You can add javascript libraries to use them for programming on the client side.
It is better to place them in the js folders of the static directory of the project. And refer to them using the src attribute in the <script> tag of the Index.html file.
For example, Demo project uses Chart.js library to create a dashboard:
<script src="/static/js/Chart.min.js"></script>
On the server side you can import python libraries to your modules.
For exapmple the mail item server module import smtplib library to send emails:
import smtplib