Expressing that a property is non-deletable (was: Function identity of non-configurable accessors)

Andrea Giammarchi andrea.giammarchi at
Wed Dec 19 08:28:30 PST 2012

didn't know about that custom possibility so yes, I guess we are OK here,
thanks for the update.


On Wed, Dec 19, 2012 at 5:54 AM, Tom Van Cutsem < at> wrote:

> 2012/12/18 Mark S. Miller <erights at>
> I see no reason why this needs to be a reflected property. As to
>> whether it is an exotic internal property or just prose, that is a
>> specification expository issue for which I defer to Allen. But the
>> spec only needs such extra state for the exotic global object. There's
>> nothing general about it.
> The deeper issue pointed out by David here, I think, is that the triplet
> {enumerable,writable,configurable} does not cover the entire possible space
> of what one might want to express about the behavior of a property.
> If I understand correctly, the concrete case here is a property that
> cannot be deleted (which is normally expressed as configurable:false), but
> which should still remain configurable.
> I think it is unpractical and unwieldy to bake new attributes in to the ES
> spec for every use case that pops up.
> However, in the context of proxies, we have previously talked about the
> usefulness of allowing handlers to accept custom attributes in their
> defineProperty trap and return custom attributes from their
> getOwnPropertyDescriptor trap.
> By analogy, it might make sense for WebIDL-specified host objects to
> return additional custom attributes to describe the behavior of special
> properties that cannot be captured by {enumerable,configurable,writable}.
> The remaining "problem", of course, is that there is no standard semantics
> for custom attributes. As mentioned by Allen in another thread, the only
> solution here is for an exotic object to properly document its interface,
> including what meaning it ascribes to any custom attributes it wishes to
> report or accept.
> Cheers,
> Tom
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