Your own test code is irrelevant then; it's testing the wrong thing(s). Try

let xs = [1];
void withoutMax(xs);
assert.deepEqual(xs,[1]);

The error in which a random array of values doesn't return the array without the last value. I'm not sure what's going on under the hood of the codewars random array testing. I can't get any of my own test code to return the error given the same input.

which error ?

This is what I did, and I think I understand that it's better to return a shallow copy that mutate an existing array. However, I don't understand why the error happens. Can someone explain?

The only thing standing between me and that 200k job is being able to spell 'function' correctly every time 😂