sortedIndex

Finds the lowest index at which a value should be inserted into an array in order to maintain its sorting order.

• Loosely check if the array is sorted in descending order.
• Use Array.prototype.findIndex() to find the appropriate index where the element should be inserted.

const sortedIndex = (arr, n) => {
  const isDescending = arr[0] > arr[arr.length - 1];
  const index = arr.findIndex(el => (isDescending ? n >= el : n <= el));
  return index === -1 ? arr.length : index;
};

sortedIndex([5, 3, 2, 1], 4); // 1
sortedIndex([30, 50], 40); // 1