[Harmony proxies] [[Construct]] semantics breaks the expectation of seeing an object as return value

David Bruant david.bruant at labri.fr
Fri Sep 23 08:25:39 PDT 2011

Le 23/09/2011 17:17, Tom Van Cutsem a écrit :
> Hi David,
> I believe the relevant use cases here were value types/value proxies, 
> which - if added in the future - would want their constructors to 
> return non-object values.
Interesting. So maybe it should be kept in mind to relax the current 
constraint of [[Construct]] having to return objects (unless types/value 
proxies are planned on being objects)



> I searched the archives but can't seem to find the relevant threads, 
> still, I'm sure it was discussed here previously.
> Cheers,
> Tom
> 2011/9/23 David Bruant <david.bruant at labri.fr 
> <mailto:david.bruant at labri.fr>>
>     Hi,
>     In ES5.1 - 13.2.2 is defined functions instances [[Construct]]. As
>     one can notice, it always returns something of type Object (as per
>     ES5.1 - 8.6). Consequently, a call to "new A()" always reliably
>     either returns an object or throws an exception.
>     Proxies semantics of [[Construct]] states that (step 5) the
>     [[Call]] internal method of constructhandler should be called. It
>     opens the door to returning things which are not objects (number,
>     boolean, undefined, ...).
>     I can't think of precedents of this for host objects and I don't
>     really see a use case in allowing "new A()" to return something
>     else than an object
>     When writing functions intended to be used as constructors, people
>     may be used of |this| being used as the to-be-returned object and
>     expect this behavior even if defining a constructor.
>     So i would be in favor of adding a type check between step 5 and 6.
>     David
>     _______________________________________________
>     es-discuss mailing list
>     es-discuss at mozilla.org <mailto:es-discuss at mozilla.org>
>     https://mail.mozilla.org/listinfo/es-discuss

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.mozilla.org/pipermail/es-discuss/attachments/20110923/43df3c3e/attachment.html>

More information about the es-discuss mailing list