How can I download a file using window.fetch?

EDIT: syg answer is better. Just use downloadjs library.

The answer I provided works well on Chrome, but on Firefox and IE you need some different variant of this code. It's better to use library for that.

I had similar problem (need to pass authorization header to download a file so this solution didn't helped).

But based on this answer you can use createObjectURL to make browser save a file downloaded by Fetch API.

getAuthToken()
    .then(token => {
        fetch("http://example.com/ExportExcel", {
            method: 'GET',
            headers: new Headers({
                "Authorization": "Bearer " + token
            })
        })
        .then(response => response.blob())
        .then(blob => {
            var url = window.URL.createObjectURL(blob);
            var a = document.createElement('a');
            a.href = url;
            a.download = "filename.xlsx";
            document.body.appendChild(a); // we need to append the element to the dom -> otherwise it will not work in firefox
            a.click();    
            a.remove();  //afterwards we remove the element again         
        });
    });

This is more shorter and efficient, no libraries only fetch API

const url ='http://sample.example.file.doc'
const authHeader ="Bearer 6Q************" 

const options = {
  headers: {
    Authorization: authHeader
  }
};
 fetch(url, options)
  .then( res => res.blob() )
  .then( blob => {
    var file = window.URL.createObjectURL(blob);
    window.location.assign(file);
  });