In general, encode integer into string, and then decode it back. The encoded sequence look completely random and suitable for shortener. This implements allow integer of range 0 to 62^6-1 = 56800235583 > 2^35

shortener_b62d6.js

const digits = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
const digitsReverse = (function(){
  var arr = digits.split('');
  var table = {};
  for(var i=0; i<arr.length; i++) table[arr[i]] = i;
  return table;
})();
const dim = 6;
const modulo = 62;
const shift = [26, 13, 47, 40, 11, 19];
const forward = [
  [6, 16, 9, 25, 32, 6],
  [16, 12, 9, 27, 13, 59],
  [55, 37, 41, 48, 53, 55],
  [31, 26, 30, 46, 44, 58],
  [27, 61, 8, 4, 43, 25],
  [35, 8, 35, 49, 19, 36]
];
const backward = [
  [2, 12, 40, 19, 34, 30],
  [31, 43, 6, 41, 51, 60],
  [50, 16, 59, 50, 57, 60],
  [5, 2, 19, 20, 47, 26],
  [21, 40, 20, 21, 38, 21],
  [28, 41, 52, 25, 28, 37]
];

function encode(id) {
  // O(dim)
  var arr = new Array(dim);
  for (let i=0; i<dim; i++){
    arr[i] = id % modulo;
    id = Math.floor(id/modulo);
  }
  // O(dim^2)
  var res = shift.concat();
  for(let i=0; i<dim; i++){
    for(let j=0; j<dim; j++){
      res[i] += forward[i][j] * arr[j];
    }
  }
  return res.map(i => digits[i%modulo]).join('');
}

function decode(hash) {
  // assert( hash.length == dim )

  // O(dim)
  var arr = new Array(dim);
  for(let i=0; i<dim; i++){
    var c = hash[i];
    arr[i] = digitsReverse[c] + modulo - shift[i];
  }

  // O(dim^2)
  var ans = 0;
  var base = 1;
  for(let i=0; i<dim; i++){
    var t = 0;
    for(let j=0; j<dim; j++){
      t += backward[i][j] * arr[j];
    }
    ans += (t%modulo) * base;
    base *= modulo;
  }

  return ans;
}