Someone correct me if I'm wrong here, this is just my hunch - but according to the fetch spec, res.json returns a promise, which are async. I'm assuming so you have the chance to handle errors as a result of malformed JSON.

Promises are used for catching errors?

Yes.

At the time that the request resolves, it has received and processed the response headers, but it may still be receiving the response body (which is represented as a stream). All the body representations (like .text() or .json()) need to wait until the body has arrived before they can resolve.

https://fetch.spec.whatwg.org/#concept-body-consume-body