"not a Date object" vs. "not an object with [[Class]] of ''Date''"

Dmitry A. Soshnikov dmitry.soshnikov at gmail.com
Sat Oct 2 14:05:01 PDT 2010

  On 03.10.2010 0:58, Dmitry A. Soshnikov wrote:
>  On 03.10.2010 0:51, Brendan Eich wrote:
>> On Oct 2, 2010, at 6:49 AM, Jorge wrote:
>>> On 02/10/2010, at 15:29, Brendan Eich wrote:
>>>> On Sep 6, 2010, at 1:43 AM, Dmitry A. Soshnikov wrote:
>>>>> For what to create a proxy? It's only for catch-traps (yes, it may 
>>>>> be used additionally to create a _catcher_ with "Array" [[Class]], 
>>>>> but without additions -- i.e. if a user wants just to inherit from 
>>>>> Array.prototype and to have all arrays' stuff -- proxies are not 
>>>>> needed to him).
>>>> Proxies are required to update length to be one greater than index 
>>>> (property name-string P such that ToString(ToUint32(P)) == P and 
>>>> ToUint32(P) is not 2^32 - 1) when setting an indexed property. 
>>>> Array's magic [[Put]] is not inherited.
>>> Why not simply spec an Array.create() ?
>>> -no need to redefine what an array is.
>> It's not clear from kangax's blog post that an array with an extra 
>> object on its prototype chain before Array.prototype is enough, but 
>> if it is, then yes: Array.create is simpler and more direct than 
>> using proxies.
>>> -no need to learn new concepts ( we're used to Object.create() 
>>> already )
>> There's definitely a new concept here. Right now you can't create an 
>> array instance whose [[Prototype]] is not some Array.prototype. 
>> Array.create changes that.
>>> -easy to grasp, expected behaviour.
>>> -it's a 3-liner that would take no more than 3 minutes to implement 
>>> in JS in any current UA.
>>> -it would just need to be in the ES specs.
>> "3-liner", "3 minutes" and "just" need demonstration. Are you writing 
>> the code and spec patches? Talk is cheap :-|. Arrays are highly 
>> optimized in modern engines (up to some array sparseness limit). 
>> Adding a prototype object shouldn't hurt if the new proto-object 
>> contains no indexed properties, though.
> Off-topic: what's wrong with the es-archive? Take a look please: 
> https://mail.mozilla.org/pipermail/es-discuss/2010-October/thread.html 
> I sent reply with implemented extended Object.create and 
> meta-constructor (meta-class) but it's not displayed there. Maybe it's 
> not reached es-discuss at all? If it didn't (by some reason), consider 
> this implementation (http://gist.github.com/607668). It allows to 
> create objects of any [[Class]] with needed inheritance chains.

Another variant btw, is just to set special internal property for the 
constructor. This property specifies the [[Class]] of objects created by 
the constructor.

function Foo() {
   this.push.apply(this, [1, 2, 3]);

Object.setObjectsClass(Foo, "Array");

var foo = new Foo();
foo.length // 3
Array.isArray(foo); // true
// etc.

> Dmitry.
>> /be
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