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

Rick Waldron waldron.rick at
Sat Sep 29 19:23:22 PDT 2012

On Sat, Sep 29, 2012 at 5:17 PM, Brendan Eich <brendan at> wrote:

> Allen Wirfs-Brock wrote:
>> However, there is a bigger issue here.  You could have asked a similar
>>  question about Map.prototype.  Why isn't Map.prototype a Map instances? Is
>> this a bug or an intentional design decision?
>> Historically, the prototype objects associated with the 9 named built-in
>> constructors were all defined to be instances of those constructors.  For
>> example, Boolean.prototype is a Boolean instance whose value is false.
>> In writing the specification for Map, I intentionally deviated from that
>> pattern.
> Failing to consult with implementors will just make editing churn. I don't
> remember much discussion on this change, if any -- we did talk about the
> general problem of prototype objects being firstborns of their class, and
> how this makes them sometimes not just basis cases but degenerate cases.
> However, Map is prototyped in SpiderMonkey and V8. In SpiderMonkey, the
> prototype is a Map, not an Object:
> js> Object.prototype.toString.**call(Map.prototype).slice(8, -1)
> "Map"

This should be considered a _must_have_. If the result were "object", only
confusion would follow.

> but you can't get, has, set, size, or iterate Map.prototype:
> js> Map.prototype.get('x')
> typein:2:0 TypeError: get method called on incompatible Map
> js> Map.prototype.has('x')
> typein:3:0 TypeError: has method called on incompatible Map
> js> Map.prototype.set('x', 42)
> typein:4:0 TypeError: set method called on incompatible Map
> js> Map.prototype.size()
> typein:5:0 TypeError: size method called on incompatible Map
> js> for (var [k, v] of Map.prototype) ;
> typein:6:13 TypeError: iterator method called on incompatible Map
> The error message is suboptimal but what's going on here is that
> Map.prototype has the SpiderMonkey equivalent of [[Class]] == "Map" (or the
> ES6 equivalent). This is important since all the builtins in ES3 + Reality
> (including RegExp; ES3 deviated there) make the prototype for built-in
> class C be of class C.
Subjectively, the examples here are exactly how I would expect (hope?) this
to behave, as the existing behaviour:

> Array.prototype.push(1)
> Array.prototype
[ 1 ]

...Has always been seemed "strange" ;)

> Your change requires implementations to provide a different built-in class
> (constructor/prototype) initialization path. That's not desirable _per se_.
> Neither IMHO is the user-facing semantic split among "old" and "new"
> constructors.
> There are two separate issues here:
> 1. Should Map.prototype be an instance (firstborn for its realm) of class
> Map?
> 2. Should Map.prototype be a key/value store that can be used or abused as
> any other Map could be?
> We should not mix these up. SpiderMonkey (and possibly V8, I haven't
> tested) says "yes" to 1 and "no" to 2.

1: yes
2: no


>     I did not specify that Map.prototype is a Map instance.  While it has
>> the methods that are applicable to Map instances it does not the internal
>> state that is necessary for those methods to actually work.  For example,
>> if you try to store a value into Map.prototype by calling its set method,
>> you will get a TypeErrior according to the specification. May.prototype can
>> not be used as a map object.
> That doesn't mean Map.prototype should not be classified as a Map per 1
> above.
>  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.
> Not so, as shown above. It's trivial in SpiderMonkey to give the prototype
> no instance state and check for that. Alternative implementation techniques
> are feasible with different trade-offs.
> Cc'ing implementors.
> /be
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