While you say the interviewer provided no other information, was it possible to ask questions? As there is only one argument to the function, I would be under the assumption that the function would always return the array sorted it one directory, most likely ascending. i.e.

fn([4,1,5]) -> [1,4,5]

But obviously that's something the interviewer should confirm for you as it affects the answer. (Also it would be very strange for the function to return asc OR desc, without any way to specify).

As such, my layman's approach would be to pass an unsorted array to the function, and then compare that again a second array I already knew was sorted -

expected = [1,4,5]
sorted = fn([4,1,5])
// compare expected and sorted...

As far as basic testing goes, the process of comparing every element against each other, and also comparing every single element to the source array does seem convoluted - compared to just comparing against a known correct list.

While you say the interviewer provided no other information, was it possible to ask questions? As there is only one argument to the function, I would be under the assumption that the function would always return the array sorted it one directory, most likely ascending. i.e.

fn([4,1,5]) -> [1,4,5]

But obviously that's something the interviewer should confirm for you as it affects the answer. (Also it would be very strange for the function to return asc OR desc, without any way to specify).

As such, my layman's approach would be to pass an unsorted array to the function, and then compare that against a second array I already knew was sorted -

expected = [1,4,5]
sorted = fn([4,1,5])
// compare expected and sorted...

As far as basic testing goes, the process of comparing every element against each other, and also comparing every single element to the source array does seem convoluted - compared to just comparing against a known correct list.