Red/Green Refactor

The Red → Green → Refactor cycle is the core cycle of TDD (Test-Driven Development).

        ┌──────────────┐
        │     RED      │
        │ Write a test │
        └──────┬───────┘
               ↓
        ┌──────────────┐
        │    GREEN     │
        │ Make it pass │
        └──────┬───────┘
               ↓
        ┌──────────────┐
        │  REFACTOR    │
        │ Improve code │
        └──────┬───────┘
               │
               └────────→ RED

1. 🔴 Red — Write a failing test

First, write a test for something that doesn’t exist yet.

Example:

test("adds two numbers", () => {
  expect(add(2, 3)).toBe(5);
});

You run it:

❌ FAIL
add() doesn't exist

That’s Red.

---

2. 🟢 Green — Make the test pass

Now write the simplest code possible to make the test pass.

function add(a, b) {
  return a + b;
}

Run the test:

✅ PASS

That’s Green.

The goal here isn’t beautiful code. It’s simply:

Make the test pass.

---

3. 🔵 Refactor — Improve the code

Now that you have a passing test, you can improve the implementation:

• remove duplication

• improve naming

• simplify logic

• reorganize code

• improve structure

While doing this, keep running the tests.

Before refactoring
      ↓
✅ Tests pass
      ↓
Refactor
      ↓
✅ Tests still pass

The important rule is:

Don’t change the behavior while refactoring.

---

Then repeat

Once you’re done:

🔴 Write another test
        ↓
🟢 Make it pass
        ↓
🔵 Refactor
        ↓
🔴 Next test
        ↓
...

Why is it useful?

It gives you a controlled development loop:

Test defines behavior
       ↓
Implementation satisfies behavior
       ↓
Tests protect behavior
       ↓
Code can safely be improved

That’s why the cycle is often summarized as:

Red → Green → Refactor → Repeat.

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Back-End Test