Le 02/07/2011 15:21, Tom Van Cutsem a écrit :
> You're right, including inherited attributes would be more consistent
> with the current ES5 behavior.
>
> Since the proxy needs to "loop over" the properties of the property
> descriptor object, what better abstraction to mimic than the for-in
> loop? I.e. it is probably easiest for programmers to figure out what's
> going on if proxies detect custom attributes by doing:
>
> for (var attr in propertyDescriptorObject) {
> ... // copy all non-standard attributes into a fresh property
> descriptor object
> }
>
> That is: custom attributes are all own or inherited, enumerable
> property names of the property descriptor object, and overridden
> attribute values get priority over inherited attribute values.
>
> Does that sound right?
So the set of attributes would be the union of standard attributes
(regardless of enumerability) and properties retrieved through for-in if
I understand well.
It sounds like if the standard attribute set changes over time, code
could break, but it also looks like an intricable problem, so I think
that this is the best trade-off.
David
>
> 2011/7/2 David Bruant <[email protected]
> <mailto:[email protected]>>
>
> Le 02/07/2011 13:53, Tom Van Cutsem a écrit :
>> 2011/7/1 David Bruant <[email protected]
>> <mailto:[email protected]>>
>>
>> Tom: about "...except that it also retains any non-standard
>> attributes
>> present in the original property descriptor passed to
>> Object.defineProperty.", what do you mean by "any"?
>> Is it Object.keys(desc)?
>> Object.enumerate(desc) (I mean the prop list enumerated over
>> with for-in)?
>> Object.getOwnPropertyNames(desc)?
>>
>>
>> I meant any own properties of the argument, regardless of
>> enumerability (i.e. as determined by Object.getOwnPropertyNames).
>> This is consistent with the modifications to
>> Object.defineProperty, step 4.b.
> Actually, in ES5 the ToPropertyDescriptor function (8.10.5) (used
> in the original Object.defineProperty step 3 (and your version
> too)) calls [[HasProperty]] and [[Get]] on the object expected to
> become a property descriptor, meaning that the prototype is climbed.
> -----
> // Currently, for ToPropertyDescriptor (and Object.defineProperty
> as a consequence),
>
> null <| {enumerable:true, configurable:false} <| {set:
> function(){}, get:function(){}}
>
> // is equivalent to
>
> {set: function(){}, get:function(){}, enumerable:true,
> configurable:false}
>
> // because it calls [[HasProperty]] and [[Get]]
> // Sorry for the harmony:proto_operator notation abuse. I'm not
> sure I can chain it.
> -----
>
> I'm afraid that if we do not climb the prototype at all now, there
> might be inconsistencies if later the spec standardized custom
> attributes.
> In the case of a custom attribute is put in the prototype now, it
> won't be passed to a proxy, while it will whenever this attribute
> becomes part of a later version of the standard (because I assume
> that for consistency sake, [[HasProperty]] and [[Get]] will be
> used too to retrieve it).
> This may create complicated bugs to notice. (think about a trap
> looping over property descriptor attributes. standardizing more
> attribute could change the number of attributes looped over)
>
> I would recommand to climb the prototype to detect custom
> attributes since that's what the ToPropertyDescriptor algorithm
> does. Of course, passing every single property is stupid since it
> would pass things like 'hasOwnProperty' (retrieved from
> Object.prototype), hence the idea of filtering out non-enumerable
> properties.
> Maybe the set of properties passed to the trap could be the union
> of Object.getOwnPropertyNames and Object.enumerate (for-in loops)?
>
> Maybe it's already time to add a custom property descriptor like
> "notAPropertyAttribute" (defauting to false, sorry for double
> negation)?
>
> Another alternative is changing ToPropertyDescriptor, but I think
> it's too late now.
>
> David
>
>
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