Node.parentElement should be maybe-null

[evmar]

lib.dom.ts defines Node.parentElement as never-null:
https://github.com/Microsoft/TypeScript/blob/95c3eccbe9f3f7e1857ce157d437e6b80b3c9c6f/lib/lib.dom.d.ts#L8749

However, it's a pretty common pattern to write loops like

while (n.parentElement != null) {
  // [...]
  n = n.parentElement;
}

to walk up the node tree.

Also parentNode, previousSibling, nextSibling, firstChild, lastChild... (?)

[mhegazy]

previousSibling, nextSibling, firstChild, lastChild... etc.. are all fair game. please feel free to send us a PR for that. You can find more information about contributing lib.d.ts fixes at https://github.com/Microsoft/TypeScript/blob/master/CONTRIBUTING.md#contributing-libdts-fixes.

parentElement is a tricky one. All elements except document have parent elements, so it is going to be annoying to most ppl. so i am not sure here what is best option, either correctness or usability.

[kitsonk]

All elements except document have parent elements,

If they are in the DOM. It is potentially a way to identified DOM fragments and other scenarios when moving parts of the DOM about. Strict is strict, (except when it isn't!). 😉

[mhegazy]

I would defer to @kitsonk opinion on this. I do not really write a lot of DOM code, so I would have limited insight here.