Solving code coverage gaps in Typescript

Code coverage is a crucial metric for maintaining code quality and ensuring robust testing practices. However, achieving and maintaining high code coverage in TypeScript projects can be challenging. Let's explore some effective strategies for identifying and fixing code coverage gaps.

Understanding Code Coverage

Code coverage measures how much of your code is executed during testing. It typically includes:

• Line coverage: Lines of code executed
• Branch coverage: Decision paths taken
• Function coverage: Functions called
• Statement coverage: Individual statements executed

Common Coverage Gaps

1. Error Handling Paths

Error handling code often goes untested because it's challenging to trigger error conditions. Here's an example:

async function fetchUserData(userId: string) {
  try {
    const response = await api.get(`/users/${userId}`)
    return response.data
  } catch (error) {
    // This block might not be covered
    logger.error('Failed to fetch user data', error)
    throw new Error('User data unavailable')
  }
}

Solution: Mock API failures and validate error handling:

test('handles API errors gracefully', async () => {
  api.get.mockRejectedValue(new Error('Network error'))
  await expect(fetchUserData('123')).rejects.toThrow('User data unavailable')
  expect(logger.error).toHaveBeenCalled()
})

2. Type Guards and Narrowing

TypeScript type guards can create branches that are hard to cover:

function processValue(value: string | number) {
  if (typeof value === 'string') {
    return value.toUpperCase()
  } else {
    // This branch might be missed
    return value.toFixed(2)
  }
}

Solution: Test both type scenarios:

describe('processValue', () => {
  it('handles string input', () => {
    expect(processValue('test')).toBe('TEST')
  })

  it('handles number input', () => {
    expect(processValue(42.123)).toBe('42.12')
  })
})

3. Async Edge Cases

Async code can have subtle paths that are easy to miss:

async function retryOperation(fn: () => Promise<any>, maxRetries = 3) {
  let attempts = 0
  while (attempts < maxRetries) {
    try {
      return await fn()
    } catch (error) {
      attempts++
      if (attempts= maxRetries) {
        throw error // This might be uncovered
      }
      await delay(1000 * attempts)
    }
  }
}

Solution: Test retry logic thoroughly:

test('retries and eventually fails', async () => {
  const operation = jest.fn().mockRejectedValue(new Error('Failed'))
  await expect(retryOperation(operation)).rejects.toThrow('Failed')
  expect(operation).toHaveBeenCalledTimes(3)
})

Best Practices for Improving Coverage

1. Use TypeScript's Strict Mode Enable strict mode to catch more potential issues:

{
  "compilerOptions": {
    "strict": true,
    "noImplicitAny": true,
    "strictNullChecks": true
  }
}

2. Leverage Test Generators Tools like ts-jest can help generate test cases:

import { generateTests } from 'ts-jest'

generateTests({
  sourceFile: 'src/utils.ts',
  targetFile: 'tests/utils.test.ts',
  coverage: true,
})

3. Monitor Coverage Trends Set up coverage reporting in your CI pipeline:

jobs:
  test:
    steps:
      - uses: actions/checkout@v2
      - name: Run tests with coverage
        run: npm test -- --coverage
      - name: Upload coverage
        uses: codecov/codecov-action@v2

Advanced Coverage Techniques

Parametric Testing

Use test.each for comprehensive coverage:

type TestCase = [input: unknown, expected: string]

const testCases: TestCase[] = [
  ['string', 'STRING'],
  [42, '42.00'],
  [null, 'INVALID'],
]

test.each(testCases)('processValue handles %p correctly', (input, expected) => {
  expect(processValue(input)).toBe(expected)
})

Mock Timers for Async Code

Control time-based operations:

beforeEach(() => {
  jest.useFakeTimers()
})

test('handles timeout correctly', async () => {
  const promise = asyncOperation()
  jest.advanceTimersByTime(5000)
  await expect(promise).rejects.toThrow('Timeout')
})

Conclusion

Achieving high code coverage requires a systematic approach:

1. Identify common patterns that create coverage gaps
2. Use TypeScript features to catch issues early
3. Implement comprehensive testing strategies
4. Monitor and maintain coverage over time

Remember that 100% coverage doesn't guarantee bug-free code, but it provides confidence in your test suite's thoroughness.