Object.getOwnPropertyDescriptor can return just about anything

Allen Wirfs-Brock allen at wirfs-brock.com
Tue Apr 29 17:49:15 PDT 2014

On Apr 30, 2014, at 8:43 AM, Jason Orendorff <jason.orendorff at gmail.com> 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
> 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}


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