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 ""

Danke you for the clarification

We're playing the code golf (https://en.wikipedia.org/wiki/Code_golf) game, tl;dr write a solution with the minimal number of characters. Yes, in real, production code, you'd use const or let (var is still valid, but most modern code uses let unless var is absolutely necessary, since it has slightly different behavior https://stackoverflow.com/a/11444416/12101554), but this is still valid JavaScript.

Use const, let or var!!!

this is considered bad practice, however

*in professional code, yes

in codegolf (crafting a solution with the fewest characters/bytes), however, anything goes

Thank you! Much appreciated.

if you omit var, const or let, the variable is automatically attached to the global object. this is considered bad practice, however

Literally just a math problem

This is just brilliant!

Clever solution, I learned something new from it, thanks.

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

you need to have the code inside of the curly braces { and }, not below.

This feels like a 10 kyu kata.