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React 17 blog post (#3335) 

* React 17 blog post

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---
title: "React v17.0"
author: [gaearon,rachelnabors]
---
Today, we are releasing React 17! We've written at length about the role of the React 17 release and the changes it contains in [the React 17 RC blog post](/blog/2020/08/10/react-v17-rc.html). This post is a brief summary of it, so if you've already read the RC post, you can skip this one.
## No New Features
The React 17 release is unusual because it doesn't add any new developer-facing features. Instead, this release is primarily focused on **making it easier to upgrade React itself**.
In particular, React 17 is a “stepping stone” release that makes it safer to embed a tree managed by one version of React inside a tree managed by a different version of React.
It also makes it easier to embed React into apps built with other technologies.
## Gradual Upgrades
**React 17 enables gradual React upgrades.** When you upgrade from React 15 to 16 (or, this time, from React 16 to 17), you would usually upgrade your whole app at once. This works well for many apps. But it can get increasingly challenging if the codebase was written more than a few years ago and isn’t actively maintained. And while it’s possible to use two versions of React on the page, until React 17 this has been fragile and caused problems with events.
We’re fixing many of those problems with React 17. This means that **when React 18 and the next future versions come out, you will now have more options**. The first option will be to upgrade your whole app at once, like you might have done before. But you will also have an option to upgrade your app piece by piece. For example, you might decide to migrate most of your app to React 18, but keep some lazy-loaded dialog or a subroute on React 17.
This doesn't mean you *have to* do gradual upgrades. **For most apps, upgrading all at once is still the best solution.** Loading two versions of React -- even if one of them is loaded lazily on demand -- is still not ideal. However, for larger apps that aren't actively maintained, this option makes sense to consider, and React 17 lets those apps not get left behind.
We've prepared an [example repository](https://github.com/reactjs/react-gradual-upgrade-demo/) demonstrating how to lazy-load an older version of React if necessary. This demo uses Create React App, but it should be possible to follow a similar approach with any other tool. We welcome demos using other tooling as pull requests.
>Note
>
>We've **postponed other changes** until after React 17. The goal of this release is to enable gradual upgrades. If upgrading to React 17 were too difficult, it would defeat its purpose.
## Changes to Event Delegation
To enable gradual updates, we've needed to make some changes to the React event system. React 17 is a major release because these changes are potentially breaking. You can check out our [versioning FAQ](/docs/faq-versioning.html#breaking-changes) to learn more about our commitment to stability.
In React 17, React will no longer attach event handlers at the `document` level under the hood. Instead, it will attach them to the root DOM container into which your React tree is rendered:
```js
const rootNode = document.getElementById('root');
ReactDOM.render(<App />, rootNode);
```
In React 16 and earlier, React would do `document.addEventListener()` for most events. React 17 will call `rootNode.addEventListener()` under the hood instead.
![A diagram showing how React 17 attaches events to the roots rather than to the document](../images/blog/react-v17-rc/react_17_delegation.png)
We've confirmed that [numerous](https://github.com/facebook/react/issues/7094) [problems](https://github.com/facebook/react/issues/8693) [reported](https://github.com/facebook/react/issues/12518) [over](https://github.com/facebook/react/issues/13451) [the](https://github.com/facebook/react/issues/4335) [years](https://github.com/facebook/react/issues/1691) [on](https://github.com/facebook/react/issues/285#issuecomment-253502585) [our](https://github.com/facebook/react/pull/8117) [issue](https://github.com/facebook/react/issues/11530) [tracker](https://github.com/facebook/react/issues/7128) related to integrating React with non-React code have been fixed by the new behavior.
If you run into issues with this change, [here's a common way to resolve them](/blog/2020/08/10/react-v17-rc.html#fixing-potential-issues).
## Other Breaking Changes
[The React 17 RC blog post](/blog/2020/08/10/react-v17-rc.html#other-breaking-changes) describes the rest of the breaking changes in React 17.
We've only had to change fewer than twenty components out of 100,000+ in the Facebook product code to work with these changes, so **we expect that most apps can upgrade to React 17 without too much trouble**. Please [tell us](https://github.com/facebook/react/issues) if you run into problems.
## New JSX Transform
React 17 supports the [new JSX transform](/blog/2020/09/22/introducing-the-new-jsx-transform.html). We've also backported support for it to React 16.14.0, React 15.7.0, and 0.14.10. Note that it is completely opt-in, and you don't have to use it. The classic JSX transform will keep working, and there are no plans to stop supporting it.
