Check if a numeric array is sorted in JavaScript

Sometimes, it's useful to know if an array is sorted in ascending or descending order. This can be especially useful in combination with other algorithms, such as binary search or sorting algorithms.

Sorting the whole array and comparing it to itself is the naive method, yet it's wildly inefficient. The more efficient way is the traditional for loop and some early returns.

Starting with the first two elements, calculate the direction of the array. If the direction changes at any point, return 0. If the array is empty or has only one element, return 0. If the direction remains the same for the whole array, return the Math.sign() of the direction to get -1 for descending order and 1 for 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