Large-Scale Refactoring with AI

The Refactoring Challenge

Large-scale refactoring involves:

• Hundreds or thousands of files
• Multiple interconnected changes
• Risk of breaking existing functionality
• Need for consistent transformation patterns

AI assistants transform this from a tedious, error-prone process into a systematic, verifiable operation.

Planning Large Refactors

Impact Analysis

claude "I want to rename the 'User' class to 'Account'
across the entire codebase. Analyze:
1. All files that reference 'User'
2. Database tables and migrations involved
3. API endpoints that need updating
4. Documentation that references 'User'
5. Risk assessment for this change"

AI produces comprehensive impact analysis:

## User → Account Rename Impact Analysis

### Direct References (147 files)
- Models: 3 files
- Services: 12 files
- Routes: 8 files
- Components: 45 files
- Tests: 67 files
- Types: 12 files

### Database Impact
- Tables: users → accounts (BREAKING: requires migration)
- Foreign keys: user_id → account_id (42 tables affected)
- Indexes: 8 need renaming

### API Impact (BREAKING CHANGES)
- GET /api/users → /api/accounts
- POST /api/users → /api/accounts
- All response bodies with "user" field

### Documentation
- README.md: 12 references
- API docs: 34 endpoints
- Contributing guide: 3 references

### Risk Assessment: HIGH
- Breaking API changes require version bump
- Database migration needs careful rollout
- Client applications need updating

### Recommendation
Consider phased approach:
1. Add Account as alias (backward compatible)
2. Migrate clients to new endpoints
3. Deprecate old names with warnings
4. Remove old names in major version

Executing Large Refactors

Codemods with AI

claude "Create a codemod to convert all class components
to functional components with hooks in our React codebase.
Handle:
- State conversion to useState
- Lifecycle methods to useEffect
- Refs to useRef
- Context consumers to useContext
- Preserve all existing behavior"

AI generates transformation script:

// scripts/class-to-function.ts
import { Transform } from 'jscodeshift';

const transform: Transform = (file, api) => {
  const j = api.jscodeshift;
  const root = j(file.source);

  // Find class components
  root.find(j.ClassDeclaration)
    .filter(path => {
      // Check if extends React.Component
      const superClass = path.node.superClass;
      return superClass &&
        (superClass.name === 'Component' ||
         superClass.property?.name === 'Component');
    })
    .forEach(path => {
      const className = path.node.id.name;
      const body = path.node.body.body;

      // Extract state
      const stateProperty = body.find(m =>
        m.type === 'ClassProperty' && m.key.name === 'state'
      );

      // Extract lifecycle methods
      const componentDidMount = body.find(m =>
        m.key.name === 'componentDidMount'
      );

      // Build functional component
      const funcComponent = j.variableDeclaration('const', [
        j.variableDeclarator(
          j.identifier(className),
          j.arrowFunctionExpression(
            [j.identifier('props')],
            buildFunctionBody(stateProperty, componentDidMount, body)
          )
        )
      ]);

      // Replace class with function
      j(path).replaceWith(funcComponent);
    });

  return root.toSource();
};

// Usage: npx jscodeshift -t scripts/class-to-function.ts src/

Batch File Updates

claude "Update all API calls from axios to fetch API.
Process all files in src/services/ and src/api/.
Preserve error handling and type safety."

// Before: axios pattern
import axios from 'axios';

const response = await axios.get('/api/users', {
  headers: { Authorization: `Bearer ${token}` }
});
const users = response.data;

// After: fetch pattern
const response = await fetch('/api/users', {
  headers: { Authorization: `Bearer ${token}` }
});
if (!response.ok) throw new Error(`HTTP ${response.status}`);
const users = await response.json();

AI tracks progress:

## Migration Progress

Files processed: 45/52
├── src/services/ - 23/23 complete ✓
├── src/api/ - 15/18 complete
│   ├── users.ts ✓
│   ├── orders.ts ✓
│   ├── payments.ts - In progress
│   ├── analytics.ts - Pending
│   └── webhooks.ts - Pending
└── src/utils/ - 7/11 complete

Errors encountered: 2
- src/api/legacy.ts - Uses axios interceptors (needs manual review)
- src/utils/retry.ts - Custom axios wrapper (skip or rewrite?)

Safe Refactoring Patterns

The Parallel Change Pattern

claude "Implement a parallel change to migrate our
authentication from JWT to session-based auth.
Maintain backward compatibility during migration."

// Step 1: Create abstraction
interface AuthProvider {
  authenticate(credentials: Credentials): Promise<AuthResult>;
  validateToken(token: string): Promise<User | null>;
  logout(userId: string): Promise<void>;
}

// Step 2: Wrap existing implementation
class JWTAuthProvider implements AuthProvider {
  // ... existing JWT logic
}

// Step 3: Create new implementation
class SessionAuthProvider implements AuthProvider {
  // ... new session-based logic
}

// Step 4: Feature flag controlled switching
class HybridAuthProvider implements AuthProvider {
  constructor(
    private jwt: JWTAuthProvider,
    private session: SessionAuthProvider
  ) {}

  async authenticate(credentials: Credentials): Promise<AuthResult> {
    // Write to both during migration
    const jwtResult = await this.jwt.authenticate(credentials);
    const sessionResult = await this.session.authenticate(credentials);

    // Return based on feature flag
    return flags.useSessionAuth ? sessionResult : jwtResult;
  }

  async validateToken(token: string): Promise<User | null> {
    // Try session first, fallback to JWT
    const sessionUser = await this.session.validateToken(token);
    if (sessionUser) return sessionUser;

    return this.jwt.validateToken(token);
  }
}

// Step 5: Gradual rollout via feature flags
// Step 6: Remove JWT provider after full migration

Branch by Abstraction

claude "Use branch by abstraction to replace our
payment processor from Stripe to Adyen without
disrupting production."

Verification and Rollback

Comprehensive Testing

claude "Generate a test suite that verifies the refactoring
preserved all existing behavior. Include:
1. Unit tests for changed functions
2. Integration tests for affected flows
3. Snapshot tests for component output
4. Contract tests for API changes"

Rollback Planning

claude "Create a rollback plan for the User→Account migration.
Include database scripts, feature flags, and verification steps."

## Rollback Plan: User → Account Migration

### Pre-requisites
- Feature flag `ACCOUNT_MIGRATION` controls new code paths
- Database maintains both user_id and account_id temporarily
- API serves both /users and /accounts endpoints

### Rollback Steps

1. **Disable Feature Flag**
   ```bash
   feature-flags set ACCOUNT_MIGRATION false

2. Verify Traffic Routing

   # Check API responses use "user" field
   curl /api/users | jq '.user'
   

3. Database Verification

   -- Ensure no orphaned accounts
   SELECT COUNT(*) FROM accounts
   WHERE user_id IS NULL;
   

4. Cache Invalidation

   redis-cli FLUSHDB
   

5. Monitor Error Rates

   • Watch dashboard for 15 minutes
   • Expected: Error rate < 0.1%

Post-Rollback Cleanup

• Keep dual-write enabled for 24 hours
• Analyze logs for migration issues
• Plan retry after fixes

## Module Summary

You've learned:
- Root cause analysis techniques with AI
- Performance optimization across the stack
- Safe large-scale refactoring patterns

Next, we'll cover production deployment workflows—taking your AI-assisted code to production safely.
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