## React Native
React Native has a separate release schedule. We currently expect the support for React 17 to land in React Native 0.65, but the exact version is subject to change. As always, you can track the release discussions on the React Native Community releases [issue tracker](https://github.com/react-native-community/releases).
## Installation {#installation}
To install React 17 with npm, run:
```bash
npm install react@17.0.0 react-dom@17.0.0
```
To install React 17 with Yarn, run:
```bash
yarn add react@17.0.0 react-dom@17.0.0
```
We also provide UMD builds of React via a CDN:
```html
<script crossorigin src="https://unpkg.com/react@17.0.0/umd/react.production.min.js"></script>
<script crossorigin src="https://unpkg.com/react-dom@17.0.0/umd/react-dom.production.min.js"></script>
```
Refer to the documentation for [detailed installation instructions](/docs/installation.html).
## Changelog {#changelog}
### React {#react}
* Add `react/jsx-runtime` and `react/jsx-dev-runtime` for the [new JSX transform](https://babeljs.io/blog/2020/03/16/7.9.0#a-new-jsx-transform-11154-https-githubcom-babel-babel-pull-11154). ([@lunaruan](https://github.com/lunaruan) in [#18299](https://github.com/facebook/react/pull/18299))
* Build component stacks from native error frames. ([@sebmarkbage](https://github.com/sebmarkbage) in [#18561](https://github.com/facebook/react/pull/18561))
* Allow to specify `displayName` on context for improved stacks. ([@eps1lon](https://github.com/eps1lon) in [#18224](https://github.com/facebook/react/pull/18224))
* Prevent `'use strict'` from leaking in the UMD bundles. ([@koba04](https://github.com/koba04) in [#19614](https://github.com/facebook/react/pull/19614))
* Stop using `fb.me` for redirects. ([@cylim](https://github.com/cylim) in [#19598](https://github.com/facebook/react/pull/19598))
### React DOM {#react-dom}
* Delegate events to roots instead of `document`. ([@trueadm](https://github.com/trueadm) in [#18195](https://github.com/facebook/react/pull/18195) and [others](https://github.com/facebook/react/pulls?q=is%3Apr+author%3Atrueadm+modern+event+is%3Amerged))
* Clean up all effects before running any next effects. ([@bvaughn](https://github.com/bvaughn) in [#17947](https://github.com/facebook/react/pull/17947))
* Run `useEffect` cleanup functions asynchronously. ([@bvaughn](https://github.com/bvaughn) in [#17925](https://github.com/facebook/react/pull/17925))
* Use browser `focusin` and `focusout` for `onFocus` and `onBlur`. ([@trueadm](https://github.com/trueadm) in [#19186](https://github.com/facebook/react/pull/19186))
* Make all `Capture` events use the browser capture phase. ([@trueadm](https://github.com/trueadm) in [#19221](https://github.com/facebook/react/pull/19221))
* Don't emulate bubbling of the `onScroll` event. ([@gaearon](https://github.com/gaearon) in [#19464](https://github.com/facebook/react/pull/19464))
* Throw if `forwardRef` or `memo` component returns `undefined`. ([@gaearon](https://github.com/gaearon) in [#19550](https://github.com/facebook/react/pull/19550))
* Remove event pooling. ([@trueadm](https://github.com/trueadm) in [#18969](https://github.com/facebook/react/pull/18969))
* Stop exposing internals that won’t be needed by React Native Web. ([@necolas](https://github.com/necolas) in [#18483](https://github.com/facebook/react/pull/18483))
* Attach all known event listeners when the root mounts. ([@gaearon](https://github.com/gaearon) in [#19659](https://github.com/facebook/react/pull/19659))
* Disable `console` in the second render pass of DEV mode double render. ([@sebmarkbage](https://github.com/sebmarkbage) in [#18547](https://github.com/facebook/react/pull/18547))
* Deprecate the undocumented and misleading `ReactTestUtils.SimulateNative` API. ([@gaearon](https://github.com/gaearon) in [#13407](https://github.com/facebook/react/pull/13407))
* Rename private field names used in the internals. ([@gaearon](https://github.com/gaearon) in [#18377](https://github.com/facebook/react/pull/18377))
* Don't call User Timing API in development. ([@gaearon](https://github.com/gaearon) in [#18417](https://github.com/facebook/react/pull/18417))
