I'm trying to follow Microsoft's guide to generate a SAS Token, and here's my script:

function createSharedAccessToken(uri, saName, saKey) {
  uri = // '...'
  saName = // '...'
  saKey = // '...'

  if (!uri || !saName || !saKey) {
    throw "Missing required parameter";
  }
  var encoded = encodeURIComponent(uri);
  var now = new Date();
  var week = 60 * 60 * 24 * 7;
  var ttl = Math.round(now.getTime() / 1000) + week;
  var signature = encoded + '\n' + ttl;
  var signatureUTF8 = utf8.encode(signature);
  var hash = crypto.createHmac('sha256', saKey).update(signatureUTF8).digest('base64');
  return 'SharedAccessSignature sr=' + encoded + '&sig=' +
    encodeURIComponent(hash) + '&se=' + ttl + '&skn=' + saName;
}

createSharedAccessToken()

But I'm getting a:

  var signatureUTF8 = utf8.encode(signature);
                      ^

ReferenceError: utf8 is not defined

Any idea on why that is happening?