On Apr 30, 2014, at 8:43 AM, Jason Orendorff <[email protected]> wrote:
> The [[Origin]] field of Property Descriptor Records is not yet
> implemented in Firefox. Eric Faust is looking at implementing it.[1]
> We noticed two interesting cases:
>
> 1. Suppose handler.getOwnPropertyDescriptor returns ({value: 0}). Then
> 9.5.5 Proxy.[[GetOwnProperty]] calls 6.2.4.6
> CompletePropertyDescriptor, and all the fields of the Property
> Descriptor Record are populated. But the object itself is not
> populated. This means Object.getOwnPropertyDescriptor will return an
> object that is missing most of the fields the caller wants to know
> about. This seems strange.
The idea is that a Proxy can define its own property descriptor formats that it
exposes to and accepts form user code. vis
Object.defineProperty/Object.getOwnPropertyDescriptors. This includes the
possibility of new property attributes and censoring built-in attributes.
For example, you might define a Proxy that exposes something like a typed array
but which did not have any attributes on indexed properties. Such a Proxy
might decided to return only {value: 0} as the descriptor for such a
zero-valued property.
>
> 2. The object returned by the handler can have getters. It can answer
> ToPropertyDescriptor's queries one way, and then say something else
> afterwards, making it look like language invariants have been broken.
Mostly, such custom descriptors are only returned to user code. I don’t
believe (it’s been while since I looked at this part of the spec, but once upon
a time I traced all these logic paths) there are any paths within actual
specified operations where a property descriptor is accessed in this manner and
that descriptor is immediately used to define another property.
There aren’t any internal invariant sensitivities that I could find. Once such
a non-standard descriptor is never directly used by any of the ordinary object
MOP operations
>
> Come to think of it, [[GetOwnProperty]] is a weird API. It computes
> two results: a set of Property Descriptor fields, and an [[Origin]]
> object. The ES language itself relies exclusively on the former.
> Scripts are only allowed to see the latter. That seems really weird to
> me.
>
Yes, that is the design. The set of fields are for internal use of specified
operations. The [[Origin]] field is there so a handler can produce a
non-standard descriptor and pass it back to user code. It is necessary, because
[[GetOwnDescriptor]] returns an internal record, not an object.
> What is an example of a concrete use case for this [[Origin]] feature?
> Is it to avoid allocating a new object here?
So a Proxy can define the property descriptors it produces and consumes. For
example, you might have a property descriptor like: {method: func,
referencesSuper: boolean}
Allen
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