name	description	license
refactor	Surgical code refactoring to improve maintainability without changing behavior. Covers extracting functions, renaming variables, breaking down god functions, improving type safety, eliminating code smells, and applying design patterns. Less drastic than repo-rebuilder; use for gradual improvements.	MIT

Refactor

Overview

Improve code structure and readability without changing external behavior. Refactoring is gradual evolution, not revolution. Use this for improving existing code, not rewriting from scratch.

When to Use

Use this skill when:

Code is hard to understand or maintain
Functions/classes are too large
Code smells need addressing
Adding features is difficult due to code structure
User asks "clean up this code", "refactor this", "improve this"

Refactoring Principles

The Golden Rules

Behavior is preserved - Refactoring doesn't change what the code does, only how
Small steps - Make tiny changes, test after each
Version control is your friend - Commit before and after each safe state
Tests are essential - Without tests, you're not refactoring, you're editing
One thing at a time - Don't mix refactoring with feature changes

When NOT to Refactor

- Code that works and won't change again (if it ain't broke...)
- Critical production code without tests (add tests first)
- When you're under a tight deadline
- "Just because" - need a clear purpose

Common Code Smells & Fixes

1. Long Method/Function

# BAD: 200-line function that does everything
- async function processOrder(orderId) {
-   // 50 lines: fetch order
-   // 30 lines: validate order
-   // 40 lines: calculate pricing
-   // 30 lines: update inventory
-   // 20 lines: create shipment
-   // 30 lines: send notifications
- }

# GOOD: Broken into focused functions
+ async function processOrder(orderId) {
+   const order = await fetchOrder(orderId);
+   validateOrder(order);
+   const pricing = calculatePricing(order);
+   await updateInventory(order);
+   const shipment = await createShipment(order);
+   await sendNotifications(order, pricing, shipment);
+   return { order, pricing, shipment };
+ }

2. Duplicated Code

# BAD: Same logic in multiple places
- function calculateUserDiscount(user) {
-   if (user.membership === 'gold') return user.total * 0.2;
-   if (user.membership === 'silver') return user.total * 0.1;
-   return 0;
- }
-
- function calculateOrderDiscount(order) {
-   if (order.user.membership === 'gold') return order.total * 0.2;
-   if (order.user.membership === 'silver') return order.total * 0.1;
-   return 0;
- }

# GOOD: Extract common logic
+ function getMembershipDiscountRate(membership) {
+   const rates = { gold: 0.2, silver: 0.1 };
+   return rates[membership] || 0;
+ }
+
+ function calculateUserDiscount(user) {
+   return user.total * getMembershipDiscountRate(user.membership);
+ }
+
+ function calculateOrderDiscount(order) {
+   return order.total * getMembershipDiscountRate(order.user.membership);
+ }

3. Large Class/Module

# BAD: God object that knows too much
- class UserManager {
-   createUser() { /* ... */ }
-   updateUser() { /* ... */ }
-   deleteUser() { /* ... */ }
-   sendEmail() { /* ... */ }
-   generateReport() { /* ... */ }
-   handlePayment() { /* ... */ }
-   validateAddress() { /* ... */ }
-   // 50 more methods...
- }

# GOOD: Single responsibility per class
+ class UserService {
+   create(data) { /* ... */ }
+   update(id, data) { /* ... */ }
+   delete(id) { /* ... */ }
+ }
+
+ class EmailService {
+   send(to, subject, body) { /* ... */ }
+ }
+
+ class ReportService {
+   generate(type, params) { /* ... */ }
+ }
+
+ class PaymentService {
+   process(amount, method) { /* ... */ }
+ }

4. Long Parameter List

# BAD: Too many parameters
- function createUser(email, password, name, age, address, city, country, phone) {
-   /* ... */
- }

# GOOD: Group related parameters
+ interface UserData {
+   email: string;
+   password: string;
+   name: string;
+   age?: number;
+   address?: Address;
+   phone?: string;
+ }
+
+ function createUser(data: UserData) {
+   /* ... */
+ }

# EVEN BETTER: Use builder pattern for complex construction
+ const user = UserBuilder
+   .email('test@example.com')
+   .password('secure123')
+   .name('Test User')
+   .address(address)
+   .build();

