Re: [ServiceWorker] Returning a FetchPromise from fetch() (#625)

@domenic I'm trying to figure out how this works, but I find the ES6 heavy going. Can you clear up a few things?

Firstly, is this how it should work?

```js
var p = fetch(url);
p.abort();
// aborts fetch for url

var p = fetch(url).then(r => r.json());
p.abort();
// aborts fetch for url and rejects promise so .json is never called
// or if fetch already resolved:
//   terminate the stream and reject .json()

var p = fetch(url1).then(_ => fetch(url2));
p.abort();
// aborts fetch for url1 and rejects promise so url2 is never fetched
// or if fetch(url1) already resolved:
//   terminate the stream for url1
//   abort the fetch for url2

var p = fetch(url1).then(_ => wait(3000)).then(_ => fetch(url2));
// aborts fetch for url1 and rejects
// or if fetch(url1) already resolved:
//   terminate the stream for url1
//   reject the 'wait' so url2 never fetches
// or if wait(3000) already resolved:
//   terminate the stream for url1
//   abort the fetch for url2

var p = Promise.resolve(fetch(url));
p.abort();
// undefined is not a function (fetchPromise has been casted to a normal promise)
```

If the above is right, we need a generic `AbortablePromise`, then a hook on that for specific abort steps which `FetchPromise` would extend.

```js
var p = fetchPromise.then(_ => "Hello");
```

For the above to work, `.then()` needs to add state (or an onabort callback) to the new promise to maintain the link back to the original fetch. This cannot be done with `@@species` since that's on the `this` object's constructor. If `@@species` was first checked on the `this` object, it could maintain state that way.

I imagine I'm missing a simpler way.

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Reply to this email directly or view it on GitHub:
https://github.com/slightlyoff/ServiceWorker/issues/625#issuecomment-75217729