```
const Koa = require('koa')
const app = new Koa()
app.use(async (ctx, next) => {
console.log(1)
ctx.body = 'hello koa'
await next()
console.log(2)
})
app.use(async (ctx, next) => {
console.log(3)
await next()
console.log(4)
})
app.use(async (ctx, next) => {
console.log(5)
})
app.listen(3031, () => {
console.log('http://localhost:3031/');
})
```
输入结果：13542,
洋葱模型：一层一层进入，一层一层出
