Validate test effectiveness with mutation testing using Stryker (TypeScript/JavaScript) and mutmut (Python). Find weak tests that pass despite code mutations. Use to improve test quality.
/plugin marketplace add secondsky/claude-skills/plugin install mutation-testing@claude-skillsThis skill is limited to using the following tools:
Expert knowledge for mutation testing - validating that your tests actually catch bugs by introducing deliberate code mutations.
# Using Bun
bun add -d @stryker-mutator/core @stryker-mutator/vitest-runner
# Using npm
npm install -D @stryker-mutator/core @stryker-mutator/vitest-runner
// 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,
}
# 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
// 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)
})
uv add --dev mutmut
# Run mutation testing
uv run mutmut run
# Show results
uv run mutmut results
# Show specific mutant
uv run mutmut show 1
# Generate HTML report
uv run mutmut html
open html/index.html
// Arithmetic Operator
// Original: a + b → a - b, a * b, a / b
// Relational Operator
// Original: a > b → a >= b, a < b, a <= b
// Logical Operator
// Original: a && b → a || b
// Boolean Literal
// Original: true → false
| Score | Quality | Action |
|---|---|---|
| 90%+ | Excellent | Maintain quality |
| 80-89% | Good | Small improvements |
| 70-79% | Acceptable | Focus on weak areas |
| < 60% | Poor | Major improvements needed |
// Before: Mutation survives
test('calculates sum', () => {
expect(sum([1, 2, 3])).toBeGreaterThan(0) // Weak!
})
// After: Mutation killed
test('calculates sum correctly', () => {
expect(sum([1, 2, 3])).toBe(6)
expect(sum([0, 0, 0])).toBe(0)
expect(sum([])).toBe(0)
})
// After: Tests boundaries
test('validates age boundaries', () => {
expect(isValidAge(18)).toBe(true) // Min valid
expect(isValidAge(17)).toBe(false) // Just below
expect(isValidAge(100)).toBe(true) // Max valid
expect(isValidAge(101)).toBe(false) // Just above
})
# 1. Ensure good coverage first
bun test --coverage
# Target: 80%+ coverage
# 2. Run mutation testing
bunx stryker run
# 3. Check report
open reports/mutation/html/index.html
# 4. Fix survived mutants
# 5. Re-run incrementally
bunx stryker run --incremental
# or: npx stryker run --incremental
vitest-testing - Unit testing frameworktest-quality-analysis - Detecting test smells