/* requesting token */

var user_token = '';

var fetch_header = new Headers({
  'Content-Type': 'application/json'
});

var fetch_body = JSON.stringfy({
  username: 'username',
  password: 'password'
});

var fetch_init = {
  method: 'POST',
  headers: fetch_header,
  body: fetch_body
};

var fetch_url = 'url/to/api/endpoint';

var fetch_req = new Request(fetch_url, fetch_init)

user_token = fetch(
  fetch_req
).then(
  res => res.json()
).then(
  resolved => resolved['token']
).catch(
  err => console.log(err)
);

/* requesting protected url */

var fetch_header = new Headers({
  'Authorization': 'JWT ${user_token}',
  'Content-Type': 'application/json'
});