I wrote a Japanese article about Array#at, but no one is saying it is what we want it to be.
To pick up on the responses to this article:
Oh no, it's not too late, we should take it back. TypeScript's number type can't determine NaN.
いやあ今なら間に合うから取り消すべきだよ/typescriptのnumber型はNanを判定できないと思う・・・ - Fushihara のブックマーク / はてなブックマーク
This is a shame
I'm not sure I'd recommend it.
【B!】【追記あり】ES2022 Array#at がちょっとおかしい - Qiita / これは残念仕様😡 手放しに推奨するのは微妙 - yasu-log のブックマーク / はてなブックマーク
I guess we shouldn't use Array#at.
Array#atは使わないほうが良さそう - door-s-dev のブックマーク / はてなブックマーク
You can also see other responses from the following site, in Japanese.