JSFiddle - React, Tailwind, and code Playground

HTML

<select id="mydropdown">
  <option value="1">test1</option>
  <option value="2" selected="selected">test2</option>
  <option value="3">test3</option>
</select>

JavaScript

var dropdown = document.getElementById("mydropdown");
var selectedIndex = dropdown.options[dropdown.selectedIndex].value;
alert(selectedIndex);