mutation-testing

Mutation Testing

<default_to_action> When validating test quality or improving test effectiveness:

1. MUTATE code (change + to -, >= to >, remove statements)
2. RUN tests against each mutant
3. VERIFY tests catch mutations (kill mutants)
4. IDENTIFY surviving mutants (tests need improvement)
5. STRENGTHEN tests to kill surviving mutants

Quick Mutation Metrics:

• Mutation Score = Killed / (Killed + Survived)
• Target: > 80% mutation score
• Surviving mutants = weak tests

Critical Success Factors:

• High coverage ≠ good tests (100% coverage, 0% assertions)
• Mutation testing proves tests actually catch bugs
• Focus on critical code paths first </default_to_action>

Quick Reference Card

When to Use

• Evaluating test suite quality
• Finding gaps in test assertions
• Proving tests catch bugs
• Before critical releases

Mutation Score Interpretation

Score	Interpretation
90%+	Excellent test quality
80-90%	Good, minor improvements
60-80%	Needs attention
< 60%	Significant gaps

Common Mutation Operators

Category	Original	Mutant
Arithmetic	a + b	a - b
Relational	x >= 18	x > 18
Logical	a && b	a || b
Conditional	if (x)	if (true)
Statement	return x	(removed)

---

How Mutation Testing Works

// Original code
function isAdult(age) {
  return age >= 18; // ← Mutant: change >= to >
}

// Strong test (catches mutation)
test('18 is adult', () => {
  expect(isAdult(18)).toBe(true); // Kills mutant!
});

// Weak test (mutation survives)
test('19 is adult', () => {
  expect(isAdult(19)).toBe(true); // Doesn't catch >= vs >
});
// Surviving mutant → Test needs boundary value

---

Using Stryker

# Install
npm install --save-dev @stryker-mutator/core @stryker-mutator/jest-runner

# Initialize
npx stryker init

Configuration:

{
  "packageManager": "npm",
  "reporters": ["html", "clear-text", "progress"],
  "testRunner": "jest",
  "coverageAnalysis": "perTest",
  "mutate": [
    "src/**/*.ts",
    "!src/**/*.spec.ts"
  ],
  "thresholds": {
    "high": 90,
    "low": 70,
    "break": 60
  }
}