Event::getResources

var calendar = new Calendar(calendarEl, {
  resources: [
    { id: 'a', title: 'Resource A' },
    { id: 'b', title: 'Resource B' }
  ],
  events: [
    { id: '1', title: 'Event 1', start: '2020-06-01', resourceId: 'a' }
  ]
});

var event = calendar.getEventById('1');
var resources = event.getResources();
var resourceIds = resources.map(function(resource) { return resource.id });

console.log(resourceIds); // [ 'a' ]