Minor type confusion in proxies proposal?

Andreas Rossberg rossberg at google.com
Fri Jul 1 05:54:48 PDT 2011

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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