sortedLastIndexBy

Returns the highest index at which value should be inserted into array in order to maintain its sort order, based on a provided iterator function.

Check if the array is sorted in descending order (loosely). Use Array.prototype.map() to apply the iterator function to all elements of the array. Use Array.prototype.reverse() and Array.prototype.findIndex() to find the appropriate last index where the element should be inserted, based on the provided iterator function.

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

sortedLastIndexBy([{ x: 4 }, { x: 5 }], { x: 4 }, (o) => o.x); // 1