* Disable console during the repeated render in Strict Mode. ([@sebmarkbage](https://github.com/sebmarkbage) in [#18547](https://github.com/facebook/react/pull/18547))
* In Strict Mode, double-render components without Hooks too. ([@eps1lon](https://github.com/eps1lon) in [#18430](https://github.com/facebook/react/pull/18430))
* Allow calling `ReactDOM.flushSync` during lifecycle methods (but warn). ([@sebmarkbage](https://github.com/sebmarkbage) in [#18759](https://github.com/facebook/react/pull/18759))
* Add the `code` property to the keyboard event objects. ([@bl00mber](https://github.com/bl00mber) in [#18287](https://github.com/facebook/react/pull/18287))
* Add the `disableRemotePlayback` property for `video` elements. ([@tombrowndev](https://github.com/tombrowndev) in [#18619](https://github.com/facebook/react/pull/18619))
* Add the `enterKeyHint` property for `input` elements. ([@eps1lon](https://github.com/eps1lon) in [#18634](https://github.com/facebook/react/pull/18634))
* Warn when no `value` is provided to `<Context.Provider>`. ([@charlie1404](https://github.com/charlie1404) in [#19054](https://github.com/facebook/react/pull/19054))
* Warn when `memo` or `forwardRef` components return `undefined`. ([@bvaughn](https://github.com/bvaughn) in [#19550](https://github.com/facebook/react/pull/19550))
* Improve the error message for invalid updates. ([@JoviDeCroock](https://github.com/JoviDeCroock) in [#18316](https://github.com/facebook/react/pull/18316))
* Exclude forwardRef and memo from stack frames. ([@sebmarkbage](https://github.com/sebmarkbage) in [#18559](https://github.com/facebook/react/pull/18559))
* Improve the error message when switching between controlled and uncontrolled inputs. ([@vcarl](https://github.com/vcarl) in [#17070](https://github.com/facebook/react/pull/17070))
* Keep `onTouchStart`, `onTouchMove`, and `onWheel` passive. ([@gaearon](https://github.com/gaearon) in [#19654](https://github.com/facebook/react/pull/19654))
* Fix `setState` hanging in development inside a closed iframe. ([@gaearon](https://github.com/gaearon) in [#19220](https://github.com/facebook/react/pull/19220))
* Fix rendering bailout for lazy components with `defaultProps`. ([@jddxf](https://github.com/jddxf) in [#18539](https://github.com/facebook/react/pull/18539))
* Fix a false positive warning when `dangerouslySetInnerHTML` is `undefined`. ([@eps1lon](https://github.com/eps1lon) in [#18676](https://github.com/facebook/react/pull/18676))
* Fix Test Utils with non-standard `require` implementation. ([@just-boris](https://github.com/just-boris) in [#18632](https://github.com/facebook/react/pull/18632))
* Fix `onBeforeInput` reporting an incorrect `event.type`. ([@eps1lon](https://github.com/eps1lon) in [#19561](https://github.com/facebook/react/pull/19561))
* Fix `event.relatedTarget` reported as `undefined` in Firefox. ([@claytercek](https://github.com/claytercek) in [#19607](https://github.com/facebook/react/pull/19607))
* Fix "unspecified error" in IE11. ([@hemakshis](https://github.com/hemakshis) in [#19664](https://github.com/facebook/react/pull/19664))
* Fix rendering into a shadow root. ([@Jack-Works](https://github.com/Jack-Works) in [#15894](https://github.com/facebook/react/pull/15894))
* Fix `movementX/Y` polyfill with capture events. ([@gaearon](https://github.com/gaearon) in [#19672](https://github.com/facebook/react/pull/19672))
* Use delegation for `onSubmit` and `onReset` events. ([@gaearon](https://github.com/gaearon) in [#19333](https://github.com/facebook/react/pull/19333))
* Improve memory usage. ([@trueadm](https://github.com/trueadm) in [#18970](https://github.com/facebook/react/pull/18970))
### React DOM Server {#react-dom-server}
* Make `useCallback` behavior consistent with `useMemo` for the server renderer. ([@alexmckenley](https://github.com/alexmckenley) in [#18783](https://github.com/facebook/react/pull/18783))
* Fix state leaking when a function component throws. ([@pmaccart](https://github.com/pmaccart) in [#19212](https://github.com/facebook/react/pull/19212))
