CustomEvent.detail

 let myDetail = customEventInstance.detail;

// add an appropriate event listener
obj.addEventListener("cat", function(e) { process(e.detail) });

// create and dispatch the event
let event = new CustomEvent("cat", {
  detail: {
    hazcheeseburger: true
  }
});
obj.dispatchEvent(event);

// Will return an object containing the hazcheeseburger property
let myDetail = event.detail;