codify.js

let alphabet = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789".split("")
let base = alphabet.length

function decode(code) {
  let c, i
  i = 0
  for (let j = 0, len = code.length; j < len; j++) {
    c = code[j]
    i = i * base + alphabet.indexOf(c)
  }
  return i
}

function encode(id) {
  let s
  if (id === 0) {
    return { code: alphabet[0] }
  }
  s = ""
  while (id > 0) {
    s += alphabet[id % base]
    id = parseInt(id / base, 10)
  }
  return s.split("").reverse().join("")
}