code: function (current, renderPriority) {
    var workInProgress = current.alternate;
    if (workInProgress === null) {
      // We use a double buffering pooling technique because we know that we'll
      // only ever need at most two versions of a tree. We pool the "other" unused
      // node that we're free to reuse. This is lazily created to avoid allocating
      // extra objects for things that are never updated. It also allow us to
      // reclaim the extra memory if needed.
      workInProgress = createFiber(current.tag, current.key, current.internalContextTag);
      workInProgress.type = current.type;
      workInProgress.stateNode = current.stateNode;
  
      {
        // DEV-only fields
        workInProgress._debugID = current._debugID;
        workInProgress._debugSource = current._debugSource;
        workInProgress._debugOwner = current._debugOwner;
      }
  
      workInProgress.alternate = current;
      current.alternate = workInProgress;
    } else {
      // We already have an alternate.
      // Reset the effect tag.
      workInProgress.effectTag = NoEffect$1;
  
      // The effect list is no longer valid.
      workInProgress.nextEffect = null;
      workInProgress.firstEffect = null;
      workInProgress.lastEffect = null;
    }
  
    workInProgress.pendingWorkPriority = renderPriority;
  
    workInProgress.child = current.child;
    workInProgress.memoizedProps = current.memoizedProps;
    workInProgress.memoizedState = current.memoizedState;
    workInProgress.updateQueue = current.updateQueue;
  
    // pendingProps is set by the parent during reconciliation.
    // TODO: Pass this as an argument.
  
    // These will be overridden during the parent's reconciliation
    workInProgress.sibling = current.sibling;
    workInProgress.index = current.index;
    workInProgress.ref = current.ref;
  
    return workInProgress;
  }
docs: This is used to create an alternate fiber to do work on.