applying-design-patterns

name	applying-design-patterns
description	Applies GoF and DDD design patterns in TypeScript: Factory, Repository, Strategy, Adapter, Decorator, Facade, Observer, Command, Specification. Use when picking the right structure for a feature or refactoring duplicated logic.

Applying Design Patterns

When to use

• Picking the right structure for a new feature
• Refactoring duplicated logic
• Need pluggable algorithms or interchangeable implementations

Core rules

1. Use patterns by name: Factory, Repository, Strategy, Adapter, Decorator, Facade, Observer, Command, Specification
2. Patterns serve the business, not the other way around
3. Prefer composition over inheritance
4. All pattern implementations must be OOP classes with explicit typing

Reference shape (TypeScript)

Factory Pattern

export class UserFactory {
  static createRegular(id: string, email: string): User {
    return User.create(id, Email.create(email), ['user']);
  }
  static createAdmin(id: string, email: string): User {
    return User.create(id, Email.create(email), ['user', 'admin']);
  }
}

Strategy Pattern

export interface PricingStrategy { calculate(base: number): number; }
export class DiscountPricing implements PricingStrategy {
  constructor(private discount: number) {}
  calculate(base: number): number { return base - this.discount; }
}

Repository Pattern

export interface OrderRepository {
  findById(id: string): Promise<Order | null>;
  save(order: Order): Promise<Order>;
}

Examples — Do

// Command pattern
class CreateOrderCommand {
  constructor(public readonly userId: string, public readonly items: OrderItem[]) {}
}
class CreateOrderHandler {
  async handle(cmd: CreateOrderCommand): Promise<Result<Order, AppError>> { ... }
}

Examples — Don't

// ❌ Over-engineering: simple logic doesn't need patterns
class UserBuilder { setName(n: string) { this.name = n; return this; } }

Checklist

• Pattern matches the problem (Factory for creation, Strategy for algorithms)
• All pattern classes are OOP with explicit types
• No over-engineering simple logic

See reference/pattern-examples.md for full patterns.