React 19 New Features and Breaking Changes

Introduction

React 19 is one of the most anticipated updates in the React ecosystem, bringing significant performance improvements, new features, and some breaking changes that developers need to be aware of. This release focuses on enhancing developer experience, optimizing rendering performance, and introducing new hooks and APIs that simplify complex use cases.

In this post, we’ll explore the key features of React 19, discuss breaking changes, and provide practical examples to help you transition smoothly. Whether you're a React developer or a tech lead evaluating the upgrade, this guide will help you understand what’s new and how to leverage these improvements effectively.

New Features in React 19

1. Server Components (Stable Release)

React 19 marks the stable release of Server Components, a feature that was previously experimental. Server Components allow parts of your React application to be rendered on the server, reducing client-side JavaScript and improving performance.

Here’s how you can define a Server Component:

// app/page.server.js  
export default function ServerComponent() {  
  return <div>This component is rendered on the server!</div>;  
}

Key benefits:

• Reduced bundle size – Only the necessary JavaScript is sent to the client.
• Faster initial load – Server-rendered content appears instantly.
• Seamless integration – Works alongside Client Components without hydration issues.

2. Actions API (Simplified Data Mutations)

The new Actions API simplifies handling form submissions and data mutations. Instead of manually managing states and effects, you can now use useActionState and useFormStatus hooks to streamline async operations.

Example of a form with Actions:

import { useActionState, useFormStatus } from 'react';  

async function submitForm(prevState, formData) {  
  const name = formData.get('name');  
  // Simulate API call  
  await new Promise(resolve => setTimeout(resolve, 1000));  
  return { message: `Hello, ${name}!` };  
}  

function SubmitButton() {  
  const { pending } = useFormStatus();  
  return <button disabled={pending}>{pending ? 'Submitting...' : 'Submit'}</button>;  
}  

export default function Form() {  
  const [state, formAction] = useActionState(submitForm, null);  
  return (  
    <form action={formAction}>  
      <input type="text" name="name" />  
      <SubmitButton />  
      {state?.message && <p>{state.message}</p>}  
    </form>  
  );  
}

3. Asset Loading Improvements

React 19 introduces built-in support for preloading assets (images, fonts, scripts) during rendering, reducing layout shifts and improving user experience. The new React.preload() function ensures resources are fetched early.

Example:

import { preload } from 'react';  

// Preload a critical image  
preload('/hero-image.jpg', { as: 'image' });  

function HomePage() {  
  return <img src="/hero-image.jpg" alt="Hero" />;  
}

Breaking Changes in React 19

1. Removal of Legacy Context API

The legacy Context API (contextTypes, childContextTypes) has been removed in favor of the modern createContext and useContext pattern. If you’re still using the old API, migrate to the new one:

// Old way (deprecated)  
class MyComponent extends React.Component {  
  static childContextTypes = { theme: PropTypes.string };  
  getChildContext() { return { theme: 'dark' }; }  
}  

// New way  
const ThemeContext = React.createContext('dark');  
function App() {  
  return (  
    <ThemeContext.Provider value="dark">  
      <ChildComponent />  
    </ThemeContext.Provider>  
  );  
}

2. Strict Mode Enhancements

React 19’s Strict Mode now detects and warns about more unsafe patterns, such as:

• Undefined ref access in effects.
• Improper cleanup in useEffect.
• Deprecated lifecycle methods.

Enable Strict Mode in your app to catch these issues early:

import { StrictMode } from 'react';  

ReactDOM.createRoot(document.getElementById('root')).render(  
  <StrictMode>  
    <App />  
  </StrictMode>  
);

Conclusion

React 19 brings exciting new features like Server Components, the Actions API, and asset preloading, making it easier to build high-performance applications. However, developers must also prepare for breaking changes, such as the removal of the legacy Context API and stricter checks in Strict Mode.

To adopt React 19 smoothly:

1. Test your app in Strict Mode to catch deprecations.
2. Migrate from legacy APIs to modern equivalents.
3. Leverage Server Components for optimized rendering.

By staying ahead of these changes, you can take full advantage of React 19’s improvements while maintaining a stable codebase. Happy coding!