Mocks, stubs and fakes

A test double replaces a real dependency in a test. A stub returns canned answers and checks nothing. A mock is a stub that also records how it was called, so the test can assert the interaction happened — "charge was called once with 20". A fake is a working, simplified implementation, such as an in-memory database or an API that keeps state.

How it works

Stubs suit tests that only need the dependency to answer. Mocks suit tests where the call itself is the behaviour being verified. Fakes suit tests that exercise sequences — create, then read, then update — where canned answers would have to anticipate every step. In HTTP testing, all three are often loosely called "mock APIs".

The misconception

That more mocking means better isolation. Over-mocked tests assert on how code is written rather than what it does, and break on every refactor while missing real integration bugs. A fake that behaves like the real service often catches more with less setup.

Try it here

This playground is a fake in the strict sense: writes persist in your sandbox, so a POST is visible to a later GET, versions advance, and stock runs out — behaviour a stub cannot have.

Sources

Mocks, stubs and test isolation — Martin Fowler

Other terms

Mock API — A stand-in for a real API that returns realistic responses without the real system behind it.
API sandbox — An isolated copy of an API where calls have no real-world effect and data can be reset.
Test fixture — The known data and state a test starts from, set up the same way on every run.
Contract testing — Testing that an API and its consumers agree on the shape of requests and responses, without running them together.
Smoke testing — A small, fast set of checks that the most important paths work at all, run before anything more thorough.
Flaky tests — A test that sometimes passes and sometimes fails without any change to the code it tests.

All glossary terms · Testing techniques · All 39 mock REST APIs

Last updated 16 October 2026