Refactoring

Systematic code improvement through smell detection, design patterns, and migration patterns.

Quick Start

Identify code smells:

Analyze this file for code smells and suggest refactoring improvements

Apply design pattern:

Refactor this code to use the Strategy pattern for the discount calculations

Migrate to TypeScript:

Help me migrate this JavaScript module to TypeScript

Capabilities

1. Code Smell Identification

Detect common code smells with specific fixes.

Smell Catalog

Smell	Symptoms	Fix
Long Method	>20 lines, multiple abstractions	Extract Method
Large Class	>300 lines, multiple responsibilities	Extract Class
Feature Envy	Method uses other class's data extensively	Move Method
Data Clump	Same data appears together repeatedly	Extract Class
Primitive Obsession	Overuse of primitives instead of objects	Replace with Object
Duplicated Code	Same logic in multiple places	Extract Method/Class
Long Parameter List	>3-4 parameters	Introduce Parameter Object
Switch Statements	Complex switch on type	Replace with Polymorphism

See references/code-smells.md for complete catalog.

---

2. Design Pattern Application

Apply Gang of Four and modern patterns.

Strategy Pattern

Replace conditional logic with interchangeable algorithms:

// BEFORE: Switch statement
function calculateDiscount(type, price) {
  switch (type) {
    case 'percentage':
      return price * 0.1;
    case 'fixed':
      return 10;
    case 'bogo':
      return price * 0.5;
    default:
      return 0;
  }
}

// AFTER: Strategy pattern
const discountStrategies = {
  percentage: (price) => price * 0.1,
  fixed: () => 10,
  bogo: (price) => price * 0.5,
};

function calculateDiscount(type, price) {
  const strategy = discountStrategies[type] || (() => 0);
  return strategy(price);
}

Observer Pattern

Decouple event producers from consumers:

// BEFORE: Tightly coupled
class Order {
  complete() {
    this.status = 'completed';
    emailService.send(this.user, 'Order complete');
    analyticsService.track('order_complete', this);
    inventoryService.update(this.items);
  }
}

// AFTER: Observer pattern
class Order extends EventEmitter {
  complete() {
    this.status = 'completed';
    this.emit('completed', this);
  }
}

// Subscribers register independently
order.on('completed', (order) => emailService.send(order.user, 'Order complete'));
order.on('completed', (order) => analyticsService.track('order_complete', order));
order.on('completed', (order) => inventoryService.update(order.items));

Factory Pattern

Centralize object creation:

// BEFORE: Scattered creation logic
function processPayment(method, amount) {
  let processor;
  if (method === 'stripe') {
    processor = new StripeProcessor(config.stripeKey);
  } else if (method === 'paypal') {
    processor = new PayPalProcessor(config.paypalId);
  }
  return processor.charge(amount);
}

// AFTER: Factory pattern
class PaymentProcessorFactory {
  static create(method) {
    const processors = {
      stripe: () => new StripeProcessor(config.stripeKey),
      paypal: () => new PayPalProcessor(config.paypalId),
      square: () => new SquareProcessor(config.squareToken),
    };

    const factory = processors[method];
    if (!factory) throw new Error(`Unknown payment method: ${method}`);
    return factory();
  }
}

function processPayment(method, amount) {
  const processor = PaymentProcessorFactory.create(method);
  return processor.charge(amount);
}

See references/design-patterns.md for all patterns.

