Migrating a Legacy React Codebase with AI Assistance

This case study details how a mid-sized fintech company successfully migrated their 5-year-old React codebase from class components to modern function components with hooks, using Sweet! CLI as their primary migration tool. The project involved over 400 components and was completed in 6 weeks with minimal disruption to ongoing development.

The Challenge

Company: FinFlow Solutions (pseudonym), a financial technology provider
Codebase: React application with 5+ years of development history
Size: 400+ class components, 150k+ lines of code
Team: 8 developers with varying React experience levels
Timeline: Needed migration completed within 2 months

Key Technical Debt Issues

• Mix of class component patterns (some using `createReactClass`, some ES6 classes)
• Inconsistent lifecycle method usage
• State management spread across components with no clear patterns
• Minimal test coverage (only 30% of components had tests)
• Dependencies on deprecated React APIs

Why Traditional Migration Approaches Were Rejected

The team initially considered three approaches:

Migration Strategy with Sweet! CLI

Phase 1: Assessment and Planning (Week 1)

The team started by having Sweet! CLI analyze the entire codebase:

sweet "Analyze our React codebase and provide:
1. Breakdown of class component types
2. Common patterns that need special handling  
3. Estimate of migration complexity per component category
4. Recommended migration order"

Key Findings:

• Components fell into 5 distinct categories by complexity
• 15% had complex lifecycle dependencies requiring special attention
• Recommendation: Start with simple presentational components, progress to complex stateful ones

Phase 2: Creating Migration Templates (Week 2)

Instead of migrating components one-by-one, the team created migration templates for each component category:

sweet "Create a migration template for simple presentational class components.
The template should:
1. Convert class to function component
2. Move props from this.props to destructured parameters  
3. Remove constructor if present
4. Convert simple state to useState
5. Update any lifecycle methods to useEffect
6. Preserve propTypes and defaultProps"

Sweet! CLI generated 5 migration templates that handled 80% of the codebase automatically.

Phase 3: Batch Migration with Validation (Weeks 3-4)

The team used Sweet! CLI's batch processing capability:

sweet "Migrate all components in src/components/ui/ using template #1.
Validate each migration by:
1. Running existing tests
2. Checking TypeScript compilation  
3. Ensuring no runtime errors in development server"

Results:

• 320 components migrated automatically (80% success rate)
• 64 components required manual review due to edge cases
• 16 components needed complete manual rewrite

Phase 4: Complex Component Handling (Week 5)

For components with complex lifecycle dependencies, the team used iterative refinement:

# First pass: Basic conversion
sweet "Convert UserDashboard class component to function component"

# Second pass: Handle specific lifecycle methods  
sweet "Convert componentDidMount and componentWillUnmount to useEffect with cleanup"

# Third pass: Optimize performance
sweet "Optimize useEffect dependencies and add useCallback where needed"

Phase 5: Testing and Validation (Week 6)

Sweet! CLI assisted with test migration and creation:

sweet "Migrate tests for PaymentForm component from Enzyme to React Testing Library.
Update assertions to work with function components."

The team also used Sweet! CLI to generate missing tests:

sweet "Generate comprehensive unit tests for the migrated AuthProvider component.
Cover all public methods and edge cases."

Results and Metrics

Quantitative Results

Qualitative Benefits

• Reduced cognitive load: Developers no longer needed to context-switch between class and function component patterns
• Improved onboarding: New hires could contribute immediately using modern React patterns
• Enhanced maintainability: Consistent code patterns made debugging and feature addition faster
• Future-proofing: Ready for React concurrent features and future updates

Lessons Learned

What Worked Well

1. Template-based approach: Creating migration templates for component categories dramatically increased efficiency
2. Incremental validation: Validating each batch before proceeding caught issues early
3. Team training: Spending the first week training the team on Sweet! CLI paid off in productivity
4. Backup strategy: Maintaining git branches for each migration phase allowed safe rollbacks

Challenges and Solutions

Recommendations for Similar Migrations

1. Start with analysis: Let Sweet! CLI assess your codebase before planning
2. Create templates: Don't migrate components individually; create templates for categories
3. Validate incrementally: Test each batch thoroughly before proceeding
4. Involve the team: Train developers on Sweet! CLI early in the process
5. Measure results: Track metrics before and after to demonstrate value

Conclusion

The migration was completed in 6 weeks (50% faster than the most optimistic manual estimate) with higher quality results. The team reported that Sweet! CLI handled approximately 85% of the migration automatically, with the remaining 15% requiring human oversight for complex edge cases.