React v18.0

March 29, 2022 by The React Team

Our latest major version includes out-of-the-box improvements like automatic batching, new APIs like startTransition, and streaming server-side rendering with support for Suspense.

Many of the features in React 18 are built on top of our new concurrent renderer, a behind-the-scenes change that unlocks powerful new capabilities. Concurrent React is opt-in — it’s only enabled when you use a concurrent feature — but we think it will have a big impact on the way people build applications.

We’ve spent years researching and developing support for concurrency in React, and we’ve taken extra care to provide a gradual adoption path for existing users. Last summer, we formed the React 18 Working Group to gather feedback from experts in the community and ensure a smooth upgrade experience for the entire React ecosystem.

In case you missed it, we shared a lot of this vision at React Conf 2021:

• In the keynote, we explain how React 18 fits into our mission to make it easy for developers to build great user experiences
• Shruti Kapoor demonstrated how to use the new features in React 18
• Shaundai Person gave us an overview of streaming server rendering with Suspense

Below is a full overview of what to expect in this release, starting with Concurrent Rendering.

What is Concurrent React?

The most important addition in React 18 is something we hope you never have to think about: concurrency. We think this is largely true for application developers, though the story may be a bit more complicated for library maintainers.

Concurrency is not a feature, per se. It’s a new behind-the-scenes mechanism that enables React to prepare multiple versions of your UI at the same time. You can think of concurrency as an implementation detail — it’s valuable because of the features that it unlocks. React uses sophisticated techniques in its internal implementation, like priority queues and multiple buffering. But you won’t see those concepts anywhere in our public APIs.

When we design APIs, we try to hide implementation details from developers. As a React developer, you focus on what you want the user experience to look like, and React handles how to deliver that experience. So we don’t expect React developers to know how concurrency works under the hood.

However, Concurrent React is more important than a typical implementation detail — it’s a foundational update to React’s core rendering model. So while it’s not super important to know how concurrency works, it may be worth knowing what it is at a high level.

A key property of Concurrent React is that rendering is interruptible. When you first upgrade to React 18, before adding any concurrent features, updates are rendered the same as in previous versions of React — in a single, uninterrupted, synchronous transaction. With synchronous rendering, once an update starts rendering, nothing can interrupt it until the user can see the result on screen.

In a concurrent render, this is not always the case. React may start rendering an update, pause in the middle, then continue later. It may even abandon an in-progress render altogether. React guarantees that the UI will appear consistent even if a render is interrupted. To do this, it waits to perform DOM mutations until the end, once the entire tree has been evaluated. With this capability, React can prepare new screens in the background without blocking the main thread. This means the UI can respond immediately to user input even if it’s in the middle of a large rendering task, creating a fluid user experience.

Another example is reusable state. Concurrent React can remove sections of the UI from the screen, then add them back later while reusing the previous state. For example, when a user tabs away from a screen and back, React should be able to restore the previous screen in the same state it was in before. In an upcoming minor, we’re planning to add a new component called <Offscreen> that implements this pattern. Similarly, you’ll be able to use Offscreen to prepare new UI in the background so that it’s ready before the user reveals it.

Concurrent rendering is a powerful new tool in React and most of our new features are built to take advantage of it, including Suspense, transitions, and streaming server rendering. But React 18 is just the beginning of what we aim to build on this new foundation.

Gradually Adopting Concurrent Features

Technically, concurrent rendering is a breaking change. Because concurrent rendering is interruptible, components behave slightly differently when it is enabled.

In our testing, we’ve upgraded thousands of components to React 18. What we’ve found is that nearly all existing components “just work” with concurrent rendering, without any changes. However, some of them may require some additional migration effort. Although the changes are usually small, you’ll still have the ability to make them at your own pace. The new rendering behavior in React 18 is only enabled in the parts of your app that use new features.

The overall upgrade strategy is to get your application running on React 18 without breaking existing code. Then you can gradually start adding concurrent features at your own pace. You can use <StrictMode> to help surface concurrency-related bugs during development. Strict Mode doesn’t affect production behavior, but during development it will log extra warnings and double-invoke functions that are expected to be idempotent. It won’t catch everything, but it’s effective at preventing the most common types of mistakes.

After you upgrade to React 18, you’ll be able to start using concurrent features immediately. For example, you can use startTransition to navigate between screens without blocking user input. Or useDeferredValue to throttle expensive re-renders.

However, long term, we expect the main way you’ll add concurrency to your app is by using a concurrent-enabled library or framework. In most cases, you won’t interact with concurrent APIs directly. For example, instead of developers calling startTransition whenever they navigate to a new screen, router libraries will automatically wrap navigations in startTransition.

