function extentSorted(x) {
    return [x[0], x[x.length - 1]];
}
The extent is the lowest & highest number in the array. With a sorted array,
the first element in the array is always the lowest while the last element is always the largest, so this calculation
can be done in one step, or constant time.

@param {Array<number>} x input
@returns {Array<number>} minimum & maximum value
@example
extentSorted([-100, -10, 1, 2, 5]); // => [-100, 5]