mutation-testing

# Mutation Testing

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

## Core Expertise

**Mutation Testing Concept**
- **Mutants**: Small code changes (mutations) introduced automatically
- **Killed**: Test fails with mutation (good - test caught the bug)
- **Survived**: Test passes with mutation (bad - weak test)
- **Coverage**: Tests execute mutated code but don't catch it
- **Score**: Percentage of mutants killed (aim for 80%+)

**What Mutation Testing Reveals**
- Tests that don't actually verify behavior
- Missing assertions or edge cases
- Overly permissive assertions
- Dead code or unnecessary logic
- Areas needing stronger tests

## TypeScript/JavaScript (Stryker)

### Installation

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

# Using npm
npm install -D @stryker-mutator/core

# For Vitest
bun add -d @stryker-mutator/vitest-runner

# For Jest
bun add -d @stryker-mutator/jest-runner
```

### Configuration

```typescript
// stryker.config.mjs
/** @type {import('@stryker-mutator/api/core').PartialStrykerOptions} */
export default {
  packageManager: 'bun',
  reporters: ['html', 'clear-text', 'progress', 'dashboard'],
  testRunner: 'vitest',
  coverageAnalysis: 'perTest',

  // Files to mutate
  mutate: [
    'src/**/*.ts',
    '!src/**/*.test.ts',
    '!src/**/*.spec.ts',
    '!src/**/*.d.ts',
  ],

  // Thresholds for CI
  thresholds: {
    high: 80,
    low: 60,
    break: 60, // Fail build if below this
  },

  // Incremental mode (faster)
  incremental: true,
  incrementalFile: '.stryker-tmp/incremental.json',

  // Concurrency
  concurrency: 4,

  // Timeout
  timeoutMS: 60000,
}
```

### Running Stryker

```bash
# Run mutation testing
npx stryker run

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

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

# With specific configuration
npx stryker run --configFile stryker.prod.config.mjs

# G