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)

Vitest Runner

Installation

# 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

// 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,
}

Bun Native Runner (bun test)

Use when projects use bun test directly (not Vitest).

Requirements