Test-Driven Development

The TDD Cycle

1. Red: Write a failing test
2. Green: Write minimal code to pass
3. Refactor: Improve code while keeping tests green

Workflows

• Write Test: Write a test that describes desired behavior
• Run Test: Verify it fails (Red)
• Implement: Write minimal code to pass
• Run Test: Verify it passes (Green)
• Refactor: Clean up while tests stay green
• Repeat: Next test case

TDD Example

Step 1: Red – Write Failing Test

describe("Calculator", () => {
  test("adds two numbers", () => {
    const calc = new Calculator();
    expect(calc.add(2, 3)).toBe(5);
  });
});

// Run: FAIL - Calculator is not defined

Step 2: Green – Minimal Implementation

class Calculator {
  add(a: number, b: number): number {
    return a + b;
  }
}

// Run: PASS

Step 3: Refactor (if needed)

Code is already clean, move to next test.

Step 4: Next Test

test("subtracts two numbers", () => {
  const calc = new Calculator();
  expect(calc.subtract(5, 3)).toBe(2);
});

// Run: FAIL - subtract is not defined

TDD Benefits

• Design Feedback: Tests reveal design issues early
• Documentation: Tests document expected behavior
• Confidence: Refactor fearlessly with test safety net
• Focus: One behavior at a time

TDD Tips

1. Start Simple: Begin with the simplest test case
2. One Assert: Each test should verify one behavior
3. Descriptive Names: Test names are documentation
4. No Logic in Tests: Tests should be obvious
5. Fast Feedback: Tests should run in milliseconds

When to Use TDD

• New features with clear requirements
• Bug fixes (write failing test first)
• Complex business logic
• API contract development

When TDD is Less Useful

• Exploratory/prototype code
• UI layout changes
• Simple CRUD operations