---

name: testing/mutation-testing description: Mutation testing with Stryker to verify test quality by introducing code mutations and measuring detection rates category: testing tags:

• testing
• mutation
• stryker
• quality
• coverage related_skills:
• testing/comprehensive-testing
• testing/vitest
• methodology/quality-gates

---

Mutation Testing

Measure test quality by introducing bugs (mutations) and verifying your tests catch them.

Quick Start

Copy# Install Stryker
npm install -D @stryker-mutator/core @stryker-mutator/vitest-runner

# Initialize configuration
npx stryker init

# Run mutation testing
npm run test:mutation

Core Concept

Mutation testing introduces small changes (mutations) to your code and runs your tests. If tests still pass, they're too weak.

Copy// Original code
function isAdult(age) {
  return age >= 18;
}

// Mutations Stryker creates:
function isAdult(age) { return age > 18; }   // >= to >
function isAdult(age) { return age <= 18; }  // >= to <=
function isAdult(age) { return false; }      // return false
function isAdult(age) { return true; }       // return true

A good test catches all mutations:

Copydescribe('isAdult', () => {
  it('returns true for age 18', () => {
    expect(isAdult(18)).toBe(true);  // Catches >= to >
  });

  it('returns false for age 17', () => {
    expect(isAdult(17)).toBe(false); // Catches >= to <=
  });

  it('returns true for age 100', () => {
    expect(isAdult(100)).toBe(true); // Catches return false
  });
});

Configuration

stryker.config.json

Copy{
  "$schema": "https://raw.githubusercontent.com/stryker-mutator/stryker/master/packages/core/schema/stryker-schema.json",
  "packageManager": "npm",
  "testRunner": "vitest",
  "mutate": [
    "src/**/*.js",
    "src/**/*.ts",
    "!src/**/*.test.js",
    "!src/**/*.spec.ts"
  ],
  "reporters": [
    "progress",
    "clear-text",
    "html",
    "json"
  ],
  "htmlReporter": {
    "fileName": "reports/mutation/index.html"
  },
  "thresholds": {
    "high": 80,
    "low": 60,
    "break": 50
  },
  "concurrency": 4,
  "timeoutMS": 60000
}

Mutation Operators

Arithmetic Operators

Copy// Original: a + b
a - b    // Plus to Minus
a * b    // Plus to Times
a / b    // Plus to Divide

Comparison Operators

Copy// Original: a > b
a >= b   // Greater to GreaterOrEqual
a < b    // Greater to Less
a <= b   // Greater to LessOrEqual
a == b   // Greater to Equal

Logical Operators

Copy// Original: a && b
a || b   // And to Or

// Original: !a
a        // Negate removal

Boundary Mutations

Copy// Original: i < 10
i <= 10  // Less to LessOrEqual
i < 11   // Boundary change
i < 9    // Boundary change

Return Value Mutations

Copy// Original: return value
return undefined;  // Remove return
return !value;     // Negate return
return "";         // Empty string
return 0;          // Zero
return null;       // Null

Understanding Results

Mutation States

Mutation Score

CopyMutation Score = (Killed / Total) * 100%

• 80%+: Excellent test quality
• 60-80%: Good, room for improvement
• 40-60%: Weak tests, many gaps
• <40%: Critical test deficiency

Improving Mutation Score

1. Boundary Testing

Copy// Weak: Only tests middle values
it('validates age', () => {
  expect(isValidAge(25)).toBe(true);
});

// Strong: Tests boundaries
it('validates age boundaries', () => {
  expect(isValidAge(0)).toBe(true);    // Min boundary
  expect(isValidAge(-1)).toBe(false);  // Below min
  expect(isValidAge(150)).toBe(true);  // Max boundary
  expect(isValidAge(151)).toBe(false); // Above max
});

2. Condition Coverage

Copy// Original
function process(a, b) {
  if (a > 0 && b > 0) {
    return 'both positive';
  }
  return 'not both positive';
}

// Weak: Only tests one path
it('processes positive', () => {
  expect(process(1, 1)).toBe('both positive');
});

// Strong: Tests all conditions
it('processes various combinations', () => {
  expect(process(1, 1)).toBe('both positive');
  expect(process(-1, 1)).toBe('not both positive'); // a negative
  expect(process(1, -1)).toBe('not both positive'); // b negative
  expect(process(0, 1)).toBe('not both positive');  // a zero
});

3. Return Value Testing

Copy// Weak: Only tests truthy
it('checks admin', () => {
  expect(isAdmin(adminUser)).toBeTruthy();
});

// Strong: Tests exact values
it('checks admin status', () => {
  expect(isAdmin(adminUser)).toBe(true);
  expect(isAdmin(regularUser)).toBe(false);
  expect(isAdmin(null)).toBe(false);
});

Incremental Mutation Testing

For large codebases, run mutations on changed files only:

Copy# Only mutate changed files
git diff --name-only origin/main | xargs npx stryker run --mutate

CI Integration

GitHub Actions

Copyname: Mutation Testing

on:
  push:
    branches: [main]
  pull_request:

jobs:
  mutation:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4

      - name: Setup Node.js
        uses: actions/setup-node@v4
        with:
          node-version: '20'
          cache: 'npm'

      - run: npm ci

      - name: Run Mutation Tests
        run: npm run test:mutation

      - name: Upload Report
        uses: actions/upload-artifact@v4
        with:
          name: mutation-report
          path: reports/mutation/

Performance Optimization

Reduce Mutation Scope

Copy{
  "mutate": [
    "src/core/**/*.js",
    "!src/core/**/*.test.js",
    "!src/core/generated/**"
  ]
}

Increase Parallelism

Copy{
  "concurrency": 8,
  "testRunner": "vitest"
}

Filter Mutators

Copy{
  "mutator": {
    "excludedMutations": [
      "StringLiteral",
      "ObjectLiteral"
    ]
  }
}

When to Use

Good Candidates

• Critical business logic
• Security-sensitive code
• Mathematical calculations
• State machines
• Validation logic

When to Skip

• Generated code
• Configuration files
• Third-party wrappers
• UI components
• Test utilities

Anti-Patterns

1. Chasing 100%: Diminishing returns above 90%
2. Ignoring Timeouts: Fix infinite loop mutations
3. Testing Everything: Focus on critical paths
4. No Baseline: Establish baseline before improving
5. Infrequent Runs: Run on every PR