We do both. We have domain based services based on nodejs which tend to be smaller for example a content management service is separate from our notification s

We do both. We have domain based services based on nodejs which tend to be smaller for example a content management service is separate from our notification service which is separate from our user authentication service which is actually provided by a third party (Auth0). In fact being able to use third party services in place of homegrown is one of the big benefits we've found of a microservice architecture. Particularly when creating prototypes and trial apps for clients. The goal is in initial launch we leverage third parties as much as we can then pull back and replace with internal services as we need. But we also have a suite of npm modules which can be shared across the services for common tasks (our ORM models being a primary use case). Maintaining the libs can be a bit of a pain point and making sure we have solid tests is crucial to making sure we don't break other services but it works pretty well.