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Discussion on: How to write unit tests

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peerreynders profile image
peerreynders

Testing is also about improving design.

The Flawed Theory Behind Unit Testing:

  • "One very common theory about unit testing is that quality comes from removing the errors that your tests catch. … It’s a nice theory, but it’s wrong."

  • "All of these techniques have been shown to increase quality. And, if we look closely we can see why: all of them force us to reflect on our code. When you write unit tests, … you scrutinize, you think, and often you prevent problems without even encountering a test failure."

  • "I like the fact that the tests we end up with using TDD give us additional leverage: they make it easier to change our code and know that it is still working without having to re-reason through the entirety of the code base."

  • "Quality is a function of thought and reflection - precise thought and reflection."

  1. Tests are supposed to enable "Refactoring Mercilessly"
  2. Code has to be structured in a certain way to be testable. That tends to drive designs towards loose coupling across internal boundaries around cohesive capabilities.

One caveat: Michael Feathers is a classicist (Detroit/Chicago style), not a mockist (London style); i.e.

"The problem that I saw with the mock object approach was that it only tested individual classes, not their interactions."

Which means overuse/abuse of mocks can undermine the beneficial design pressures that tests can bring to bear; typically mocks are OK for dependencies you don't control but a classicist would generally not mock code that is under their control.

Now people will often quote DHH's Test-induced design damage to support that TDD doesn't improve design. However that is from the perspective of a framework author. A framework (or library) imposes design pressures of it's own which often compete with those of the application domain.

With a framework a problem is always solved on the framework's terms somehow coercing the various application concerns in there. So TDD will always place frameworks (like Rails, React) with the Horrible Outside World.

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marcinwosinek profile image
Marcin Wosinek Author

Well said! Thanks for your comment.