---

3. Migration Patterns

Class Components to Hooks (React)

// BEFORE: Class component
class UserProfile extends React.Component {
  state = { user: null, loading: true };

  componentDidMount() {
    this.fetchUser();
  }

  componentDidUpdate(prevProps) {
    if (prevProps.userId !== this.props.userId) {
      this.fetchUser();
    }
  }

  async fetchUser() {
    this.setState({ loading: true });
    const user = await api.getUser(this.props.userId);
    this.setState({ user, loading: false });
  }

  render() {
    const { user, loading } = this.state;
    if (loading) return <Spinner />;
    return <Profile user={user} />;
  }
}

// AFTER: Hooks with custom hook extraction
function useUser(userId) {
  const [user, setUser] = useState(null);
  const [loading, setLoading] = useState(true);

  useEffect(() => {
    setLoading(true);
    api.getUser(userId)
      .then(setUser)
      .finally(() => setLoading(false));
  }, [userId]);

  return { user, loading };
}

function UserProfile({ userId }) {
  const { user, loading } = useUser(userId);
  if (loading) return <Spinner />;
  return <Profile user={user} />;
}

Callbacks to Async/Await

// BEFORE: Callback hell
function processFile(filename, callback) {
  fs.readFile(filename, 'utf8', (err, content) => {
    if (err) return callback(err);
    parseData(content, (err, data) => {
      if (err) return callback(err);
      transformData(data, (err, transformed) => {
        if (err) return callback(err);
        saveData(transformed, callback);
      });
    });
  });
}

// AFTER: Async/await
async function processFile(filename) {
  const content = await fs.promises.readFile(filename, 'utf8');
  const data = await parseDataAsync(content);
  const transformed = await transformDataAsync(data);
  return await saveDataAsync(transformed);
}

CommonJS to ES Modules

// BEFORE: CommonJS
const express = require('express');
const { Router } = require('express');
const utils = require('./utils');

module.exports = { router, handler };

// AFTER: ES Modules
import express, { Router } from 'express';
import * as utils from './utils.js';

export { router, handler };

---

4. JavaScript to TypeScript Migration

Step-by-Step Process

1. Install TypeScript and configure
2. Rename files .js -> .ts
3. Add type annotations progressively
4. Fix type errors
5. Enable stricter options gradually

tsconfig.json for Migration

{
  "compilerOptions": {
    "target": "ES2020",
    "module": "NodeNext",
    "moduleResolution": "NodeNext",
    "strict": false,
    "allowJs": true,
    "checkJs": false,
    "noImplicitAny": false,
    "strictNullChecks": false,
    "outDir": "./dist",
    "rootDir": "./src"
  },
  "include": ["src/**/*"]
}

Progressive Type Addition

// Phase 1: Basic types
function greet(name) { // Keep as any initially
  return `Hello, ${name}`;
}

// Phase 2: Add parameter types
function greet(name: string) {
  return `Hello, ${name}`;
}

// Phase 3: Add return types
function greet(name: string): string {
  return `Hello, ${name}`;
}

// Phase 4: Handle edge cases
function greet(name: string | null): string {
  return `Hello, ${name ?? 'Guest'}`;
}

See references/typescript-migration.md for complete guide.

---

5. Performance Refactoring

Memoization

// BEFORE: Recalculates every render
function ExpensiveComponent({ data }) {
  const processed = expensiveCalculation(data);
  return <div>{processed}</div>;
}

// AFTER: Memoized
function ExpensiveComponent({ data }) {
  const processed = useMemo(
    () => expensiveCalculation(data),
    [data]
  );
  return <div>{processed}</div>;
}

Lazy Loading

// BEFORE: Eager loading
import HeavyComponent from './HeavyComponent';

function App() {
  return <HeavyComponent />;
}

// AFTER: Lazy loading
const HeavyComponent = React.lazy(() => import('./HeavyComponent'));

function App() {
  return (
    <Suspense fallback={<Loading />}>
      <HeavyComponent />
    </Suspense>
  );
}

Database Query Optimization

// BEFORE: N+1 query
const users = await User.findAll();
for (const user of users) {
  user.orders = await Order.findAll({ where: { userId: user.id } });
}

// AFTER: Eager loading
const users = await User.findAll({
  include: [{ model: Order }]
});

---

6. Security Hardening

Input Validation

// BEFORE: No validation
app.post('/user', (req, res) => {
  db.query(`INSERT INTO users (name) VALUES ('${req.body.name}')`);
});