### React Test Renderer {#react-test-renderer}
* Improve `findByType` error message. ([@henryqdineen](https://github.com/henryqdineen) in [#17439](https://github.com/facebook/react/pull/17439))
### Concurrent Mode (Experimental) {#concurrent-mode-experimental}
* Revamp the priority batching heuristics. ([@acdlite](https://github.com/acdlite) in [#18796](https://github.com/facebook/react/pull/18796))
* Add the `unstable_` prefix before the experimental APIs. ([@acdlite](https://github.com/acdlite) in [#18825](https://github.com/facebook/react/pull/18825))
* Remove `unstable_discreteUpdates` and `unstable_flushDiscreteUpdates`. ([@trueadm](https://github.com/trueadm) in [#18825](https://github.com/facebook/react/pull/18825))
* Remove the `timeoutMs` argument. ([@acdlite](https://github.com/acdlite) in [#19703](https://github.com/facebook/react/pull/19703))
* Disable `<div hidden />` prerendering in favor of a different future API. ([@acdlite](https://github.com/acdlite) in [#18917](https://github.com/facebook/react/pull/18917))
* Add `unstable_expectedLoadTime` to Suspense for CPU-bound trees. ([@acdlite](https://github.com/acdlite) in [#19936](https://github.com/facebook/react/pull/19936))
* Add an experimental `unstable_useOpaqueIdentifier` Hook. ([@lunaruan](https://github.com/lunaruan) in [#17322](https://github.com/facebook/react/pull/17322))
* Add an experimental `unstable_startTransition` API. ([@rickhanlonii](https://github.com/rickhanlonii) in [#19696](https://github.com/facebook/react/pull/19696))
* Using `act` in the test renderer no longer flushes Suspense fallbacks. ([@acdlite](https://github.com/acdlite) in [#18596](https://github.com/facebook/react/pull/18596))
* Use global render timeout for CPU Suspense. ([@sebmarkbage](https://github.com/sebmarkbage) in [#19643](https://github.com/facebook/react/pull/19643))
* Clear the existing root content before mounting. ([@bvaughn](https://github.com/bvaughn) in [#18730](https://github.com/facebook/react/pull/18730))
* Fix a bug with error boundaries. ([@acdlite](https://github.com/acdlite) in [#18265](https://github.com/facebook/react/pull/18265))
* Fix a bug causing dropped updates in a suspended tree. ([@acdlite](https://github.com/acdlite) in [#18384](https://github.com/facebook/react/pull/18384) and [#18457](https://github.com/facebook/react/pull/18457))
* Fix a bug causing dropped render phase updates. ([@acdlite](https://github.com/acdlite) in [#18537](https://github.com/facebook/react/pull/18537))
* Fix a bug in SuspenseList. ([@sebmarkbage](https://github.com/sebmarkbage) in [#18412](https://github.com/facebook/react/pull/18412))
* Fix a bug causing Suspense fallback to show too early. ([@acdlite](https://github.com/acdlite) in [#18411](https://github.com/facebook/react/pull/18411))
* Fix a bug with class components inside SuspenseList. ([@sebmarkbage](https://github.com/sebmarkbage) in [#18448](https://github.com/facebook/react/pull/18448))
* Fix a bug with inputs that may cause updates to be dropped. ([@jddxf](https://github.com/jddxf) in [#18515](https://github.com/facebook/react/pull/18515) and [@acdlite](https://github.com/acdlite) in [#18535](https://github.com/facebook/react/pull/18535))
* Fix a bug causing Suspense fallback to get stuck. ([@acdlite](https://github.com/acdlite) in [#18663](https://github.com/facebook/react/pull/18663))
* Don't cut off the tail of a SuspenseList if hydrating. ([@sebmarkbage](https://github.com/sebmarkbage) in [#18854](https://github.com/facebook/react/pull/18854))
* Fix a bug in `useMutableSource` that may happen when `getSnapshot` changes. ([@bvaughn](https://github.com/bvaughn) in [#18297](https://github.com/facebook/react/pull/18297))
* Fix a tearing bug in `useMutableSource`. ([@bvaughn](https://github.com/bvaughn) in [#18912](https://github.com/facebook/react/pull/18912))
* Warn if calling setState outside of render but before commit. ([@sebmarkbage](https://github.com/sebmarkbage) in [#18838](https://github.com/facebook/react/pull/18838))
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