koa ctx.throw([status], [msg], [properties]) 的 properties 怎么用？

Question

You may optionally pass apropertiesobject which is merged into the error as-is, useful for decorating machine-friendly errors which are reported to the requester upstream.

ctx.throw(401, 'access_denied', { user: user });

Koa 文档的这句完全看不懂。