Test-Driven Development

The TDD Cycle

• Red: Write a failing test

• Green: Write minimal code to pass

• 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

• Start Simple: Begin with the simplest test case

• One Assert: Each test should verify one behavior

• Descriptive Names: Test names are documentation

• No Logic in Tests: Tests should be obvious

• 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