JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id="avans" /><br />
<input type="text" id="dop_avans" /><br />
<p id="p"></p>

CSS

#p { background: #aaa}

JavaScript

$(document).ready(function(){
    $('#avans, #dop_avans').keyup(function(){
        var a = $('#avans').val();
        var b = $('#dop_avans').val();
        $('#p').html(a+b);
    });
});