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 No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin '' 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