Spies

As the name suggests, spies spy on a dependency. They wrap themselves around the real object and observe its behavior. Strictly speaking, we are not simulating the object but rather recording all the interactions with the underlying object we are spying on.

Spies are used in very specific contexts, such as when it is much easier to use the real implementation than a mock but you still want to assert how the method under test interacts with the dependency. Spies are less common in the wild.