JSFiddle - React, Tailwind, and code Playground

HTML

<form>

 <select id="dropDown1">
    <option value = "A">A</option>
    <option value = "B">B</option>
    <option value = "C">C</option>
 </select>

<!-- <select id="dropDown2">
    <option value="You chose A">You chose A</option>
    <option value="You chose B">You chose B</option>
    <option value="You chose C">You chose C</option>
</select> -->
<input type="text" id="dropDown2" value="1" readonly>
</form>

JavaScript

$(function() {
    $('#dropDown1').change(function(){
    	let v1 = $('#dropDown1').val();
      var test = (function(){
      	switch(v1) {
        	case "A" : return "1";
          case "B" : return "2";
          case "C" : return "3";
        }
      });
      $('#dropDown2').val(test);
    });
});