The ariaSetSize property of the Element interface reflects the value of the aria-setsize attribute, which defines the number of items in the current set of listitems or treeitems.

A string containing an integer.

In this example the aria-setsize attribute on the element with an ID of tab-id is set to "3", to inform a device that there are currently 3 tabs in the group. Using ariaSetSize we update the value to "4".

<button
  role="tab"
  aria-selected="true"
  aria-setsize="3"
  aria-controls="tabpanel-id"
  id="tab-id">
  Tab label
</button>

let el = document.getElementById("tab-id");
console.log(el.ariaSetSize); // 3
el.ariaSetSize = "4";
console.log(el.ariaSetSize); // 4

• ARIA: tab role