resolve()/reject() on Promise subclasses and @@species

Claude Pache claude.pache at
Fri Oct 30 17:19:19 UTC 2015

> Le 29 oct. 2015 à 03:51, Boris Zbarsky <bzbarsky at MIT.EDU> a écrit :
> I was just implementing subclassing of Promise in Gecko, when I realized that given a Promise subclass MyPromise these two calls:
>  MyPromise.race([])
>  MyPromise.all([])
> will take MyPromise[@@species] into account when creating the return value, but these two calls:
>  MyPromise.resolve()
>  MyPromise.reject()
> will not; they will invoke MyPromise itself, not MyPromise[@@species].
> This is because and do the whole @@species thing but and do not.
> Is this behavior intentional?  If so, I'd really like to understand the reason for it.
> -Boris

Relevant discussion: <>

As I understand, the specified behaviour is an accident of history. Previously, `Promise.resolve()` and `Promise.reject()` used the species pattern in order to determine the constructor to be used, just as `Promise.all()` and `Promise.race()`. Then, at the last minute, it was decided that it wasn’t the best semantics for `Promise.resolve()`, and it was corrected. For `Promise.all()` and `Promise.race()`, the motivation wasn’t stronger than the lack of time.

Should it be corrected before @@species is widely implemented? I think so. The arguments I give are:

* Overall consistency in the language. Except for the two offending Promise static methods, all uses of `@@species` in the ES2015 spec (15 uses) are for the following pattern: Starting from one instance, one constructs a derived object for that instance. (The effective lookup of the @@species property is factored in the SpiecesConstructor and ArraySpeciesCreate abstract operations.)

* Also, in static methods like `Promise.all` and `Promise.race`, a constructor is explicitly provided by the user: simply use it. Compare with what is done for arrays:

	Array.from(someArray) // use the Array constructor explicitly provided.
	Array.of(x, y, z) // ditto

	someArray.slice(0) // compute the constructor to be used through some algorithm involving @@species.
	[].concat(x, y, z) // ditto


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