Mutation Testing with Stryker for Meaningful Coverage

100% code coverage does not guarantee tests catch bugs. Tests that run code without asserting results will have perfect coverage but detect nothing.

Use Stryker mutation testing to verify that test assertions actually catch code changes.

# Install
npm install --save-dev @stryker-mutator/core @stryker-mutator/jest-runner

# stryker.config.json
{
  "testRunner": "jest",
  "reporters": ["html", "clear-text", "progress"],
  "coverageAnalysis": "perTest",
  "mutate": ["src/**/*.ts", "!src/**/*.test.ts"]
}

# Run mutation testing
npx stryker run

// Stryker mutates your code (e.g., changes + to -) and runs tests
// If tests still pass after the mutation, it's a 'surviving mutant'
// Surviving mutants = test gaps

// Example mutation:
function add(a: number, b: number): number {
  return a + b; // Stryker mutates to: return a - b;
}

// Test must assert the exact result:
test('add returns correct sum', () => {
  expect(add(2, 3)).toBe(5); // Catches the mutation
  // NOT: expect(add(2, 3)).toBeDefined(); // Misses the mutation
});

// Mutation score = (killed mutants / total mutants) * 100
// Aim for 70-80%+ for critical business logic

Mutation testing introduces deliberate bugs (mutations) and checks if tests catch them. A high mutation score means your tests are actually sensitive to behavior changes, not just coverage.