Minor type confusion in proxies proposal?

Andreas Rossberg rossberg at google.com
Sat Jul 2 10:20:31 PDT 2011

Hi Tom.

On 2 July 2011 13:50, Tom Van Cutsem <tomvc.be at gmail.com> wrote:
> Hi Andreas,
> First, you're right about the typing issue:
> In ES5, for values of type Object, the signature for [[DefineOwnProperty]]
> would be:
> [[DefineOwnProperty]](P: a property name, Desc: an internal property
> descriptor, Throw: a boolean)
> On trapping proxies, that signature would need to change to:
> [[DefineOwnProperty]](P: a property name, Desc: an Object, Throw: a boolean)

I don't think such a change is consistent. [[DefineOwnProperty]] is
invoked in a number of places in the spec, and I think in many of them
the type of the receiver is not distinguished and may well be a proxy,
so a proxy may then receive both kinds of descriptors. Moreover, I
think it would be a mistake to make the appropriate case distinction
everywhere -- you really want the internal method to have the same
signature in all cases.

> With that, I believe the strawman is otherwise internally consistent. In
> [[DefineOwnProperty]] step 5, what will be passed to the user-defined
> "defineProperty" trap is a proper Object, not an internal descriptor.
> I did clarify the note you referred to, to be more explicit in this regard.
> I don't see an alternative to changing the signature of
> [[DefineOwnProperty]]. It can't just receive an internal descriptor, as it
> doesn't preserve any non-standard attributes.

How about simply bypassing [[DefineOwnProperty]] in
Object.defineProperty for proxies, as I suggested in my reply to
David? That seems to be the only place where additional objects can
occur, or am I wrong?


> 2011/7/1 Andreas Rossberg <rossberg at google.com>
>> On 1 July 2011 12:12, Andreas Rossberg <rossberg at google.com> wrote:
>> > I believe there is some "type" confusion in the proxy proposal spec
>> > wrt property descriptors and their reification into attributes
>> > objects.
>> >
>> > 1. In a note on the def of [[DefineOwnProperty]] for proxies, the
>> > proposal says:
>> >
>> > "The Desc argument to this trap is a property descriptor object
>> > validated by ToPropertyDescriptor, except that it also retains any
>> > non-standard attributes present in the original property descriptor
>> > passed to Object.defineProperty. See the semantics of the modified
>> > Object.defineProperty built-in, below."
>> >
>> > That seems fishy, since according to ES5 8.10:
>> >
>> > "Values of the Property Descriptor type are records composed of named
>> > fields where each field‘s name is an attribute name and its value is a
>> > corresponding attribute value as specified in 8.6.1."
>> >
>> > In particular, I take this to mean that property descriptors are not
>> > objects (but abstract records), and that there cannot be any fields
>> > whose name is not an attribute name. (In fact, in V8 we currently
>> > encode property descriptors using objects, but the encoding is
>> > different from the reified attributes object representation, and not
>> > quite compatible with the idea of adding arbitrary other fields.)
>> I forgot to say: step 5 of the definition invokes the defineProperty
>> trap of the handler passing Desc as the second argument. But the
>> handler expects a reified attributes object.
>> > 2. In the modified definition of Object.defineProperty, the proposal
>> > says in step 4.c:
>> >
>> > "Call the [[DefineOwnProperty]] internal method of O with arguments
>> > name, descObj, and true."
>> >
>> > This is passing descObj, which in fact is _not_ a descriptor, but its
>> > reification as an attributes object.
>> >
>> > /Andreas
>> >
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