Red-Green-Refactor Methodology

You are following the RED-GREEN-REFACTOR cycle for test-driven development. Every new feature, bug fix, or behavior change starts with a failing test.

The Cycle

1. RED Phase, Write a Failing Test

1. Understand the requirement, what specific behavior must exist?
2. Write one test asserting that behavior
3. Run the test, it MUST fail (red)
4. Verify the failure reason, not a syntax error, but a missing implementation

The test should be focused on ONE behavior, named descriptively, and use clear assertions.

Executable example (Jest):

// calculateTotal.test.js
const { calculateTotal } = require('./calculateTotal');

describe('calculateTotal', () => {
  it('should apply 10% discount when total exceeds 100', () => {
    const items = [{ price: 60 }, { price: 60 }]; // total = 120
    expect(calculateTotal(items)).toBe(108); // 120 * 0.90
  });
});

Running this now produces: Cannot find module './calculateTotal', correct RED state.

---

2. GREEN Phase, Make the Test Pass

Write the minimum code needed to pass the test. Don't add anything extra.

// calculateTotal.js
function calculateTotal(items) {
  const total = items.reduce((sum, item) => sum + item.price, 0);
  return total > 100 ? total * 0.9 : total;
}
module.exports = { calculateTotal };

Run the test, it passes. GREEN achieved. Stop here; resist adding more logic.

---

3. REFACTOR Phase, Improve the Code

With a passing test as your safety net, clean up the implementation. Run tests after every change.

// calculateTotal.js, refactored for clarity
const DISCOUNT_THRESHOLD = 100;
const DISCOUNT_RATE = 0.9;

function calculateTotal(items) {
  const subtotal = items.reduce((sum, { price }) => sum + price, 0);
  return subtotal > DISCOUNT_THRESHOLD ? subtotal * DISCOUNT_RATE : subtotal;
}
module.exports = { calculateTotal };

Test still passes, GREEN maintained. Constants now communicate intent.