Function useOptionalSyncExternalStore

useOptionalSyncExternalStore<T>(
    subscribe:
        | ((onStoreChange: () => void) => () => void)
        | null
        | undefined,
    getSnapshot: (() => T) | null | undefined,
): T | null
Like useSyncExternalStore, but for a store that may be absent (null/undefined).
Type Parameters
- T
  The snapshot value type.
Parameters
- subscribe: ((onStoreChange: () => void) => () => void) | null | undefined
- getSnapshot: (() => T) | null | undefined
Returns T | null

Remarks
React forbids calling useSyncExternalStore conditionally, so observing a sometimes-there store otherwise forces per-call-site null sentinels (you cannot early-return, nor pass null.subscribe). Pass the store's subscribe and snapshot getter — or null/undefined when there is no store; returns the snapshot, or null when absent. The snapshot doubles as the SSR snapshot (null with no store), keeping it SSR-safe.

subscribe/getSnapshot MUST be stable references (e.g. bound class-field methods): an inline arrow changes identity every render and would resubscribe each time. Reading them off a stable instance via optional chaining (store?.subscribe) is the intended usage.