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

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.