Sun Jul 4 11:13:42 PDT 2010
an inference from [[Class]]; if [[Class]] is x then [action]. However,
from a programmer's perspective, `Object.prototype.toString.call(x)`
cannot discriminate between objects originating in the host
environment (what is currently defined as "host object") and native
objects. Mostly they shouldn't care, but should follow the
specification approach to derive [[Class]] based inference, however
they can't follow that because that is not compatible with existing
implementations (notably IE versions and Opera (which copied IE)), and
making an "isStrict" supported global flag is not going to provide a
closely related inference about various types of host objects across a
wide range of implementations in the wild.
Internet Explorer 9 host objects seem to be absent of problems seen in
previous versions of IE but there are still host objects that are
callable, have [[Class]] "Object", and are not implemented with native
semantics. Two examples of such object are alert and addEventListener.
>> Because that allows `alert` to be any native ECMAScript object
>> (Function, Object, etc), while still letting it be defined as a host
>> object and not violating that spec.
> "that spec"? What specification demands that alert be a host object? I have
I cited 4.3.8; the definition of host object.
> not heard of any. This might be a consequence of the upcoming WebIDL-to-ES5
> language bindings, but I have seen no draft and so have no idea. My own
> preference would be for these language bindings to result in alert being a
> native Function, but that's an argument for a different standards committee
> If an implementation's alert is a Function, then it is a native object and
> its [[Class]] must be "Function". It can still be an object provided by a
> host environment whose [[Call]] behavior is written in C++. This simply
> makes it a host provided native built-in object (4.3.7), not a host object.
No, alert is not a defined by ECMAScript specification. It is
therefore host object, by definition of "host object" in ES5, s 4.3.8.
>> Iff, however, following my
>> proposed amendment, `alert` had [[Class]] "Object", and it was not a
>> native ES object (as in IE versions), then it would be a specification
>> > But it is also used from JS. Host objects are exempt from most of the
>> > specific behaviors specified for specific kinds of native objects. Were
>> > a
>> > host object to be able to allege to be a kind of native object without
>> > behaving as that kind of native object behaves, that would be bad.
>> This is not in the spec:
>> "without behaving as that kind of native object behaves"
> See 4.3.6.
That is where native object is defined.
>> While the specification does not preclude the possibility that a host
>> object may be implemented with native semantics, it nonetheless
>> defines a host object:
>> | 4.3.8
>> | host object
>> | object supplied by the host environment to complete the
>> | execution environment of ECMAScript.
>> | NOTE Any object that is not native is a host object.
>> And that means that `alert`, `window`, `document`, XMLHttpRequest, are
>> all host objects. Whether or not those objects are implemented as
>> native ECMAScript objects is another matter altogether.
> This is the crux. The language there is indeed poorly phrased. But native
> objects are not host objects.
Where is that stated anywhere in the specification?
> Indeed, it is so poorly phrased that perhaps we should add an errata to
> clean this up. Sigh. cc'ing es5-discuss.
>> It seems the the spec is wrong and that you have misinterpreted it. I
>> believe that instead it should be written:
> Text missing?
Yes, as I wrote before:
| The value of the [[Class]] internal property of any non-native host
| object must be any String value except one of..
The other possibility is that you have misinterpreted the definition
of host object to mean something along the lines of "any object that
is not a native object".
And going by that, referring to the example, `alert` could be a native
object and would then not be a host object.
>> > (alert instanceof Function) is not a reliable test in either direction.
>> No of course not.
>> > host object as well as a native non-function is perfectly free to
>> > inherit
>> > from Function.prototype and thus pass this test. And an actual function
>> > be an instance of Function constructor from another frame and so fail
>> > the
>> > test. But yes, iff alert is indeed a native function, it's [[Class]]
>> > be "Function".
>> Therein lies a contradiction: A host object here may be a function.
> No it may not. A host object may be callable and it may inherit from
> Function.prototype, but it may not be a function.
Once again, ES5 does not preclude the possibility that a host object
may be implemented with native semantics.
>> Yet because it is a host object, that same object's [[Class]] must not
>> be "Function", and yet again, since it is a function, and any function
>> must have [[Class]] "Function", then this object's [[Class]] must be
>> >> However according to ES5 specs, any host object must not be withing
>> > the set of values that are not allowable and so the assertion could be
>> >> made that if any object has a [[Class]] that is one of those values,
>> >> then the object is not a host object.
>> > Yes, that is intentional.
>> Then it will fail today, as
>> - will result "[object Object]" or "[object Function]"
> 1) No current implementations claim ES5 conformance.
> 2) On ES5 conformant implementations, alert may well be a function in which
> case the second result will remain correct.
The only way that could be anything other than a contradiction is if
you have redefined "host object" to be as I suggested above (I'll
repeat that one more time).
`alert` is a host object by definition of host object in s. 4.3.8. It
seems as if you have misinterpreted that definition as: "any object
that is not a native object". Is that how you read it?
That is a significantly different definition than either definition in
Eds 3 or 5.
| A host object is any object supplied by the host
| environment to complete the execution environment of
| ECMAScript. Any object that is not native is a host object.
| object supplied by the host environment to complete
| the execution environment of ECMAScript.
| NOTE Any object that is not native is a host object.
Perhaps this is a case of denying the antecedent?
1. if (not native) then host object.
2. alert is native,
therefore, alert is not host object. (false conclusion)
If the definition of host object is correct then the clause that
mentions "[[Class]]" property must change as I initially suggested.
However, if what appears to be your definition of host object was also
what was intended by the editor, then that definition should be
If a host object is defined as being any object supplied by the host
environment that is not a native object, then the spec should state
so. If, however, a host object may be a native object -- and that is
how it is currently defined, then for any host object that is not a
native object, that object's [[Class]] property must not be in the set
of class values for native types.
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