How to Upgrade to the React 18 Release Candidate

Tue, 08 Mar 2022 00:00:00 GMT

Our next major version, React 18, is available today as a Release Candidate (RC). As we shared at React Conf, React 18 introduces features powered by our new concurrent renderer, with a gradual adoption strategy for existing applications. In this post, we will guide you through the steps for upgrading to React 18.

If you’d like to help us test React 18, follow the steps in this upgrade guide and report any issues you encounter so we can fix them before the stable release.

Note for React Native users: React 18 will ship in React Native with the New React Native Architecture. For more information, see the React Conf keynote here.

Installing

To install the latest React 18 RC, use the @rc tag:

npm install react@rc react-dom@rc

Or if you’re using yarn:

yarn add react@rc react-dom@rc

Updates to Client Rendering APIs

When you first install React 18, you will see a warning in the console:

ReactDOM.render is no longer supported in React 18. Use createRoot instead. Until you switch to the new API, your app will behave as if it’s running React 17. Learn more: https://reactjs.org/link/switch-to-createroot

React 18 introduces a new root API which provides better ergonomics for managing roots. The new root API also enables the new concurrent renderer, which allows you to opt-into concurrent features.

// Before
import { render } from 'react-dom';
const container = document.getElementById('app');
render(<App tab="home" />, container);

// After
import { createRoot } from 'react-dom/client';
const container = document.getElementById('app');
const root = createRoot(container);
root.render(<App tab="home" />);

We’ve also changed unmountComponentAtNode to root.unmount:

// Before
unmountComponentAtNode(container);

// After
root.unmount();

We’ve also removed the callback from render, since it usually does not have the expected result when using Suspense:

// Before
const container = document.getElementById('app');
ReactDOM.render(<App tab="home" />, container, () => {
  console.log('rendered');
});

// After
function AppWithCallbackAfterRender() {
  useEffect(() => {
    console.log('rendered');
  });

return <App tab="home" />
}

const container = document.getElementById('app');
const root = ReactDOM.createRoot(container);
root.render(<AppWithCallbackAfterRender />);

Note: There is no one-to-one replacement for the old render callback API — it depends on your use case. See the working group post for Replacing render with createRoot for more information.

Finally, if your app uses server-side rendering with hydration, upgrade hydrate to hydrateRoot:

// Before
import { hydrate } from 'react-dom';
const container = document.getElementById('app');
hydrate(<App tab="home" />, container);

// After
import { hydrateRoot } from 'react-dom/client';
const container = document.getElementById('app');
const root = hydrateRoot(container, <App tab="home" />);
// Unlike with createRoot, you don't need a separate root.render() call here.

For more information, see the working group discussion here.

Updates to Server Rendering APIs

In this release, we’re revamping our react-dom/server APIs to fully support Suspense on the server and Streaming SSR. As part of these changes, we’re deprecating the old Node streaming API, which does not support incremental Suspense streaming on the server.

Using this API will now warn:

renderToNodeStream: Deprecated ⛔️️

Instead, for streaming in Node environments, use:

renderToPipeableStream: New ✨

We’re also introducing a new API to support streaming SSR with Suspense for modern edge runtime environments, such as Deno and Cloudflare workers:

renderToReadableStream: New ✨

The following APIs will continue working, but with limited support for Suspense:

renderToString: Limited ⚠️
renderToStaticMarkup: Limited ⚠️

Finally, this API will continue to work for rendering e-mails:

renderToStaticNodeStream

For more information on the changes to server rendering APIs, see the working group post on Upgrading to React 18 on the server, a deep dive on the new Suspense SSR Architecture, and Shaundai Person’s talk on Streaming Server Rendering with Suspense at React Conf 2021.

Automatic Batching

React 18 adds out-of-the-box performance improvements by doing more batching by default. Batching is when React groups multiple state updates into a single re-render for better performance. Before React 18, we only batched updates inside React event handlers. Updates inside of promises, setTimeout, native event handlers, or any other event were not batched in React by default:

// Before React 18 only React events were batched

function handleClick() {
  setCount(c => c + 1);
  setFlag(f => !f);
  // React will only re-render once at the end (that's batching!)
}

setTimeout(() => {
  setCount(c => c + 1);
  setFlag(f => !f);
  // React will render twice, once for each state update (no batching)
}, 1000);

Starting in React 18 with createRoot, all updates will be automatically batched, no matter where they originate from. This means that updates inside of timeouts, promises, native event handlers or any other event will batch the same way as updates inside of React events:

// After React 18 updates inside of timeouts, promises,
// native event handlers or any other event are batched.

function handleClick() {
  setCount(c => c + 1);
  setFlag(f => !f);
  // React will only re-render once at the end (that's batching!)
}

setTimeout(() => {
  setCount(c => c + 1);
  setFlag(f => !f);
  // React will only re-render once at the end (that's batching!)
}, 1000);

This is a breaking change, but we expect this to result in less work rendering, and therefore better performance in your applications. To opt-out of automatic batching, you can use flushSync:

import { flushSync } from 'react-dom';

function handleClick() {
  flushSync(() => {
    setCounter(c => c + 1);
  });
  // React has updated the DOM by now
  flushSync(() => {
    setFlag(f => !f);
  });
  // React has updated the DOM by now
}

For more information, see the Automatic batching deep dive.

New APIs for Libraries

In the React 18 Working Group we worked with library maintainers to create new APIs needed to support concurrent rendering for use cases specific to their use case in areas like styles, external stores, and accessibility. To support React 18, some libraries may need to switch to one of the following APIs:

useId is a new hook for generating unique IDs on both the client and server, while avoiding hydration mismatches. This solves an issue that already exists in React 17 and below, but it’s even more important in React 18 because of how our streaming server renderer delivers HTML out-of-order. For more information see the useId post in the working group.
useSyncExternalStore is a new hook that allows external stores to support concurrent reads by forcing updates to the store to be synchronous. This new API is recommended for any library that integrates with state external to React. For more information, see the useSyncExternalStore overview post and useSyncExternalStore API details.
useInsertionEffect is a new hook that allows CSS-in-JS libraries to address performance issues of injecting styles in render. Unless you’ve already built a CSS-in-JS library we don’t expect you to ever use this. This hook will run after the DOM is mutated, but before layout effects read the new layout. This solves an issue that already exists in React 17 and below, but is even more important in React 18 because React yields to the browser during concurrent rendering, giving it a chance to recalculate layout. For more information, see the Library Upgrade Guide for