// AFTER: Validated and parameterized
import { z } from 'zod';

const userSchema = z.object({
  name: z.string().min(1).max(100),
  email: z.string().email(),
});

app.post('/user', (req, res) => {
  const { name, email } = userSchema.parse(req.body);
  db.query('INSERT INTO users (name, email) VALUES (?, ?)', [name, email]);
});

Output Encoding

// BEFORE: XSS vulnerable
element.innerHTML = userContent;

// AFTER: Encoded
element.textContent = userContent;
// Or use DOMPurify for HTML
element.innerHTML = DOMPurify.sanitize(userContent);

---

7. Accessibility Improvements

Semantic HTML

<!-- BEFORE: Non-semantic -->
<div class="button" onclick="submit()">Submit</div>

<!-- AFTER: Semantic -->
<button type="submit">Submit</button>

ARIA Attributes

<!-- BEFORE: No accessibility -->
<div class="modal">
  <div class="content">Modal content</div>
</div>

<!-- AFTER: Accessible -->
<div role="dialog" aria-modal="true" aria-labelledby="modal-title">
  <h2 id="modal-title">Modal Title</h2>
  <div class="content">Modal content</div>
</div>

Focus Management

// BEFORE: No focus management
function openModal() {
  modal.classList.add('open');
}

// AFTER: Manages focus
function openModal() {
  previouslyFocused = document.activeElement;
  modal.classList.add('open');
  modal.querySelector('[autofocus]')?.focus();
}

function closeModal() {
  modal.classList.remove('open');
  previouslyFocused?.focus();
}

---

8. Internationalization (i18n) Preparation

String Externalization

// BEFORE: Hardcoded strings
function Welcome({ name }) {
  return <h1>Welcome, {name}!</h1>;
}

// AFTER: Externalized
function Welcome({ name }) {
  const { t } = useTranslation();
  return <h1>{t('welcome', { name })}</h1>;
}

// translations/en.json
{
  "welcome": "Welcome, {{name}}!"
}

// translations/es.json
{
  "welcome": "Bienvenido, {{name}}!"
}

Date/Number Formatting

// BEFORE: Locale-unaware
const formatted = `$${price.toFixed(2)}`;
const date = new Date().toLocaleDateString();

// AFTER: Locale-aware
const formatted = new Intl.NumberFormat(locale, {
  style: 'currency',
  currency: 'USD'
}).format(price);

const date = new Intl.DateTimeFormat(locale, {
  dateStyle: 'long'
}).format(new Date());

---

9. Monolith to Microservices Extraction

Identify Bounded Contexts

1. Map domain concepts and relationships
2. Identify natural boundaries (user, order, payment)
3. Check for shared data access patterns
4. Look for independent scaling needs

Strangler Fig Pattern

1. Create new microservice
2. Route new functionality to microservice
3. Gradually migrate existing functionality
4. Eventually remove old code from monolith

API Gateway Setup

// Gateway routing
const routes = {
  '/users/*': 'http://user-service:3001',
  '/orders/*': 'http://order-service:3002',
  '/payments/*': 'http://payment-service:3003',
};

app.all('*', (req, res) => {
  const service = findService(req.path);
  proxy.web(req, res, { target: service });
});

---

10. Micro-Frontend Extraction

Module Federation Setup

// webpack.config.js (shell app)
module.exports = {
  plugins: [
    new ModuleFederationPlugin({
      name: 'shell',
      remotes: {
        userApp: 'userApp@http://localhost:3001/remoteEntry.js',
        orderApp: 'orderApp@http://localhost:3002/remoteEntry.js',
      },
    }),
  ],
};

// webpack.config.js (user micro-frontend)
module.exports = {
  plugins: [
    new ModuleFederationPlugin({
      name: 'userApp',
      filename: 'remoteEntry.js',
      exposes: {
        './UserProfile': './src/components/UserProfile',
      },
    }),
  ],
};

