Detecting the "type" of a DOM object

Allen Wirfs-Brock allen at
Mon Jun 25 11:39:30 PDT 2012

On Jun 25, 2012, at 11:10 AM, David Bruant wrote:

> Le 25/06/2012 17:49, Allen Wirfs-Brock a écrit :
>> On Jun 24, 2012, at 5:51 AM, Brendan Eich wrote:
>>> David Bruant wrote:
>>>> Instead of adding a new [[instanceofHint]] internal property, maybe the
>>>> [[NativeBrand]] could be reused.
>>> +1 on that, which suggests [[NativeBrandMatch]] instead of [[InstanceofMatch]].
>> No, the whole point of transitioning from [[Class]] to [[NativeBrand]] was to ensure that [[NativeBrand]] was a closed ended, non-extensible set of specification internal nominal type tags that are used within the ES specification primarily to specify various legacy behaviors (certain built-in methods only work on specific built-in objects, etc.)
>> Making [[NativeBrand]] extensible would be exactly equivalent to the current [[Class]] and would have the same issues.  For example, it conflates default toString behavior with the internal nominal types.
> From the latest draft:
> 4) If O has a [[NativeBrand]] internal property, let tag be the
> corresponding value from the Table 27.
> 5) Else if O is a host object, let tag be an implementation defined
> string value. This value may not be "Array", "Boolean", "Date", "Error",
> "Function", "JSON", "Math", "Number", "Object", "RegExp", or "String".
> 6) Else, let tag be the string value "Object".
> Non-spec objects cannot define a [[NativeBrand]], but can have a "tag"
> (which should probably be mentionned in Table 9, no?). This sounds very
> subtle to me.

This is just a transitional form of the specification of this method and not its final ES6 specification.

This version is essentially a refactoring that replaces [[Class]] with [[InternalBrand]] and moves the restriction that had previously applied to [[Class]] string values from the prose of 8.6.2 into this algorithm.

The next step is to specify a more explicit extension mechanism. In an early message on the original thread I said I favored making {}.toString extensional via a private named property.  I still do, although there are other possible extension mechanism. 
> Also, I don't mind if a new [[InstanceOfHint]] property (that can take
> any value) is added, but the difference between that and leveraging
> [[NativeBrand]] (so that non-spec objects can take any value with some
> exceptions) is also very subtle and I am not sure justified.

I really want to be explicit the extension points:
    1) Here is how you extend {}.toSting...
    2) Here is how you extend the instanceof operator...


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