Summary

React 19 introduces runtime and JSX improvements that require a few breaking changes. Most apps should be unaffected, but follow a staged upgrade: first move to React 18.3 (adds warnings), enable the modern JSX transform, update packages/types, run the React 19 codemods, and then migrate removed APIs (createRoot/hydrateRoot, removed legacy APIs, test-utils changes).

Key Points

• Recommended upgrade path:

  • Upgrade to React 18.3 to surface deprecation warnings before moving to React 19.
  • Ensure the new JSX transform is enabled (required in React 19).
  • Install React 19 and React DOM 19: npm install --save-exact react@^19.0.0 react-dom@^19.0.0 (or yarn add --exact ...).
  • If you use TypeScript: update types with npm install --save-exact @types/react@^19.0.0 @types/react-dom@^19.0.0.

• Use codemods to automate changes:

  • Run the React 19 codemod recipe: npx codemod @latest react/19/migration-recipe (runs replace-reactdom-render, replace-string-ref, replace-act-import, replace-use-form-state, prop-types-typescript).
  • Individual codemods available in the react-codemod repo (e.g., npx codemod @latest react/prop-types-typescript, npx codemod @latest react/19/replace-string-ref).

• Major breaking changes to handle:

  • Error handling: errors thrown in render are no longer re-thrown; uncaught errors are reported to window.reportError and caught errors to console.error. Use createRoot/hydrateRoot options onUncaughtError and onCaughtError for custom reporting.
  • Removed/changed React APIs: propTypes checks removed from React (migrate to TypeScript or external prop-types), defaultProps removed for function components (use ES6 defaults), legacy context (contextTypes/getChildContext) removed, string refs removed (use callback or createRef), module pattern factories and createFactory removed.
  • React DOM changes: ReactDOM.render, ReactDOM.hydrate, ReactDOM.unmountComponentAtNode, and ReactDOM.findDOMNode removed. Migrate to createRoot(...) + root.render(...), hydrateRoot(...), root.unmount(), and DOM refs respectively.
  • Testing updates: act moved to the react package (import { act } from 'react'). Many react-dom/test-utils helpers were removed; prefer testing-library approaches and update tests accordingly.

• Practical checklist for engineers:

  • Run your test suite after enabling the new JSX transform.
  • Run the codemod recipe and review changes in PRs.
  • Replace deprecated APIs listed above and update error-reporting hooks if you relied on re-thrown render errors.
  • Update type packages and fix any TypeScript signature changes.

Useful commands (examples)

• Install React 19: npm install --save-exact react@^19.0.0 react-dom@^19.0.0
• Update types: npm install --save-exact @types/react@^19.0.0 @types/react-dom@^19.0.0
• Run codemod recipe: npx codemod @latest react/19/migration-recipe
• Codemods for targeted migrations: npx codemod @latest react/19/replace-string-ref, npx codemod @latest react/19/replace-reactdom-render, npx codemod @latest react/prop-types-typescript.

Next steps

• Test your app thoroughly and report issues when testing React 19. See the React 19 release notes and react-codemod repo for details and additional codemods.