React 18 vs React 19 (RC): Key Differences and Migration Tips with Examples

React, the popular JavaScript library for building user interfaces, continues to evolve with each new release. In this blog post, we'll explore the key differences between React 18 and the upcoming React 19 (currently in Release Candidate stage), provide examples of new features, and offer migration tips for developers using React with Vite.

Table of Contents

1. Introduction
2. Current Status of React 19
3. Key Differences with Examples
   • Improved Server-Side Rendering
   • Enhanced Concurrent Rendering
   • New Hooks and APIs
   • Performance Optimizations
4. Migration Tips
5. Using React 19 RC with Vite
6. Conclusion

Introduction

React 18 introduced significant changes, including automatic batching, new APIs for concurrent rendering, and transitions. React 19, while still in development, aims to build upon these foundations with further improvements and new features.

Current Status of React 19

As of September 2024, React 19 is in the Release Candidate (RC) stage. It's feature-complete and ready for testing but not yet recommended for production use. Features and APIs may still change before the final release.

Key Differences with Examples

Let's dive into the key improvements and new features expected in React 19, with examples and comparisons to React 18 where applicable.

Improved Server-Side Rendering

1. Enhanced Streaming SSR

React 19 aims to optimize streaming SSR further. While the API might remain similar to React 18, the performance improvements should be noticeable.

Example (similar in both React 18 and 19):

// server.js
import { renderToPipeableStream } from 'react-dom/server';

app.get('/', (req, res) => {
  const { pipe } = renderToPipeableStream(<App />, {
    bootstrapScripts: ['/client.js'],
    onShellReady() {
      res.statusCode = 200;
      res.setHeader('Content-type', 'text/html');
      pipe(res);
    },
  });
});

1. Refined Selective Hydration

React 19 is expected to improve the selective hydration introduced in React 18.

Example in React 19 (syntax might be similar to React 18, but with improved behavior):

import { Suspense } from 'react';

function App() {
  return (
    <Suspense fallback={<Loading />}>
      <MainContent />
      <Suspense fallback={<SidebarLoading />}>
        <Sidebar />
      </Suspense>
    </Suspense>
  );
}

In this example, React 19 might provide smoother hydration, prioritizing the MainContent component while the Sidebar loads.

1. Server Components

React 19 is expected to include a more stable implementation of Server Components.

Example of a Server Component in React 19:

// Note: This syntax is speculative and may change
'use server';

import { db } from './database';

async function UserProfile({ userId }) {
  const user = await db.user.findUnique({ where: { id: userId } });
  return <div>{user.name}</div>;
}

export default UserProfile;

In this example, the UserProfile component runs on the server, allowing direct database access without exposing sensitive information to the client.

Enhanced Concurrent Rendering

1. Improved Suspense

React 19 is enhancing the Suspense component with better fallback handling.

React 18 example:

function ProfilePage({ userId }) {
  return (
    <Suspense fallback={<h1>Loading profile...</h1>}>
      <ProfileDetails userId={userId} />
      <Suspense fallback={<h2>Loading posts...</h2>}>
        <ProfileTimeline userId={userId} />
      </Suspense>
    </Suspense>
  );
}

Potential React 19 improvement (speculative):

function ProfilePage({ userId }) {
  return (
    <Suspense
      fallback={<h1>Loading profile...</h1>}
      primaryContent={<ProfileDetails userId={userId} />}
    >
      <ProfileTimeline userId={userId} />
    </Suspense>
  );
}

In this speculative React 19 example, the primaryContent prop might allow developers to specify which content should be prioritized during loading.

1. Extended Automatic Render Batching

React 18 introduced automatic batching for setState and hooks. React 19 might extend this to more scenarios.

React 18 example:

function Counter() {
  const [count, setCount] = useState(0);

  function handleClick() {
    setCount(c => c + 1); // Does not re-render yet
    setCount(c => c + 1); // Does not re-render yet
    // React will only re-render once at the end (that's batching!)
  }

  return <button onClick={handleClick}>{count}</button>;
}

React 19 might extend this batching to more scenarios, potentially including asynchronous operations.

1. Fine-tuned Priority-based Rendering

React 19 might introduce more granular control over rendering priorities.

