text: const koa = require('koa');
  const static = require('koa-static');
  const router = require('koa-router');
  
  let server = new koa();
  server.listen(8080);
  
  server.use(async (ctx, next)=>{
    console.log('aa');
  
    await next();
    console.log('bbb');
  });
  
  server.use(async (ctx, next) => {
    console.log(1111);
  });
  
  /*
  可以像写同步的方式去写异步
  output:
  aa
  1111
  bbb
  */
  