const nearestLargestNumber = (arr, idx1) => {
  const num = arr[idx1];
  let nearestLargest;
  const findIdxOfNearestLargest = (idxOfNearestLargest, item, idx2) => {
    if (item > num &&
       (idxOfNearestLargest === null || nearestLargest > item )) {
      nearestLargest = item;
      idxOfNearestLargest = idx2;
    }
    return idxOfNearestLargest;
  };

  return arr.reduce(findIdxOfNearestLargest, null);
};

console.log(nearestLargestNumber([4, 1, 3, 5, 6], 0));