It may take some time for libraries to upgrade to be concurrent compatible. We’ve provided new APIs to make it easier for libraries to take advantage of concurrent features. In the meantime, please be patient with maintainers as we work to gradually migrate the React ecosystem.

For more info, see our previous post: How to upgrade to React 18.

Suspense in Data Frameworks

In React 18, you can start using Suspense for data fetching in opinionated frameworks like Relay, Next.js, Hydrogen, or Remix. Ad hoc data fetching with Suspense is technically possible, but still not recommended as a general strategy.

In the future, we may expose additional primitives that could make it easier to access your data with Suspense, perhaps without the use of an opinionated framework. However, Suspense works best when it’s deeply integrated into your application’s architecture: your router, your data layer, and your server rendering environment. So even long term, we expect that libraries and frameworks will play a crucial role in the React ecosystem.

As in previous versions of React, you can also use Suspense for code splitting on the client with React.lazy. But our vision for Suspense has always been about much more than loading code — the goal is to extend support for Suspense so that eventually, the same declarative Suspense fallback can handle any asynchronous operation (loading code, data, images, etc).

Server Components is Still in Development

Server Components is an upcoming feature that allows developers to build apps that span the server and client, combining the rich interactivity of client-side apps with the improved performance of traditional server rendering. Server Components is not inherently coupled to Concurrent React, but it’s designed to work best with concurrent features like Suspense and streaming server rendering.

Server Components is still experimental, but we expect to release an initial version in a minor 18.x release. In the meantime, we’re working with frameworks like Next.js, Hydrogen, and Remix to advance the proposal and get it ready for broad adoption.

What’s New in React 18

New Feature: Automatic Batching

Batching is when React groups multiple state updates into a single re-render for better performance. Without automatic batching, 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. With automatic batching, these updates will be batched automatically:

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

// After: updates inside of timeouts, promises,
// native event handlers or any other event are batched.
setTimeout(() => {
  setCount(c => c + 1);
  setFlag(f => !f);
  // React will only re-render once at the end (that's batching!)
}, 1000);

For more info, see this post for Automatic batching for fewer renders in React 18.

New Feature: Transitions

A transition is a new concept in React to distinguish between urgent and non-urgent updates.

• Urgent updates reflect direct interaction, like typing, clicking, pressing, and so on.
• Transition updates transition the UI from one view to another.

Urgent updates like typing, clicking, or pressing, need immediate response to match our intuitions about how physical objects behave. Otherwise they feel “wrong”. However, transitions are different because the user doesn’t expect to see every intermediate value on screen.

For example, when you select a filter in a dropdown, you expect the filter button itself to respond immediately when you click. However, the actual results may transition separately. A small delay would be imperceptible and often expected. And if you change the filter again before the results are done rendering, you only care to see the latest results.

Typically, for the best user experience, a single user input should result in both an urgent update and a non-urgent one. You can use startTransition API inside an input event to inform React which updates are urgent and which are “transitions”:

import { startTransition } from 'react';

// Urgent: Show what was typed
setInputValue(input);

// Mark any state updates inside as transitions
startTransition(() => {
  // Transition: Show the results
  setSearchQuery(input);
});

Updates wrapped in startTransition are handled as non-urgent and will be interrupted if more urgent updates like clicks or key presses come in. If a transition gets interrupted by the user (for example, by typing multiple characters in a row), React will throw out the stale rendering work that wasn’t finished and render only the latest update.

• useTransition: a hook to start transitions, including a value to track the pending state.
• startTransition: a method to start transitions when the hook cannot be used.

Transitions will opt in to concurrent rendering, which allows the update to be interrupted. If the content re-suspends, transitions also tell React to continue showing the current content while rendering the transition content in the background (see the Suspense RFC for more info).

See docs for transitions here.

New Suspense Features

Suspense lets you declaratively specify the loading state for a part of the component tree if it’s not yet ready to be displayed:

<Suspense fallback={<Spinner />}>
  <Comments />
</Suspense>

Suspense makes the “UI loading state” a first-class declarative concept in the React programming model. This lets us build higher-level features on top of it.

We introduced a limited version of Suspense several years ago. However, the only supported use case was code splitting with React.lazy, and it wasn’t supported at all when rendering on the server.

In React 18, we’ve added support for Suspense on the server and expanded its capabilities using concurrent rendering features.

Suspense in React 18 works best when combined with the transition API. If you suspend during a transition, React will prevent already-visible content from being replaced by a fallback. Instead, React will delay the render until enough data has loaded to prevent a bad loading state.

For more, see the RFC for Suspense in React 18.

