The ariaPosInSet property of the Element interface reflects the value of the aria-posinset attribute, which defines an element's number or position in the current set of listitems or treeitems.

A string containing an integer.

In this example the aria-posinset attribute on the element with an ID of article2 is set to "2". Using ariaPosInSet we update the value to "3".

<article id="article1" aria-posinset="1">…</article>
<article id="article2" aria-posinset="2">…</article>

let el = document.getElementById("article2");
console.log(el.ariaPosInSet); // "2"
el.ariaPosInSet = "3";
console.log(el.ariaPosInSet); // "3"