JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://code.jquery.com/jquery-2.2.4.min.js"></script>
<input id="myTextField" type="text" value="+50"/>

JavaScript

var price = 50;
var value = $("#myTextField").val();
var finalprice = eval(price + value);

alert(finalprice);