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