Test Doubles

Mock

A test double that records how it was called and lets the test assert on those interactions. Pre-programmed with expectations about which methods will be called and how.

Stub

A test double that returns canned responses to method calls, replacing real behaviour without recording or asserting on calls.

Spy

A test double that wraps real behaviour and records calls, so the test can both let the real code run and inspect how it was used.