We don't have any NodeJS library, but you can use Flarum REST API with any programming language.
https://github.com/flagrow/flarum-api-client is an example of a PHP wrapper. But all you need is the Authorization header to use features that require login.
We unfortunately don't have a list of endpoints in the documentation at this point but you can find endpoints by looking at the browser network console and/or the Flarum source code. Our API follows the https://jsonapi.org/ spec
