Mutation Testing Setup

You are a test quality engineer. The user wants to set up mutation testing to verify that their test suite can catch code changes (mutations) and validate test effectiveness.

What to check first

• Run npm list to verify testing framework is installed (Jest, Mocha, Vitest, etc.)
• Check if your project has a package.json with a test script defined
• Ensure your test suite runs successfully with npm test before adding mutation testing

Steps

1. Install Stryker (the industry-standard mutation testing framework) with npm install --save-dev @stryker-mutator/core @stryker-mutator/typescript-checker for TypeScript projects, or @stryker-mutator/javascript-mutator for JavaScript
2. Initialize Stryker configuration by running npx stryker init which generates a stryker.config.mjs file with sensible defaults for your test runner
3. Configure the testRunner property in stryker.config.mjs to match your framework: use "jest" for Jest, "mocha" for Mocha, or "vitest" for Vitest
4. Set the files array in the config to include your source files: ["src/**/*.ts", "!src/**/*.test.ts"] (exclude test files)
5. Define mutate patterns to specify which files should be mutated: ["src/**/*.ts", "!src/**/*.d.ts", "!src/**/*.test.ts"]
6. Set thresholds in config to enforce mutation score requirements: { high: 80, medium: 60, low: 0 } for minimum acceptable scores
7. Run mutation tests with npx stryker run to generate a report showing which mutations were killed (caught by tests) vs survived (missed)
8. Review the HTML report in reports/mutation/index.html to identify weak test coverage areas and add assertions to catch more mutations

Code

// stryker.config.mjs
export default {
  // Test runner configuration
  testRunner: "jest", // or "mocha", "vitest"
  
  // Source files to mutate
  mutate: [
    "src/**/*.ts",
    "!src/**/*.test.ts",
    "!src/**/*.spec.ts",
    "!src/**/*.d.ts"
  ],
  
  // Test files
  files: [
    "src/**/*.ts",
    "!src/**/*.test.ts",
    "!src/**/*.spec.ts"
  ],
  
  // Mutation testing configuration
  concurrency: 4,
  timeoutMS: 5000,
  maxTestRunnerReuse: 3,
  
  // Thresholds for mutation score
  thresholds: {
    high: 80,
    medium: 60,
    low: 0
  },
  
  // Reporters for output

Note: this example was truncated in the source. See the GitHub repo for the latest full version.

Common Pitfalls

• Treating this skill as a one-shot solution — most workflows need iteration and verification
• Skipping the verification steps — you don't know it worked until you measure
• Applying this skill without understanding the underlying problem — read the related docs first

When NOT to Use This Skill

• When a simpler manual approach would take less than 10 minutes
• On critical production systems without testing in staging first
• When you don't have permission or authorization to make these changes

How to Verify It Worked

• Run the verification steps documented above
• Compare the output against your expected baseline
• Check logs for any warnings or errors — silent failures are the worst kind

Production Considerations

• Test in staging before deploying to production
• Have a rollback plan — every change should be reversible
• Monitor the affected systems for at least 24 hours after the change