Last insertion index in sorted array based on function

OSCC · JavaScript, Array · Oct 22, 2020

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

  • Loosely check if the array is sorted in descending order.
  • 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

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