html

<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>

js

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