unknownDuplicates.js

function findDupsMiss(arr) {

  let missingNum = [];
  let newArr = [];
  arr = arr.sort((a, b) => a - b);
  let dup = [...new Set(arr)];
  for (let y = 1; y < dup.length; y++) {
    if (dup[y] - dup[y - 1] != 1) missingNum.push(dup[y] - 1)
  }
  for (let i = 0; i < arr.length; i++) {
    for (let j = i + 1; j < arr.length; j++) {
      if (arr[i] === arr[j]) newArr.push(arr[i])
    }
  }
  missingNum.push(newArr);
  return missingNum;
}