The ariaExpanded property of the Element interface reflects the value of the aria-expanded attribute, which indicates whether a grouping element owned or controlled by this element is expanded or collapsed.

A string with one of the following values:

"true"

The grouping element this element owns or controls is expanded.

"false"

The grouping element this element owns or controls is collapsed.

"undefined"

The element does not own or control a grouping element that is expandable.

In this example the aria-expanded attribute on the element with an ID of animal is set to "false". Using ariaExpanded we update the value to "true".

<div class="animals-combobox">
  <label for="animal">Animal</label>
  <input
    id="animal"
    type="text"
    role="combobox"
    aria-autocomplete="list"
    aria-expanded="false"
    aria-haspopup="true" />
  <button id="animals-button" tabindex="-1" aria-label="Open">▽</button>
  <ul id="animals-listbox" role="listbox" aria-label="Animals">
    <li id="animal-cat" role="option">Cat</li>
    <li id="animal-dog" role="option">Dog</li>
  </ul>
</div>

let el = document.getElementById("animal");
console.log(el.ariaExpanded); // false
el.ariaExpanded = "true";
console.log(el.ariaExpanded); // true