Potential React 19 example (speculative):

import { useDeferredValue, startTransition } from 'react';

function SearchResults({ query }) {
  const deferredQuery = useDeferredValue(query);

  return (
    <>
      <div>Searching for: {query}</div>
      <Suspense fallback={<Spinner />}>
        <Results query={deferredQuery} />
      </Suspense>
    </>
  );
}

function handleSearch(input) {
  startTransition(() => {
    setSearchQuery(input);
  });
}

In this example, React 19 might provide more fine-grained control over how different parts of the UI update in response to user input.

New Hooks and APIs

1. useEvent Hook

React 19 is expected to introduce the useEvent hook to solve stale closure problems.

React 18 problem:

function ChatRoom({ roomId }) {
  const [message, setMessage] = useState('');

  function handleSend() {
    // This might use a stale `roomId` if the component re-renders
    sendMessage(roomId, message);
  }

  return <button onClick={handleSend}>Send</button>;
}

Potential React 19 solution with useEvent:

function ChatRoom({ roomId }) {
  const [message, setMessage] = useState('');

  const handleSend = useEvent(() => {
    // This will always use the current `roomId`
    sendMessage(roomId, message);
  });

  return <button onClick={handleSend}>Send</button>;
}

1. Improved Context API

React 19 may include improvements to the Context API to address performance concerns.

React 18 example:

const ThemeContext = React.createContext('light');

function App() {
  const [theme, setTheme] = useState('light');
  return (
    <ThemeContext.Provider value={theme}>
      <Header />
      <Main />
      <Footer />
    </ThemeContext.Provider>
  );
}

Potential React 19 improvement (speculative):

const ThemeContext = React.createContext('light', (prev, next) => prev === next);

function App() {
  const [theme, setTheme] = useState('light');
  return (
    <ThemeContext.Provider value={theme}>
      <Header />
      <Main />
      <Footer />
    </ThemeContext.Provider>
  );
}

In this speculative example, the context might include a comparison function to prevent unnecessary re-renders.

Performance Optimizations

While many performance optimizations happen under the hood, some might be visible to developers:

1. Improved Diffing Algorithm

React 19 is expected to optimize the reconciliation process. This might not require changes to your code but could result in faster updates for complex UIs.

1. Memory Usage Improvements

React 19 may include optimizations to reduce memory usage. Again, this might not require code changes but could improve performance, especially for large applications.

1. Better Tree Shaking

React 19 might improve tree shaking capabilities. This could result in smaller bundle sizes when using build tools like Vite.

Example vite.config.js that might better leverage React 19's tree shaking:

import { defineConfig } from 'vite'
import react from '@vitejs/plugin-react'

export default defineConfig({
  plugins: [react()],
  build: {
    rollupOptions: {
      output: {
        manualChunks(id) {
          if (id.includes('node_modules')) {
            return 'vendor';
          }
        }
      }
    }
  }
})

Migration Tips

1. Stay Informed: Keep an eye on the official React blog and documentation for updates.
2. Experiment in Non-Production Environments: Try the React 19 RC in development or staging environments.
3. Review Deprecated APIs: Check the documentation for any deprecated APIs and plan updates accordingly.
4. Leverage New Features Gradually: Implement new features in non-critical parts of your application first.
5. Optimize Rendering: Review your component structure and use of Suspense boundaries.
6. Comprehensive Testing: Thoroughly test your application, especially areas relying on React's internal APIs.

Using React 19 RC with Vite

To experiment with the React 19 Release Candidate using Vite:

1. Create a new Vite project:

   npm create vite@latest my-react-19-rc-app -- --template react

1. Navigate to the project directory:

   cd my-react-19-rc-app

1. Install the React 19 RC versions:

   npm install react@rc react-dom@rc

1. Update your vite.config.js:

   import { defineConfig } from 'vite'
   import react from '@vitejs/plugin-react'

   export default defineConfig({
     plugins: [react()],
     esbuild: {
       jsxInject: `import React from 'react'`
     },
     optimizeDeps: {
       include: ['react', 'react-dom']
     }
   })

1. Start the development server:

   npm run dev

Remember, using the RC version in production is not recommended.

Conclusion

While React 19 is still in the Release Candidate stage, it promises exciting improvements and new features. From enhanced server-side rendering to new hooks and performance optimizations, there's much to explore in React 19.

As the release date approaches, stay tuned to the official React documentation and community resources for the most up-to-date information. By staying informed and gradually adopting new features as they become stable, you'll be well-positioned to leverage the improvements in React 19 for your projects.