function (
  a, // positive base10 encoded integer
  b, // placeholder for result
  c  // placeholder for modulo
) {

  for (
    a = a !== +a || a % 1 ? -1 : a, b = ""; // if not a base10 integer, 'a' will be '-1'
                                            // for example, '.5 % 1 == .5' but '1 % 1 == 0'
    a >= 0; // also prevents the user to use negative base10 integers
    a = Math.floor(a / 62) || -1 // using a bitwise hack here will fail with great numbers
  )

    // a%62 -> 0-61
    // 0-9   | 36-61 | 10-35
    // 48-57 | 65-90 | 97-121
    // 0-9   | A-Z   | a-z

    b = String.fromCharCode(((c = a % 62) > 9 ? c > 35 ? 29 : 87 : 48) + c) + b;

  return b // will return either an empty or a base62-encoded string

}