Function flushSync

packages/react-noop-renderer/src/createReactNoop.js:1116–1145

(fn: () => R)

  }

  function flushSync<R>(fn: () => R): R {
    if (__DEV__) {
      if (NoopRenderer.isAlreadyRendering()) {
        console.error(
          'flushSync was called from inside a lifecycle method. React cannot ' +
            'flush when React is already rendering. Consider moving this call to ' +
            'a scheduler task or micro task.',
        );
      }
    }
    if (disableLegacyMode) {
      const previousTransition = ReactSharedInternals.T;
      const preivousEventPriority = currentEventPriority;
      try {
        ReactSharedInternals.T = null;
        currentEventPriority = DiscreteEventPriority;
        if (fn) {
          return fn();
        } else {
          return undefined;
        }
      } finally {
        ReactSharedInternals.T = previousTransition;
        currentEventPriority = preivousEventPriority;
        NoopRenderer.flushSyncWork();
      }
    } else {
      return NoopRenderer.flushSyncFromReconciler(fn);
    }
  }

  function onRecoverableError(error) {
    // TODO: Turn this on once tests are fixed