pattern-detection-walkthroughs

Pattern Detection Walkthroughs

Diagnostic Flow

Each walkthrough follows four steps:

1. Smell Detection (detect-code-smells)
2. Anti-Pattern Check (anti-patterns-catalog)
3. Refactoring Selection (refactor skills)
4. Design Pattern (prevent recurrence)

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Walkthrough 1: God Class → Extract Class + Facade

Smell: Large Class — 15+ methods across auth, profiles, email, billing.

// BEFORE: UserManager does everything
class UserManager {
  private db: Database; private mailer: Mailer; private stripe: StripeClient;
  login(email: string, password: string): Session { /* ... */ }
  logout(sessionId: string): void { /* ... */ }
  updateProfile(userId: string, data: ProfileData): User { /* ... */ }
  sendWelcomeEmail(userId: string): void { /* ... */ }
  createSubscription(userId: string, plan: string): Subscription { /* ... */ }
  processRefund(userId: string, amount: number): void { /* ... */ }
  // ... 8 more methods
}

Anti-Pattern: God Object — changes to billing break auth; every dev edits same file.

Refactoring: Extract Class per responsibility (refactor-moving-features).

// AFTER: focused services
class AuthService { login(...) { } logout(...) { } }
class ProfileService { updateProfile(...) { } deleteAccount(...) { } }
class NotificationService { sendWelcomeEmail(...) { } sendInvoice(...) { } }
class BillingService { createSubscription(...) { } processRefund(...) { } }

Pattern: Facade — single entry point for coordinated multi-service flows.

class UserFacade {
  constructor(private auth: AuthService, private profile: ProfileService,
    private notifications: NotificationService, private billing: BillingService) {}

  async registerUser(email: string, password: string, plan: string): Promise<User> {
    const session = await this.auth.login(email, password);
    await this.billing.createSubscription(session.userId, plan);
    await this.notifications.sendWelcomeEmail(session.userId);
    return this.profile.updateProfile(session.userId, { email });
  }
}