Refactor Skill

Safely restructure code without changing behavior.

Golden Rule

Refactoring changes HOW code works, not WHAT it does.

All tests should pass before AND after refactoring.

Process

1. Ensure Test Coverage

Before refactoring, verify tests exist:

bash

# Run existing tests
npm run test

# Check coverage for the code you're refactoring
npm run test -- --coverage src/module-to-refactor/

If coverage is low, add tests first before refactoring.

2. Make Small, Incremental Changes

Each step should be:

•A single, focused change
•Independently committable
•Passing all tests

Bad: One massive commit that changes everything Good: Series of small commits, each improving the code

3. Run Tests After Each Change

bash

# Quick feedback loop
npm run test -- --watch src/module/

4. Commit Frequently

bash

git commit -m "refactor(auth): extract token validation to separate function"
git commit -m "refactor(auth): rename validateToken to verifyJWT"
git commit -m "refactor(auth): move JWT logic to dedicated service"

Common Refactoring Patterns

Extract Function

When: Code block does one specific thing

typescript

// Before
function processOrder(order) {
  // 20 lines calculating total
  // 15 lines validating inventory
  // 10 lines sending notifications
}

// After
function processOrder(order) {
  const total = calculateTotal(order);
  validateInventory(order.items);
  sendOrderNotifications(order);
}

Extract Module/Class

When: Related functions should be grouped

code

// Before: utils.ts with 50 functions

// After:
// date-utils.ts - date formatting functions
// string-utils.ts - string manipulation
// validation-utils.ts - validators

Rename for Clarity

When: Names don't describe purpose

typescript

// Before
const d = new Date();
function proc(x) { ... }

// After
const createdAt = new Date();
function processPayment(transaction) { ... }

Replace Conditionals with Polymorphism

When: Switch/if chains based on type

typescript

// Before
function calculateArea(shape) {
  if (shape.type === 'circle') return Math.PI * shape.radius ** 2;
  if (shape.type === 'rectangle') return shape.width * shape.height;
}

// After
class Circle {
  calculateArea() { return Math.PI * this.radius ** 2; }
}
class Rectangle {
  calculateArea() { return this.width * this.height; }
}

Remove Duplication (DRY)

When: Same code appears in multiple places

typescript

// Before: Same validation in 3 files

// After: Shared validation utility
import { validateEmail } from '@/utils/validation';

Simplify Conditionals

When: Complex boolean logic

typescript

// Before
if (user && user.subscription && user.subscription.active && !user.banned) {
  // ...
}

// After
function canAccessPremiumContent(user) {
  if (!user) return false;
  if (user.banned) return false;
  return user.subscription?.active ?? false;
}

if (canAccessPremiumContent(user)) {
  // ...
}

Refactoring Checklist

• Tests pass before starting
• Identified specific improvements to make
• Making one change at a time
• Running tests after each change
• Committing after each successful change
• Not adding new features (refactor only)
• Not fixing bugs (separate concern)

Warning Signs to Stop

•Tests start failing unexpectedly
•Scope is growing beyond original plan
•You're tempted to "also fix this bug"
•Changes are getting hard to explain

When in doubt, commit what you have and reassess.

Code Smells to Look For

Smell	Refactoring
Long function	Extract functions
Long parameter list	Introduce parameter object
Duplicate code	Extract shared function
Large class	Split into focused classes
Feature envy	Move method to appropriate class
Primitive obsession	Create domain types
Deep nesting	Extract functions, early returns