Skill: Coverage Analysis

Overview

This skill performs deep coverage analysis:

• Line coverage
• Branch coverage
• Function coverage
• Statement coverage
• Coverage trends
• Uncovered code identification

Purpose

Ensure comprehensive test coverage. Use this skill:

• During development
• Before code review
• For coverage reports
• To identify testing gaps

Execution Flow

1. DETECT test framework
2. RUN tests with coverage
3. PARSE coverage data
4. ANALYZE coverage patterns
5. IDENTIFY gaps
6. GENERATE detailed report

Input Parameters

Parameter	Type	Description	Required
threshold	number	Minimum coverage %	No
type	string	line, branch, function, all	No
diff	boolean	Compare with base branch	No

Coverage Tools

Language	Tool	Output
JavaScript	Jest, Vitest	lcov, json
Python	coverage.py	xml, html
Java	JaCoCo	xml, html
Go	go test -cover	text, html
.NET	coverlet	json, opencover

Configuration

Jest

// jest.config.js
module.exports = {
    collectCoverage: true,
    coverageDirectory: 'coverage',
    coverageReporters: ['text', 'lcov', 'html', 'json-summary'],
    coverageThreshold: {
        global: {
            branches: 80,
            functions: 80,
            lines: 80,
            statements: 80,
        },
    },
    collectCoverageFrom: [
        'src/**/*.{ts,tsx}',
        '!src/**/*.spec.{ts,tsx}',
        '!src/**/*.d.ts',
    ],
};

Vitest

// vitest.config.ts
import { defineConfig } from 'vitest/config';

export default defineConfig({
    test: {
        coverage: {
            provider: 'v8',
            reporter: ['text', 'json', 'html', 'lcov'],
            exclude: ['**/*.spec.ts', '**/*.test.ts'],
            thresholds: {
                lines: 80,
                functions: 80,
                branches: 80,
                statements: 80,
            },
        },
    },
});

Python

# .coveragerc
[run]
source = src
branch = True
omit = tests/*

[report]
exclude_lines =
    pragma: no cover
    if TYPE_CHECKING:
    raise NotImplementedError
fail_under = 80

[html]
directory = htmlcov

Output Format

# Coverage Analysis Report

**Date:** [timestamp]
**Framework:** Jest with v8 coverage

## Coverage Summary

| Type | Coverage | Target | Status |
|------|----------|--------|--------|
| **Lines** | **85%** | 80% | ✅ Pass |
| **Branches** | **78%** | 80% | ❌ Fail |
| **Functions** | **92%** | 80% | ✅ Pass |
| **Statements** | **86%** | 80% | ✅ Pass |

## Coverage by Directory

| Directory | Lines | Branches | Functions |
|-----------|-------|----------|-----------|
| src/services | 92% | 85% | 95% |
| src/components | 88% | 75% | 90% |
| src/utils | 72% | 65% | 80% |
| src/api | 95% | 90% | 100% |

## Files Below Threshold

### 1. src/utils/validators.ts

| Type | Coverage | Missing |
|------|----------|---------|
| Lines | 65% | 14 lines |
| Branches | 55% | 9 branches |
| Functions | 70% | 3 functions |

**Uncovered Lines:** 23-28, 45-52, 78-82

```typescript
// Line 23-28: Not covered
export function validateEmail(email: string): boolean {
    if (!email) return false;  // ❌ Not covered
    const regex = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
    return regex.test(email);  // ❌ Not covered
}

Suggested Tests:

describe('validateEmail', () => {
    it('should return false for empty email', () => {
        expect(validateEmail('')).toBe(false);
    });

    it('should return true for valid email', () => {
        expect(validateEmail('test@example.com')).toBe(true);
    });
});

2. src/components/Button.tsx

Type	Coverage	Missing
Lines	70%	8 lines
Branches	60%	4 branches

Uncovered: Loading state, disabled variant

Branch Coverage Details

File	Branch	Coverage
validators.ts	if (!email)	❌ False path
validators.ts	if (!phone)	❌ Both paths
Button.tsx	if (loading)	❌ True path
Button.tsx	if (disabled)	❌ True path

Coverage Trend

Date	Lines	Branches	Change
2026-02-15	82%	75%	-
2026-02-16	84%	77%	+2%
2026-02-17	85%	78%	+1%

Recommendations

1. Priority: Add tests for validators.ts (14 uncovered lines)
2. Priority: Cover Button loading/disabled states
3. Medium: Improve utils directory coverage to 80%+

Artifacts

• HTML Report: coverage/lcov-report/index.html
• LCOV: coverage/lcov.info
• JSON: coverage/coverage-final.json

## Example Usage

### Example 1: Full Coverage Analysis

USER: /coverage-analysis

SKILL OUTPUT:

Coverage Analysis Report

Coverage Summary

Type	Coverage	Target
Lines	85%	80% ✅
Branches	78%	80% ❌
Functions	92%	80% ✅
Statements	86%	80% ✅

Files Below Threshold

src/utils/validators.ts

• Lines: 65%
• Missing: 14 lines
• Top uncovered: validateEmail(), validatePhone()

src/components/Button.tsx

• Lines: 70%
• Missing: loading state, disabled variant

Recommendation

Add 8 tests to reach 80% branch coverage:

1. validateEmail empty test
2. validateEmail valid test
3. validatePhone empty test
4. Button loading state test
5. Button disabled state test ...

### Example 2: Coverage Diff

USER: /coverage-analysis --diff main

SKILL OUTPUT:

Coverage Diff Report

Base: main Current: feature/new-auth

Coverage Change

Type	Base	Current	Change
Lines	82%	85%	+3% ✅
Branches	75%	78%	+3% ✅
Functions	90%	92%	+2% ✅

New Code Coverage

File	New Lines	Covered	Coverage
auth.service.ts	45	40	89%
login.component.ts	32	28	88%

Regression Alert

⚠️ Coverage decreased in:

• user.service.ts: 90% → 85%

Missing tests for new error handling code.

### Example 3: Threshold Enforcement

USER: /coverage-analysis --threshold 90

SKILL OUTPUT:

Coverage Analysis Report

Threshold: 90%

Results

Type	Coverage	Status
Lines	85%	❌ Fail
Branches	78%	❌ Fail
Functions	92%	✅ Pass
Statements	86%	❌ Fail

❌ Build Failed: Coverage below threshold

Required to Pass

Need 5% more line coverage (10 lines):

• validators.ts: 4 lines
• Button.tsx: 3 lines
• Form.tsx: 3 lines

## Running Coverage

### Jest
```bash
jest --coverage

Vitest

vitest run --coverage

Python

pytest --cov=src --cov-report=html

Go

go test -coverprofile=coverage.out
go tool cover -html=coverage.out

CI/CD Integration

# .github/workflows/coverage.yml
name: Coverage

on: [push, pull_request]

jobs:
    coverage:
        runs-on: ubuntu-latest
        steps:
            - uses: actions/checkout@v4
            - uses: actions/setup-node@v4
            - run: npm ci
            - run: npm test -- --coverage
            - uses: codecov/codecov-action@v3
              with:
                  files: ./coverage/lcov.info

Best Practices

1. Set Thresholds - Enforce minimum coverage
2. Track Trends - Monitor coverage over time
3. Review Gaps - Regularly review uncovered code
4. Exclude Generated - Don't cover generated code
5. Focus on Critical - Prioritize business logic

Integration

This skill integrates with:

• mutation-testing: Combined quality metrics
• test-generation: Generate missing tests
• Codecov/SonarQube for trend tracking