5. Feature Envy

# BAD: Method that uses another object's data more than its own
- class Order {
-   calculateDiscount(user) {
-     if (user.membershipLevel === 'gold') {
+       return this.total * 0.2;
+     }
+     if (user.accountAge > 365) {
+       return this.total * 0.1;
+     }
+     return 0;
+   }
+ }

# GOOD: Move logic to the object that owns the data
+ class User {
+   getDiscountRate(orderTotal) {
+     if (this.membershipLevel === 'gold') return 0.2;
+     if (this.accountAge > 365) return 0.1;
+     return 0;
+   }
+ }
+
+ class Order {
+   calculateDiscount(user) {
+     return this.total * user.getDiscountRate(this.total);
+   }
+ }

6. Primitive Obsession

# BAD: Using primitives for domain concepts
- function sendEmail(to, subject, body) { /* ... */ }
- sendEmail('user@example.com', 'Hello', '...');

- function createPhone(country, number) {
-   return `${country}-${number}`;
- }

# GOOD: Use domain types
+ class Email {
+   private constructor(public readonly value: string) {
+     if (!Email.isValid(value)) throw new Error('Invalid email');
+   }
+   static create(value: string) { return new Email(value); }
+   static isValid(email: string) { return /^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(email); }
+ }
+
+ class PhoneNumber {
+   constructor(
+     public readonly country: string,
+     public readonly number: string
+   ) {
+     if (!PhoneNumber.isValid(country, number)) throw new Error('Invalid phone');
+   }
+   toString() { return `${this.country}-${this.number}`; }
+   static isValid(country: string, number: string) { /* ... */ }
+ }
+
+ // Usage
+ const email = Email.create('user@example.com');
+ const phone = new PhoneNumber('1', '555-1234');

7. Magic Numbers/Strings

# BAD: Unexplained values
- if (user.status === 2) { /* ... */ }
- const discount = total * 0.15;
- setTimeout(callback, 86400000);

# GOOD: Named constants
+ const UserStatus = {
+   ACTIVE: 1,
+   INACTIVE: 2,
+   SUSPENDED: 3
+ } as const;
+
+ const DISCOUNT_RATES = {
+   STANDARD: 0.1,
+   PREMIUM: 0.15,
+   VIP: 0.2
+ } as const;
+
+ const ONE_DAY_MS = 24 * 60 * 60 * 1000;
+
+ if (user.status === UserStatus.INACTIVE) { /* ... */ }
+ const discount = total * DISCOUNT_RATES.PREMIUM;
+ setTimeout(callback, ONE_DAY_MS);

8. Nested Conditionals

# BAD: Arrow code
- function process(order) {
-   if (order) {
-     if (order.user) {
-       if (order.user.isActive) {
-         if (order.total > 0) {
-           return processOrder(order);
+         } else {
+           return { error: 'Invalid total' };
+         }
+       } else {
+         return { error: 'User inactive' };
+       }
+     } else {
+       return { error: 'No user' };
+     }
+   } else {
+     return { error: 'No order' };
+   }
+ }

# GOOD: Guard clauses / early returns
+ function process(order) {
+   if (!order) return { error: 'No order' };
+   if (!order.user) return { error: 'No user' };
+   if (!order.user.isActive) return { error: 'User inactive' };
+   if (order.total <= 0) return { error: 'Invalid total' };
+   return processOrder(order);
+ }

# EVEN BETTER: Using Result type
+ function process(order): Result<ProcessedOrder, Error> {
+   return Result.combine([
+     validateOrderExists(order),
+     validateUserExists(order),
+     validateUserActive(order.user),
+     validateOrderTotal(order)
+   ]).flatMap(() => processOrder(order));
+ }

9. Dead Code

# BAD: Unused code lingers
- function oldImplementation() { /* ... */ }
- const DEPRECATED_VALUE = 5;
- import { unusedThing } from './somewhere';
- // Commented out code
- // function oldCode() { /* ... */ }

# GOOD: Remove it
+ // Delete unused functions, imports, and commented code
+ // If you need it again, git history has it

10. Inappropriate Intimacy

# BAD: One class reaches deep into another
- class OrderProcessor {
-   process(order) {
-     order.user.profile.address.street;  // Too intimate
-     order.repository.connection.config;  // Breaking encapsulation
+   }
+ }

# GOOD: Ask, don't tell
+ class OrderProcessor {
+   process(order) {
+     order.getShippingAddress();  // Order knows how to get it
+     order.save();  // Order knows how to save itself
+   }
+ }

