Function performUnitOfWork

packages/react-reconciler/src/ReactFiberWorkLoop.js:2990–3032

(unitOfWork: Fiber)

}

function performUnitOfWork(unitOfWork: Fiber): void {
  // The current, flushed, state of this fiber is the alternate. Ideally
  // nothing should rely on this, but relying on it here means that we don't
  // need an additional field on the work in progress.
  const current = unitOfWork.alternate;

  let next;
  if (enableProfilerTimer && (unitOfWork.mode & ProfileMode) !== NoMode) {
    startProfilerTimer(unitOfWork);
    if (__DEV__) {
      next = runWithFiberInDEV(
        unitOfWork,
        beginWork,
        current,
        unitOfWork,
        entangledRenderLanes,
      );
    } else {
      next = beginWork(current, unitOfWork, entangledRenderLanes);
    }
    stopProfilerTimerIfRunningAndRecordDuration(unitOfWork);
  } else {
    if (__DEV__) {
      next = runWithFiberInDEV(
        unitOfWork,
        beginWork,
        current,
        unitOfWork,
        entangledRenderLanes,
      );
    } else {
      next = beginWork(current, unitOfWork, entangledRenderLanes);
    }
  }

  unitOfWork.memoizedProps = unitOfWork.pendingProps;
  if (next === null) {
    // If this doesn't spawn new work, complete the current work.
    completeUnitOfWork(unitOfWork);
  } else {
    workInProgress = next;
  }
}

function replaySuspendedUnitOfWork(unitOfWork: Fiber): void {
  // This is a fork of performUnitOfWork specifcally for replaying a fiber that