Coverage Standards Skill

This skill covers test coverage standards and analysis.

When to Use

Use this skill when:

• Setting coverage thresholds
• Analyzing coverage reports
• Improving test coverage
• Configuring CI coverage gates

Core Principle

COVERAGE IS A MINIMUM, NOT A GOAL - 80% coverage is a floor, not a ceiling. Focus on testing critical paths.

Coverage Thresholds

Minimum Requirements

Vitest Configuration

// vitest.config.ts
export default defineConfig({
  test: {
    coverage: {
      provider: 'v8',
      thresholds: {
        lines: 80,
        functions: 80,
        branches: 80,
        statements: 80,
      },
    },
  },
});

Coverage Metrics Explained

Lines Coverage

Percentage of executable lines that were run during tests.

function example(x: number): number {
  if (x > 0) {         // Line 1
    return x * 2;      // Line 2 - only covered if x > 0
  }
  return 0;            // Line 3 - only covered if x <= 0
}

Branch Coverage

Percentage of decision branches (if/else, switch, ternary) that were taken.

function example(x: number): string {
  // Two branches: true and false
  if (x > 0) {
    return 'positive';  // Branch 1
  } else {
    return 'non-positive';  // Branch 2
  }
}

// Need both tests for 100% branch coverage
test('positive', () => expect(example(1)).toBe('positive'));
test('non-positive', () => expect(example(0)).toBe('non-positive'));

Function Coverage

Percentage of functions that were called at least once.

Statement Coverage

Percentage of statements that were executed.

Coverage Exclusions

Valid Exclusions

// vitest.config.ts
export default defineConfig({
  test: {
    coverage: {
      exclude: [
        'node_modules/',
        'dist/',
        '**/*.test.ts',
        '**/__tests__/**',
        '**/__mocks__/**',
        '**/*.d.ts',
        '**/types/**',
        '**/index.ts',  // Re-export files
        '**/generated/**',  // Generated code
      ],
    },
  },
});

Inline Exclusions

/* v8 ignore next */
function debugOnly(): void {
  console.log('debug');
}

/* v8 ignore start */
function untestableCode(): void {
  // Platform-specific code
}
/* v8 ignore stop */

Coverage Reports

Report Types

export default defineConfig({
  test: {
    coverage: {
      reporter: [
        'text',      // Terminal output
        'html',      // HTML report
        'json',      // JSON for tools
        'lcov',      // For coverage services
        'cobertura', // For CI systems
      ],
      reportsDirectory: './coverage',
    },
  },
});

Viewing Reports

# Generate coverage
npm run test:coverage

# View HTML report
open coverage/index.html

Improving Coverage

Identify Gaps

1. Run coverage report
2. Check HTML report for red (uncovered) lines
3. Prioritize critical paths
4. Add tests for uncovered branches

Common Uncovered Patterns

Error Handling

// Often uncovered
function fetchData(): Promise<Data> {
  try {
    return await api.get('/data');
  } catch (error) {
    // This branch often uncovered
    throw new ApiError('Failed to fetch');
  }
}

// Test the error path
it('should throw on API error', async () => {
  vi.mocked(api.get).mockRejectedValue(new Error('Network'));
  await expect(fetchData()).rejects.toThrow('Failed to fetch');
});

Edge Cases

function divide(a: number, b: number): number {
  if (b === 0) {
    throw new Error('Division by zero');  // Often uncovered
  }
  return a / b;
}

// Test edge case
it('should throw for division by zero', () => {
  expect(() => divide(1, 0)).toThrow('Division by zero');
});

Guard Clauses

function processUser(user: User | null): string {
  if (!user) {
    return 'No user';  // Test this branch
  }
  return user.name;
}

Coverage Anti-Patterns

Testing Implementation Details

// ❌ Bad - tests internal state
it('should set internal flag', () => {
  service.process();
  expect(service._internalFlag).toBe(true);
});

// ✅ Good - tests behavior
it('should produce expected output', () => {
  const result = service.process();
  expect(result).toEqual(expected);
});

Coverage Without Assertions

// ❌ Bad - covers code but doesn't verify
it('should run without errors', () => {
  const result = processData(input);
  // No assertions!
});

// ✅ Good - verifies behavior
it('should transform data correctly', () => {
  const result = processData(input);
  expect(result).toEqual(expectedOutput);
});

Chasing 100% Coverage

// Not everything needs testing
/* v8 ignore next */
if (process.env.DEBUG) {
  console.log('Debug info');
}

CI Integration

GitHub Actions

- name: Run tests with coverage
  run: npm run test:coverage

- name: Upload coverage to Codecov
  uses: codecov/codecov-action@v4
  with:
    files: ./coverage/lcov.info
    fail_ci_if_error: true

Coverage Comments on PRs

- name: Coverage Report
  uses: davelosert/vitest-coverage-report-action@v2

Best Practices Summary

1. 80% is minimum - Aim higher for critical code
2. Test behavior - Not just for coverage numbers
3. Exclude generated code - Don't inflate metrics
4. Cover error paths - Critical for reliability
5. Review coverage drops - Investigate regressions
6. Don't chase 100% - Focus on value
7. Use coverage in CI - Catch regressions early

Code Review Checklist

• Coverage thresholds configured
• All coverage metrics meet 80%
• Error paths tested
• Edge cases covered
• No coverage-only tests
• Valid exclusions documented
• CI coverage gate enabled