React 19 - Key Changes

This skill focuses on what changed in React 19. Not a complete React reference.

Coming from React 16/17?

If upgrading from pre-18 versions, these changes accumulated and are now mandatory:

Change	Introduced	React 19 Status
`createRoot` / `hydrateRoot`	React 18	Required (`ReactDOM.render` removed)
Concurrent rendering	React 18	Foundation for all R19 features
Automatic batching	React 18	Default behavior
`useId`, `useSyncExternalStore`	React 18	Stable, commonly used
Hooks (no classes for new code)	React 16.8	Only path for new features
`createContext` (not legacy)	React 16.3	Required (legacy Context removed)
Error Boundaries	React 16	Now with better error callbacks

Migration path: Upgrade to React 18.3 first (shows deprecation warnings), then to 19.