Refactoring Legacy Code with Google Antigravity: AI-Powered Modernization Strategies

Legacy code doesn't have to be a burden. Google Antigravity's Gemini 3 excels at understanding old patterns and modernizing them safely. This guide shows you how to tackle legacy codebases confidently.

Understanding Legacy Code

What Makes Code "Legacy"?

• Outdated patterns or frameworks
• Missing documentation
• No tests
• Original developers unavailable
• Fear of making changes

Gemini 3's Advantage

Large context window (1M tokens) means Gemini 3 can:

• Understand entire legacy systems
• Trace dependencies across files
• Identify patterns and anti-patterns
• Suggest safe modernization paths

Refactoring Strategies

Strategy 1: Incremental Modernization

Don't rewrite everything at once.

You: "Analyze this file and identify the top 3 improvements that would have the biggest impact with lowest risk"

Gemini 3 response:

## Analysis: legacy-user-service.js

### Top 3 Improvements:

1. **Add TypeScript types (Low risk, High impact)**
   - Create types.ts with interfaces
   - Add JSDoc for now, migrate to .ts later
   - Risk: None - JS is valid TS

2. **Extract database queries (Medium risk, High impact)**
   - Move raw SQL to repository pattern
   - Enables testing with mocks
   - Risk: Moderate - need to test each query

3. **Replace callbacks with async/await (Medium risk, Medium impact)**
   - Already using Node 18, so supported
   - Makes error handling clearer
   - Risk: Moderate - need comprehensive tests first

Strategy 2: Strangler Fig Pattern

Gradually replace legacy with new code.

// Old code - keep working
class LegacyUserService {
  getUser(id, callback) {
    db.query(`SELECT * FROM users WHERE id = ${id}`, callback);
  }
}

// New code - modern implementation
class UserService {
  async getUser(id: string): Promise<User> {
    return this.repository.findById(id);
  }
}

// Facade - route to appropriate service
class UserServiceFacade {
  async getUser(id: string): Promise<User> {
    if (featureFlags.useNewUserService) {
      return newService.getUser(id);
    }
    return promisify(legacyService.getUser)(id);
  }
}