┌───────┐
          │  E2E  │  Few, slow, expensive
         ─┴───────┴─
        ┌───────────┐
        │Integration│  Some, medium speed
       ─┴───────────┴─
      ┌───────────────┐
      │     Unit      │  Many, fast, cheap
     ─┴───────────────┴─

// Arrange - Set up test data and conditions
const user = createTestUser({ email: 'test@example.com' });
const service = new UserService(mockDb);

// Act - Execute the code under test
const result = await service.createUser(user);

// Assert - Verify the expected outcome
expect(result.id).toBeDefined();
expect(result.email).toBe('test@example.com');

component.test.ts       # Unit tests
component.spec.ts       # Alternative convention
component.integration.ts # Integration tests
component.e2e.ts        # End-to-end tests

// Pattern: should [expected behavior] when [condition]
describe('UserService', () => {
  describe('createUser', () => {
    it('should create user when valid data provided', () => {});
    it('should throw ValidationError when email is invalid', () => {});
    it('should throw DuplicateError when email already exists', () => {});
  });
});

describe('User Registration', () => {
  describe('given a valid email and password', () => {
    describe('when the user submits the form', () => {
      it('then creates a new account', () => {});
      it('then sends a welcome email', () => {});
    });
  });
});

// Mock external API
const mockApi = {
  getUser: jest.fn().mockResolvedValue({ id: '1', name: 'Test' })
};

// Inject mock
const service = new UserService(mockApi);
const result = await service.getUser('1');

// Verify interaction
expect(mockApi.getUser).toHaveBeenCalledWith('1');
expect(result.name).toBe('Test');

// factories/user.ts
export function createTestUser(overrides = {}) {
  return {
    id: randomUUID(),
    email: `test-${Date.now()}@example.com`,
    name: 'Test User',
    createdAt: new Date(),
    ...overrides
  };
}

// In tests
const user = createTestUser({ name: 'Custom Name' });

// fixtures/users.ts
export const validUser = {
  email: 'valid@example.com',
  password: 'SecurePass123!'
};

export const invalidEmails = [
  'no-at-sign',
  '@no-local.com',
  'no-domain@',
  'spaces in@email.com'
];

// BAD - vague assertion
expect(result).toBeTruthy();

// GOOD - specific assertion
expect(result.success).toBe(true);
expect(result.data.id).toBe('expected-id');

// BAD - multiple unrelated assertions
it('should process order', () => {
  expect(order.id).toBeDefined();
  expect(order.total).toBe(100);
  expect(emailService.send).toHaveBeenCalled();
  expect(inventory.reduce).toHaveBeenCalled();
});

// GOOD - focused tests
it('should assign an order ID', () => {
  expect(order.id).toBeDefined();
});

it('should calculate correct total', () => {
  expect(order.total).toBe(100);
});

it('should send confirmation email', () => {
  expect(emailService.send).toHaveBeenCalled();
});

expect.extend({
  toBeValidEmail(received) {
    const pass = /^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(received);
    return {
      pass,
      message: () => `expected ${received} to be a valid email`
    };
  }
});

// Usage
expect('user@example.com').toBeValidEmail();

// 100% coverage but useless test
it('should run without error', () => {
  processOrder(order); // No assertions!
});

# Run tests in parallel
vitest --pool=threads
jest --maxWorkers=4
pytest -n auto
go test -parallel 4

// BAD - shared state
let counter = 0;
it('test 1', () => { counter++; });
it('test 2', () => { expect(counter).toBe(1); }); // Fragile!

// GOOD - isolated state
beforeEach(() => { counter = 0; });
it('test 1', () => { counter++; expect(counter).toBe(1); });
it('test 2', () => { counter++; expect(counter).toBe(1); });