JSFiddle - React, Tailwind, and code Playground

HTML

<select id="dropdown">
    <option value="Red">Red</option>
    <option value="Green">Green</option>
    <option value="Blue">Blue</option>
</select>
<br>
<button id="fire" type="button" onclick="runThis()">Show dropdown items</button>

JavaScript

$("#fire").click(function () {
    var e = document.createEvent("MouseEvents");
    e.initMouseEvent("mousedown");
    $("#dropdown")[0].dispatchEvent(e);
});