Base58 (and other) Encoding and Decoding in Javascript

base58.js

/* 
 * (c) 2012 inflammable/raromachine
 * Licensed under the MIT License.
 * ---
 * this is a refactored version of https://gist.github.com/inflammable/2929362
 */

const alphabet = '123456789abcdefghijkmnopqrstuvwxyzABCDEFGHJKLMNPQRSTUVWXYZ'
const base = alphabet.length

function encode(value) {
  if (typeof value !== 'number') {
    value = parseInt(value, 10)

    if (Number.isNaN(value)) {
      throw '.encode() expects an int'
    }
  }

  let encoded = ''
  while (value) {
    const remainder = value % base
    value = Math.floor(value / base)
    encoded = alphabet[remainder].toString() + encoded
  }

  return encoded
}

function decode(str) {
  if (typeof str !== 'string') {
    throw '.decode() only accepts strings'
  }

  let decoded = 0
  while(str) {
    const alphabetPosition = alphabet.indexOf(str[0])
    const power = str.length - 1
    decoded += alphabetPosition * Math.pow(base, power)
    str = str.substring(1)
  }

  return decoded
}