More than 10 years after the progress promises discussions and we still don't have a great pattern for progress signals...

My choices seem to be either: createX() returns a promise for a not-yet-initialized X object, which fires progress events and has a ready promise for when it's fully usable. Or, createX({ progressCallback }), which only fulfills once the X is fully ready.

The latter is a bit nicer but I can find zero precedent for it on the web platform...

fetch("test", {
  method: "POST",
  observer => {
    observer.onprogress = () => {
      console.log("finally");
    }
  }
});

const client = new XMLHttpRequest();
client.open("POST", "test");
client.onprogress = () => {
  console.log("been doing this for a long time");
}

It's not terrible, I think what people might prefer is an overloaded return promise. But subclassing promises is very much uncharted territory and prolly has its own issues.

As in, naively I would expect this:

const observer = fetch("test", { method: "POST" });
observer.onprogress = () => {
  console.log("finally");
}

If body is a Document, then set this’s request body to body, serialized, converted, and UTF-8 encoded.