--- name: testing-patterns model: standard category: testing description: Unit, integration, and E2E testing patterns with framework-specific guidance. Use when asked to "write tests", "add test coverage", "testing strategy", "test this function", "create test suite", "fix flaky tests", or "improve test quality". version: 1.0 ---

Testing Patterns

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

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Testing Pyramid

| Level | Ratio | Speed | Cost | Confidence | Scope | |-------|-------|-------|------|------------|-------| | Unit | ~70% | ms | Low | Low (isolated) | Single function/class | | Integration | ~20% | seconds | Medium | Medium | Module boundaries, APIs, DB | | E2E | ~10% | minutes | High | High (realistic) | Full user workflows |

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

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Unit Testing Patterns

Core Patterns

| Pattern | When to Use | Structure | |---------|------------|-----------| | Arrange-Act-Assert | Default for all unit tests | Setup, Execute, Verify | | Given-When-Then | BDD-style, behavior-focused | Precondition, Action, Outcome | | Parameterized | Same logic, multiple inputs | Data-driven test cases | | Snapshot | UI components, serialized output | Compare against saved baseline | | Property-Based | Mathematical invariants | Generate random inputs, assert properties |

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.

| Double | Purpose | When to Use | Example | |--------|---------|-------------|---------| | Stub | Returns canned data | Control indirect input | jest.fn().mockReturnValue(42) | | Mock | Verifies interactions | Assert something was called | expect(mock).toHaveBeenCalledWith('arg') | | Spy | Wraps real implementation | Observe without replacing | jest.spyOn(service, 'save') | | Fake | Working simplified impl | Need realistic behavior | In-memory database, fake HTTP server |

// 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

| Strategy | Approach | Trade-off | |----------|----------|-----------| | Transaction rollback | Wrap each test in a transaction, rollback after | Fast, but hides commit bugs | | Fixtures/seeds | Load known data before suite | Predictable, but brittle if schema changes | | Factory functions | Generate data programmatically | Flexible, but more setup code | | Testcontainers | Spin up real DB in Docker | Realistic, but slower startup |

// 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: '[email protected]' });
  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: '[email protected]' })
      .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

| Mock | Don't Mock | |------|-----------| | HTTP APIs, external services | Pure functions | | Database (in unit tests) | Your own domain logic | | File system, network | Data transformations | | Time/Date (Date.now) | Simple calculations | | Environment variables | Internal class methods |

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(),
);

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Framework Quick Reference

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