JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>
<div>
  <input id="TextBox" type="text" />
</div>

<br />

<div>
  <select id="SelectBox" multiple>
    <option value="volvo">Volvo</option>
    <option value="saab">Saab</option>
    <option value="mercedes">Mercedes</option>
    <option value="audi">Audi</option>
  </select>
</div>

JavaScript

$("#SelectBox").change(function(){
	countSelectedItems();
});

function countSelectedItems(){
	var count = $("#SelectBox:selected").length;
  alert(count);
}