useSyncExternalStore

The useSyncExternalStore hook lets a component read and subscribe to a store that lives outside React — a browser API, a global singleton, a third-party state library. You give it a subscribe function (React calls it to be notified of changes) and a getSnapshot function (React calls it to read the current value). The optional getServerSnapshot supplies a value during server rendering so the markup matches on hydration. Below, two components subscribe to real browser state: your online/offline status and the viewport width.