The ariaSetSize property of the ElementInternals interface reflects the value of the aria-setsize attribute, which defines the number of items in the current set of listitems or treeitems.
Note: Setting aria attributes on ElementInternals allows default semantics to be defined on a custom element. These may be overwritten by author-defined attributes, but ensure that default semantics are retained should the author delete those attributes, or fail to add them at all. For more information see the Accessibility Object Model explainer.