react redux router 小白疑惑列表

server/index.js
成功代码

const User = require('./models/user');
const router=require('koa-router')();
router.post('/user/login',async (ctx,next) => {
    ctx.body = await User.findOne({username:'test'}).exec();
    next();
})
const app=new Koa();
app
    .use(cors())
    .use(bodyparser())
    .use(static(path.resolve(__dirname,'build')))
    .use(router.routes())
    .use(async (ctx,next) => {
        console.log(`\n\n\n${ctx.method} ${ctx.url} ${new Date().toLocaleString()}`);
        next();
        console.log(`-----------------${new Date().toLocaleString()}------------\n`,ctx.body);
    });

失败，ctx.body=undefined

app
    .use(cors())
    .use(bodyparser())
    .use(static(path.resolve(__dirname,'build')))
    .use(async (ctx,next) => {
        console.log(`\n\n\n${ctx.method} ${ctx.url} ${new Date().toLocaleString()}`);
        next();
        console.log(`-----------------${new Date().toLocaleString()}------------\n`,ctx.body);
    })
    .use(router.routes());

区别在于中间件routers的位置，but why？