When to Use

Apply when implementing new features, fixing bugs, or refactoring code. TDD ensures tests drive design and all code is covered.

Patterns

Pattern 1: Red-Green-Refactor Cycle

code

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/TestDrivenDevelopment.html

Pattern 2: Test Structure (AAA)

typescript

// 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);
});

Pattern 3: One Assertion Per Test

typescript

// 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 */ });

Pattern 4: Test Naming Convention

typescript

// 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', () => {});
});

Pattern 5: Outside-In TDD

code

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

Source: https://martinfowler.com/bliki/TestDrivenDevelopment.html

Anti-Patterns

•Test after code - Loses design benefits; tests become afterthought
•Testing implementation - Test behavior, not internal methods
•Large test steps - Keep RED-GREEN cycles small (minutes, not hours)
•Skipping refactor - Technical debt accumulates; refactor is mandatory

Verification Checklist

• Test written BEFORE implementation
• Test fails for the right reason (RED)
• Minimal code written to pass (GREEN)
• Code refactored, tests still pass
• Each test covers one behavior
• Test names describe expected behavior