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What’s new actually in React v17.0


It has been two and a half years since React v16 was first released. The dev team promises that update v17 is incredibly important for the future of React but claims that no new features are being added. So what exactly does that mean? And how can React claim to be releasing a major new version without adding new features?

Gradual React updates

Past React updates have always caused deprecations of older versions. This could make it incredibly difficult for teams to upgrade their React versions, especially when working with large or neglected codebases. So, the React team has decided that v17 will lay the groundwork for a new method of adopting updates. When React v18 is released, developers will be able to choose to only upgrade parts of their app and keep other parts running on v17. React advises against this if you’re application is actively maintained or in large-scale production, but it can help teams who don’t actively maintain their application or who don’t need to migrate certain components to the newer version. In the future, new React versions won’t leave these apps behind or demand massive migrations.

To enable this ability, the React team has spent some time reworking the event system. These updates are what caused React v17 to be a major release since it could potentially break many applications.

Events are no longer attached to the document

Contrary to what you might think, React doesn’t attach events to individual elements. Instead, React binds one handler per event type to the document node. This approach increases performance in applications with large element trees and also enables features like replaying events.

In the background, React can capture an event and determine which component to call before propagating the event up the component tree. However, in native Javascript, the event has already propagated to the document level. While this may not sound like an issue, this system breaks when running multiple instances of React. If a React app nested inside of another one has stopped the propagation of the event, the parent React app will still capture the event. This was a major headache for developers looking to nest different versions of React. You may not think that this happens in production, but the Atom code editor encountered this issue four years ago!

React will now attach event handlers to the root node of the React app. This change will allow the safe nesting of React trees within each other. Granted, all React instances will have to be running on version 17 or higher for this to work. This update also means that React apps are easier to embed inside of other frameworks like jQuery. React will now stop events from propagating to jQuery as one would expect.

Optimizing effect cleanup

It is fairly common to return a cleanup function within a useEffect method to stop any code running within the function. In the past, this cleanup function would run synchronously before the next frame was drawn. In reality, most apps don’t require screen updates to be delayed will running cleanup methods. So they will now run asynchronously after React has rendered the next frame. This should result in faster switch times between components. You can still use the useLayoutEffect method to run a cleanup method that blocks the render loop.

No more event pooling

Event pooling is incredibly confusing and doesn’t provide any optimizations on modern browsers. Instead, it can result in some strange bugs when working with multiple components that receive the same event. One event handler might set the event to be null, and thus the other components using the event would run into a null variable error. In short, newer React version will allow event fields to be accessed whenever you need them.

React v17 may not be the feature-packed update React developers were hoping for. It is instead more of a piece-of-mind update. So while we may not be getting cool new components or hooks, these additions will pay off down the road in later updates. You can try React v17 now by running npm install react@17.0.0-rc.0 react-dom@17.0.0-rc.0 in your React project.

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