isSorted

Checks if a numeric array is sorted.

• Calculate the ordering direction for the first pair of adjacent array elements.
• Return 0 if the given array is empty, only has one element or the direction changes for any pair of adjacent array elements.
• Use Math.sign() to covert the final value of direction to -1 (descending order) or 1 (ascending order).

const isSorted = arr => {
  if (arr.length <= 1) return 0;
  const direction = arr[1] - arr[0];
  for (let i = 2; i < arr.length; i++) {
    if ((arr[i] - arr[i - 1]) * direction < 0) return 0;
  }
  return Math.sign(direction);
};

isSorted([0, 1, 2, 2]); // 1
isSorted([4, 3, 2]); // -1
isSorted([4, 3, 5]); // 0
isSorted([4]); // 0