React 17 Vs 18: What Changed And Why to Consider Upgrading

React is still a great tool for building UIs, especially in modern web apps where interactivity and speed matter. With React 18 out, developers have new features, big performance wins and concurrent rendering.

In this article, we’ll go over the differences between 17 and 18, the benefits of upgrading, and real-world examples from experience and expertise.

Why the Jump from React 17 to React 18 Matters

React 17 focused heavily on improving the development process and compatibility issues without introducing breaking changes. It acted as a stepping stone for future updates. React 18, a major release, introduces concurrent rendering features, automatic batching, and new features that dramatically improve how we handle user interactions, server-side rendering, and performance optimizations.

What’s New in React 18?

React 18 brings updates aimed at improved performance and a smoother user experience. Below are the new features that differentiate it from React 17.

1. Concurrent Rendering

This is the cornerstone of React 18. It enables React to interrupt rendering work and switch between tasks, allowing multiple tasks to be handled efficiently.

The key difference between React 17 and React 18 is that React 17 used a synchronous rendering model. React 18 adopts concurrent rendering, which is more adaptive to user input and improves load responsiveness.

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Benefits:

• Better load times for the entire page

• React can pause and resume rendering

• Prepares the groundwork for future concurrent features

2. Automatic Batching

In React 17, only events like clicks or form submissions triggered batching. React 18 expands this to cover multiple tasks, reducing the number of renders.

React 17:

1setCount(c => c + 1);
2setFlag(f => !f);
3// Triggers two renders

​

React 18:

1import { flushSync } from 'react-dom';
2
3flushSync(() => {
4  setCount(c => c + 1);
5  setFlag(f => !f);
6});
7// Batched into a single render

​

You get performance improvements and fewer re-render cycles in more scenarios using automatic batching.

3. New Root API

React 18 introduces a new way to mount your app: using createRoot instead of ReactDOM.render, which is required to enable concurrent rendering.

1import ReactDOM from 'react-dom/client';
2const root = ReactDOM.createRoot(document.getElementById('root'));
3root.render(<App />);

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This is mandatory if you plan to use concurrent or new rendering features.

4. Start Transition API

For user interactions that don’t require urgent updates, you can now use startTransition() to tell React they’re low-priority:

1import { startTransition } from 'react';
2
3startTransition(() => {
4  setSearchQuery(input);
5});

​

This improves responsiveness during high-priority user input while background transitions are handled separately.

5. React 18 and Server-side Rendering

With server-side rendering, React 18 brings streaming capabilities through pipeToNodeWritable and renderToPipeableStream. This results in better performance enhancements for SSR apps.

1import { renderToPipeableStream } from 'react-dom/server';
2
3const stream = renderToPipeableStream(<App />, {
4  onShellReady() {
5    stream.pipe(response);
6  },
7});

​

React 17 supported SSR but lacked the streaming abilities now native to React 18.

Strict Mode in React 17 vs. 18

The strict mode in React 18 behaves differently. React 18 intentionally mounts components twice in development mode to help identify issues early. This might catch side effects in child components, enabling future-proofing.

1<React.StrictMode>
2  <App />
3</React.StrictMode>

​

Strict mode helps simulate concurrent rendering behavior even in development, aiding in identifying potential bugs.

Improved Support for Suspense

React 18 improves experimental suspense hook and introduces streaming SSR with Suspense on the server.

You can now use Suspense boundaries for data fetching:

1<Suspense fallback={<Loading />}>
2  <ProfileDetails />
3</Suspense>

​

This aligns well with react-router v6.4+ for data fetching and loading states across different pages in react applications.

Code Splitting and Dynamic Imports

React 18 fully supports code splitting through lazy loading and dynamic imports, improving performance optimizations.

1const LazyComponent = React.lazy(() => import('./MyComponent'));

​

Pair it with Suspense:

1<Suspense fallback={<div>Loading...</div>}>
2  <LazyComponent />
3</Suspense>

​

This ensures the entire page doesn't wait for one component to load.

React 17 vs. 18: Key Differences in Summary

React 18 brings a full suite of new features to improve user interfaces and make apps more responsive under heavy load.

Feature	React 17	React 18
Rendering Model	Synchronous rendering only	Concurrent rendering for better responsiveness
Root API	ReactDOM.render()	createRoot() via react-dom for concurrent capabilities
Batching of Updates	Manual batching for events like clicks	Automatic batching across multiple tasks and async calls
Suspense Support	Limited support (client-side only)	Experimental suspense hook + full streaming SSR support
Server-side Rendering	Basic SSR without streaming	Improved server-side rendering with renderToPipeableStream
Strict Mode Behavior	Normal mount lifecycle	Strict mode double-invokes lifecycle methods in dev for error handling
Concurrent Features	Unavailable	Includes startTransition, automatic batching, and selective rendering
Support for New APIs	Legacy APIs	Supports new features like useId, improved context API, and modern SSR
Performance Improvements	Incremental over earlier versions	Advanced performance optimizations and support for handling user interactions smoothly

Updating Your React App to React 18

1. Update react and react-dom in package.json:

1"react": "^18.2.0",
2"react-dom": "^18.2.0"

​

2. Replace ReactDOM.render() with createRoot():

1import { createRoot } from 'react-dom/client';
2const root = createRoot(document.getElementById('root'));
3root.render(<App />);

​

3. Test thoroughly in strict mode to catch bug fixes related to side effects and re-render behaviors.

4. Review your project structure and event listeners to ensure they align with the new features and concurrent mode.

Benefits of React 18 for Real-World React Applications

• Better performance optimizations through automatic batching

• Real-time concurrent rendering for interactive experiences

• Reliable error handling with double mounting

• Support for modern server-side rendering

• Fewer compatibility issues with the new Root API

• Improved state variables handling in asynchronous transitions

Whether managing a complex React native app or working with react-router, React 18 offers significant performance improvements and a more refined development process.

Frequently Used Patterns Improved in React 18

1. Return Multiple Elements

1return (
2  <>
3    <Header />
4    <Main />
5    <Footer />
6  </>
7);

​

React 18 continues to support fragments natively, simplifying structure without additional wrappers.

2. App Component with Function Signature

1function App() {
2  return <MainLayout />;
3}

​

Use the same function app pattern with updated React APIs.

Final Thoughts on React 17 vs. 18

The key differences between React 17 and 18 go beyond syntax. The shift toward concurrent rendering, automatic batching, and an updated API is not just about syntax changes but strategic performance improvements and future-proofing your application.

React 18 builds on the stability of React 17 with several bug fixes, new hooks, and expanded react-dom capabilities. The React team has focused on smoother user experiences, streamlined data fetching, and better support for low-level caching strategies. The official documentation provides migration paths to ensure a seamless transition for developers upgrading from previous versions.

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