I have written up a checklist to use for unit-level Test-Driven Development, to make sure I do not skip steps while writing code, at a very low level of the development process. Ideally I will soon internalize this process to the point that I would recognize smells as soon as they show up the first time. This checklist is also applicable to the outer cycle of Acceptance TDD, but the Green part becomes much longer and it comprehends writing other tests. Ignore this paragraph if this get you confused.
TDD is described by a basic red-green-refactor cycle, constantly repeatead to add new features or fix bugs. I do not want to descend too much in object-oriented design in this post as you may prefer different techniques than me, so I will insist on the best practices to apply as soon as possible in the development of tests and production code. The checklist is written in the form of questions we should ask ourselves while going through the different phases, and that are often overlooked for the perceived simplicity of this cycle.
The development of every new feature should start with a failing test.
- Have you checked in the code in your remote or local repository? In case the code breaks, a revert is faster than a rewrite.
- Have you already written some production code? If so, comment it or (best) delete it to not be implicitly tied to an Api while writing the test.
- Have you chosen the right unit to expand? The modified class should be the one that remains more cohesive after the change, and often in new classes should be introduced instead of accomodating functionalites in existing ones.
- Does the test fail? If not, rewrite the test to expose the lack of functionality.
- Does a subset of the test already fail? Is so, you can remove the surplus part of the test, avoiding verbosity; it can come back in different test methods.
- Does the test prescribe overly specific assertions or expectations? If so, lessen the mock expectations by not checking method calls order or how many times a method is called; improve the assertions by substituting equality matches with matches over properties of the result object.
- Does the test name describe its intent? Make sure it is not tied to implementation details and works as low-level documentation.
- How much can you change in an hypothetical implementation without breaking the test (making it brittle)?
- Is the failure message expressive about what is broken? Make sure it describes where the failing functionality resides, highlighting the right location if it breaks in the future.
- Are magic numbers and strings expressed as constants? Is there repeated code? Test code refactoring is easy when done early and while a test fails, since in this paradigm it is more important to keep it failing then to keep it passing.
Enough production code should be written to make the test pass.
- Does the production code make the test pass? (Plainly obvious)
- Does a subset of the production code make the test pass? If so, you can comment or (best) remove the unnecessary production code. Any more lines you write are untested lines you'll have to read and maintain in the future.
- Every other specific action will be taken in the Refactor phase.
Improve the structure of the code to ease future changes and maintenance.
- Does repeated code exist in the current class?
- Is the name of the class under test appropriate?
- Do the public and protected method names describe their intent? Are they readable? Rename refactorings are between the most powerful ones.
- Does repeated code exist in different classes? Is there a missing domain concept? You can extract abstract classes or refactor towards composition. At this high-level the refactoring should be also applied to the unit tests, and there are many orthogonal techniques you can apply so I won't describe them all here.
Feel free to add insights and items on the list in the comments. I value very much feedback from other TDDers.