en.javascript.info/2-ui/1-document/03-dom-navigation/3-navigation-links-which-null/solution.md

1. Yes, true. The element elem.lastChild is always the last one, it has no nextSibling, so if there are children, then yes.
2. No, wrong, because elem.children[0] is the first child among elements. But there may be non-element nodes before it. So previousSibling may be a text node.

Please note that for both cases if there are no children, then there will be an error. For instance, if elem.lastChild is null, we can't access elem.lastChild.nextSibling.