The ariaValueNow property of the Element interface reflects the value of the aria-valuenow attribute, which defines the current value for a range widget.

A string which contains a number.

In this example the aria-valuenow attribute on the element with an ID of slider is set to "1". Using ariaValueNow we update the value to "2".

<div
  role="slider"
  aria-valuenow="1"
  aria-valuemin="1"
  aria-valuemax="7"
  aria-valuetext="Sunday"></div>

let el = document.getElementById("slider");
console.log(el.ariaValueNow); // 1
el.ariaValueNow = "2";
console.log(el.ariaValueNow); // 2