Test Doubles: Mocks, Stubs, Spies

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Jest Mock Functions(DOCUMENTATION)

Introduction

Test doubles replace real dependencies during testing. Understanding the difference between mocks, stubs, and spies helps you choose the right tool.

Key Concepts

Types of test doubles:

  • Stub: Returns pre-programmed responses
  • Spy: Records calls while using real implementation
  • Mock: Stubs with verification of how it was called

Deep Dive

Creating Stubs

typescript
// Stub - just returns a value
const getUserStub = jest.fn().mockResolvedValue({
  id: '1',
  name: 'John',
});

Creating Spies

typescript
// Spy - calls real implementation and records
const consoleSpy = jest.spyOn(console, 'log');

await doSomething();

expect(consoleSpy).toHaveBeenCalledWith('Expected message');
consoleSpy.mockRestore();

Full Mock with Verification

typescript
const mockPrisma = {
  user: {
    findUnique: jest.fn(),
    create: jest.fn(),
  },
};

jest.mock('@/lib/prisma', () => ({ prisma: mockPrisma }));

it('creates user with correct data', async () => {
  await createUser({ name: 'John' });
  
  expect(mockPrisma.user.create).toHaveBeenCalledWith({
    data: { name: 'John' },
  });
});

Mock Return Values

typescript
mock.mockReturnValue(42);           // Sync
mock.mockResolvedValue({ data });   // Async success
mock.mockRejectedValue(new Error());
 mock.mockImplementation((x) => x * 2);

Summary

Use stubs when you only need controlled return values, spies when you want to observe real behavior, and full mocks when you need both controlled behavior and call verification.

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