React 19 Migration Guide - AI-Optimized Technical Reference

Critical Breaking Changes

Removed APIs (Hard Failures)

• ReactDOM.render: Deleted completely - throws TypeError: ReactDOM.render is not a function
• findDOMNode: Removed - causes DOM access failures
• String refs: Removed - components crash with undefined property errors
• Legacy Context API: Removed - apps using contextTypes/getChildContext fail completely
• PropTypes: Removed from function components

Behavioral Changes

• Error Handling: Errors go to window.reportError instead of re-throwing (breaks Sentry integration)
• Strict Mode: Enhanced strictness surfaces hidden bugs
• JSX Transform: Requires new JSX transform from 2020
• useRef TypeScript: Requires initial value argument

Pre-Migration Assessment

Failure Detection Strategy

npm install react@18.3 react-dom@18.3
npm start
# Check browser console for warnings

Risk Assessment:

• Zero warnings = Migration likely successful (1-2 days)
• 10-50 warnings = Moderate complexity (1-2 weeks)
• 50+ warnings = High risk, clear calendar (2-6 weeks)

Dependency Compatibility Matrix

Library	Minimum Version	Status	Migration Impact
React Router	v7+	v6 breaks on concurrent rendering	High
Material-UI	v6+	v5 ref forwarding broken	High
Redux	Redux Toolkit 2.x	Old Redux incompatible	Medium
Testing Library	v16+	async act() changed	Medium
Create React App	Deprecated	Must migrate to Vite/Next.js	High

Migration Implementation

Phase 1: Automated Fixes (80% Coverage)

# Core migration
npx codemod@latest react/19/migration-recipe

# TypeScript updates
npx types-react-codemod@latest preset-19 ./src

What Codemods Fix:

• ReactDOM.render → createRoot
• String refs → callback refs
• Import path updates
• Basic TypeScript type changes

What Codemods Miss:

• Legacy Context API (manual conversion required)
• PropTypes cleanup
• Complex ref forwarding patterns
• Custom error boundary updates

Phase 2: Manual Code Changes

Legacy Context Migration (Critical)

// Before (breaks completely)
static childContextTypes = { theme: PropTypes.string };
getChildContext() { return { theme: 'dark' }; }

// After (required)
const ThemeContext = React.createContext('light');
<ThemeContext.Provider value="dark">{children}</ThemeContext.Provider>

Error Handling Configuration

const root = createRoot(container, {
  onUncaughtError: (error, errorInfo) => {
    Sentry.captureException(error, { extra: errorInfo });
  },
  onCaughtError: (error, errorInfo) => {
    console.error('Caught error:', error, errorInfo);
  }
});

useRef Type Updates (TypeScript)

// Before (breaks)
const ref = useRef<HTMLInputElement>();

// After (required)
const ref = useRef<HTMLInputElement>(null);

Deployment Strategy

Risk Mitigation Approach

1. Feature Flag Implementation: Gradual rollout with instant killswitch
2. Staging Environment: Must mirror production exactly (Node version, data, traffic)
3. Rollback Plan: Package.json revert + redeploy (5-10 minute recovery)

Critical Monitoring Points

• Error rates (watch for 5% → 50% spikes in 3 minutes)
• Bundle size changes (should decrease with React Compiler)
• Memory leak detection (ref forwarding performance issues)
• Server-side rendering compatibility

Resource Requirements

Time Investment by App Size

• Small app: 1-2 days (ideal) / 1 week (realistic)
• Medium app: 2-4 weeks
• Enterprise app: 2-6 weeks (double all estimates)

Expertise Requirements

• React internals knowledge: Essential for error boundary updates
• TypeScript proficiency: Required for type system changes
• Build tool configuration: Needed for JSX transform updates
• Error monitoring setup: Critical for production stability

Common Failure Modes

High-Risk Scenarios

1. Third-party library incompatibility: Can block entire migration for months
2. Legacy Context API usage: Requires complete architectural changes
3. Custom error boundary logic: Silent failures in production
4. Testing framework integration: Jest spy counts and timing assertions break

Breaking Points

• UI Performance: Concurrent rendering changes component timing
• Memory Usage: Ref forwarding patterns can cause leaks under load
• Error Reporting: Monitoring tools lose visibility without configuration updates
• Bundle Size: Some libraries bundle both React versions during transition

Success Criteria

Validation Checklist

• Zero console errors in production
• Error monitoring tools receiving data correctly
• Performance metrics maintained or improved
• Full test suite passing
• Rollback procedure tested and documented

Performance Expectations

• Bundle size: 10-15% reduction with React Compiler
• Initial load: Improved with better SSR
• Runtime performance: Automatic optimizations from compiler
• Memory usage: Should be stable or improved

Recovery Procedures

Immediate Response (Production Failures)

1. Feature flag killswitch: Instant traffic redirect to React 18
2. Package rollback: 5-10 minute recovery time
3. API compatibility maintenance: Keep endpoints backward compatible

Post-Incident Analysis

• Document specific failure patterns
• Update migration checklist
• Adjust monitoring thresholds
• Plan incremental retry strategy

Long-term Optimization

React Compiler Integration

• Enable experimental compiler for automatic memoization
• Remove manual React.memo, useMemo, useCallback where appropriate
• Monitor performance impact before removing optimizations

Asset Loading Improvements

• Implement preload and preinit for critical resources
• Leverage enhanced Suspense capabilities
• Adopt server component patterns where applicable

This migration represents a forced breaking change with significant operational risk but substantial long-term performance benefits when executed correctly.