An important part of HTTP servers is content negotiation. Perhaps the most important is Accept-Encoding and Content-Encoding, which negotiates whether to compress content. This is the only Accept header supported by most CDNs.

In Koa, this.request.acceptsEncodings() does all the content negotiation for you. Remember, if you compress your body, you should set the Content-Encoding header. View more about content negotiation in the official docs.

You do not want to do anything like if (~this.request.headers['accept-encoding'].indexOf('json')). These headers are very complex, and this type of logic is not specification-compliant. Use the this.request.accepts()-type methods.

Exercise

Using this.request.acceptsEncodings(), either send hello world gzipped or not gzipped (identity). For the purposes of this test, always set the Content-Encoding header, even if it's just identity.