Microcosm of Koa's flow control

MiniKoa.js

/*
   MiniKoa ^.^
 - In the real Koa, the module Koa-composer takes on the recursive behaviour.
 - This still works with async functions because await will automatically convert resolved values into promises.
 - The most notable difference of course is that this doesn't respond to an http request instead it just runs instantly with whatever context object you give it. 
*/

class MiniKoa {
	constructor() {
		this.middleware = [];
	}
	callback (ctx, index = 0) {
		const isLast = index === this.middleware.length - 1;
		let next = isLast ? () => {} : this.callback.bind(this, ctx, index + 1);
		return this.middleware[index](ctx, next);
	}
	use(func) {
		if(typeof func === 'function') this.middleware.push(func);
		else throw new TypeError("Middleware must be of type Function");
	}
}

const app = new MiniKoa();

app.use(async function(ctx, next) {
	console.log(`${ctx.downstream} #1`);
	await next();
	console.log(`${ctx.upstream} #1`);
});

app.use(async function(ctx, next) {
	console.log(`${ctx.downstream} #2`);
	await next();
	console.log(`${ctx.upstream} #2`);
});

app.use(async function(ctx, next) {
	console.log(`${ctx.downstream} #3`);
	await next();
	console.log(`${ctx.upstream} #3`);
});

app.callback({upstream: 'upstream', downstream: 'downstream'});