JSFiddle - React, Tailwind, and code Playground

HTML

<input id="quantity" size="30" type="number" value="1"/>
 <p id="total" value="29.99">29.99</p>

JavaScript

$(document).ready(function() {
    var price = parseFloat($("#total").text());        
    function updatePrice() {
        var sum, num = parseInt($("#quantity").val(),10);
        console.log(num);
        if(num) {
	        sum = num * price;
	        $("#total").text(sum.toFixed(2));
        }
    }
    $(document).on("change, mouseup, keyup", "#quantity", updatePrice);
});