Mock vs Stub — Explained with Examples

Mocks, stubs, fakes, and spies are test doubles that replace real dependencies in tests, each serving a different purpose in verification and isolation.

Test doubles are stand-ins for real objects during testing. The term was popularized by Gerard Meszaros in xUnit Test Patterns. Each type has a specific role.

The Four Types

Stub — provides canned answers to calls made during the test. Used when you need a consistent return value.

// Stub — returns fixed data
const userServiceStub = {
  getUser: async (id) => ({ id, name: 'Alice', role: 'admin' })
};

Mock — records calls and verifies that specific interactions happened. Used when you need to assert behavior.

// Mock — verifies interactions
const emailServiceMock = {
  send: jest.fn().mockResolvedValue(true)
};

// Test asserts that send() was called with expected args
test('sends welcome email on registration', async () => {
  await registerUser('alice@example.com', emailServiceMock);
  expect(emailServiceMock.send).toHaveBeenCalledWith(
    'alice@example.com',
    expect.stringContaining('Welcome')
  );
});

Fake — a working implementation but simplified (e.g., in-memory database instead of real PostgreSQL).

// Fake — lightweight in-memory database
class InMemoryUserRepo {
  constructor() { this.users = new Map(); }
  async create(user) {
    this.users.set(user.email, user);
    return user;
  }
  async findByEmail(email) {
    return this.users.get(email);
  }
}

Spy — wraps a real object and records calls without replacing behavior.

// Spy — wraps real logger, records calls
const logger = new Logger();
const spy = jest.spyOn(logger, 'info');

doSomething(logger);
expect(spy).toHaveBeenCalledWith('Operation completed');

Real-World Analogy

Consider a flight simulator for pilot training:

• Stub — the control panel always shows altitude 30,000 ft (fixed data)
• Mock — records that the pilot pressed the “landing gear up” button (behavior verification)
• Fake — a simplified physics engine that simulates flight but isn’t real aircraft software (working but simplified)
• Spy — a real altimeter with a recording of what it displayed (wraps real, records)

When to Use Each

Related Terms

Unit Testing, TDD, Integration Testing