JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8/themes/dot-luv/jquery-ui.css">
<div>
    <input id="number" type="text" />

    <select id="Selection" name="D1"> 
        <option value="1">Plus 1</option>
        <option value="-1">Minus 1</option>
    </select>

    <input id="result" type="text" />
</div>

JavaScript

$('#Selection').change(calc);
$("#number").keyup(calc);
function calc() {
  $('#result').val(
    parseInt($('#number').val(), 10) + parseInt($("#Selection").val(), 10)
  );
}