testing-tdd-workflow

RED:     Write failing test (test what, not how)
GREEN:   Write minimal code to pass
REFACTOR: Improve code, keep tests green
REPEAT:  Next behavior

// Source: https://martinfowler.com/bliki/GivenWhenThen.html
it('should calculate total with discount', () => {
  // Arrange (Given)
  const cart = new Cart();
  cart.add({ price: 100, quantity: 2 });

  // Act (When)
  const total = cart.calculateTotal(0.1); // 10% discount

  // Assert (Then)
  expect(total).toBe(180);
});

// Source: https://blog.cleancoder.com/uncle-bob/2014/12/17/TheCyclesOfTDD.html
// GOOD: Single behavior per test
it('should add item to cart', () => {
  cart.add(item);
  expect(cart.items).toContain(item);
});

it('should update cart count', () => {
  cart.add(item);
  expect(cart.count).toBe(1);
});

// BAD: Multiple behaviors
it('should add item and update count', () => { /* multiple asserts */ });

// Format: should [expected behavior] when [condition]
describe('Cart', () => {
  it('should return 0 when cart is empty', () => {});
  it('should apply discount when code is valid', () => {});
  it('should throw error when quantity is negative', () => {});
});

1. Start with acceptance test (user story)
2. Discover collaborators through failing test
3. Write unit tests for collaborators
4. Implement from inside out
5. Acceptance test passes