Avoiding terrible assumptions by testing real behaviour

When writing tests, many developers want to remove the complexity of the system, focusing on the details at hand. To achieve this, we often reach for tools like mocking libraries. These libraries makes it easy to simulate dependencies and quickly test different scenarios. This allows you to focus on the details of the specific component you are testing.

While this may feel productive, that focus and speed can lead you into a trap. By testing implementation details, you are making assumptions on how the solution is built – assumptions that may not hold up over time. In this talk I will explore why testing behaviour, rather than implementation, is a better strategy in the long term.