JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" placeholder="n" id="n">
<span id="calc"></span>

JavaScript

var k = 2;
    document.getElementById('n').addEventListener('keyup', function(e) {
        var input = document.getElementById('n');
        var val, calc = document.getElementById('calc');
        val = input.value * k;
        if (!isNaN(val)) {
            calc.innerHTML = val;
        }
        else {
            calc.innerHTML = 'введите число, пожалуйста';
        }
    }, false);