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;
};
More recently Michael Jackson had a project called Mach which was a similar idea, but it's no longer active either: https://github.com/mjackson/mach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
I went on to implement it in Swift (https://github.com/danneu/hansel) and then seriously improved on it in Kotlin (https://github.com/danneu/kog).
It was fun but hard to really make a convincing upgrade to Koa once you consider the rest of the ecosystem. For example, since Koa exposes Node's req/res, then you can still use existing Node/Express middleware.