Test Coverage Analyzer

This skill helps identify gaps in test coverage and suggests specific test cases to improve code quality.

When to Use This Skill

• User asks to analyze test coverage
• User wants to improve test coverage
• User requests suggestions for missing tests
• Working on increasing code coverage percentage
• User mentions "coverage gaps", "untested code", or "test suggestions"

Instructions

1. Detect Testing Framework

Identify the testing framework(s) used in the project:

JavaScript/TypeScript:

• Jest: jest in package.json or jest.config.js
• Mocha: mocha in package.json or .mocharc
• Vitest: vitest in package.json or vitest.config.js
• Jasmine: jasmine in package.json

Python:

• pytest: pytest in requirements.txt or pytest.ini
• unittest: Standard library, check for test files
• coverage.py: coverage in requirements.txt

Ruby:

• RSpec: rspec in Gemfile
• Minitest: Standard library

Go:

• Built-in: go test (coverage via go test -cover)

Java:

• JUnit: Look for JUnit in pom.xml or build.gradle
• JaCoCo: Coverage tool configuration

Use Glob and Grep to find configuration files.

2. Locate Coverage Reports

Find existing coverage reports:

Common locations:

• coverage/ directory
• .nyc_output/ (NYC for Node.js)
• htmlcov/ (coverage.py for Python)
• target/site/jacoco/ (JaCoCo for Java)
• Coverage files: lcov.info, coverage.json, .coverage

If no coverage report exists:

• Guide user to run coverage: npm test -- --coverage, pytest --cov, etc.
• Wait for report generation before analyzing

3. Parse Coverage Data

Extract coverage information:

From lcov.info:

• Lines covered vs total
• Functions covered vs total
• Branches covered vs total
• Uncovered line numbers by file

From coverage.json (Jest):

• Statement coverage percentage
• Branch coverage percentage
• Function coverage percentage
• Line coverage percentage
• Uncovered lines per file

From .coverage (Python):

• Use coverage report or coverage json
• Missing line ranges
• Excluded lines

From HTML reports:

• Read summary statistics
• Identify files with low coverage

4. Identify Coverage Gaps

Prioritize files/functions that need testing:

High priority:

• Business logic with 0% coverage
• Public APIs and exported functions
• Error handling paths (catch blocks, error callbacks)
• Edge cases and boundary conditions
• Critical paths (authentication, payment, data validation)

Medium priority:

• Utility functions with partial coverage
• Private functions called by public APIs
• Configuration and initialization code

Lower priority:

• Simple getters/setters
• Type definitions
• Auto-generated code
• Third-party code

5. Analyze Untested Code Paths

For each file with coverage gaps:

1. Read the source file to understand the code structure
2. Identify untested code:
   • Uncovered lines and line ranges
   • Untested conditional branches (if/else)
   • Uncovered error handlers
   • Missing function coverage
3. Categorize missing tests:
   • Happy path tests
   • Error/exception tests
   • Edge case tests
   • Integration tests

6. Generate Test Suggestions

For each coverage gap, suggest specific test cases:

Format:

File: src/utils/validator.js (42% coverage)

Missing Coverage:
- Lines 15-18: Email validation error path
- Lines 23-25: Empty input handling
- Lines 30-35: Edge case for special characters

Suggested Tests:
1. Test email validation with invalid format (covers lines 15-18)
2. Test validator with empty string input (covers lines 23-25)
3. Test special character handling in names (covers lines 30-35)

Test case details should include:

• Test description
• Input data
• Expected output/behavior
• Lines/branches covered

7. Create Test Stubs (Optional)

If user wants, generate test file stubs:

• Use appropriate test framework syntax
• Include describe/test blocks with TODO comments
• Add example test structure
• Reference the template in templates/test-template.js (or appropriate extension)

Example (Jest):

describe('Validator', () => {
  describe('validateEmail', () => {
    it('should reject invalid email format', () => {
      // TODO: Test email validation error path (lines 15-18)
      const result = validateEmail('invalid-email');
      expect(result.valid).toBe(false);
    });

    it('should handle empty string input', () => {
      // TODO: Test empty input handling (lines 23-25)
    });
  });
});

8. Calculate Coverage Metrics

Provide summary statistics:

• Overall coverage percentage
• Coverage by file/directory
• Functions/methods without tests
• Critical files with low coverage
• Coverage trend (if historical data available)

9. Prioritize Recommendations

Order suggestions by:

1. Impact: Critical business logic first
2. Risk: High-risk areas (security, data integrity)
3. Complexity: Complex logic needs more tests
4. Ease: Quick wins (simple tests for high coverage gain)

10. Output Format

Present findings in a clear, actionable format:

Coverage Analysis Summary
=========================

Overall Coverage: 67%
- Statements: 65%
- Branches: 58%
- Functions: 72%
- Lines: 67%

Files Needing Attention (sorted by priority):

1. src/payment/processor.js (23% coverage) - HIGH PRIORITY
   - Missing: Error handling for payment failures
   - Missing: Retry logic tests
   - Missing: Transaction validation

2. src/auth/validator.js (45% coverage) - MEDIUM PRIORITY
   - Missing: Invalid token handling
   - Missing: Expired session tests

Suggested Test Cases:
[Detailed suggestions for each file]

Best Practices

1. Focus on meaningful coverage: 100% coverage isn't always necessary
2. Test behavior, not implementation: Suggest tests that verify functionality
3. Prioritize critical paths: Authentication, payments, data validation first
4. Consider test types: Unit, integration, and e2e where appropriate
5. Check existing tests: Don't suggest tests that already exist
6. Realistic test data: Use plausible inputs in examples
7. Coverage thresholds: Suggest minimum coverage targets (e.g., 80%)

Running Coverage Reports

Provide commands based on detected framework:

Jest: npm test -- --coverage or jest --coverage Vitest: npm test -- --coverage or vitest run --coverage Mocha + NYC: nyc mocha pytest: pytest --cov=src --cov-report=html Go: go test -cover ./... or go test -coverprofile=coverage.out JUnit + JaCoCo: mvn test jacoco:report

Supporting Files

• scripts/parse-coverage.sh: Helper script to extract coverage data
• templates/test-template.js: Test file template for JavaScript
• templates/test-template.py: Test file template for Python
• templates/test-template.go: Test file template for Go