mutation-testing

# Mutation Testing

Expert knowledge for mutation testing - validating that your tests actually catch bugs by introducing deliberate code mutations.

## Core Concept

- **Mutants**: Small code changes introduced automatically
- **Killed**: Test fails with mutation (good - test caught the bug)
- **Survived**: Test passes with mutation (bad - weak test)
- **Score**: Percentage of mutants killed (aim for 80%+)

## TypeScript/JavaScript (Stryker)

### Installation

```bash
# Using Bun
bun add -d @stryker-mutator/core @stryker-mutator/vitest-runner

# Using npm
npm install -D @stryker-mutator/core @stryker-mutator/vitest-runner
```

### Configuration

```typescript
// stryker.config.mjs
export default {
  packageManager: 'bun',
  reporters: ['html', 'clear-text', 'progress'],
  testRunner: 'vitest',
  coverageAnalysis: 'perTest',
  mutate: ['src/**/*.ts', '!src/**/*.test.ts'],
  thresholds: { high: 80, low: 60, break: 60 },
  incremental: true,
}
```

### Running Stryker

```bash
# Run mutation testing
bunx stryker run

# Incremental mode (only changed files)
bunx stryker run --incremental

# Specific files
bunx stryker run --mutate "src/utils/**/*.ts"

# Open HTML report
open reports/mutation/html/index.html
```

### Example: Weak Test

```typescript
// Source code
function calculateDiscount(price: number, percentage: number): number {
  return price - (price * percentage / 100)
}

// ❌ WEAK: Test passes even if we mutate calculation
test('applies discount', () => {
  expect(calculateDiscount(100, 10)).toBeDefined() // Too weak!
})

// ✅ STRONG: Test catches mutation
test('applies discount correctly', () => {
  expect(calculateDiscount(100, 10)).toBe(90)
  expect(calculateDiscount(100, 20)).toBe(80)
  expect(calculateDiscount(50, 10)).toBe(45)
})
```

## Python (mutmut)

### Installation

```bash
uv add --dev mutmut
```

### Running mutmut

```bash
# Run mutation testing
uv run mutmut run

# Show results
uv run mutmut results

# Show specific mutant
uv run mutmut show 1