Quite possibly the smallest base 64 utility out there.

base64.js

// base64.decode(base64.encode('💩'));

// Browsers
const base64 = {
  encode: str => btoa(unescape(encodeURIComponent(str))),
  decode: str => decodeURIComponent(escape(atob(str)))
};

/* ES3 compat version
var base64 = {
  encode: function (str) {
    return btoa(unescape(encodeURIComponent(str)));
  },
  decode: function (str) {
    return decodeURIComponent(escape(atob(str)));
  }
};
*/

// NodeJS
const base64 = {
  encode: str => Buffer.from(str).toString('base64'),
  decode: str => Buffer.from(str, 'base64').toString()
};

As single module for both browsers and node

var base64 = {
  encode: typeof btoa === 'function' ?
    function (str) {
      return btoa(unescape(encodeURIComponent(str)));
    } :
    function (str) {
      return Buffer.from(str).toString('base64');
    },
  decode: typeof atob === 'function' ?
    function (str) {
      return decodeURIComponent(escape(atob(str)));
    } :
    function (str) {
      return Buffer.from(str, 'base64').toString();
    } 
};

try { module.exports = base64; } catch(o_O) {}