JSFiddle - React, Tailwind, and code Playground

HTML

<form>
    <h3>Lenght:</h3>
    <input id="len" type="text"> 
    <div id="result"> </div>
</form>

JavaScript

$("#len").keyup(function(){
    var r = $('#len').val()/(2*Math.PI);        
    $('#result').text('Area: ' + Math.PI*r*r);            
});