sortedIndexBy

Finds the lowest index at which a value should be inserted into an array in order to maintain its sorting order, based on the provided iterator function.

• 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, based on the iterator function fn.

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

sortedIndexBy([{ x: 4 }, { x: 5 }], { x: 4 }, o => o.x); // 0