Testing Patterns

> Write tests that catch bugs, not tests that pass. — Confidence through coverage, speed through isolation.

Testing Pyramid

> Rule: If your E2E tests outnumber your unit tests, invert the pyramid.

Unit Testing Patterns

Core Patterns

Arrange-Act-Assert (AAA)

The default structure for every unit test. Clear separation of setup, execution, and verification makes tests readable and maintainable.

// Clean AAA structure
test('calculates order total with tax', () => {
  // Arrange
  const items = [{ price: 10, qty: 2 }, { price: 5, qty: 1 }];
  const taxRate = 0.08;

  // Act
  const total = calculateTotal(items, taxRate);

  // Assert
  expect(total).toBe(27.0);
});

Test Doubles

Use the right type of test double for the situation. Each serves a different purpose.

// Stub — control indirect input
const getUser = jest.fn().mockResolvedValue({ id: 1, name: 'Alice' });

// Spy — observe without replacing
const spy = jest.spyOn(logger, 'warn');
processInvalidInput(data);
expect(spy).toHaveBeenCalledWith('Invalid input received');

// Fake — lightweight substitute
class FakeUserRepo implements UserRepository {
  private users = new Map<string, User>();
  async save(user: User) { this.users.set(user.id, user); }
  async findById(id: string) { return this.users.get(id) ?? null; }
}

Parameterized Tests

Use parameterized tests when the same logic needs verification with multiple inputs. This eliminates copy-paste tests while providing comprehensive coverage.

// Vitest/Jest
test.each([
  ['hello', 'HELLO'],
  ['world', 'WORLD'],
  ['', ''],
  ['123abc', '123ABC'],
])('toUpperCase(%s) returns %s', (input, expected) => {
  expect(input.toUpperCase()).toBe(expected);
});

# pytest
@pytest.mark.parametrize("input,expected", [
    ("hello", "HELLO"),
    ("world", "WORLD"),
    ("", ""),
])
def test_to_upper(input, expected):
    assert input.upper() == expected

// Go — table-driven tests (idiomatic)
func TestAdd(t *testing.T) {
    tests := []struct {
        name     string
        a, b     int
        expected int
    }{
        {"positive", 2, 3, 5},
        {"zero", 0, 0, 0},
        {"negative", -1, -2, -3},
    }
    for _, tc := range tests {
        t.Run(tc.name, func(t *testing.T) {
            if got := Add(tc.a, tc.b); got != tc.expected {
                t.Errorf("Add(%d,%d) = %d, want %d", tc.a, tc.b, got, tc.expected)
            }
        })
    }
}

Integration Testing Patterns

Database Testing Strategies

// Transaction rollback pattern (Prisma)
beforeEach(async () => {
  await prisma.$executeRaw`BEGIN`;
});
afterEach(async () => {
  await prisma.$executeRaw`ROLLBACK`;
});

test('creates user in database', async () => {
  const user = await createUser({ name: 'Alice', email: 'a@b.com' });
  const found = await prisma.user.findUnique({ where: { id: user.id } });
  expect(found?.name).toBe('Alice');
});

API Testing

// Supertest (Node.js)
import request from 'supertest';
import { app } from '../src/app';

describe('POST /api/users', () => {
  it('creates a user and returns 201', async () => {
    const res = await request(app)
      .post('/api/users')
      .send({ name: 'Alice', email: 'alice@test.com' })
      .expect(201);

    expect(res.body).toMatchObject({
      id: expect.any(String),
      name: 'Alice',
    });
  });

  it('returns 400 for invalid email', async () => {
    await request(app)
      .post('/api/users')
      .send({ name: 'Alice', email: 'not-an-email' })
      .expect(400);
  });
});

Mocking Best Practices

Mock Boundaries, Not Implementations

The fundamental rule: mock at system boundaries (external APIs, databases, file systems) and never mock internal domain logic.

// BAD — mocking internal implementation
jest.mock('./utils/formatDate');  // Breaks on refactor

// GOOD — mocking external boundary
jest.mock('./services/paymentGateway');  // Third-party API is the boundary

When to Mock vs Not Mock

Dependency Injection for Testability

Structure code so dependencies can be swapped in tests. This is the single most impactful pattern for testable code.

// Injectable dependencies — easy to test
class OrderService {
  constructor(
    private paymentGateway: PaymentGateway,
    private inventory: InventoryService,
    private notifier: NotificationService,
  ) {}

  async placeOrder(order: Order): Promise<OrderResult> {
    const stock = await this.inventory.check(order.items);
    if (!stock.available) return { status: 'out_of_stock' };

    const payment = await this.paymentGateway.charge(order.total);
    if (!payment.success) return { status: 'payment_failed' };

    await this.notifier.send(order.userId, 'Order confirmed');
    return { status: 'confirmed', id: payment.transactionId };
  }
}

// In tests — inject fakes
const service = new OrderService(
  new FakePaymentGateway(),
  new FakeInventory({ available: true }),
  new FakeNotifier(),
);

Framework Quick Reference

Test Quality Checklist

> Naming convention: test_[unit]_[scenario]_[expected result] or should [do X] when [condition Y]

Coverage Strategy

When to Aim for What

What NOT to Test

Test Organization

src/
├── services/
│   ├── order.service.ts
│   └── order.service.test.ts      # Co-located unit tests
├── api/
│   └── routes/
│       └── orders.ts
tests/
├── integration/
│   ├── api/
│   │   └── orders.test.ts         # API integration tests
│   └── db/
│       └── order.repo.test.ts     # DB integration tests
├── e2e/
│   ├── pages/                     # Page objects
│   │   └── checkout.page.ts
│   └── specs/
│       └── checkout.spec.ts       # E2E specs
└── helpers/
    ├── factories.ts               # Test data factories
    └── setup.ts                   # Global test setup

> Rule: Co-locate unit tests with source. Separate integration and E2E tests into dedicated directories.

Anti-Patterns

NEVER Do

1. NEVER test implementation details instead of behavior — tests must verify what the code does, not how it does it
2. NEVER use sleep() in tests — use explicit waits, polling, events, or assertions that auto-retry
3. NEVER share mutable state between tests — each test sets up and tears down its own state
4. NEVER write assert-free tests — a test that asserts nothing proves nothing
5. NEVER mock internal domain logic — only mock at system boundaries (network, DB, filesystem, clock)
6. NEVER skip tests without a linked issue and a plan to re-enable — skipped tests rot into permanent gaps
7. NEVER leave a test suite in a failing state — fix it or remove it with justification before moving on
8. NEVER chase 100% coverage as a goal — coverage percentage is a tool, not a target; strong assertions on critical paths beat weak assertions everywhere

Summary

> Remember: Tests are a safety net — a fast, trustworthy suite lets you refactor fearlessly and ship with confidence.