React 17.0 release candidate has been published two and a half years since the previous major release of React. In spite of this lengthy timeline, this version comes with no new developer-facing features. It is good to note that there are plenty of new React features coming in the future, just not in this release.
The main focus of this release is to make it easier to upgrade React itself. React 17 enables gradual React upgrades rather than the all-or-nothing approach which has been used since inception. As the library grows this situation seems to be getting more problematic depending on the situation you are in.
With this new option, you will now be able to upgrade your whole app at once or opt to upgrade your app piece by piece or even have a mix of the two. This would allow you to upgrade your entire app for example to React 18 while lazy-loading just what you need from React 17.
Because of the nature of this new version, there are potential breaking changes, especially within the React Event System.
Check out this announcement post to get more details about this new React 17.0 release candidate version. If you are new to React or you just want to dive in you can visit the docs page here to learn how to use the library.