Refactor

Refactoring Assistant

Identify code smells, suggest improvements, and guide safe refactoring following best practices.

When to Activate

Explicit Triggers

• User says "refactor this"
• User says "重构代码"
• User says "improve this code"
• User says "clean up this code"

Implicit Triggers

• User mentions code is hard to understand
• User asks about code quality
• User mentions technical debt

NOT Activated For

• New feature implementation
• Bug fixes (unless refactoring is needed)
• Simple formatting changes

Code Smells to Detect

1. Long Functions/Methods

Smell: Function > 50 lines Fix: Extract smaller functions

// Before: 80-line function
function processOrder(order) {
  // validation (20 lines)
  // calculation (30 lines)
  // database save (20 lines)
  // notification (10 lines)
}

// After: Extracted functions
function processOrder(order) {
  validateOrder(order);
  const total = calculateTotal(order);
  saveOrder(order, total);
  notifyCustomer(order);
}

2. Large Files

Smell: File > 800 lines Fix: Split by responsibility

// Before: user-service.ts (1200 lines)
UserService
  - authentication
  - profile management
  - permissions
  - notifications

// After: Split into focused files
auth-service.ts (300 lines)
profile-service.ts (250 lines)
permission-service.ts (200 lines)
notification-service.ts (150 lines)

3. Duplicated Code

Smell: Same logic in multiple places Fix: Extract to shared function

// Before: Duplicated validation
function createUser(data) {
  if (!data.email || !data.email.includes('@')) {
    throw new Error('Invalid email');
  }
  // ...
}

function updateUser(id, data) {
  if (!data.email || !data.email.includes('@')) {
    throw new Error('Invalid email');
  }
  // ...
}

// After: Extracted validation
function validateEmail(email) {
  if (!email || !email.includes('@')) {
    throw new Error('Invalid email');
  }
}

function createUser(data) {
  validateEmail(data.email);
  // ...
}

function updateUser(id, data) {
  validateEmail(data.email);
  // ...
}

4. Deep Nesting

Smell: Nesting > 4 levels Fix: Early returns, extract functions

// Before: Deep nesting
function processData(data) {
  if (data) {
    if (data.isValid) {
      if (data.user) {
        if (data.user.isActive) {
          // actual logic here
        }
      }
    }
  }
}

// After: Early returns
function processData(data) {
  if (!data) return;
  if (!data.isValid) return;
  if (!data.user) return;
  if (!data.user.isActive) return;

  // actual logic here
}

5. Magic Numbers/Strings

Smell: Hardcoded values without explanation Fix: Named constants

// Before: Magic numbers
function calculateDiscount(price) {
  if (price > 100) {
    return price * 0.1;
  }
  return 0;
}

// After: Named constants
const DISCOUNT_THRESHOLD = 100;
const DISCOUNT_RATE = 0.1;

function calculateDiscount(price) {
  if (price > DISCOUNT_THRESHOLD) {
    return price * DISCOUNT_RATE;
  }
  return 0;
}

6. God Objects/Classes

Smell: Class with too many responsibilities Fix: Split by Single Responsibility Principle

// Before: God class
class UserManager {
  authenticate() { }
  validateEmail() { }
  sendNotification() { }
  generateReport() { }
  processPayment() { }
}

// After: Focused classes
class AuthService {
  authenticate() { }
}

class EmailValidator {
  validate() { }
}

class NotificationService {
  send() { }
}

class ReportGenerator {
  generate() { }
}

class PaymentProcessor {
  process() { }
}

7. Long Parameter Lists

Smell: Function with > 4 parameters Fix: Parameter object

// Before: Too many parameters
function createUser(
  email: string,
  name: string,
  age: number,
  address: string,
  phone: string,
  role: string
) { }

// After: Parameter object
interface UserData {
  email: string;
  name: string;
  age: number;
  address: string;
  phone: string;
  role: string;
}

function createUser(userData: UserData) { }

8. Mutable State

Smell: Direct mutation of objects Fix: Immutable updates

// Before: Mutation
function updateUser(user, newEmail) {
  user.email = newEmail;
  return user;
}

// After: Immutable
function updateUser(user, newEmail) {
  return {
    ...user,
    email: newEmail
  };
}

Refactoring Techniques

1. Extract Function

Move code block into a named function

2. Extract Variable

Replace complex expression with named variable

3. Inline Function

Replace function call with function body (if trivial)

4. Rename

Give better names to variables, functions, classes

5. Move Function

Move function to more appropriate class/module

6. Replace Conditional with Polymorphism

Use inheritance/interfaces instead of if/switch

7. Introduce Parameter Object

Group related parameters into object

8. Replace Magic Number with Constant

Name hardcoded values

Safe Refactoring Process

1. Ensure Tests Exist

# Run tests before refactoring
npm test

# Check coverage
npm run coverage

2. Make Small Changes

• One refactoring at a time
• Commit after each successful change
• Keep tests green

3. Run Tests After Each Change

# After each refactoring step
npm test

4. Use Automated Tools

• IDE refactoring features (rename, extract, move)
• Linters (ESLint, Pylint, golangci-lint)
• Code formatters (Prettier, Black, gofmt)

Refactoring Checklist

Before refactoring:

• Tests exist and pass
• Understand the code's purpose
• Identify specific code smell
• Plan refactoring approach

During refactoring:

• Make one change at a time
• Run tests after each change
• Keep commits small and focused
• Preserve existing behavior

After refactoring:

• All tests still pass
• Code is more readable
• Complexity reduced
• No new bugs introduced

When NOT to Refactor

• Deadline pressure: Refactor after delivery
• Unclear requirements: Understand first, refactor later
• No tests: Write tests first
• Working code in production: If it works and isn't causing issues, leave it

Refactoring Priorities

High Priority:

• Security vulnerabilities
• Performance bottlenecks
• Code causing frequent bugs
• Code blocking new features

Medium Priority:

• Hard-to-understand code
• Duplicated logic
• Large files/functions

Low Priority:

• Minor style inconsistencies
• Slightly verbose code
• Personal preferences

Related Resources

• Always have tests before refactoring
• Make small, incremental changes
• Keep tests green throughout
• Use IDE refactoring tools when available
• Commit frequently during refactoring