JSFiddle - React, Tailwind, and code Playground

HTML

<form action="post">
    <input id="a" />
    <input id="b" />
    <button id="calcular_btn" onclick="return calcular();">X</button>
</form>

JavaScript

function calcular() {
    var $a = $('#a');
    var $b = $('#b');
    
    $a.val($a.val() * $b.val());
    
    return false;
}