js-keygen/base64url.js

/*jslint browser: true, sloppy: true */
//adapted from https://tools.ietf.org/html/draft-ietf-jose-json-web-signature-08#appendix-C

function base64urlEncode(arg) {
  var s = window.btoa(arg); // Regular base64 encoder
  s = s.split('=')[0]; // Remove any trailing '='s
  s = s.replace(/\+/g,  '-'); // 62nd char of encoding
  s = s.replace(/\//g, '_'); // 63rd char of encoding
  return s;
}

function base64urlDecode(s) {
  s = s.replace(/-/g, '+'); // 62nd char of encoding
  s = s.replace(/_/g, '/'); // 63rd char of encoding
  switch (s.length % 4) { // Pad with trailing '='s
  case 0: // No pad chars in this case
    break;
  case 2: // Two pad chars
    s += "==";
    break;
  case 3: // One pad char
    s += "=";
    break;
  default:
    throw "Illegal base64url string!";
  }
  return window.atob(s); // Standard base64 decoder
}