New Client and Server Rendering APIs

In this release we took the opportunity to redesign the APIs we expose for rendering on the client and server. These changes allow users to continue using the old APIs in React 17 mode while they upgrade to the new APIs in React 18.

React DOM Client

These new APIs are now exported from react-dom/client:

• createRoot: New method to create a root to render or unmount. Use it instead of ReactDOM.render. New features in React 18 don’t work without it.
• hydrateRoot: New method to hydrate a server rendered application. Use it instead of ReactDOM.hydrate in conjunction with the new React DOM Server APIs. New features in React 18 don’t work without it.

Both createRoot and hydrateRoot accept a new option called onRecoverableError in case you want to be notified when React recovers from errors during rendering or hydration for logging. By default, React will use reportError, or console.error in the older browsers.

See docs for React DOM Client here.

React DOM Server

These new APIs are now exported from react-dom/server and have full support for streaming Suspense on the server:

• renderToPipeableStream: for streaming in Node environments.
• renderToReadableStream: for modern edge runtime environments, such as Deno and Cloudflare workers.

The existing renderToString method keeps working but is discouraged.

See docs for React DOM Server here.

New Strict Mode Behaviors

In the future, we’d like to add a feature that allows React to add and remove sections of the UI while preserving state. For example, when a user tabs away from a screen and back, React should be able to immediately show the previous screen. To do this, React would unmount and remount trees using the same component state as before.

This feature will give React apps better performance out-of-the-box, but requires components to be resilient to effects being mounted and destroyed multiple times. Most effects will work without any changes, but some effects assume they are only mounted or destroyed once.

To help surface these issues, React 18 introduces a new development-only check to Strict Mode. This new check will automatically unmount and remount every component, whenever a component mounts for the first time, restoring the previous state on the second mount.

Before this change, React would mount the component and create the effects:

* React mounts the component.
  * Layout effects are created.
  * Effects are created.

With Strict Mode in React 18, React will simulate unmounting and remounting the component in development mode:

* React mounts the component.
  * Layout effects are created.
  * Effects are created.
* React simulates unmounting the component.
  * Layout effects are destroyed.
  * Effects are destroyed.
* React simulates mounting the component with the previous state.
  * Layout effects are created.
  * Effects are created.

See docs for ensuring reusable state here.

New Hooks

useId

useId is a new hook for generating unique IDs on both the client and server, while avoiding hydration mismatches. It is primarily useful for component libraries integrating with accessibility APIs that require unique IDs. This solves an issue that already exists in React 17 and below, but it’s even more important in React 18 because of how the new streaming server renderer delivers HTML out-of-order. See docs here.

Note

useId is not for generating keys in a list. Keys should be generated from your data.

useTransition

useTransition and startTransition let you mark some state updates as not urgent. Other state updates are considered urgent by default. React will allow urgent state updates (for example, updating a text input) to interrupt non-urgent state updates (for example, rendering a list of search results). See docs here

useDeferredValue

useDeferredValue lets you defer re-rendering a non-urgent part of the tree. It is similar to debouncing, but has a few advantages compared to it. There is no fixed time delay, so React will attempt the deferred render right after the first render is reflected on the screen. The deferred render is interruptible and doesn’t block user input. See docs here.

useSyncExternalStore

useSyncExternalStore is a new hook that allows external stores to support concurrent reads by forcing updates to the store to be synchronous. It removes the need for useEffect when implementing subscriptions to external data sources, and is recommended for any library that integrates with state external to React. See docs here.

Note

useSyncExternalStore is intended to be used by libraries, not application code.

useInsertionEffect

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. See docs here.

Note

useInsertionEffect is intended to be used by libraries, not application code.

How to Upgrade

See How to Upgrade to React 18 for step-by-step instructions and a full list of breaking and notable changes.

