Why is the response object from JavaScript fetch API a promise?

Because somtimes we need a precise control for the loading process (from recieving the first piece of data to recieving the last one).

In actual world, json may not be a good example cause it's reletively samll. But imaging a situation where a large picture is loaded gruadually (from mosaic to clear). In that case, it is too late to inform the program when the data recieving has done completely.

Since fetch() is a relatively low level api, otherwise you could use axios or so on.

If your question is "why does response.json() return a promise?" then @Bergi provides the clue in comments: "it waits for the body to load".

If your question is "why isn't response.json an attribute?", then that would have required fetch to delay returning its response until the body had loaded, which might be OK for some, but not everyone.

This polyfill should get you what you want:

var fetchOk = api => fetch(api)
  .then(res => res.ok ? res : res.json().then(err => Promise.reject(err)));

then you can do:

fetchOk(API)
  .then(response => response.json())
  .catch(err => console.log(err));

The reverse cannot be polyfilled.