On Thu, Jul 17, 2014 at 9:28 AM, Tobie Langel <[email protected]>
wrote:
> Hi all,
>
> [Jake Archibald recently pointed out][1] that the promise returned by
> `Promise.race` is rejected if any of the promises passed to `Promise.race`
> get rejected before one was resolved. So in the following example:
>
> ```
> var p = Promise.race([
> new Promise(function(_, reject) { reject(new Error("boom!")); }),
> new Promise(function(resolve) { setTimeout(resolve, 1000, "foo"); })
> ]);
> ```
>
> `p` will be rejected with error `"boom!"` (instead of being resolved with
> value "foo").
>
> Although this seems to fit some use case, it doesn't work when the aim is
> to resolve `p` with whichever promise resolves first and only reject `p`
> when all promises have failed. For example, when doing a cache lookup and
> racing it against the network, you want to display the data from whichever
> source resolves first, and only display a fallback resource when neither
> the cache nor the network are available.
>
> In the same thread, [Tab Atkins mentioned][2] `Promise.any` fulfills this
> use case and was briefly discussed within TC39 before being abandoned.
>
> I'd imagine a pseudo implementation would look something like the below,
> though you might want to surface all of the errors somehow.
>
> ```
> Promise.any = function(promises) {
> return new Promise(function(resolve, reject) {
> var count = promises.length, resolved = false;
> promises.forEach(function(p) {
> Promise.resolve(p).then(function(value) {
> resolved = true;
> count--;
> resolve(value);
> }, function() {
> count--;
> if (count === 0 && !resolved) {
> reject(new Error("No promises resolved successfully."))
> }
> });
> });
> });
> };
> ```
>
> Would TC39 be willing to (re)consider adding something like this to the
> spec?
>
For ES7, sure, this is something we can consider. ES6 promises
intentionally have a small API surface -- a much smaller one than I expect
for ES7 promises. As for what should be added, that will of course be an
extended discussion. Happy to see it start here.
>
> Thanks,
>
> --tobie
>
> ---
> [1]:
> https://github.com/slightlyoff/ServiceWorker/issues/359#issuecomment-48605442
> [2]:
> https://github.com/slightlyoff/ServiceWorker/issues/359#issuecomment-49056388
>
> _______________________________________________
> es-discuss mailing list
> [email protected]
> https://mail.mozilla.org/listinfo/es-discuss
>
>
--
Cheers,
--MarkM
_______________________________________________
es-discuss mailing list
[email protected]
https://mail.mozilla.org/listinfo/es-discuss
