Base64.js

base64.js

function encode(s) {
  var c = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=",
      o = [];
  for (var i = 0, n = s.length; i < n;) {
    var c1 = s.charCodeAt(i++),
        c2 = s.charCodeAt(i++),
        c3 = s.charCodeAt(i++);
    o.push(c.charAt(c1 >> 2));
    o.push(c.charAt(((c1 & 3) << 4) | (c2 >> 4)));
    o.push(c.charAt(i < n + 2 ? ((c2 & 15) << 2) | (c3 >> 6) : 64));
    o.push(c.charAt(i < n + 1 ? c3 & 63 : 64));
  }
  return o.join("");
}

Note that some browsers support the built-in functions atob and btoa for decoding and encoding, respectively.