapped.log(\`\${this.prefix} \${message}\`); }
}

// Stack decorators
const logger = new PrefixLogger( new TimestampLogger(new ConsoleLogger()), '[INFO]');
logger.log('Server started');
// [INFO] [2026-04-10T...] Server started

// Facade — simplify a complex subsystem
class OrderFacade { constructor( private inventory: InventoryService, private payment: PaymentService, private shipping: ShippingService, private email: EmailService, ) {} async placeOrder(cart: Cart, card: CreditCard, address: Address) { await this.inventory.reserve(cart.items); await this.payment.charge(card, cart.total); const tracking = await this.shipping.dispatch(cart.items, address); await this.email.sendConfirmation(cart.userId, tracking); return tracking; }
}