Test Coverage

Analyze test coverage, identify gaps, and generate missing tests to reach 80%+ coverage.

Step 1: Detect Test Framework

Indicator	Coverage Command
jest.config.* or package.json jest	npx jest --coverage --coverageReporters=json-summary
vitest.config.*	npx vitest run --coverage
pytest.ini / pyproject.toml pytest	pytest --cov=src --cov-report=json
Cargo.toml	cargo llvm-cov --json
pom.xml with JaCoCo	mvn test jacoco:report
go.mod	go test -coverprofile=coverage.out ./...

Step 2: Analyze Coverage Report

1. Run the coverage command
2. Parse the output (JSON summary or terminal output)
3. List files below 80% coverage, sorted worst-first
4. For each under-covered file, identify:
   • Untested functions or methods
   • Missing branch coverage (if/else, switch, error paths)
   • Dead code that inflates the denominator

Step 3: Generate Missing Tests

For each under-covered file, generate tests following this priority:

1. Happy path — Core functionality with valid inputs
2. Error handling — Invalid inputs, missing data, network failures
3. Edge cases — Empty arrays, null/undefined, boundary values (0, -1, MAX_INT)
4. Branch coverage — Each if/else, switch case, ternary

Test Generation Rules

• Place tests adjacent to source: foo.ts → foo.test.ts (or project convention)
• Use existing test patterns from the project (import style, assertion library, mocking approach)
• Mock external dependencies (database, APIs, file system)
• Each test should be independent — no shared mutable state between tests
• Name tests descriptively: test_create_user_with_duplicate_email_returns_409

Step 4: Verify

1. Run the full test suite — all tests must pass
2. Re-run coverage — verify improvement
3. If still below 80%, repeat Step 3 for remaining gaps

Step 5: Report

Show before/after comparison:

Coverage Report
──────────────────────────────
File                   Before  After
src/services/auth.ts   45%     88%
src/utils/validation.ts 32%    82%
──────────────────────────────
Overall:               67%     84%  ✅

Focus Areas

• Functions with complex branching (high cyclomatic complexity)
• Error handlers and catch blocks
• Utility functions used across the codebase
• API endpoint handlers (request → response flow)
• Edge cases: null, undefined, empty string, empty array, zero, negative numbers