Elegant Architecture

Before writing any code:
- List all features/functionalities needed
- Estimate code volume for each module
- Identify shared components
- Map dependencies between modules

When estimated lines > 200:
- Convert file to folder with index
- Split by sub-functionality
- Extract shared utilities

Example transformation:

# Before (user.ts - 400+ lines)
user.ts

# After (user/ folder)
user/
├── index.ts        # Public exports
├── types.ts        # Interfaces, types
├── validation.ts   # Input validation
├── repository.ts   # Data access
└── service.ts      # Business logic

// Define contracts before implementation
interface UserService {
  create(input: CreateUserInput): Promise<User>;
  findById(id: string): Promise<User | null>;
  update(id: string, input: UpdateUserInput): Promise<User>;
  delete(id: string): Promise<void>;
}

interface UserRepository {
  save(user: User): Promise<User>;
  findById(id: string): Promise<User | null>;
  findByEmail(email: string): Promise<User | null>;
  delete(id: string): Promise<void>;
}

For each module:
1. Create type definitions
2. Implement core logic
3. Add error handling
4. Write tests
5. Verify line count < 200

// ❌ Avoid: Mock everything
const mockRepo = jest.fn();
const service = new UserService(mockRepo);

// ✅ Prefer: Real implementations
const testDb = createTestDatabase();
const repo = new UserRepository(testDb);
const service = new UserService(repo);

// Test actual behavior
const user = await service.create({ email: 'test@example.com' });
const found = await service.findById(user.id);
expect(found).toEqual(user);

src/
├── modules/
│   ├── auth/
│   │   ├── index.ts
│   │   ├── types.ts
│   │   ├── service.ts
│   │   └── middleware.ts
│   ├── user/
│   │   ├── index.ts
│   │   ├── types.ts
│   │   ├── service.ts
│   │   └── repository.ts
│   └── order/
│       ├── index.ts
│       ├── types.ts
│       ├── service.ts
│       └── repository.ts
├── shared/
│   ├── database/
│   ├── errors/
│   └── utils/
└── index.ts

// Decouple components via constructor injection
class OrderService {
  constructor(
    private readonly orderRepo: OrderRepository,
    private readonly userService: UserService,
    private readonly paymentGateway: PaymentGateway
  ) {}

  async createOrder(userId: string, items: OrderItem[]): Promise<Order> {
    const user = await this.userService.findById(userId);
    if (!user) throw new NotFoundError('User', userId);

    const order = Order.create(user, items);
    await this.paymentGateway.charge(user, order.total);
    return this.orderRepo.save(order);
  }
}

// Wire up in composition root
const orderService = new OrderService(
  new PostgresOrderRepository(db),
  new UserService(userRepo),
  new StripePaymentGateway(stripeClient)
);

// Complex object creation
class NotificationFactory {
  create(type: NotificationType, data: NotificationData): Notification {
    switch (type) {
      case 'email':
        return new EmailNotification(data, this.emailClient);
      case 'sms':
        return new SmsNotification(data, this.smsClient);
      case 'push':
        return new PushNotification(data, this.pushClient);
      default:
        throw new Error(`Unknown notification type: ${type}`);
    }
  }
}

// Replaceable algorithms
interface PricingStrategy {
  calculate(order: Order): Money;
}

class StandardPricing implements PricingStrategy {
  calculate(order: Order): Money {
    return order.items.reduce((sum, item) => sum.add(item.price), Money.zero());
  }
}

class DiscountPricing implements PricingStrategy {
  constructor(private readonly discount: Percentage) {}

  calculate(order: Order): Money {
    const standard = new StandardPricing().calculate(order);
    return standard.subtract(standard.multiply(this.discount));
  }
}

class OrderProcessor {
  constructor(private pricing: PricingStrategy) {}

  setPricing(strategy: PricingStrategy) {
    this.pricing = strategy;
  }

  process(order: Order): ProcessedOrder {
    const total = this.pricing.calculate(order);
    return { ...order, total };
  }
}

1. Identify logical boundaries
2. Create folder with same name as file
3. Move related code to separate files
4. Create index.ts for public exports
5. Update imports in dependent files

module/
├── index.ts          # Public API exports
├── types.ts          # Interfaces, types, enums
├── constants.ts      # Configuration, magic values
├── utils.ts          # Helper functions
├── service.ts        # Business logic
├── repository.ts     # Data access
├── validation.ts     # Input validation
└── errors.ts         # Custom errors

## Pre-Implementation
- [ ] Requirements analyzed
- [ ] Code volume estimated
- [ ] File structure designed
- [ ] Interfaces defined
- [ ] Dependencies mapped

## Implementation
- [ ] Each file < 200 lines
- [ ] Single responsibility per module
- [ ] Dependencies injected
- [ ] Error handling complete
- [ ] No hardcoded values

## Testing
- [ ] Real implementations used
- [ ] No mocks for core logic
- [ ] Edge cases covered
- [ ] Integration tests exist

## Review
- [ ] Architecture documented
- [ ] Public APIs clear
- [ ] No circular dependencies
- [ ] Easy to extend

❌ God files (500+ lines doing everything)
❌ Mocking everything in tests
❌ Coding before planning
❌ Tight coupling between modules
❌ Hardcoded configuration
❌ Circular dependencies
❌ Unclear module boundaries