React 18 to React 19 Migration Guide - AI Summary

Configuration

Prerequisites

• Node.js: 16+ minimum, 18+ recommended (Node 14 will cause weird errors)
• TypeScript: 4.7+ required (older versions throw fits about useRef changes)
• JSX Transform: New transform required (automatic runtime)

Production-Ready Settings

# Exact versions to avoid surprise breakage
npm install --save-exact react@^19.1.1 react-dom@^19.1.1 @types/react@^19.0.0 @types/react-dom@^19.0.0

# JSX Transform Configuration
{
  "presets": [
    ["@babel/preset-react", { "runtime": "automatic" }]
  ]
}

Critical Bundler Requirements

• Webpack 4: Don't attempt migration - upgrade to Webpack 5 first
• Vite: JSX automatic by default
• Next.js: Automatic since v11
• Create React App: Supported since v4.0

Resource Requirements

Timeline Reality Check

Project Size	Official Estimate	Actual Time Required
Small (<50 components)	2-4 hours	2-3 days (TypeScript issues)
Medium (50-200 components)	1-2 days	1-2 weeks (dependency hell)
Large (200+ components)	3-5 days	2-3 weeks (legacy API cleanup)
Enterprise	1-2 weeks	2-4+ weeks (testing, CI, deployment)

Expertise Requirements

• Essential: Deep React knowledge, TypeScript proficiency
• Critical: Debugging skills for cryptic dependency errors
• Helpful: Experience with codemods and AST transformations

Cost Factors

• Hidden Time Sink: Fixing third-party dependencies using React internals
• Testing Overhead: All tests require updates (act imports, assertion changes)
• CI/CD Updates: Node.js version bumps, timeout adjustments

Critical Warnings

Breaking Changes That Will Explode Your App

Removed APIs (Zero Tolerance)

// These will crash your app immediately
ReactDOM.render(<App />, container); // ❌ REMOVED
ReactDOM.unmountComponentAtNode(container); // ❌ REMOVED
useRef(); // ❌ TypeScript error - requires argument

Silent Failures (More Dangerous)

// These fail silently in production
Button.propTypes = { ... }; // ❌ IGNORED on function components
findDOMNode(component); // ❌ REMOVED

Dependency Compatibility Hell

Libraries Known to Break:

• React Router: Needs v6.8+
• Styled Components: v5.3.6+ required
• Formik: Notorious for React internals usage
• react-beautiful-dnd: Often broken, check latest
• Any library using SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED

Detection Command:

grep -r "SECRET_INTERNALS" node_modules/*/
npm ls react # Shows version conflicts

TypeScript Breaking Changes

// Before: Permissive
const ref = useRef(); // Worked by accident

// After: Strict enforcement
const ref = useRef<HTMLDivElement>(null); // Required

Testing Framework Carnage

• react-test-renderer: Deprecated - migrate to Testing Library
• act import moved: From react-dom/test-utils to react
• Hydration error messages changed: Update error assertions

Decision Criteria

When to Upgrade

Green Light Indicators:

• React 18.3 upgrade successful (includes React 19 warnings)
• All dependencies React 19 compatible
• TypeScript 4.7+ in use
• Modern JSX transform enabled

Red Flag Warnings:

• Critical dependencies not updated in 6+ months
• Webpack 4 still in use
• Node.js 14 or below
• Heavy reliance on deprecated APIs

Worth It Despite Issues

Benefits That Justify Pain:

• Actions API: Eliminates manual form state management
• React Compiler: Automatic memoization and performance
• Better error messages: Improved debugging experience
• Server Components: Enhanced SSR capabilities

Not Worth It If:

• App works fine and team has limited React expertise
• Multiple critical dependencies abandoned
• Tight deadline with no testing time

Implementation Strategy

Mandatory Pre-Migration Steps

1. Upgrade to React 18.3 first (reveals breaking changes)
2. Backup everything (git branch + database if using SSR)
3. Update Node.js and bundler (avoid double-debugging)
4. Audit dependencies (identify React internals users)

Migration Execution Order

# 1. Core React upgrade
npm install --save-exact react@^19.1.1 react-dom@^19.1.1

# 2. Run automated codemods
npx codemod@latest react/19/migration-recipe
npx types-react-codemod@latest preset-19 ./src

# 3. Manual fixes (high failure rate)
# - useRef arguments
# - Ref callback return types
# - PropTypes removal
# - Test imports

# 4. Verification
npm test
npm run lint
npm run typecheck

Emergency Rollback Strategy

{
  "overrides": {
    "broken-library": { "react": "^18.3.0" }
  }
}

Failure Modes and Solutions

Common Breakage Patterns

Third-Party Dependency Explosion

Symptoms: Cannot read properties of undefined, module resolution errors
Root Cause: Libraries hacking React internals
Solution: Update libraries or use package overrides for temporary fixes

TypeScript Type System Rebellion

Symptoms: useRef argument errors, ref callback type mismatches
Root Cause: Stricter type enforcement in React 19
Solution: Run type codemods, add explicit type annotations

Form Submission Conflicts

Symptoms: Double submissions, pending state conflicts
Root Cause: Mixing useActionState with manual state management
Solution: Choose one pattern - either Actions API or manual handling

Performance Degradation

Symptoms: Slower rendering, increased re-renders
Root Cause: Strict Mode behavior changes exposing side effects
Solution: Move side effects from render to useEffect

Testing Infrastructure Failures

Act Import Breakage: Update import { act } from 'react'
Renderer Deprecation: Replace react-test-renderer with Testing Library
Hydration Assertion Failures: Update error message expectations

Production Deployment Risks

CI Node.js Version: Update to 18+ (CI often lags behind)
Bundle Size Increase: Check for duplicate React versions
SSR Hydration Errors: Add proper client-only guards

Success Metrics

Verification Checklist

• App loads without console errors
• All forms submit correctly
• Tests pass with new imports
• TypeScript compiles without errors
• Bundle size maintained or reduced
• Performance metrics stable
• SSR hydration clean

Performance Indicators

• React DevTools Profiler shows no new performance regressions
• Core Web Vitals maintained
• Memory usage stable
• Bundle analysis shows no duplicate React versions

This migration is worth doing for the improved DX and future-proofing, but requires significant planning and testing time. Don't underestimate the effort required.