testing-patterns

Testing Patterns and Anti-Patterns

Overview

Classify test doubles, recognize test smells, apply testing strategies, and avoid TDD anti-patterns. Use when writing tests, reviewing test coverage in a PR, or diagnosing slow/fragile/misleading test suites.

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Test Double Taxonomy

Google's preference order: Real > Fake > Stub > Mock

Double	What it does	Verifies calls?	Has logic?	Use when
Dummy	Passed but never used	No	No	Satisfying unused required params
Stub	Returns preset values	No	Minimal	Controlling outputs for a specific path
Spy	Records calls, delegates to real	Yes (after)	Yes	Verifying call count/args without replacing behavior
Mock	Pre-programmed expectations	Yes (during)	No	Strict interaction verification — use sparingly
Fake	Simplified working impl	No	Yes	Replacing slow/external systems (in-memory DB, fake API)

// DUMMY
const dummyLogger: Logger = {} as Logger;

// STUB
const stubUserRepo = { findById: (_id: string) => Promise.resolve({ id: "1", name: "Alice" }) };

// SPY
const sendEmailSpy = jest.spyOn(emailService, "send");
await notifyUser(user);
expect(sendEmailSpy).toHaveBeenCalledWith(user.email, expect.any(String));

// MOCK
const mockGateway = { charge: jest.fn().mockResolvedValue({ status: "ok" }) };
await checkout(cart, mockGateway);
expect(mockGateway.charge).toHaveBeenCalledWith(cart.total);

// FAKE
class FakeUserRepository implements UserRepository {
  private store = new Map<string, User>();
  async findById(id: string) { return this.store.get(id) ?? null; }
  async save(user: User) { this.store.set(user.id, user); }
}

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Test Smells

1. The Liar

Passes but does not verify claimed behavior. Assertions always evaluate to true.

// SMELL: expect(result).toBeDefined();  // FIX: expect(result).toBe(90);

2. Excessive Setup

50+ lines before first assertion. Unit under test has too many dependencies. Fix: Factory function accepting only test-relevant params.