koa ajax 返回数据

想问koa ajax是怎么返回数据的?
之前是使用express可以用res.writeHead设置头, res.write设置返回内容, 但是koa没有了这些方法, 要怎么返回呢?

我直接this.body = 并没有效果

Fetch API cannot load http://192.168.33.1:8888/indie/song/1. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://127.0.0.1:8888' is therefore not allowed access. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.

初学koa不是很懂, es6正在啃, 但项目来的急啊, 先求方法解决一下

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