JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id="quantity" onkeyup="updateTotal();"/>
<span id="totalPrice"></span>

JavaScript

var unitPrice = 50;

function updateTotal() {
    var quantity = parseInt(document.getElementById('quantity').value, 10);
    if(isNaN(quantity)) quantity = 0;
    var totalPrice = unitPrice * quantity;
    document.getElementById('totalPrice').innerText = totalPrice;
}