---name: testing-best-practicesdescription: Unit testing, integration testing, and test-driven development principles. Use when writing tests, reviewing test code, improving test coverage, or setting up testing strategy. Triggers on "write tests", "review tests", "testing best practices", or "TDD".license: MITmetadata: author: agent-skillsversion: 2.0.0---

Testing Best Practices

Unit testing, integration testing, and TDD principles for reliable, maintainable test suites. Contains 34 rules across 7 categories using TypeScript with Jest/Vitest.

Metadata

• Version: 2.0.0
• Rule Count: 34 rules across 7 categories
• License: MIT

When to Apply

Reference these guidelines when:

• Writing unit or integration tests
• Reviewing test code quality
• Improving test coverage strategy
• Setting up testing infrastructure
• Debugging flaky or slow tests

Rule Categories by Priority

Priority	Category	Impact	Prefix
1	Test Structure	CRITICAL	struct-
2	Test Isolation	CRITICAL	iso-
3	Assertions	HIGH	assert-
4	Test Data	HIGH	data-
5	Mocking	MEDIUM	mock-
6	Coverage	MEDIUM	cov-
7	Performance	LOW	perf-

Quick Reference

1. Test Structure (CRITICAL)

• struct-aaa-pattern - Arrange, Act, Assert pattern
• struct-descriptive-names - Descriptive test names
• struct-one-assertion - One logical assertion per test
• struct-describe-it - Organized test suites with describe/it
• struct-given-when-then - BDD style when appropriate
• struct-setup-teardown - Proper setup and teardown

2. Test Isolation (CRITICAL)

• iso-independent-tests - Tests run independently
• iso-no-shared-state - No shared mutable state
• iso-deterministic - Same result every run
• iso-no-order-dependency - Run in any order
• iso-cleanup - Clean up after tests
• iso-test-doubles - Strategic use of test doubles

3. Assertions (HIGH)

• assert-specific - Use specific assertions
• assert-meaningful-messages - Helpful failure messages
• assert-expected-actual - Expected value first
• assert-no-magic-numbers - Use named constants
• assert-custom-matchers - Custom matchers for domain logic

4. Test Data (HIGH)

• data-factories - Use factories for test data
• data-builders - Builder pattern for complex objects
• data-faker - Generate realistic fake data
• data-minimal - Minimal test data
• data-realistic - Realistic edge cases
• data-fixtures - Manage test fixtures

5. Mocking (MEDIUM)

• mock-boundaries - Mock only at boundaries
• mock-verify-interactions - Verify important mock calls
• mock-minimal - Don't over-mock
• mock-realistic - Realistic mock behavior with MSW

6. Coverage (MEDIUM)

• cov-meaningful - Focus on meaningful coverage
• cov-edge-cases - Cover edge cases and boundary values
• cov-unhappy-paths - Test error scenarios
• cov-not-100-percent - 100% coverage isn't the goal

7. Performance (LOW)

• perf-fast-unit - Keep unit tests fast (<50ms each)
• perf-parallel - Run tests in parallel
• perf-test-organization - Organize tests for fast feedback

Essential Guidelines

AAA Pattern (Arrange, Act, Assert)

it('calculates total with discount', () => {
  // Arrange
  const cart = new ShoppingCart();
  cart.addItem({ name: 'Book', price: 20 });
  cart.applyDiscount(0.1);

  // Act
  const total = cart.getTotal();

  // Assert
  expect(total).toBe(18);
});

Descriptive Test Names

// ✅ Describes behavior and scenario
describe('UserService.register', () => {
  it('creates user with hashed password', () => {});
  it('throws ValidationError when email is invalid', () => {});
  it('sends welcome email after successful registration', () => {});
});

Test Isolation

// ✅ Each test sets up its own data
beforeEach(() => {
  mockRepository = { save: vi.fn(), find: vi.fn() };
  service = new OrderService(mockRepository);
});

How to Use

Read individual rule files for detailed explanations:

rules/struct-aaa-pattern.md
rules/iso-independent-tests.md
rules/mock-boundaries.md
rules/cov-meaningful.md

References

• Vitest Documentation
• Jest Documentation
• Testing Library
• MSW (Mock Service Worker)

Full Compiled Document

For the complete guide with all rules expanded: AGENTS.md