Function flushSyncFromReconciler

packages/react-reconciler/src/ReactFiberWorkLoop.js:1815–1853

(fn: (() => R) | void)

declare function flushSyncFromReconciler<R>(fn: () => R): R;
declare function flushSyncFromReconciler(void): void;
export function flushSyncFromReconciler<R>(fn: (() => R) | void): R | void {
  // In legacy mode, we flush pending passive effects at the beginning of the
  // next event, not at the end of the previous one.
  if (
    pendingEffectsStatus !== NO_PENDING_EFFECTS &&
    !disableLegacyMode &&
    pendingEffectsRoot.tag === LegacyRoot &&
    (executionContext & (RenderContext | CommitContext)) === NoContext
  ) {
    flushPendingEffects();
  }

  const prevExecutionContext = executionContext;
  executionContext |= BatchedContext;

  const prevTransition = ReactSharedInternals.T;
  const previousPriority = getCurrentUpdatePriority();

  try {
    setCurrentUpdatePriority(DiscreteEventPriority);
    ReactSharedInternals.T = null;
    if (fn) {
      return fn();
    } else {
      return undefined;
    }
  } finally {
    setCurrentUpdatePriority(previousPriority);
    ReactSharedInternals.T = prevTransition;

    executionContext = prevExecutionContext;
    // Flush the immediate callbacks that were scheduled during this batch.
    // Note that this will happen even if batchedUpdates is higher up
    // the stack.
    if ((executionContext & (RenderContext | CommitContext)) === NoContext) {
      flushSyncWorkOnAllRoots();
    }
  }
}

// If called outside of a render or commit will flush all sync work on all roots
// Returns whether the the call was during a render or not