TDD Mastery

Core Cycle: Red-Green-Refactor

1. Red - Write a failing test that defines the desired behavior
2. Green - Write the minimum code to make the test pass
3. Refactor - Clean up while keeping tests green

Never write production code without a failing test first. Each cycle should take 2-10 minutes.

Test Structure

Use the Arrange-Act-Assert pattern consistently:

Arrange: Set up test data and dependencies
Act:     Execute the behavior under test
Assert:  Verify the expected outcome

Name tests as test_<unit>_<scenario>_<expected_result> or it("should <behavior> when <condition>").

Jest / Vitest Patterns

describe("OrderService", () => {
  it("should apply discount when order exceeds threshold", () => {
    const order = createOrder({ items: [{ price: 150, qty: 1 }] });
    const result = applyDiscount(order, { threshold: 100, percent: 10 });
    expect(result.total).toBe(135);
  });

  it("should throw when applying discount to empty order", () => {
    const order = createOrder({ items: [] });
    expect(() => applyDiscount(order, defaultDiscount)).toThrow(EmptyOrderError);
  });
});

Use vi.fn() / jest.fn() for mocks. Prefer dependency injection over module mocking. Use beforeEach for shared setup, never share mutable state between tests.

pytest Patterns

@pytest.fixture
def db_session():
    session = create_test_session()
    yield session
    session.rollback()

def test_create_user_stores_hashed_password(db_session):
    user = UserService(db_session).create(email="a@b.com", password="secret")
    assert user.password_hash != "secret"
    assert verify_password("secret", user.password_hash)

@pytest.mark.parametrize("input,expected", [
    ("", False),
    ("short", False),
    ("ValidPass1!", True),
])
def test_password_validation(input, expected):
    assert validate_password(input) == expected