$ npm install koa-before-response

const koa = require('koa');
const Router = require('koa-router');
const beforeResponse = require('koa-before-response');
const app = new koa();

app.use(async function(ctx, next) {
  ctx.startTime = 'foo';
  await next();
  ctx.endTime = 'bar';
});

const router = new Router();
router.get('/test', async (ctx) => {
  ctx.body = { foo: 'bar'}
})

app.use(beforeResponse({
  beforeResponse: function(ctx) {
    console.log('hooked, ctx.endTime:', ctx.endTime);
  }
}));

app.use(router.routes());
app.listen(5000);