---

Refactoring Workflow

Safe Refactoring Process

1. IDENTIFY
   - Run static analysis
   - Review code metrics
   - Collect team feedback

2. TEST
   - Ensure tests exist for affected code
   - Add tests if coverage is low
   - Run full test suite

3. REFACTOR
   - Make small, incremental changes
   - Commit frequently
   - Keep tests passing

4. VERIFY
   - Run tests after each change
   - Review for regressions
   - Check performance if relevant

5. DOCUMENT
   - Update comments/docs if needed
   - Note breaking changes
   - Update changelog

Refactoring Checklist

## Pre-flight
- [ ] Tests exist and pass
- [ ] Code coverage >= 80% for affected area
- [ ] Feature branch created

## During Refactoring
- [ ] Making atomic commits
- [ ] Tests passing after each change
- [ ] No functionality changes

## Post-flight
- [ ] All tests pass
- [ ] Manual smoke test completed
- [ ] Code review requested

---

Hook Integration

PreToolUse Hook - Refactoring Safety Checks

Before applying refactorings, verify safety:

{
  "hooks": {
    "PreToolUse": [{
      "matcher": "Edit",
      "command": "check-refactoring-safety.sh"
    }]
  }
}

Script example:

#!/bin/bash
# check-refactoring-safety.sh

FILE="$1"

# Check if tests exist
TEST_FILE=$(echo "$FILE" | sed 's/src/tests/' | sed 's/\.ts$/.test.ts/')
if [ ! -f "$TEST_FILE" ]; then
  echo "WARNING: No test file found for $FILE"
  echo "Consider adding tests before refactoring"
fi

# Check test coverage
coverage=$(npm test -- --coverage --collectCoverageFrom="$FILE" 2>/dev/null | grep -oP '\d+(?=%)')
if [ "$coverage" -lt 70 ]; then
  echo "WARNING: Low test coverage ($coverage%) for $FILE"
  echo "Consider adding tests before refactoring"
fi

# Check for uncommitted changes
if ! git diff --quiet "$FILE"; then
  echo "WARNING: Uncommitted changes in $FILE"
  echo "Commit changes before refactoring"
fi

Hook response pattern:

interface RefactoringSafetyCheck {
  safe: boolean;
  warnings: Array<{
    type: 'no_tests' | 'low_coverage' | 'uncommitted_changes';
    message: string;
    file: string;
  }>;
  suggestions: string[];
}

PostToolUse Hook - Verify Refactoring

After refactoring, run verification:

{
  "hooks": {
    "PostToolUse": [{
      "matcher": "Edit",
      "command": "verify-refactoring.sh"
    }]
  }
}

CI/CD Integration

GitHub Actions

name: Refactoring Verification
on: [pull_request]

jobs:
  verify:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
        with:
          fetch-depth: 0

      - name: Check for behavior changes
        run: |
          npm test -- --coverage
          # Compare coverage before/after
          git diff origin/main --stat

      - name: Type check
        run: npx tsc --noEmit

      - name: Lint
        run: npm run lint

      - name: Complexity check
        run: |
          npx escomplex src/ --format json > after.json
          git checkout origin/main
          npx escomplex src/ --format json > before.json
          # Compare complexity metrics

Pre-commit Hook

#!/bin/bash
# .git/hooks/pre-commit

# Ensure tests pass before committing refactoring
npm test -- --passWithNoTests || {
  echo "Tests failed - commit aborted"
  exit 1
}

# Check for console.log/debugger
if git diff --cached | grep -E "(console\.(log|debug)|debugger)"; then
  echo "Remove debug statements before committing"
  exit 1
fi

Reference Files

• references/code-smells.md - Complete code smell catalog
• references/design-patterns.md - GoF patterns with modern examples
• references/typescript-migration.md - JS to TS migration guide
• patterns/migration-guides.md - Detailed migration patterns