JavaScript Number::toString and parseInt for bases above 36.

base.js

function encode(input, base, map){
  var num = input, arr = [], item;
  if(typeof base != "number" && typeof base == "string" || Array.isArray(base)){
    map = base;
    base = map.length;
  }
  if(typeof base != "number") throw "Base must be a number";
  if(typeof base < 2) throw "Base must be greater than 1";
  while(num){
    item = num % base;
    if(typeof map == "string" || Array.isArray(map)) item = map[item];
    if(typeof map == "function") item = map(item);
    arr[arr.length] = item;
    num = Math.floor(num / base);
  }
  return arr.reverse();
}
function decode(input, base, map){
  var num = 0, arr = input, item;
  if(typeof base != "number" && typeof base == "string" || Array.isArray(base)){
    map = base;
    base = map.length;
  }
  if(typeof base != "number") throw "Base must be a number";
  if(typeof base < 2) throw "Base must be greater than 1";
  var i = 0;
  while(arr.length){
    item = arr.shift();
    if(typeof map == "string" || Array.isArray(map)) item = map.indexOf(item);
    if(typeof map == "function") item = map(item);
    num += item * Math.pow(base, arr.length);
    if(i++ >= 1000) throw "Overflow!";
  }
  return num;
}
var dict = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
encode(10, 16, dict).join(""); // "a"
encode(1000000, dict).join(""); // "4c92"
decode([1, 0], 16); // 16
decode([1, 1], 2); // 3
decode([1, 0], "0123456789"); // 10