Test Patterns

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

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: [] });

Benefits:

• Reduces test setup boilerplate
• Makes test intent clearer
• Easy to create variations

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	Purpose	When to Use
Stub	Returns canned data	Need predictable inputs
Mock	Verifies interactions	Testing side effects
Spy	Records calls	Partial mocking
Fake	Working implementation	Need realistic behavior

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)