In this release, React is dropping support for Internet Explorer, which is going out of support on June 15, 2022. More background on the act testing API and related changes is available in the working group. For example, the next version of React Testing Library has built-in support for React 18 without any additional configuration. This can be useful for end-to-end tests that simulate a full browser environment.Įventually, we expect testing libraries will configure this for you automatically. You can also set the flag to false to tell React that act isn’t needed. React will log helpful warnings if you forget to wrap an update with act. The purpose of the flag is to tell React that it’s running in a unit test-like environment. 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.īefore this change, React would mount the component and create the effects: To help surface these issues, React 18 introduces a new development-only check to Strict Mode. Most effects will work without any changes, but some effects assume they are only mounted or destroyed once. This feature will give React better performance out-of-the-box, but requires components to be resilient to effects being mounted and destroyed multiple times. To do this, React would unmount and remount trees using the same component state as before. For example, when a user tabs away from a screen and back, React should be able to immediately show the previous screen. In the future, we’d like to add a feature that allows React to add and remove sections of the UI while preserving state. React 18 also introduces new APIs for concurrent rendering such as startTransition, useDeferredValue and useId, which we share more about in the release post. For more information, see the Library Upgrade Guide for. 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. This hook will run after the DOM is mutated, but before layout effects read the new layout. Unless you’ve already built a CSS-in-JS library we don’t expect you to ever use this. useInsertionEffect is a new hook that allows CSS-in-JS libraries to address performance issues of injecting styles in render. For more information, see the useSyncExternalStore overview post and useSyncExternalStore API details. This new API is recommended for any library that integrates with state external to React.
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