TDD Cycle Skill

Execute ONE Red-Green-Refactor cycle following testing best practices.

Prerequisites

Reference: See testing-reference.md (in this folder) for complete testing patterns and guidelines.

The Cycle

RED Phase (Write Failing Test)

1. Write ONE failing test:

   • Use Given-When-Then structure (see testing-reference.md)
   • Format: it('should [behavior] when [condition]', () => { ... })
   • Check for test utilities FIRST - use existing factories/builders when available
   • Avoid manually constructing complex test data

2. Run the test - verify it fails for the RIGHT reason:

   # Run your project's test command
   npm test path/to/test
   pytest path/to/test.py
   

   (Adjust to your project's test framework)

3. Verify failure message makes sense - if not, improve the test

GREEN Phase (Make It Pass)

4. Write minimum code to make the test pass:

   • Follow your project's architecture and conventions
   • Write only what's needed to pass the test
   • Don't add extra features "while you're there"

5. Run the test again - verify it passes

6. Run full test suite - verify no regressions:

   # Run your full test suite
   npm test
   pytest
   

REFACTOR Phase (Improve Structure)

7. Look for improvements (only if needed):

   • Remove duplication
   • Improve naming for clarity
   • Extract methods/functions
   • Simplify conditionals
   • Improve type definitions

8. Run tests after EACH refactoring - must stay green

9. Commit the change:

   # Use feat: for new features
   git commit -m "feat: [description]"
   
   # Use fix: for bug fixes
   git commit -m "fix: [description]"
   
   # Use test: for test additions
   git commit -m "test: [description]"
   

Critical Rules

• ✅ Write ONE test at a time (never batch multiple tests)
• ✅ Test MUST fail before implementation (verify RED phase)
• ✅ Write MINIMUM code to pass (resist gold-plating)
• ✅ Run tests after EVERY change
• ✅ Use test utilities (factories/builders) when available
• ✅ Follow Given-When-Then with blank lines, NO comments

Example Cycle

// RED: Write failing test
describe('ShoppingCart', () => {
  it('should calculate total when items are added', () => {
    const cart = new ShoppingCart();
    const item1 = { id: '1', price: 10.00 };
    const item2 = { id: '2', price: 15.50 };

    cart.addItem(item1);
    cart.addItem(item2);

    expect(cart.getTotal()).toBe(25.50);
  });
});

// GREEN: Implement minimum code to pass
// REFACTOR: Improve code quality while tests stay green

When to Repeat

After completing one RED-GREEN-REFACTOR cycle:

• If there's more functionality to add, start a new cycle
• If the feature is complete, mark the todo as completed
• Each cycle should be small and focused on one behavior