http4k VS rack-test

You named Express as an example for a good framework - I'd say both http4k and ktor come close to it. Spring Boot would really be on the other end and I met lots of JS/TS devs that didn't even want to touch it. I did have the same impression than you though: Documentation for ktor is not great at all.

http4k has excellent documentation and very simple concepts.

I couple of years ago I was lucky to use http4k, a server as a function web library for Kotlin. It was such a wonderful change compared to every other technologies available in both Java and Kotlin. It's simple.

Testing becomes so much easier too, as one can instantiate a the whole web routing aspect, without having to bind it to a port and having to send real http requests.

If strongly suggest people to take a look at it. It's not perfect, but it's a lot simpler than other frameworks and libraries. And it's a shift in some of the current mentality of using heavy frameworks (such as spring boot) which blow up anyone's cognitive load.

https://github.com/http4k/http4k

http4k with the contract, format-jackson, and server-undertow modules

In this part, we’ll set up our testing environment so that we can test our Rails API using minitest with minitest/spec. We’ll look at the differences between traditional style unit tests and spec-style tests, or specs. I’ll demonstrate why you should use minitest-rails. We’ll look at using rack-test for testing our API. We’ll even create our own generator to generate API specs.

The "app-as-function" has some very concrete applications though, even outside languages like Haskell. The [rack-test](https://github.com/rack/rack-test) gem (and similar languages) work exactly like this: they run the "app" part without the "web server" part to enable much easier testing. Rack middlewares similarly treat the enclosed app as just a function taking some specified input and returning some output value.