name: test-driven-development category: core version: 1.0.0 description: RED-GREEN-REFACTOR cycle for writing bulletproof code with strong test coverage author: obra/superpowers verified: true tags: [testing, tdd, development, quality]

Test-Driven Development (TDD)

Overview

Master the RED-GREEN-REFACTOR cycle to build software with guaranteed test coverage and fewer bugs.

When to Use This Skill

✅ Building new features from scratch ✅ Refactoring existing code with safety nets ✅ Fixing bugs with regression prevention ✅ Working on critical systems requiring high confidence

The TDD Cycle

1. RED Phase

Write a failing test that demonstrates the desired behavior:

• Write the MINIMAL test case for new functionality
• Run the test and confirm it FAILS (red)
• Don't write implementation code yet

2. GREEN Phase

Write the MINIMAL code to make the test pass:

• Implement just enough to pass the test
• Don't worry about perfect code yet
• Run the test and confirm it PASSES (green)

3. REFACTOR Phase

Improve the code while keeping tests green:

• Clean up duplication
• Improve names and structure
• Optimize for readability
• Run tests after each change to ensure they still pass

Example Workflow

// RED: Write failing test
test('calculates total with tax', () => {
  expect(calculateTotal(100, 0.1)).toBe(110);
});

// GREEN: Write minimal implementation
function calculateTotal(amount, taxRate) {
  return amount + (amount * taxRate);
}

// REFACTOR: Improve if needed (tests still pass!)

Best Practices

• Keep tests small and focused
• One assertion per test when possible
• Test behaviors, not implementation details
• Run tests frequently (every few minutes)
• Never skip the RED phase - it proves the test works

Anti-Patterns to Avoid

❌ Writing tests after the code ❌ Testing multiple behaviors in one test ❌ Mocking too much (test real behavior) ❌ Skipping tests when "in a hurry"

Integration with Claude

When using Claude Code, say:

• "Let's use TDD to build [feature]"
• "Help me write tests for [behavior]"
• "Refactor this code using TDD principles"

Claude will automatically:

• Follow RED-GREEN-REFACTOR sequence
• Keep tests focused and clear
• Suggest edge cases you might miss
• Ensure tests remain green during refactoring