How is the client and server part of the application connected?

Question

JavaScript

Hello.
Mastering NodeJS and web application development.
I sketched the back-end part on NodeJS + Mongoose with all the pluses I needed, and sketched the front-end part on React.
How to connect them now among themselves in one operating application?
Do I need to use react as a templating engine instead of handlebars or what?

Answer 1

Obviously online. REST or websocket.