https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21TypedData%21Plugin%21DataType%21ItemList.php/function/ItemList%3A%3AappendItem/11.x
function ItemList::appendItem | Drupal API
functionitemlistdrupalapi
https://developer.mozilla.org/en-US/docs/Web/API/SVGPointList/appendItem
SVGPointList: appendItem() method - Web APIs | MDN
The appendItem() method of the SVGPointList interface adds a DOMPoint to the end of the list.
web apismethodmdn