hard - return updated auth token + parsed json.js

const checkStatus = (response) => {
  if (response.status >= 200 && response.status < 300) return response
  const error = new Error(`Error ${response.status}: ${response.statusText}`)
  error.response = response
  throw error
}

const parseResponse = (response) => {
  const authToken = response.headers.get(`Authorization`)
  return response.json().then(json => ({ authToken, json }))
}

const getAuthTokenAndJSON = () => {
  return fetch(url, {opts})
    .then(checkStatus)
    .then(parseResponse)
}

// and call thusly:

getAuthTokenAndJSON()
  .then({ authToken, json } => {
    // now you have both the authToken and json
  }
  .catch(err => console.warn(err.message)) // now the error contains the whole response object too if you need it