Your solution seems correct, but did not handle the edge case below.

n being the length of the string array, if n = 0 or k > n or k <= 0 return ""

This kata asks you to solve sudoku that require guessing and to throw errors if there's more than one solution.

Yes

because I don't know JS destructuration well enough. It does something, but it's not necessary, actually

Yes I do.

All other comparisons do not just check for undefined. == Would give a false positive on null. if ( ! a ) Would give false positives on null, "", false and 0.

Actually, I might use Object.is, but I find === easier to write.

I sincerely hope you're not implying I should be using ==. == Should be taken out and shot, hung, quartered and then really hurt. There are exceedingly few reasons to use == instead of ===, and not having uncoercing >, >=, <, <=, like === and unlike ==, is the single worst design decision in JavaScript. I like JavaScript, but I wish it were strongly and statically typed ( and that is one of the reasons I like Haskell ).

Not an issue, and that's for you to discover. Please, don't post solutions like that.

JavaScript is a pathway to many abilities some consider to be... unnatural.