[[Get]] and GetValue can return Reference Type

Allen Wirfs-Brock allen at wirfs-brock.com
Tue Jun 12 09:11:22 PDT 2012

On Jun 11, 2012, at 11:03 PM, Yusuke Suzuki wrote:

> This issue is derived from Esprima issue 81
> http://code.google.com/p/esprima/issues/detail?id=81
> Hello everyone,
> According to the ECMA262 5.1th, [[Call]] of host object may return Reference Type.
> So, all values using raw [[Call]] result may be Reference Type.
> For example, [[Get]] may return Reference Type if getter is a host object that [[Call]] returns Reference Type.
> http://ecma-international.org/ecma-262/5.1/#sec-8.12.3
> As the result, GetValue may return Reference Type because that use result value of [[Get]]
> http://ecma-international.org/ecma-262/5.1/#sec-8.7.1
> And, for example, if we define getter of object, getter function is host object that [[Call]] return Reference Type,
>     (10, obj.getter)
> returns Reference type, so,
>     (10, obj.getter) = 10;
> is valid because GetValue is performed, but GetValue result is also Reference Type.
> In the ECMA262 5.1th, GetValue is assumed that doesn't return Reference Type, for exmaple,
>     var obj = {
>         get v() {
>            // this v function is host object and returns Reference;
>            // we can define this v using host object and defineProperty
>            return Reference;
>         }
>     };
> And,
>     typeof obj.v
> Then, typeof should treat Reference Type also, but in 11.4.3, Reference Type is not considered.
> http://ecma-international.org/ecma-262/5.1/#sec-11.4.3
> I think this is a bug of spec and GetValue and [[Get]] should not return Reference type, is it right?
> To fix this, I think following 3 way are good.
> perform GetValue on [[Call]] result in the spec
> re-define [[Get]] and GetValue can return Reference Type. This needs to re-define all the spec part using [[Get]], GetValue and others
> remove feature that [[Call]] can return Reference Type
> Because all modern engines don't consider that [[Call]] result is Reference and this feature makes big performance-regression, personally I think 3 is better.

Well, if your implementation doesn't provide a way for  host functions to return Reference values then you don't have to worry about it...

But, I agree that this seems to be an issue  and that it was introduced into the spec. when accessor properties were added.

I also think the preferable way to fix it would be to remove the [[Call]] returning Reference feature.  The last time we discussed this, I believe there was some effort made to keep HTML5 from enshrining legacy IE behavior that required this.  Does anybody remember? If it is only legacy IE that depends upon this feature then I'd think we could drop it.  Any implementation that wants [[Call]] returning References could still provide them as a non-standard extension.


> Regards,
> Yusuke Suzuki
> _______________________________________________
> es-discuss mailing list
> es-discuss at mozilla.org
> https://mail.mozilla.org/listinfo/es-discuss

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