const useSyncExternalStore:()

in packages/recoil/core/Recoil_ReactMode.js [34:83]

• 33 lines of code
• 10 McCabe index (conditional complexity)

const useSyncExternalStore: <T>(
  subscribe: (() => void) => () => void,
  getSnapshot: () => T,
  getServerSnapshot?: () => T,
) => T =
  // flowlint-next-line unclear-type:off
  (React: any).useSyncExternalStore ??
  // flowlint-next-line unclear-type:off
  (React: any).unstable_useSyncExternalStore;

type ReactMode =
  | 'TRANSITION_SUPPORT'
  | 'SYNC_EXTERNAL_STORE'
  | 'MUTABLE_SOURCE'
  | 'LEGACY';

/**
 * mode: The React API and approach to use for syncing state with React
 * early: Re-renders from Recoil updates occur:
 *    1) earlier
 *    2) in sync with React updates in the same batch
 *    3) before transaction observers instead of after.
 * concurrent: Is the current mode compatible with Concurrent Mode and useTransition()
 */
function reactMode(): {mode: ReactMode, early: boolean, concurrent: boolean} {
  // NOTE: This mode is currently broken with some Suspense cases
  // see Recoil_selector-test.js
  if (gkx('recoil_transition_support')) {
    return {mode: 'TRANSITION_SUPPORT', early: true, concurrent: true};
  }

  if (gkx('recoil_sync_external_store') && useSyncExternalStore != null) {
    return {mode: 'SYNC_EXTERNAL_STORE', early: true, concurrent: false};
  }

  if (
    gkx('recoil_mutable_source') &&
    useMutableSource != null &&
    typeof window !== 'undefined' &&
    !window.$disableRecoilValueMutableSource_TEMP_HACK_DO_NOT_USE
  ) {
    return gkx('recoil_suppress_rerender_in_callback')
      ? {mode: 'MUTABLE_SOURCE', early: true, concurrent: true}
      : {mode: 'MUTABLE_SOURCE', early: false, concurrent: false};
  }

  return gkx('recoil_suppress_rerender_in_callback')
    ? {mode: 'LEGACY', early: true, concurrent: false}
    : {mode: 'LEGACY', early: false, concurrent: false};
}