JSFiddle - React, Tailwind, and code Playground

HTML

<select id="select1" onchange="findIndex()">
    <option>I</option>
    <option>Love</option>
    <option>my</option>
    <option>granny</option>
</select>

JavaScript

function findIndex() {
    var e = document.getElementById("select1");
    alert(e.selectedIndex);
}