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

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+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`.

2-ui/1-document/03-dom-navigation/3-navigation-links-which-null/task.md

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+importance: 5
+
+---
+
+# The sibling question
+
+If `elem` -- is an arbitrary DOM element node...
+
+- Is it true that `elem.lastChild.nextSibling` is always `null`?
+- Is it true that `elem.children[0].previousSibling` is always `null` ?