Extract Method Refactoring

Before and After

# Before: One long function
- function printReport(users) {
-   console.log('USER REPORT');
-   console.log('============');
-   console.log('');
-   console.log(`Total users: ${users.length}`);
-   console.log('');
-   console.log('ACTIVE USERS');
-   console.log('------------');
-   const active = users.filter(u => u.isActive);
-   active.forEach(u => {
-     console.log(`- ${u.name} (${u.email})`);
-   });
-   console.log('');
-   console.log(`Active: ${active.length}`);
-   console.log('');
-   console.log('INACTIVE USERS');
-   console.log('--------------');
-   const inactive = users.filter(u => !u.isActive);
-   inactive.forEach(u => {
-     console.log(`- ${u.name} (${u.email})`);
-   });
-   console.log('');
-   console.log(`Inactive: ${inactive.length}`);
- }

# After: Extracted methods
+ function printReport(users) {
+   printHeader('USER REPORT');
+   console.log(`Total users: ${users.length}\n`);
+   printUserSection('ACTIVE USERS', users.filter(u => u.isActive));
+   printUserSection('INACTIVE USERS', users.filter(u => !u.isActive));
+ }
+
+ function printHeader(title) {
+   const line = '='.repeat(title.length);
+   console.log(title);
+   console.log(line);
+   console.log('');
+ }
+
+ function printUserSection(title, users) {
+   console.log(title);
+   console.log('-'.repeat(title.length));
+   users.forEach(u => console.log(`- ${u.name} (${u.email})`));
+   console.log('');
+   console.log(`${title.split(' ')[0]}: ${users.length}`);
+   console.log('');
+ }

Introducing Type Safety

From Untyped to Typed

# Before: No types
- function calculateDiscount(user, total, membership, date) {
-   if (membership === 'gold' && date.getDay() === 5) {
-     return total * 0.25;
-   }
-   if (membership === 'gold') return total * 0.2;
-   return total * 0.1;
- }

# After: Full type safety
+ type Membership = 'bronze' | 'silver' | 'gold';
+
+ interface User {
+   id: string;
+   name: string;
+   membership: Membership;
+ }
+
+ interface DiscountResult {
+   original: number;
+   discount: number;
+   final: number;
+   rate: number;
+ }
+
+ function calculateDiscount(
+   user: User,
+   total: number,
+   date: Date = new Date()
+ ): DiscountResult {
+   if (total < 0) throw new Error('Total cannot be negative');
+
+   let rate = 0.1; // Default bronze
+
+   if (user.membership === 'gold' && date.getDay() === 5) {
+     rate = 0.25; // Friday bonus for gold
+   } else if (user.membership === 'gold') {
+     rate = 0.2;
+   } else if (user.membership === 'silver') {
+     rate = 0.15;
+   }
+
+   const discount = total * rate;
+
+   return {
+     original: total,
+     discount,
+     final: total - discount,
+     rate
+   };
+ }

Design Patterns for Refactoring

Strategy Pattern

# Before: Conditional logic
- function calculateShipping(order, method) {
-   if (method === 'standard') {
-     return order.total > 50 ? 0 : 5.99;
-   } else if (method === 'express') {
-     return order.total > 100 ? 9.99 : 14.99;
+   } else if (method === 'overnight') {
+     return 29.99;
+   }
+ }

# After: Strategy pattern
+ interface ShippingStrategy {
+   calculate(order: Order): number;
+ }
+
+ class StandardShipping implements ShippingStrategy {
+   calculate(order: Order) {
+     return order.total > 50 ? 0 : 5.99;
+   }
+ }
+
+ class ExpressShipping implements ShippingStrategy {
+   calculate(order: Order) {
+     return order.total > 100 ? 9.99 : 14.99;
+   }
+ }
+
+ class OvernightShipping implements ShippingStrategy {
+   calculate(order: Order) {
+     return 29.99;
+   }
+ }
+
+ function calculateShipping(order: Order, strategy: ShippingStrategy) {
+   return strategy.calculate(order);
+ }

Chain of Responsibility

# Before: Nested validation
- function validate(user) {
-   const errors = [];
-   if (!user.email) errors.push('Email required');
+   else if (!isValidEmail(user.email)) errors.push('Invalid email');
+   if (!user.name) errors.push('Name required');
+   if (user.age < 18) errors.push('Must be 18+');
+   if (user.country === 'blocked') errors.push('Country not supported');
+   return errors;
+ }

