Testing Strategy

Test Pyramid

        /  E2E  \          ~10% — Slow, brittle, expensive
       /  Integ  \         ~20% — Medium speed, real dependencies
      /   Unit    \        ~70% — Fast, isolated, numerous
     /______________\

If the pyramid inverts (more E2E than unit), treat it as a structural problem.

Unit Test Principles

FIRST Properties

What to Unit Test

• Pure functions and their edge cases
• Business logic and calculations
• State transitions and validation rules
• Data transformations and mappings
• Error handling paths
• Boundary conditions (empty, null, max, min, off-by-one)

What NOT to Unit Test

• Framework configuration or boilerplate
• Simple getters/setters with no logic
• Third-party library internals
• Private methods directly (test through public interface)

Arrange-Act-Assert

describe("calculateDiscount", () => {
  it("applies 10% discount for orders over $100", () => {
    // Arrange
    const order = { items: [{ price: 120, quantity: 1 }] };
    // Act
    const result = calculateDiscount(order);
    // Assert
    expect(result.discount).toBe(12);
    expect(result.total).toBe(108);
  });
});

class TestCalculateDiscount:
    def test_applies_10_percent_for_orders_over_100(self):
        order = Order(items=[Item(price=120, quantity=1)])
        result = calculate_discount(order)
        assert result.discount == 12
        assert result.total == 108

Integration Test Scope

describe("UserRepository", () => {
  let db: Database;
  beforeAll(async () => { db = await createTestDatabase(); await db.migrate(); });
  afterAll(async () => { await db.close(); });
  beforeEach(async () => { await db.truncateAll(); });

  it("finds user by email", async () => {
    await db.insert("users", { email: "test@example.com", name: "Test User" });
    const user = await userRepo.findByEmail("test@example.com");
    expect(user).not.toBeNull();
    expect(user.name).toBe("Test User");
  });
});

Integration Test Rules

• Use a dedicated test database, never production data
• Reset state between tests (truncate tables, clear queues)
• Use test containers or in-memory databases where possible
• Test the happy path and the most critical error paths

E2E Test Selection Criteria

Include: Critical revenue paths (signup, checkout, payment), authentication flows, core user journeys (3-5 things your product must do), cross-system workflows.

Exclude: Edge cases (cover in unit tests), UI-only validation, admin features, scenarios covered by lower-level tests.

test("user can complete checkout", async ({ page }) => {
  await page.goto("/login");
  await page.fill('[name="email"]', "user@test.com");
  await page.fill('[name="password"]', "password123");
  await page.click('button[type="submit"]');
  await page.goto("/products/widget-1");
  await page.click("text=Add to Cart");
  await page.goto("/cart");
  await page.click("text=Checkout");
  await page.fill('[name="card"]', "4242424242424242");
  await page.click("text=Place Order");
  await expect(page.locator(".order-confirmation")).toBeVisible();
});

Mocking Guidelines

When to Mock

When NOT to Mock

// Good: mock the HTTP client, not the service
const mockHttpClient = { get: jest.fn().mockResolvedValue({ data: { id: 1, name: "Test" } }) };
const service = new UserService(mockHttpClient);

// Bad: mocking the method you are testing
jest.spyOn(service, "getUser").mockResolvedValue({ id: 1 });

Test Fixtures and Factories

Prefer factories over shared fixtures to keep tests independent.

function buildUser(overrides: Partial<User> = {}): User {
  return {
    id: randomUUID(),
    email: `user-${Date.now()}@test.com`,
    name: "Test User",
    role: "member",
    createdAt: new Date(),
    ...overrides,
  };
}
const admin = buildUser({ role: "admin" });

• Generate unique values to prevent cross-test contamination
• Use factories with sensible defaults and explicit overrides
• Keep test data minimal; only set fields relevant to the assertion
• Never share mutable state between tests

Property-Based Testing

Test invariants with randomly generated inputs.

import fc from "fast-check";

test("sort is idempotent", () => {
  fc.assert(fc.property(fc.array(fc.integer()), (arr) => {
    expect(sort(sort(arr))).toEqual(sort(arr));
  }));
});

test("encode/decode roundtrip", () => {
  fc.assert(fc.property(fc.string(), (input) => {
    expect(decode(encode(input))).toBe(input);
  }));
});

Use for: serialization roundtrips, sorting invariants, mathematical properties, parsers, encoding.

Snapshot Testing

Appropriate: Serialized output formats, generated config files, stable component render output. Harmful: Large frequently-changing outputs, UI with dynamic content, testing logic or behavior.

• Review snapshot diffs carefully; never blindly update
• Keep snapshots small and focused
• Use inline snapshots for short outputs

Coverage Metrics

• Track coverage trends over time, not just absolute numbers
• Never game coverage with meaningless tests
• 100% coverage does not mean bug-free; focus on meaningful assertions
• Exclude generated code, vendor code, and config from metrics

Testing Anti-Patterns

Test Naming Conventions

// describe/it style
describe("ShoppingCart")
  it("calculates total with tax for items in cart")

// given/when/then style
test("given an empty cart, when adding an item, then total reflects item price")

// should style
test("should return 404 when user is not found")

• Name describes the scenario and expected outcome
• Name reads as a sentence or specification
• Failed test name tells you what broke without reading the code