Must built-in prototypes also be valid instances? (Was: Why DataView.prototype object's [[Class]] is "Object"?)

Allen Wirfs-Brock allen at
Sat Sep 29 17:16:23 PDT 2012

On Sep 29, 2012, at 4:20 PM, Yusuke Suzuki wrote:

> Thanks for your reply.
> Why did I do this?  Because, we are defining a significant number new built-in "classes" and it is going to be increasingly hard to define meaningful instance state for all such prototypes.  It is also arguably undesirable for most of these prototypes.  Does any really think it is a good idea for Map.prototype to be globally available storage bin into which anybody can store and retrieve key/value pairs?  Finally, note that the prototype objects created by ES6 class declarations typically are not valid instances of the class.  Programmers would have to go out of their way to initialize them with per instance state and there is really no good reason why somebody would do that.
> Agreed.
> I think it is time to recognize that the built-in ES constructors have always presented  a class-like structure where it really is unnecessary and even arguably undesirable for their associated prototype objects to also act as instance objects.  It's time to abandon that precedent and starting with the new ES6 built-ins to specify prototypes simply as objects that are containers of the methods shared by instances. In general, they should not be usable as instance objects.
> Current Map constructor allows `class DerivedMap extends Map { }` and it is very nice for ECMAScripters.
> So I have an another question. Do you have a plan to change prototype of ArrayBuffer and other objects to new style prototype, constructor of them to new style like a Map constructor, and Class check(such as [[Class]] is "DataView") to internal property check(such as object.[[HasProperty]]("DataView")) ?
> Probably because at least ArrayBuffer has no internal specialized method, I think transforming to subclassable constructor is easy.

My intention, subject to feedback here and from TC39,  is to follow the pattern I used for Map as much as possible.  However, TypedArray object are all ready implemented by all major browsers to that may limit how we apply it to them.

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