Red-Green-Refactor Methodology

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;