JSFiddle - React, Tailwind, and code Playground

HTML

<input type=text />
<span data-result></span>

JavaScript

var $result = $('[data-result]');
$('input[type=text]').on('keyup', function () {
    var num = Number(this.value),
        res = 'Please type some numbers';
    if (this.value && !isNaN(num)) {
        res = num * 2;
    }
    $result.text(res);
}).trigger('keyup');