test-coverage

Test Coverage Skill

Overview

The test-coverage skill provides comprehensive on-demand test coverage analysis for your codebase. It identifies untested code paths, measures coverage metrics, finds test gaps, evaluates test quality, and provides actionable recommendations for improving test coverage across all supported technology stacks.

When to Use:

• Measuring current test coverage
• Identifying untested critical code paths
• Finding gaps before deployment
• Improving test suite quality
• Compliance requirements (80% coverage)
• Refactoring with confidence
• Onboarding new team members

Technology Coverage:

• React/TypeScript/JavaScript (Jest, Vitest, React Testing Library)
• Go (go test with coverage)
• Rust (cargo test with tarpaulin)
• Python (pytest with coverage.py)
• Full-stack applications

Coverage Metrics

1. Line Coverage

Definition: Percentage of executable lines that are executed by tests.

Example:

function divide(a: number, b: number): number {
  if (b === 0) {              // Line 1: COVERED
    throw new Error('Div 0'); // Line 2: NOT COVERED
  }
  return a / b;               // Line 3: COVERED
}

// Test only covers normal case
test('divides numbers', () => {
  expect(divide(10, 2)).toBe(5);
});

// Line Coverage: 66% (2/3 lines)

Interpretation:

• 100%: All lines executed (ideal for critical code)
• 80-99%: Good coverage, some edge cases missed
• 60-79%: Moderate, needs improvement
• <60%: Poor, significant gaps

2. Branch Coverage

Definition: Percentage of decision branches (if/else, switch, ternary) that are tested.

Example:

function getUserStatus(user: User): string {
  if (user.isActive) {     // Branch 1: true (COVERED)
    if (user.isPremium) {  // Branch 2: true (NOT COVERED)
      return 'premium';    // NOT COVERED
    }
    return 'active';       // COVERED
  }
  return 'inactive';       // Branch 1: false (NOT COVERED)
}

// Test only covers active non-premium
test('returns status', () => {
  expect(getUserStatus({ isActive: true, isPremium: false })).toBe('active');
});

// Branch Coverage: 33% (1/3 branches)
// Line Coverage: 60% (3/5 lines)