Unit Testing

Test individual functions, methods, and classes in isolation from dependencies.

When to use me

Use this skill when:

• Writing new code to ensure it works correctly
• Refactoring existing code without breaking functionality
• During continuous integration to catch regressions early
• Following Test-Driven Development (TDD) practices

What I do

• Identify testable units (functions, methods, classes)
• Set up test fixtures and mock dependencies
• Write test cases covering normal, edge, and error conditions
• Run unit tests and report pass/fail status
• Measure code coverage for tested units
• Integrate with CI/CD pipelines for automated testing

Examples

# Run unit tests with common frameworks
npm test                    # Node.js with Jest/Mocha
python -m pytest tests/     # Python with pytest
go test ./...              # Go built-in testing
cargo test                 # Rust with Cargo
dotnet test               # .NET with xUnit/NUnit

# Run specific test files
npm test -- src/utils.test.js
pytest tests/test_auth.py

# Run with coverage reporting
npm test -- --coverage
pytest --cov=src tests/

Output format

✓ Test Suite: UserService
  ✓ should create user with valid data
  ✓ should reject duplicate email
  ✗ should handle database connection errors (failed)
    Expected: "Connection error handled"
    Received: "Unhandled database error"

Coverage: 92% (15/16 lines)
Failed: 1 test

Notes

• Unit tests should be fast (milliseconds per test)
• Mock external dependencies (databases, APIs, filesystem)
• Aim for high code coverage (80-90%+)
• Tests should be independent and order-agnostic
• Follow AAA pattern: Arrange, Act, Assert
• Use test doubles (mocks, stubs, spies) appropriately