fetch('/wp-json/jwt-auth/v1/token', {
  headers: new Headers({
    'Content-Type': 'application/x-www-form-urlencoded'
  }),
  method:'POST',
  body: 'username=yourname&password=yourpass'
})
  .then(res => res.json())
  .then(json => {
    if(json.token) {
      localStorage.token = json.token
    } else {
      const error = new Error(json.message)
      error.code = json.code
      throw error
    }
  })
  .catch(reason => console.warn(reason))