Correct about Fusion.js being based on koa. We find that this fits really nicely within the plugin system. The diagram is complex, but we inject the render phase into the middleware stack. Everything before `await next()` is pre-render, and everything after `await next()` is post-render. It makes it very easy to reason about the lifecycle of a plugin, as everything is in one place.
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More recently Michael Jackson had a project called Mach which was a similar idea, but it's no longer active either: https://github.com/mjackson/mach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
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.
