red-green-refactor

1# Red-Green-Refactor Methodology
2 
3You are following the RED-GREEN-REFACTOR cycle for test-driven development. Every new feature, bug fix, or behavior change starts with a failing test.
4 
5## The Cycle
6 
7### 1. RED Phase — Write a Failing Test
8 
91. **Understand the requirement** — what specific behavior must exist?
102. **Write one test** asserting that behavior
113. **Run the test** — it MUST fail (red)
124. **Verify the failure reason** — not a syntax error, but a missing implementation
13 
14The test should be focused on ONE behavior, named descriptively, and use clear assertions.
15 
16**Executable example (Jest):**
17```js
18// calculateTotal.test.js
19const { calculateTotal } = require('./calculateTotal');
20 
21describe('calculateTotal', () => {
22  it('should apply 10% discount when total exceeds 100', () => {
23    const items = [{ price: 60 }, { price: 60 }]; // total = 120
24    expect(calculateTotal(items)).toBe(108); // 120 * 0.90
25  });
26});
27```
28Running this now produces: `Cannot find module './calculateTotal'` — correct RED state.
29 
30---
31 
32### 2. GREEN Phase — Make the Test Pass
33 
34Write the **minimum code** needed to pass the test. Don't add anything extra.
35 
36```js
37// calculateTotal.js
38function calculateTotal(items) {
39  const total = items.reduce((sum, item) => sum + item.price, 0);
40  return total > 100 ? total * 0.9 : total;
41}
42module.exports = { calculateTotal };
43```
44Run the test — it passes. GREEN achieved. Stop here; resist adding more logic.
45 
46---
47 
48### 3. REFACTOR Phase — Improve the Code
49…