Mutation Testing: Why Code Coverage Lies

1Mutation Score = (Killed Mutants / Total Mutants) × 100
2
3Example:
4  Total mutants: 150
5  Killed: 120
6  Survived: 30
7  Mutation Score: 80%
8
9Target: >75% for business logic, >60% for general code

1# Install Stryker
2npm install --save-dev @stryker-mutator/core @stryker-mutator/jest-runner
3
4# Initialize configuration
5npx stryker init
6
7# Run mutation testing
8npx stryker run
9
10# Run on a specific file
11npx stryker run --mutate src/utils/pricing.ts

1// Arithmetic Operator Mutation
2return a + b;  // → a - b, a * b, a / b, a % b
3
4// Test that kills it:
5expect(add(3, 2)).toBe(5);  // 3 - 2 = 1, not 5 ✓
6
7// Logical Operator Mutation
8if (isAdmin && isVerified)  // → isAdmin || isVerified
9
10// Test that kills it:
11expect(canAccess(true, false)).toBe(false);  // || would give true ✓
12
13// Relational Operator Mutation
14if (age >= 18)  // → age > 18, age <= 18, age === 18
15
16// Test that kills boundary:
17expect(isAdult(18)).toBe(true);  // > 18 would give false ✓
18
19// Return Value Mutation
20return price * discount;  // → return undefined
21
22// Test that kills it:
23expect(calculate(100, 0.2)).toBe(20);  // undefined !== 20 ✓

1// stryker.config.mjs
2export default {
3  testRunner: 'jest',
4  coverageAnalysis: 'perTest',
5  mutate: ['src/**/*.ts', '!src/**/*.spec.ts', '!src/**/__tests__/**'],
6  thresholds: { high: 80, low: 60, break: 50 },
7  reporters: ['html', 'clear-text'],
8  timeoutMS: 10000,
9  concurrency: 4,
10};
11
12// Run:
13// npx stryker run
14// npx stryker run --mutate src/pricing.ts
15
16// CI (nightly, not every PR — too slow):
17// schedule:
18//   - cron: '0 2 * * *'  # 2am daily
19// jobs:
20//   mutation:
21//     steps:
22//       - run: npx stryker run --reporters dashboard

1File: src/utils/pricing.ts
2Mutation score: 73% (22/30 killed)
3
4Survived mutants:
5Line 15 | ArithmeticOperator | return price * (1 - discount) → return price * (1 + discount)
6Line 22 | RelationalOperator | if (qty > 0) → if (qty >= 0)
7Line 31 | StringLiteral     | throw Error('Invalid') → throw Error('')

1// For each survived mutant, add a test:
2// Line 15 survived: add assertion on return value
3test('calculates correct discounted price', () => {
4  expect(applyDiscount(100, 0.2)).toBe(80); // kills arithmetic mutant
5});
6
7// Line 22 survived: add boundary test
8test('quantity validation: zero is invalid', () => {
9  expect(isValidQuantity(0)).toBe(false); // kills >= mutant
10  expect(isValidQuantity(1)).toBe(true);
11});
12
13// Line 31 survived: assert specific error message
14test('throws specific error message', () => {
15  expect(() => validateCoupon('FAKE')).toThrow('Invalid');
16  // toThrow('') would also pass — use specific message
17});

1// Equivalent mutant example:
2function initCounter() {
3  count = 0;  // Stryker mutates to: count = 1
4}
5// If count is never read in tests, both versions behave identically
6// This is an equivalent mutant — safe to ignore
7
8// Ignore annotation:
9function initCounter() {
10  // Stryker:disable-next-line:ArithmeticOperator
11  count = 0; // No arithmetic mutations applied to this line
12}
13
14// Ignore a block:
15// Stryker:disable ArithmeticOperator,StringLiteral
16function legacyCode() {
17  return 'legacy string' + (x + y);
18}
19// Stryker:enable ArithmeticOperator,StringLiteral
20
21// Document why you're ignoring:
22// Stryker:disable-next-line:RelationalOperator -- Arrays cannot have negative length, equivalent mutant
23if (items.length < 0) { /* unreachable */ }