The ariaValueMax property of the Element interface reflects the value of the aria-valuemax attribute, which defines the maximum allowed value for a range widget.

A string which contains a number.

In this example the aria-valuemax attribute on the element with an ID of slider is set to "7". Using ariaValueMax we update the value to "6".

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

let el = document.getElementById("slider");
console.log(el.ariaValueMax); // 7
el.ariaValueMax = "6";
console.log(el.ariaValueMax); // 6