sort.js

var arr = [
  1,
  5,
  3,
  [4, 2, 1]
];

function sortLevel(list) {
  list.forEach(function(item) {
    if (Array.isArray(item)) {
      sortLevel(item);
    }
  });
  return list.sort(function(a, b) {
    var as = a.toString();
    var bs = b.toString();
    if (as === bs) {
      return 0;
    }
    if (as > bs) {
      return 1;
    } else {
      return -1;
    }
  });
}

console.log(sortLevel(arr));