On Sat, Dec 3, 2011 at 8:27 AM, Axel Rauschmayer <[email protected]> wrote:
> I’m still trying to understand why some of the built-in constructors can’t be
> extended. The following describes my current understanding. If you would,
> please let me know where/if I’m wrong.
>
> - The following built-in constructors can’t be easily extended:
> Array
> Boolean
> Date
> Function
> Number
> Object
> RegExp
> String
> Error and sub-constructors
>
> - Reason (Array): Array instances need special support to update the length
> property depending on what elements are added. But that specialness should go
> away with Allen’s proposal for overriding element access via [].
Arrays have a special [[Put]], but Strings have a special [[Get]] too,
so Array isn't the only one with a custom internal method.
> - Reason (all): The constructors can’t be invoked as functions while handing
> in `this` – be it that they create new instances, be it that they do
> something else – they can’t be made to add properties to an existing
> instance. This prevents the pattern SuperConstructor.call(this, arg1, arg2,
> ...) from working.
That would be messy.
If the String constructor could add "String-ness" to an existing
object, what would it do to a Number object (which already have a
[[PrimitiveValue]] property), or an Array (which might already have a
"0" property), or a something that's already a String object, or ...
the possibilities are staggering :) Would it change the [[Class]] of
the obejct?
It's going to be hard to make that safe.
What you need (if you need it) is a way to create a, say, Date object
with a custom [[Prototype]].
That's why the common example given is Array.create - working
similarly to Object.create, but creating Array objects (with class
"Array" and Array behavior), just like Object.create creates Object
objects. Ofcourse this needs a .create on all the special
constructors.
Another option is to make a more generalized
Object.createByClass("Array", proto) that creates an object with class
"Array" and, since "Array" is a known class, also adds Array nature to
the created object. Doing Object.createByClass("arglebargle",
Object.prototype) just gets you an object where
Object.prototype.toString returns "[object arglebargle]'.
/L
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