Test-Driven Development

The Red-Green-Refactor Cycle

1. RED: Write a failing test that defines expected behavior
2. GREEN: Write minimal code to make the test pass
3. REFACTOR: Improve code quality while keeping tests green

Critical Rules

• Never write production code without a failing test first
• Only write enough code to make the current test pass
• Refactor only when tests are green
• Run tests after every change

Test Structure (AAA Pattern)

// Arrange - Set up test data and conditions
// Act - Execute the code under test
// Assert - Verify the expected outcome

Test Naming Convention

test_[unit]_[scenario]_[expected_result]

Examples:
- test_calculateTotal_emptyCart_returnsZero
- test_userLogin_invalidPassword_throwsAuthError
- test_parseDate_invalidFormat_returnsNull

Test Doubles

Coverage Targets

• Unit tests: >90% line coverage
• Integration tests: Critical paths covered
• E2E tests: Happy paths + key error scenarios

Anti-Patterns to Avoid

• Testing implementation details (test behavior, not internals)
• Overly specific assertions (leads to brittle tests)
• Test interdependence (each test must be isolated)
• Testing trivial code (getters/setters)
• Ignoring flaky tests (fix or remove immediately)

TDD for Different Contexts

API Endpoints

1. Test request validation
2. Test business logic
3. Test response format
4. Test error handling

UI Components

1. Test rendering with props
2. Test user interactions
3. Test state changes
4. Test accessibility

Database Operations

1. Test CRUD operations
2. Test constraints/validation
3. Test transactions
4. Test edge cases (null, empty)