findUserId() {
  if(accessToken === undefined) {
    this.getAccessToken();
  }
  console.log(accessToken);
  let userId;
  return fetch(`https://api.spotify.com/v1/me`, {'Authorization': `Bearer ${accessToken}`}}
    ).then(response => {return response.json()}
    ).then(jsonResponse => {
        return jsonResponse.id;
    });
}

// use as
findUserId().then(id => {
  console.log(id)
})