Code Coverage Traps: 100% Coverage Does Not Mean No Bugs

Teams chase 100% code coverage but have tests that cover lines without asserting behavior. High coverage gives false confidence while bugs in integration paths go undetected.

Focus on meaningful coverage metrics and mutation testing rather than line coverage alone.

// BAD: Covers the line but asserts nothing
test('formatDate exists', () => {
  formatDate(new Date()); // Called but not asserted
});

// GOOD: Tests actual behavior with edge cases
test('formatDate formats correctly', () => {
  expect(formatDate(new Date('2024-01-15'))).toBe('Jan 15, 2024');
  expect(formatDate(new Date('2024-12-31'))).toBe('Dec 31, 2024');
});

test('formatDate handles invalid date', () => {
  expect(formatDate(new Date('invalid'))).toBe('Invalid date');
});

// Coverage config: exclude non-testable files
// jest.config.js
export default {
  collectCoverageFrom: [
    'src/**/*.ts',
    '!src/**/*.d.ts',
    '!src/types/**',
    '!src/**/index.ts', // re-exports only
  ],
  coverageThreshold: {
    global: { branches: 80, functions: 90 },
  },
};

Line coverage measures which code ran, not whether it was correct. A test can execute every line while making no assertions. Branches coverage is more meaningful; mutation testing is the strongest signal.