node-base58/src/base58.js

var alphabet = "123456789abcdefghijkmnopqrstuvwxyzABCDEFGHJKLMNPQRSTUVWXYZ";
var base = alphabet.length;

// Create a lookup table to fetch character index
var alphabetLookup = alphabet.split("").reduce(function(lookup, char, index) {
  lookup[char] = index;
  return lookup;
}, {});

function assertNonNegativeSafeInteger(val) {
  if (
    typeof val !== "number" ||
    isNaN(val) ||
    val < 0 ||
    val > Number.MAX_SAFE_INTEGER ||
    Math.floor(val) !== val
  ) {
    throw new Error("Value passed is not a non-negative safe integer.");
  }
}

function assertString(str) {
  if (typeof str !== "string") {
    throw new Error("Value passed is not a string.");
  }
}

function assertBase58Character(character) {
  if (alphabetLookup[character] === undefined) {
    throw new Error("Value passed is not a valid Base58 string.");
  }
}

exports.encode = function(num) {
  var str = "";
  var modulus;

  num = Number(num);

  assertNonNegativeSafeInteger(num);

  while (num >= base) {
    modulus = num % base;
    str = alphabet[modulus] + str;
    num = Math.floor(num / base);
  }

  return alphabet[num] + str;
};

exports.decode = function(str) {
  assertString(str);

  return str.split("").reverse().reduce(function(num, character, index) {
    assertBase58Character(character);
    return num + alphabetLookup[character] * Math.pow(base, index);
  }, 0);
};