# After: Chain of responsibility
+ abstract class Validator {
+   abstract validate(user: User): string | null;
+   setNext(validator: Validator): Validator {
+     this.next = validator;
+     return validator;
+   }
+   validate(user: User): string | null {
+     const error = this.doValidate(user);
+     if (error) return error;
+     return this.next?.validate(user) ?? null;
+   }
+ }
+
+ class EmailRequiredValidator extends Validator {
+   doValidate(user: User) {
+     return !user.email ? 'Email required' : null;
+   }
+ }
+
+ class EmailFormatValidator extends Validator {
+   doValidate(user: User) {
+     return user.email && !isValidEmail(user.email) ? 'Invalid email' : null;
+   }
+ }
+
+ // Build the chain
+ const validator = new EmailRequiredValidator()
+   .setNext(new EmailFormatValidator())
+   .setNext(new NameRequiredValidator())
+   .setNext(new AgeValidator())
+   .setNext(new CountryValidator());

Refactoring Steps

Safe Refactoring Process

1. PREPARE
   - Ensure tests exist (write them if missing)
   - Commit current state
   - Create feature branch

2. IDENTIFY
   - Find the code smell to address
   - Understand what the code does
   - Plan the refactoring

3. REFACTOR (small steps)
   - Make one small change
   - Run tests
   - Commit if tests pass
   - Repeat

4. VERIFY
   - All tests pass
   - Manual testing if needed
   - Performance unchanged or improved

5. CLEAN UP
   - Update comments
   - Update documentation
   - Final commit

Refactoring Checklist

Code Quality

Structure

Related code is together
Clear module boundaries
Dependencies flow in one direction
No circular dependencies

Type Safety

Types defined for all public APIs
No any types without justification
Nullable types explicitly marked

Testing

Refactored code is tested
Tests cover edge cases
All tests pass

Common Refactoring Operations

Operation	Description
Extract Method	Turn code fragment into method
Extract Class	Move behavior to new class
Extract Interface	Create interface from implementation
Inline Method	Move method body back to caller
Inline Class	Move class behavior to caller
Pull Up Method	Move method to superclass
Push Down Method	Move method to subclass
Rename Method/Variable	Improve clarity
Introduce Parameter Object	Group related parameters
Replace Conditional with Polymorphism	Use polymorphism instead of switch/if
Replace Magic Number with Constant	Named constants
Decompose Conditional	Break complex conditions
Consolidate Conditional	Combine duplicate conditions
Replace Nested Conditional with Guard Clauses	Early returns
Introduce Null Object	Eliminate null checks
Replace Type Code with Class/Enum	Strong typing
Replace Inheritance with Delegation	Composition over inheritance

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Refactor

Overview

When to Use

Refactoring Principles

The Golden Rules

When NOT to Refactor

Common Code Smells & Fixes

1. Long Method/Function

2. Duplicated Code

3. Large Class/Module

4. Long Parameter List

5. Feature Envy

6. Primitive Obsession

7. Magic Numbers/Strings

8. Nested Conditionals

9. Dead Code

10. Inappropriate Intimacy

Extract Method Refactoring

Before and After

Introducing Type Safety

From Untyped to Typed

Design Patterns for Refactoring

Strategy Pattern

Chain of Responsibility

Refactoring Steps

Safe Refactoring Process

Refactoring Checklist

Code Quality

Structure

Type Safety

Testing

Common Refactoring Operations

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SKILL.md

Latest commit

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SKILL.md

File metadata and controls

Refactor

Overview

When to Use

Refactoring Principles

The Golden Rules

When NOT to Refactor

Common Code Smells & Fixes

1. Long Method/Function

2. Duplicated Code

3. Large Class/Module

4. Long Parameter List

5. Feature Envy

6. Primitive Obsession

7. Magic Numbers/Strings

8. Nested Conditionals

9. Dead Code

10. Inappropriate Intimacy

Extract Method Refactoring

Before and After

Introducing Type Safety

From Untyped to Typed

Design Patterns for Refactoring

Strategy Pattern

Chain of Responsibility

Refactoring Steps

Safe Refactoring Process

Refactoring Checklist

Code Quality

Structure

Type Safety

Testing

Common Refactoring Operations