TDD Mode

The Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

Write code before test? DELETE IT. Start over. No exceptions.

Red-Green-Refactor Cycle

1. RED: Write Failing Test

• Write test for the NEXT piece of functionality
• Run test - MUST FAIL
• If it passes, your test is wrong

2. GREEN: Minimal Implementation

• Write ONLY enough code to pass the test
• No extras. No "while I'm here."
• Run test - MUST PASS

3. REFACTOR: Clean Up

• Improve code quality
• Run tests after EVERY change
• Must stay green

4. REPEAT

• Next failing test
• Continue cycle

Enforcement Rules

If You See	Action
Code written before test	STOP. Delete code. Write test first.
Test passes on first run	Test is wrong. Fix it to fail first.
Multiple features in one cycle	STOP. One test, one feature.
Skipping refactor	Go back. Clean up before next feature.

Commands

Before each implementation:

npx jest --watchAll  # Should have ONE new failure
# or: npx vitest run, pytest, go test ./...

After implementation:

npx jest --watchAll  # New test should pass, all others still pass

Worked Example: Adding a

capitalize

Function

RED Phase — Write the Failing Test

// src/strings.test.js
const { capitalize } = require('./strings');

describe('capitalize', () => {
  it('should uppercase the first letter and lowercase the rest', () => {
    expect(capitalize('hello')).toBe('Hello');
  });

  it('should handle single character strings', () => {
    expect(capitalize('h')).toBe('H');
  });

  it('should return empty string for empty input', () => {
    expect(capitalize('')).toBe('');
  });
});

Run tests — they fail because

strings.js

does not export

capitalize

:

FAIL  src/strings.test.js
  ● capitalize › should uppercase the first letter and lowercase the rest
    TypeError: capitalize is not a function

GREEN Phase — Minimal Implementation

// src/strings.js
function capitalize(str) {
  if (!str) return '';
  return str.charAt(0).toUpperCase() + str.slice(1).toLowerCase();
}

module.exports = { capitalize };

Run tests — all three pass:

PASS  src/strings.test.js
  capitalize
    ✓ should uppercase the first letter and lowercase the rest
    ✓ should handle single character strings
    ✓ should return empty string for empty input

REFACTOR Phase — Clean Up

Extract a guard clause for clarity, ensure tests still pass:

function capitalize(str) {
  if (str.length === 0) return '';
  return str[0].toUpperCase() + str.slice(1).toLowerCase();
}

Tests still green. Move to the next failing test.

Output Format

When guiding TDD:

## TDD Cycle: [Feature Name]

### RED Phase
Test: [test code]
Expected failure: [what error you expect]
Actual: [run result showing failure]

### GREEN Phase
Implementation: [minimal code]
Result: [run result showing pass]

### REFACTOR Phase
Changes: [what was cleaned up]
Result: [tests still pass]

Remember: The discipline IS the value. Shortcuts destroy the benefit.