$: objectsById = currentObjects.reduce((items, item) => {
  items[item.id] = item;
  return items;
}, {});

function onEvent(newObjects) {
  let objectsToReplace = {};
  let objectsToAdd = [];
  for (const obj of newObjects) {
    if (items[obj.id]) {
      objectsToReplace[obj.id] = obj;
    } else {
      objectsToAdd.push(obj);
    }
  }
  const newObjects = currentObjects.map((obj) => {
    return objectsToReplace[obj.id]
      ? objectsToReplace[obj.id] 
      : obj;
  });
  newObjects.push(objectsToAdd);
  currentObjects = newObjects;
}