mutation-testing

Mutation testing measures test quality by introducing code changes (mutants) and running your test suite against each one. A higher kill rate—the percentage of mutants caught by tests—indicates stronger test coverage and reveals which code paths lack adequate assertions.

Mutation score quantifies test effectiveness as a percentage: how many intentional code mutations your tests detected. A low score signals weak assertions and untested paths; a high score confirms your tests catch real bugs and edge cases.

Run mutation testing to identify surviving mutants—code changes your tests missed. Review the assertion gaps in those areas, add targeted tests to kill the survivors, and repeat until your mutation score reaches acceptable levels.

Yes. Mutation testing applies to TypeScript and JavaScript projects using mutation tooling like Stryker. The Skill applies mutation analysis during test quality evaluation and pre-release risk assessment for these language ecosystems.

Run mutation testing during test quality evaluation, gap analysis before release, and when assessing assertion coverage. It's most valuable for critical code paths where weak tests carry high risk of undetected bugs reaching production.

Code coverage measures which lines execute during tests; mutation testing measures whether tests actually catch bugs. High coverage doesn't guarantee effective tests—mutation analysis reveals weak assertions that coverage misses.