function sumAll(arr) {
  let max = Math.max(arr[0], arr[1]);
  let min = Math.min(arr[0], arr[1]);
  let sumBetween = 0;
  for (let i = min; i <= max; i++) {
    sumBetween += i;
  }
  return sumBetween;
}

sumAll([1, 4]);