Function reconcileChildrenIterator

packages/react-reconciler/src/ReactChildFiber.js:1432–1602

(
    returnFiber: Fiber,
    currentFirstChild: Fiber | null,
    newChildren: ?Iterator<mixed>,
    lanes: Lanes,
  )

  }

  function reconcileChildrenIterator(
    returnFiber: Fiber,
    currentFirstChild: Fiber | null,
    newChildren: ?Iterator<mixed>,
    lanes: Lanes,
  ): Fiber | null {
    if (newChildren == null) {
      throw new Error('An iterable object provided no iterator.');
    }

    let resultingFirstChild: Fiber | null = null;
    let previousNewFiber: Fiber | null = null;

    let oldFiber = currentFirstChild;
    let lastPlacedIndex = 0;
    let newIdx = 0;
    let nextOldFiber = null;

    let knownKeys: Set<string> | null = null;

    let step = newChildren.next();
    for (
      ;
      oldFiber !== null && !step.done;
      newIdx++, step = newChildren.next()
    ) {
      if (oldFiber.index > newIdx) {
        nextOldFiber = oldFiber;
        oldFiber = null;
      } else {
        nextOldFiber = oldFiber.sibling;
      }
      const newFiber = updateSlot(returnFiber, oldFiber, step.value, lanes);
      if (newFiber === null) {
        // TODO: This breaks on empty slots like null children. That's
        // unfortunate because it triggers the slow path all the time. We need
        // a better way to communicate whether this was a miss or null,
        // boolean, undefined, etc.
        if (oldFiber === null) {
          oldFiber = nextOldFiber;
        }
        break;
      }

      if (__DEV__) {
        knownKeys = warnOnInvalidKey(
          returnFiber,
          newFiber,
          step.value,
          knownKeys,
        );
      }

      if (shouldTrackSideEffects) {
        if (oldFiber && newFiber.alternate === null) {
          // We matched the slot, but we didn't reuse the existing fiber, so we
          // need to delete the existing child.
          deleteChild(returnFiber, oldFiber);