Setting array.length for unit test

If I am running a unit test that is testing the logic based only on the length of a passed in array, should I be concerned about setting the length of it explicitly?

For example:

function foo(arrayFoo) {
  return arrayFoo.length > 10 ? 'too long' : 'just right';
}

And for the test:

let testArray = [];
testArray.length = 11;

const expected = 'too long'
const actual = foo(testArray);

expect(actual).toBe(expected);

I understand that this is creating an array of undefineds but is there a better way or am I just being paranoid?