Exploring React 19: Features, Deprecations, and Breaking Changes

React 19 is here, bringing a wealth of new features, improvements, and breaking changes. This release redefines how developers handle state management, forms, and server-side rendering while deprecating legacy APIs. If you’re planning to upgrade, buckle up for a transformative journey. This guide will walk you through the highlights of React 19, with tips to ensure a smooth transition.

Highlights of React 19

New Features

1. Actions and `useActionState`

The startTransition API now supports async functions, referred to as "Actions." Actions can manage state updates, handle side effects like fetch(), and include error handling. They streamline transitions with coordinated state updates and UI rendering.

The new useActionState hook complements this by providing access to Action states, including pending and final states. It accepts a reducer for granular control, making it an essential tool for form interactions and complex state flows.

2. Optimistic Updates with `useOptimistic`

useOptimistic enables developers to set temporary state changes while a transition is ongoing, providing a smoother user experience. The state reverts or updates automatically once the async operation finishes.

3. `use` API

React 19 introduces the use API, which allows promises or contexts to be read during render. This can simplify server data fetching workflows but comes with the restriction that use can only be called within a render function.

4. `ref` as a Prop

You can now pass refs as props directly, eliminating the need for forwardRef. This change simplifies component composition and makes working with refs more intuitive.

5. Improved Suspense

Suspense now supports sibling pre-warming, which commits fallback components immediately when a sibling suspends. This enhancement boosts performance and user experience in data-heavy applications.

React DOM Client Enhancements

1. Form Actions

Forms in React are smarter with <form> action props, enabling better integration with useFormStatus. Submitting a form automatically resets its state for uncontrolled components.

2. Document Metadata and Resource Optimization

React 19 natively supports rendering document metadata, such as <title> or <meta> tags, in the component tree. Additionally, new APIs like preinit, preload, and preconnect improve resource discovery and loading times.

3. Async Scripts

You can now render async scripts anywhere in the component tree. React handles ordering and deduplication, streamlining third-party script integration.

React DOM Server

1. New Prerender APIs

The prerender and prerenderToNodeStream APIs enhance server-side rendering (SSR) by supporting streaming environments like Node.js. These APIs await data loading before generating HTML, making SSR more robust.

2. Stable React Server Components (RSC)

Server Components are now stable, allowing libraries to target React 19 as a peer dependency. This aligns with the Full-stack React architecture and enables seamless integration with frameworks like Next.js.

Deprecations and Breaking Changes

Deprecations

element.ref access: Deprecated in favor of element.props.ref.
react-test-renderer: Logs deprecation warnings; consider migrating to React Testing Library.
Legacy APIs: APIs like contextTypes, defaultProps for functions, and string refs are officially deprecated.

Breaking Changes

JSX Transform Requirement The new JSX transform is mandatory in React 19. This enables features like refs as props and improves overall performance.
Error Handling Changes Uncaught errors are now reported to window.reportError, while errors caught by boundaries are logged via console.error. New methods like onUncaughtError and onCaughtError allow customization.
Removed APIs
- ReactDOM.render and ReactDOM.hydrate: Replaced by ReactDOM.createRoot and ReactDOM.hydrateRoot.
- defaultProps for functions: Use ES6 default parameters instead.
- Legacy Context: Use the modern contextType API.
- react-dom/test-utils: Replaced with act from React core.
Removed UMD Builds UMD builds are no longer supported. Use ESM-based CDNs for script-tag usage, such as esm.sh.

Preparing for the Upgrade

1. Upgrade to React 18.3 First

React 18.3 introduces deprecation warnings for APIs removed in React 19. This intermediate step helps identify potential issues before the full upgrade.

2. Codemods and Migration Tools

Use React codemods for automating repetitive updates, such as refactoring deprecated APIs and adjusting TypeScript types.

3. TypeScript Adjustments

React 19 includes stricter TypeScript typings. For example:

ReactChild → React.ReactElement | number | string
VoidFunctionComponent → FunctionComponent

Refactor your code to align with these updates.

Tips for a Smooth Transition

Leverage Strict Mode: React 19 introduces stricter enforcement of best practices. Running your app in Strict Mode can reveal hidden bugs.
Test Early and Often: Given the breaking changes, thorough testing is critical. Transitioning from react-test-renderer to React Testing Library ensures compatibility with concurrent rendering.
Review Server Rendering: If your app uses SSR, test against React 19’s new prerender APIs and ensure smooth integration of server components.

Why React 19 Matters

React 19 is a significant leap forward, refining the developer experience while enabling powerful capabilities for modern applications. Whether you're excited about async Actions, improved Suspense, or server-side rendering enhancements, this release is packed with features to help developers build faster, more resilient apps.

Ready to dive in? Start with the React 19 Upgrade Guide and explore the full release notes.

Happy coding! 🚀