convert from XMLHttpRequest to Fetch – Blancer.com Tutorials and projects

How can I convert the following javascript based on XMLHttpRequest to Fetch?
This is the sample code provided by Trix-editor for uploading images from here
I want to migrate to Fetch for a few reasons:

I want to be consistent across my codebase and use fetch()
for my other API calls.
I’m a newbie and read the XMLHttpRequest doc but don’t understand it. Fetch just seems more intuitive to me for some reason so another reason to convert this.
Plus, this code does not work for me but I don’t know how to
troubleshoot. I have a Flask route that uploads the file (the one specified as HOST) but the tags aren’t being saved in Trix-editor.

(function() {
var HOST = “http://localhost:5000/_upload/”

addEventListener(“trix-attachment-add”, function(event) {
if (event.attachment.file) {
uploadFileAttachment(event.attachment)
}
})

function uploadFileAttachment(attachment) {
uploadFile(attachment.file, setProgress, setAttributes)
```
 function setProgress(progress) {
   attachment.setUploadProgress(progress)
 }

 function setAttributes(attributes) {
   attachment.setAttributes(attributes)
 }
```
}

function uploadFile(file, progressCallback, successCallback) {
var key = createStorageKey(file)
var formData = createFormData(key, file)
var xhr = new XMLHttpRequest()
xhr.open(“POST”, HOST, true)
```
 xhr.upload.addEventListener("progress", function(event) {
   var progress = event.loaded / event.total * 100
   progressCallback(progress)
 })

 xhr.addEventListener("load", function(event) {
   if (xhr.status == 204) {
     var attributes = {
       url: HOST + key,
       href: HOST + key + "?content-disposition=attachment"
     }
     successCallback(attributes)
   }
 })

 xhr.send(formData)
```
}

function createStorageKey(file) {
var date = new Date()
var day = date.toISOString().slice(0,10)
var name = date.getTime() + “-” + file.name
return [ “tmp”, day, name ].join(“/”)
}

function createFormData(key, file) {
var data = new FormData()
data.append(“key”, key)
data.append(“Content-Type”, file.type)
data.append(“file”, file)
return data
}
})();