Koa is an improvement over Express, but I think it would neat if the JavaScript ecosystem adopted an even more functional middleware style, like Python's WSGI, Ruby's Rack, Clojure's Ring, etc.

Conceptually it's simple: middleware is a function that typically accepts a downstream "app", and returns a new "app" which accepts a "request" and returns a "response":

    const middleware = (app) =>
      async (request) => {
        // do stuff with request
        const response = await app(request);
        // do stuff with response
        return response;
      };

I worked on this idea way back in ~2009 (creatively called "JSGI" for the interface spec and "Jack" for an implementation) but promises weren't really a thing in JS, and async/await definitely wasn't a thing, so it was awkward.

More recently Michael Jackson had a project called Mach which was a similar idea, but it's no longer active either: https://github.com/mjackson/mach

As an aside, one of my favorite aspects of this style is having symmetric interfaces for HTTP servers and clients. You could do neat things like use a cache middleware for both a client and server, or write a simple HTTP proxy in a couple lines.

Anyway, with the addition of promises, async/await, and async iterators to JS I'm starting to dust off these old ideas. prototype here https://github.com/tlrobinson/twosixonesix