If a document has no activeElement then it cannot have a selection. Furthermore, isTextField and isNumberInput access the passed in element completely unguarded, so the assumption here was always that activeElement was defined.

range is not a boolean so we cast it to a boolean to appease the return value.

nonTextInputs is a string[] and because element.type could be undefined | string it doesn't fit in the box of a string[]. Checking for it's presence first turns it into a string which we can then pass to the includes function on the next line.

Pretty new to this. Why is Partial needed?

Partial is needed because the Element itself could be any HTML element that we're testing to check to see if it's an input, textarea or contentEditable Element. I wasn't sure how else to express that the Input part is optional and I think this is what @sirreal suggested as a potential solution to this problem (though I may be mis remembering, I can't find the original comment where that was suggested.

I believe this was introduced because we have some type that HTMLInputElement extends, but we don't know that it's an HTMLInputElement.
Partial<HTMLInputElement> doesn't seem ideal to me, it's typically used when we have an interface with optional properties. This usage is really that we have an Element (I believe, could be another type) and want to use it safely if it satisfies HTMLInputElement.

In my opinion, a better approach would be to accept the element type we expect to pass in (Element?), and use a type guard to narrow our element to our desired type. I implemented a type guard like this in #29220 to handle this type of case:

@sarayourfriend I believe you're thinking of this comment: #30030 (comment)

Right, maybe just HTMLElement? We should allow any element to be checked with this function.

Aha, yes, I did try to create a type guard but got stuck on contentEditable... I'll revisit this though and see if I can make it work with a type guard.

Meaning: sometimes the flexibility is nice so you don't have to check the type before calling a function that's being strict about the type. I think this is generally good for all functions in this package.