Superior generic middleware composition

event-middleware.js

const compose = (...fns) => fns.reduce((f, g) => (...args) => f(g(...args)));

const listeners = {};

const on = (ev, ...handlers) => {
	if (!listeners[ev]) listeners[ev] = [];
	listeners[ev].push(compose(...handlers)(x => x));
};

const emit = (ev, ctx) => {
	const list = listeners[ev];
	if (list) list.forEach(listener => listener(ctx));
};

on(
	"hello",
	next => ctx => {
		ctx.a = true;
		next(ctx);
	},
	next => ctx => {
		ctx.b = true;
		next(ctx);
	},
	next => ctx => {
		ctx.respond({ success: ctx.a && ctx.b });
	},
);

emit("hello", { respond: x => console.log(x) });