How to follow upload progress with fetch()

_fetch-form.md

How can we follow the upload progress with fetch() ?

Simple: We can't.
Not right now anyway, fetch() does not support that, but good old XMLHttpRequest does though.

The function below behaves much like fetch(), it will return a Promise that will resolve into a Response object.

Just pass a progress function in the options parameter.

fetch-form.js

async function fetchForm(form, options = {}) 
{
    var method = options.method || form.getAttribute('method') || 'get';
    var action = options.url || form.getAttribute('action');
    var data   = new FormData(form);
    var xhr    = new XMLHttpRequest(); 

    return new Promise(function(success, failure) 
    {
        xhr.responseType = 'blob';
        xhr.onreadystatechange = function() 
        {
            if (xhr.readyState != 4) { // done
               return; 
            }

            var response = new Response(xhr.response, { 
                url         : xhr.responseURL, 
                status      : xhr.status, 
                statusText  : xhr.statusText
            });
            
            success(response);
        }        

        xhr.addEventListener('error', () => 
        { 
            failure( new TypeError('Failed to fetch') ) 
        });

        if (options.progress) {
            xhr.addEventListener('progress', options.progress);
        }

        xhr.open(method, action, true);
        xhr.send(data);
    });
}

how-to-use-example.js

function progress(e) 
{
    console.log( e.loaded + " loaded of " + e.total );
}

var form = document.getElementById('my-form');
fetchForm(form, { progress });

What is url : xhr.responseURL, meant to be doing?