Library › Book 22 › Code Coverage and Mutation Testing
Code Coverage and Mutation Testing
19.1🔒Code Coverage: What It Measures and What It Does NotCode coverage measures which parts of the code are executed when your tests run. It answers: "Which lines of code have at least one test…
19.2🔒The Weak Assertion ProblemCoverage measures execution, not verification. A test that runs code but does not assert the correct behavior is theater, not testing.
19.3🔒When Coverage Numbers Lie
19.4🔒Healthy Use of Coverage Metrics- Use coverage to find blind spots, not to prove quality - Track coverage trends, not absolute numbers - Require minimum coverage for…
19.5🔒Mutation Testing: The True Measure of Test QualityMutation testing measures test effectiveness by deliberately introducing bugs (mutations) into your code and checking whether your tests…
19.6🔒Mutation ScoreKEY FORMULA: Mutation Score = (Killed Mutants / Total Mutants) x 100
19.7🔒Common Mutation Operators
19.8🔒Mutation Testing Tools
19.9🔒Practical Considerations- Mutation testing is slow. It runs your test suite once per mutation. A suite with 100 tests and 500 mutations means 50,000 test…
19.10🔒Q&ASelf-Assessment Quiz: Chapter 191. What does code coverage measure? What does it NOT measure? 2. Give an example of a test with 100% coverage that catches zero bugs. 3…
19.11🔒Key Takeaways- Code coverage measures execution, not verification -- high coverage does not guarantee quality - Mutation testing is the gold standard…
19.12🔒Hands-On ExercisesExercise 19.1 (Beginner): Calculate your project's code coverage. Review 5 tests in a high-coverage area -- are they genuinely testing…
19.13🔒Q&AResume phrasing- Introduced mutation testing (Stryker) for the payment processing module, revealing that despite 92% line coverage, only 68% of mutations…
19.14🔒Q&ACover letter framingI understand that code coverage measures execution, not verification. High coverage with weak assertions is test theater -- it looks good…
19.15🔒Q&AInterview framing"I approach coverage metrics with healthy skepticism. I use line coverage to find blind spots -- uncovered code that needs tests -- but I…
19.16🔒Q&AWhat not to say- "We have 90% coverage so our tests are great" -- coverage measures execution, not verification; 90% coverage with no assertions catches…
19.17🔒Q&AQuestion 1Prompt: A developer shows you a module with 95% line coverage and asks you to approve it. What questions do you ask before approving? What…
19.18🔒Q&AQuestion 2Prompt: Your team has never used mutation testing. How do you introduce it without overwhelming them? What a strong answer should cover…
19.19🔒Q&AQuestion 3Prompt: How do you set appropriate coverage targets that are context-dependent rather than team-wide? What a strong answer should cover…