Code Coverage and Quality Metrics

What Code Coverage Measures

Code coverage measures what percentage of your source code is executed when your tests run.

Four types of coverage:

• Line coverage — Which lines of code were executed?
• Branch coverage — Which branches (if/else, ternary) were taken?
• Function coverage — Which functions were called?
• Statement coverage — Which statements executed?

Branch coverage is the most meaningful — it catches logic that is only wrong in certain conditions.

The Coverage Trap

100% coverage does not mean zero bugs.

Coverage measures execution, not correctness. A test that calls a function and asserts nothing will count as coverage. Coverage is a floor, not a ceiling.

Practical Coverage Targets

• Auth, payments, data mutations: 95%+
• Core business logic: 90%+
• API routes: 85%+
• UI components: 70%+
• Configuration files and generated code: Skip

Mutation Testing: Coverage Done Right

Mutation testing answers: "If I introduce a bug, does a test fail?" It automatically introduces small bugs (mutations) and runs your tests. If tests still pass with a bug present, they're not actually catching that bug. Mutation score is a better quality metric than line coverage.

Key Takeaways

• Branch coverage is more meaningful than line coverage — it catches conditional logic bugs
• Set coverage thresholds that fail CI if coverage drops — this creates accountability
• 100% coverage does not mean zero bugs — coverage measures execution, not correctness
• Focus high coverage (95%+) on critical paths: authentication, payments, data mutations
• Mutation testing is the gold standard — it verifies that your tests would actually catch bugs