Koa: setTimeout response problem

[koalex]

This example return 404.

const Koa = require('koa');
const app = new Koa();

app.use(ctx => {

  setTimeout(function () {
    ctx.body = 'Hello Koa';
  }, 1000);

});

app.listen(3000);

[magicdawn]

try

app.use(ctx => new Promise(resolve =>{
  setTimeout(function () {
    ctx.body = 'Hello Koa';
        resolve()
  }, 1000);
}));

[zzzgit]

https://github.com/koajs/koa/issues/1166**

[pyuyu]

why

try

app.use(ctx => new Promise(resolve =>{
  setTimeout(function () {
      ctx.body = 'Hello Koa';
        resolve()
  }, 1000);
}));

owing to what? event-loop?