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name: test-patterns description: Applies proven testing patterns — Arrange-Act-Assert (AAA), Given-When-Then, Test Data Builders, Object Mother, parameterized tests, fixtures, spies, and test doubles — to help write maintainable, reliable, and readable test suites. Use when the user asks about writing unit tests, integration tests, or end-to-end tests; structuring test cases or test suites; applying TDD or BDD practices; working with mocks, stubs, spies, or fakes; improving test coverage or reducing flakiness; or needs guidance on test organization, naming conventions, or assertions in frameworks like Jest, Vitest, pytest, or similar. version: 1.0.0 triggers:

• test patterns
• testing best practices
• how to write tests
• test structure
• arrange act assert tags:
• testing
• patterns
• quality
• best-practices difficulty: intermediate estimatedTime: 10 relatedSkills:
• testing/red-green-refactor
• testing/anti-patterns

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Test Patterns

You are applying proven testing patterns to write maintainable, reliable tests. These patterns help ensure tests are readable, focused, and trustworthy.

Pattern Selection Guide

Use this to choose the right pattern for your situation:

• Structuring a single test? → Arrange-Act-Assert (AAA) or Given-When-Then
• Writing behavior/feature specs? → Given-When-Then (BDD style)
• Repeating test setup data? → Test Data Builders
• Many variations of complex objects? → Object Mother
• Testing same logic with many inputs? → Parameterized Tests
• Shared setup/teardown across tests? → Test Fixtures
• Verifying a dependency was called? → Spy
• Replacing an external dependency? → Test Doubles (Stub / Mock / Fake)

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Pattern Combination Workflows

Patterns rarely stand alone — here's how to combine them for common scenarios:

Unit tests (isolated logic): Fixtures for setup → AAA structure → Stubs/Mocks for dependencies → Parameterized Tests for multiple input cases

Integration tests (service + external dependencies): Fixtures for setup → AAA structure → Fakes for external services (e.g. in-memory DB) → Spies to verify interaction points

BDD / feature specs: Given-When-Then → Object Mother or Test Data Builders for scenario data → Fakes for infrastructure

High-variation logic (validators, calculators, formatters): Parameterized Tests → Test Data Builders to construct each case → AAA structure within each case

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Core Pattern: Arrange-Act-Assert (AAA)

Structure every test with three distinct phases:

// Arrange - Set up test data and dependencies
const user = createTestUser({ role: 'admin' });
const service = new UserService(mockRepository);

// Act - Execute the code under test
const result = await service.updateRole(user.id, 'member');

// Assert - Verify the expected outcome
expect(result.role).toBe('member');
expect(mockRepository.save).toHaveBeenCalledWith(user);

Guidelines:

• Keep sections visually separated (blank lines or comments)
• Arrange should be minimal - only what's needed for this test
• Act should be a single operation
• Assert should verify one logical concept

Pattern: Given-When-Then (BDD Style)

For behavior-focused tests:

describe('Shopping Cart', () => {
  describe('when adding an item', () => {
    it('should increase the item count', () => {
      // Given
      const cart = new Cart();

      // When
      cart.add({ id: '1', quantity: 2 });

      // Then
      expect(cart.itemCount).toBe(2);
    });
  });
});

Pattern: Test Data Builders

Create flexible test data without repetition:

// Builder function
function createTestOrder(overrides = {}) {
  return {
    id: 'order-123',
    status: 'pending',
    items: [],
    total: 0,
    ...overrides
  };
}

// Usage
const completedOrder = createTestOrder({ status: 'completed', total: 99.99 });
const emptyOrder = createTestOrder({ items: [] });

Pattern: Object Mother

Factory for complex test objects:

class TestUserFactory {
  static admin() {
    return new User({ role: 'admin', permissions: ALL_PERMISSIONS });
  }

  static guest() {
    return new User({ role: 'guest', permissions: [] });
  }

  static withSubscription(tier) {
    return new User({ subscription: { tier, active: true } });
  }
}

Pattern: Parameterized Tests

Test multiple cases efficiently:

describe('isValidEmail', () => {
  const validCases = [
    'user@example.com',
    'user.name@domain.co.uk',
    'user+tag@example.org'
  ];

  const invalidCases = [
    '',
    'not-an-email',
    '@no-local.com',
    'no-domain@'
  ];

  test.each(validCases)('should accept valid email: %s', (email) => {
    expect(isValidEmail(email)).toBe(true);
  });

  test.each(invalidCases)('should reject invalid email: %s', (email) => {
    expect(isValidEmail(email)).toBe(false);
  });
});

Pattern: Test Fixtures

Reusable test setup:

describe('OrderService', () => {
  let service;
  let mockPaymentGateway;
  let mockInventory;

  beforeEach(() => {
    mockPaymentGateway = createMockPaymentGateway();
    mockInventory = createMockInventory();
    service = new OrderService(mockPaymentGateway, mockInventory);
  });

  afterEach(() => {
    jest.clearAllMocks();
  });
});

Pattern: Spy on Dependencies

Verify interactions without implementation:

it('should send notification on order completion', async () => {
  const notifySpy = jest.spyOn(notificationService, 'send');

  await orderService.complete(orderId);

  expect(notifySpy).toHaveBeenCalledWith({
    type: 'order_completed',
    orderId: orderId
  });
});

Pattern: Test Doubles

Choose the right type:

| Type | When to Use | |------|-------------| | Stub | Need predictable, canned return values | | Mock | Need to assert a dependency was called correctly | | Spy | Partial mocking — observe calls on a real object | | Fake | Need a working lightweight substitute (e.g. in-memory DB) |

Pattern: Test Isolation

Ensure tests don't affect each other:

1. Fresh instances - Create new objects in each test
2. Reset mocks - Clear mock state between tests
3. Clean up - Remove side effects (files, database rows)
4. No shared mutable state - Avoid global variables

Naming Conventions

Test names should describe:

• What is being tested
• Under what conditions
• What the expected outcome is

Good examples:

• shouldReturnEmptyArrayWhenNoItemsExist
• throwsErrorWhenUserNotAuthenticated
• calculatesDiscountForPremiumMembers

Test Organization

src/
  services/
    UserService.ts
    UserService.test.ts    # Co-located tests

tests/
  integration/
    api.test.ts            # Integration tests
  e2e/
    checkout.spec.ts       # End-to-end tests

Verification Checklist

For each test:

• [ ] Single responsibility (tests one thing)
• [ ] Clear AAA or GWT structure
• [ ] Descriptive name
• [ ] Fast execution (< 100ms for unit tests)
• [ ] Deterministic (no flakiness)
• [ ] Independent (runs in any order)