Dispatch event with data

Perhaps you are looking for event.detail

new CustomEvent('eventName', {'detail': data})

Instead of data use x and in event listener you can access x using event.detail

function getSelectionBounds() {
  var x = (bounds["x"].toFixed(2));
  var y = "xyz";
  var selectionFired = new CustomEvent("selectionFired", {
    "detail": {"x":x,"y":y }
  });

  document.dispatchEvent(selectionFired);
};

document.addEventListener("selectionFired", function (e) {
  alert(e.detail.x+"   "+e.detail.y);
});

Unfortunately CustomEvent not work on IE (before Edge), but you can simply use the normal Event.

Just consider that also Event is an object, and so you can add all custom attributes you need:

 event.X = "foo";

On this way you can check your custom field on the callback function joned to the event.

NB. For old IE version you need this polyfill for new Event missed function:

var event;
if(typeof(Event) === 'function') {
  event = new Event(EVENT_NAME);
}else{
  event = document.createEvent('Event');
  event.initEvent(EVENT_NAME, false, false);
}