Please, add details on the assumptions about the input array such as it containing only positive integers, without repetition, or add some tests.

Also precise input array could be large.

I agree, some solutions return 1 for [2, 2, 2]...

Maybe you could prevent usage of some Array functions by redefining them ?

Also I was not sure from description if we could or not use at least some functions like concat which seems quite inevitable...

const sortedDigits = n => { let arr = n.toString().split(''); arr.sort((a, b) => b - a); return arr; };

could be

const sortedDigits = n => n.toString().split('').sort((a, b) => b - a);