JSFiddle - React, Tailwind, and code Playground

HTML

<input id='one' type='text' value='12,34' /><br>
<input id='two' type='text' value='43,21' /><br>
<input id='result' type='text' /><br>
<div id='click'>Click to add</div>

JavaScript

$('#click').click(function() {
    var one = parseFloat($('#one').val().replace(',','.'));
    var two = parseFloat($('#two').val().replace(',','.'));
    var result = one + two;
    $('#result').val(result.toString().replace('.',','));
});