browser-base64url-arraybuffer.js

function base642ab(base64) {
  const str = window.atob(base64);
  const len = str.length;
  const bytes = new Uint8Array(len);
  for (let i = 0; i < len; i++) {
    bytes[i] = str.charCodeAt(i);
  }
  return bytes.buffer;
}

function ab2base64(ab) {
  const str = String.fromCharCode.apply(null, new Uint8Array(ab))
  return window.btoa(str);
}

function base64url2ab(base64url) {
  return base642ab(base64url2base64(base64url));
}

function ab2base64url(ab) {
  const str = String.fromCharCode.apply(null, new Uint8Array(ab))
  return base642base64url(window.btoa(str));  
}

function base642base64url(base64) {
  return base64.replace(/\+/g, '-').replace(/\//g, '_').replace(/=*$/g, '')
}

function base64url2base64(base64url) {
  let base64 = base64url.replace(/-/g, '+').replace(/_/g, '/');
  const padding = base64.length % 4;
  if (padding > 0) {
    return base64 + '===='.slice(padding);
  } 
  return base64;
}