didact-fiber.js

function cloneChildFibers(parentFiber) {
  const oldFiber = parentFiber.alternate;
  if (!oldFiber.child) {
    return;
  }

  let oldChild = oldFiber.child;
  let prevChild = null;
  while (oldChild) {
    const newChild = {
      type: oldChild.type,
      tag: oldChild.tag,
      stateNode: oldChild.stateNode,
      props: oldChild.props,
      partialState: oldChild.partialState,
      alternate: oldChild,
      parent: parentFiber
    };
    if (prevChild) {
      prevChild.sibling = newChild;
    } else {
      parentFiber.child = newChild;
    }
    prevChild = newChild;
    oldChild = oldChild.sibling;
  }
}