Mocks vs stubs vs fakes — what’s the difference?

Deep dive

Pick the right substitute:

• Stub: replace a dependency with canned outputs.
• Mock: assert calls, arguments, or call order.
• Fake: simplified but functional implementation.

Fakes are often more robust than mocks for integration-style tests.

Examples

Payment provider:

Stub: returns success
Mock: assert charge() called once with amount=100
Fake: in-memory ledger that records balances

Common pitfalls

• Overusing mocks, making tests brittle.
• Fakes that diverge from real behavior.
• Stubs that hide important errors.

Interview follow-ups

• When do you prefer a fake over a mock?
• How do you keep fakes in sync with real services?
• What is a spy and how is it different?