Changelog

React

• Add useTransition and useDeferredValue to separate urgent updates from transitions. (#10426, #10715, #15593, #15272, #15578, #15769, #17058, #18796, #19121, #19703, #19719, #19724, #20672, #20976 by @acdlite, @lunaruan, @rickhanlonii, and @sebmarkbage)
• Add useId for generating unique IDs. (#17322, #18576, #22644, #22672, #21260 by @acdlite, @lunaruan, and @sebmarkbage)
• Add useSyncExternalStore to help external store libraries integrate with React. (#15022, #18000, #18771, #22211, #22292, #22239, #22347, #23150 by @acdlite, @bvaughn, and @drarmstr)
• Add startTransition as a version of useTransition without pending feedback. (#19696 by @rickhanlonii)
• Add useInsertionEffect for CSS-in-JS libraries. (#21913 by @rickhanlonii)
• Make Suspense remount layout effects when content reappears. (#19322, #19374, #19523, #20625, #21079 by @acdlite, @bvaughn, and @lunaruan)
• Make <StrictMode> re-run effects to check for restorable state. (#19523 , #21418 by @bvaughn and @lunaruan)
• Assume Symbols are always available. (#23348 by @sebmarkbage)
• Remove object-assign polyfill. (#23351 by @sebmarkbage)
• Remove unsupported unstable_changedBits API. (#20953 by @acdlite)
• Allow components to render undefined. (#21869 by @rickhanlonii)
• Flush useEffect resulting from discrete events like clicks synchronously. (#21150 by @acdlite)
• Suspense fallback={undefined} now behaves the same as null and isn’t ignored. (#21854 by @rickhanlonii)
• Consider all lazy() resolving to the same component equivalent. (#20357 by @sebmarkbage)
• Don’t patch console during first render. (#22308 by @lunaruan)
• Improve memory usage. (#21039 by @bgirard)
• Improve messages if string coercion throws (Temporal.*, Symbol, etc.) (#22064 by @justingrant)
• Use setImmediate when available over MessageChannel. (#20834 by @gaearon)
• Fix context failing to propagate inside suspended trees. (#23095 by @gaearon)
• Fix useReducer observing incorrect props by removing the eager bailout mechanism. (#22445 by @josephsavona)
• Fix setState being ignored in Safari when appending iframes. (#23111 by @gaearon)
• Fix a crash when rendering ZonedDateTime in the tree. (#20617 by @dimaqq)
• Fix a crash when document is set to null in tests. (#22695 by @SimenB)
• Fix onLoad not triggering when concurrent features are on. (#23316 by @gnoff)
• Fix a warning when a selector returns NaN. (#23333 by @hachibeeDI)
• Fix a crash when document is set to null in tests. (#22695 by @SimenB)
• Fix the generated license header. (#23004 by @vitaliemiron)
• Add package.json as one of the entry points. (#22954 by @Jack)
• Allow suspending outside a Suspense boundary. (#23267 by @acdlite)
• Log a recoverable error whenever hydration fails. (#23319 by @acdlite)

React DOM

• Add createRoot and hydrateRoot. (#10239, #11225, #12117, #13732, #15502, #15532, #17035, #17165, #20669, #20748, #20888, #21072, #21417, #21652, #21687, #23207, #23385 by @acdlite, @bvaughn, @gaearon, @lunaruan, @rickhanlonii, @trueadm, and @sebmarkbage)
• Add selective hydration. (#14717, #14884, #16725, #16880, #17004, #22416, #22629, #22448, #22856, #23176 by @acdlite, @gaearon, @salazarm, and @sebmarkbage)
• Add aria-description to the list of known ARIA attributes. (#22142 by @mahyareb)
• Add onResize event to video elements. (#21973 by @rileyjshaw)
• Add imageSizes and imageSrcSet to known props. (#22550 by @eps1lon)
• Allow non-string <option> children if value is provided. (#21431 by @sebmarkbage)
• Fix aspectRatio style not being applied. (#21100 by @gaearon)
• Warn if renderSubtreeIntoContainer is called. (#23355 by @acdlite)

React DOM Server

• Add the new streaming renderer. (#14144, #20970, #21056, #21255, #21200, #21257, #21276, #22443, #22450, #23247, #24025, #24030 by @sebmarkbage)
• Fix context providers in SSR when handling multiple requests. (#23171 by @frandiox)
• Revert to client render on text mismatch. (#23354 by @acdlite)
• Deprecate renderToNodeStream. (#23359 by @sebmarkbage)
• Fix a spurious error log in the new server renderer. (#24043 by @eps1lon)
• Fix a bug in the new server renderer. (#22617 by @shuding)
• Ignore function and symbol values inside custom elements on the server. (#21157 by @sebmarkbage)

React DOM Test Utils

• Throw when act is used in production. (#21686 by @acdlite)
• Support disabling spurious act warnings with global.IS_REACT_ACT_ENVIRONMENT. (#22561 by @acdlite)
• Expand act warning to cover all APIs that might schedule React work. (#22607 by @acdlite)
• Make act batch updates. (#21797 by @acdlite)
• Remove warning for dangling passive effects. (#22609 by @acdlite)

React Refresh

• Track late-mounted roots in Fast Refresh. (#22740 by @anc95)
• Add exports field to package.json. (#23087 by @otakustay)

Server Components (Experimental)

• Add Server Context support. (#23244 by @salazarm)
• Add lazy support. (#24068 by @gnoff)
• Update webpack plugin for webpack 5 (#22739 by @michenly)
• Fix a mistake in the Node loader. (#22537 by @btea)
• Use globalThis instead of window for edge environments. (#22777 by @huozhi)