Mutation Testing Setup

Set up mutation testing to verify test quality

Set up mutation testing to verify test quality.

Instructions

1. Install Stryker Mutator:

npm install -D @stryker-mutator/core @stryker-mutator/jest-runner @stryker-mutator/typescript-checker
npx stryker init    # interactive setup

2. Configure stryker.config.mjs:

export default {
  mutator: { excludedMutations: [] },
  testRunner: 'jest',
  checkers: ['typescript'],
  reporters: ['html', 'progress'],
  coverageAnalysis: 'perTest',
  mutate: ['src/**/*.ts', '!src/**/*.test.ts', '!src/**/*.d.ts'],
};

3. Run mutation testing:

npx stryker run

4. Interpret results:

   • Killed: mutation was caught by a test (good)
   • Survived: mutation was NOT caught (you need better tests)
   • No coverage: no test covers this code at all
   • Timeout: mutation caused an infinite loop (usually killed)

5. For each surviving mutant, write a test that catches it.

Target: mutation score > 80%

Focus on killing mutants in critical business logic — not in boilerplate.