how koa uses composed middleware

fn = compose(this.middleware);

if (!this.listeners('error').length) this.on('error', this.onerror);

const handleRequest = (req, res) => {
  res.statusCode = 404;
  const ctx = this.createContext(req, res);
  const onerror = err => ctx.onerror(err);
  const handleResponse = () => respond(ctx);
  onFinished(res, onerror);
  fn(ctx).then(handleResponse).catch(onerror);
};

return handleRequest