const options = {
  method: 'GET',
  headers: {
    accept: 'application/json',
    authorization: 'Basic MzMwOTcxOTQ6ZTZmNzFiMDc5ODRjMWNiMmE2ZWI5MGMwZjI1ODA4NTA='
  }
};

fetch('https://acuityscheduling.com/api/v1/calendars', options)
  .then(response => response.json())
  .then(response => console.log(response))
  .catch(err => console.error(err));