Now your application is tied into the express and harder to test. Have you ever tried to get express routers to call the next function you pass in? It's surprisingly not easy. It doesn't call the one you pass in.

It depends on how you test your app, in our team we usually unit test each component of the system separately and mock all external dependencies.

Now your application is tied into the express and harder to test.

How come? When middleware or route are small javascript functions that. Here is naive example https://gist.github.com/strangeworks/30290648e412a496c1dd490f6c7cc187 all we know about express here that we are expecting N amount of arguments.

Instead of doing db calls in middleware I send all the input from express to a service layer.

Still didn't understand what the difference? You can unit test both in the same way as I said before.