The structure of a test
As we saw earlier, a unit could be a feature if we are doing BDD, or it could be a component if we are doing traditional TDD. So, what does a unit test look like? From a very high level point of view, a unit test is like the following image:
You can see that the test is acting as the unit. The term "act" means that the test is performing a single operation on the unit through its public API.
Then, the test must check or assert the result of the operation. In this phase, we need to check whether the actual return value is as we expect, but we also need to check whether the side effects are the expected ones. A side effect is ...