JWT with HS256 (HMAC SHA-256) algorithm in JS

jwt.js

function base64UrlEncode(str) {
  return btoa(str).replace('+', '-').replace('/', '_').replace(/=+$/, '');
}

async function HMACSHA256(key, message){
  const g = str => new Uint8Array([...unescape(encodeURIComponent(str))].map(c => c.charCodeAt(0))),
  k = g(key),
  m = g(message),
  c = await crypto.subtle.importKey('raw', k, { name: 'HMAC', hash: 'SHA-256' },true, ['sign']),
  s = await crypto.subtle.sign('HMAC', c, m);
  [...new Uint8Array(s)].map(b => b.toString(16).padStart(2, '0')).join('');
  return base64UrlEncode(String.fromCharCode(...new Uint8Array(s)));
}

async function jwt(header, payload, key) {
  const prefix = `${base64UrlEncode(JSON.stringify(header))}.${base64UrlEncode(JSON.stringify(payload))}`;
  const suffix = await HMACSHA256(key, prefix);
  return `${prefix}.${suffix}`;
}

//Example:
(async function() {
  const header = {
    'typ': 'JWT',
    'alg': 'HS256'
  };

  const payload = {
    'sub': 'helton',
    'iss': 'my-company'
  };

  const key = 'my-very-secret-key';

  console.log('jwt', await jwt(header, payload, key));
})();