Encode and decode byte arrays to and from base64url encoding in JavaScript.

base64url.js

function base64EncodeURL(byteArray) {
  return btoa(Array.from(new Uint8Array(byteArray)).map(val => {
    return String.fromCharCode(val);
  }).join('')).replace(/\+/g, '-').replace(/\//g, '_').replace(/\=/g, '');
}

function base64DecodeURL(b64urlstring) {
  return new Uint8Array(atob(b64urlstring.replace(/-/g, '+').replace(/_/g, '/')).split('').map(val => {
    return val.charCodeAt(0);
  }));
}