React 18's useSyncExternalStore Offers a Cleaner Way to Read RxJS BehaviorSubjects

React 18 introduced useSyncExternalStore, a built-in hook that provides a straightforward way to connect RxJS BehaviorSubjects to React components without third-party libraries or manual useEffect subscriptions. The hook accepts a subscribe function and a getSnapshot function, mapping directly onto a BehaviorSubject's existing subscribe and getValue methods. Developers can build a reusable adapter hook by wrapping these two functions with useCallback to prevent unnecessary resubscriptions on every render. A key pitfall is defining the subscribe function inline without memoization, which causes React to tear down and recreate the RxJS subscription on every render, risking missed state updates. The same primitive is used internally by state management libraries like Redux, Zustand, and Jotai, making it a reliable foundation for external store integrations.