Mastery of TDD
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A collection of references and practices to build your mastery of TDD.
Once the red–green–refactor rhythm is second nature, the craft shifts from following the cycle to making better design decisions inside it. The guides below dig into that next layer: spotting the test smells that signal a design problem, breaking dependencies so units can be tested in isolation, applying SOLID principles to keep code malleable, choosing the refactoring that pays off most, and building test data and vocabulary that keep your suite readable as it grows. Work through them alongside the harder katas in the catalog and the lessons will stick.
Common mistakes in applying TDD. Recognize the patterns that signal your tests need attention.
Mocks, stubs, fakes — what to use when. Learn to isolate units and break tight coupling in your tests.
SOLID principles made simple. Understand how each principle shapes testable, maintainable code.
How to identify your next refactoring. Prioritize changes that deliver the most design improvement.
Builders, Object Mothers, and Scenario Factories. Turn test setup into a composable, domain-language API agents and humans can read.
Keep the language of the business, the code, and the tests converged. A practical guide to stopping vocabulary drift.
Where to start
The mastery guides don’t need to be read in order, but this sequence builds on itself:
- Test Smells. Learn to see what’s wrong with a test before learning the techniques that fix it.
- Breaking Dependencies. Most hard to test code is hard because of what it depends on. Mocks, stubs, and fakes each solve a different version of that problem.
- SOLID Explained and Refactoring Priorities together. SOLID tells you what good structure looks like; the priorities tell you which improvement to make first during the refactor step.
- Test Data Builders and Ubiquitous Language in Tests once your suite is large enough that setup code and inconsistent naming start to slow you down.
Practice at this level
Mastery shows up on problems with more moving parts. Katas such as Gilded Rose and Tennis Refactoring start from messy existing code that you have to change without breaking its behavior, while Bank OCR, Mars Rover, and Event Sourcing reward careful design as the requirements grow. Add a rule from the Constraint List to any of them to make the design pressure explicit.