html

<button
  aria-label="Close"
  aria-description="A longer description of the function of this element"
  id="close-button">
  X
</button>

js

let el = document.getElementById("close-button");
console.log(el.ariaDescription); // "A longer description of the function of this element"
el.ariaDescription = "A different description";
console.log(el.ariaDescription); // "A different description"