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Returns the lowest 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.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;