Example how to using external libraries in react js..

using_external_libraries_in_reactjs.txt

Try following steps:

including the external js file link in your /public/index.html
use the external library with prefix window.
for example, JQuery.

put following line in your /public/index.html
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
use it in your project:
window.$("#btn1").click(function(){
    alert("Text: " + $("#test").text());
});