I know that fetch is using promises and both of them allow you to do AJAX requests to a server. I have read that fetch has some extra features, which aren't available in XMLHttpRequest (and in the fetch polyfill, since it's based on XHR).
What extra capabilities does the fetch API have?
There are a few things that you can do with fetch and not with XHR:
There are a couple of things that you can do with XHR that you can't do yet with fetch, but they're going to be available sooner or later (read the "Future improvements" paragraph here: https://hacks.mozilla.org/2015/03/this-api-is-so-fetching/):
This article https://jakearchibald.com/2015/thats-so-fetch/ contains a more detailed description.