Go to your static folder and there initialize your npm project.
cd flask_app/static
$ npm init
after installing and saving npm packages you can serve them like this:
<script src="https://stackoverflow.com/questions/24514936/{{ url_for("static', filename="node_modules/toastr/toastr.js")}}"></script>
credits to: https://codeburst.io/creating-a-full-stack-web-application-with-python-npm-webpack-and-react-8925800503d9