I think the idea of detecting if you have initialized or noninitialized
instance at runtime is very unhappy idea.
You know when it is called as an initializer (2, 4 below) and when not
(1 below). 3 below is legacy way of doing super - it can be simply replaced.
And there is also no breaking compatibility cost - so far, there was no
API to distinguish this; so once it is added, it can be safely specified
that "initializer" context is only 2 and 4.
Herby
Axel Rauschmayer wrote:
[Referring to Allen’s slides:
http://wiki.ecmascript.org/lib/exe/fetch.php?id=meetings%3Ameeting_jan_29_2013&cache=cache&media=meetings:subclassing_builtins.pdf
<http://wiki.ecmascript.org/lib/exe/fetch.php?id=meetings%3Ameeting_jan_29_2013&cache=cache&media=meetings:subclassing_builtins.pdf>
]
Is this really true?
I can see four ways of invoking a constructor function C:
1. As a function: C(...)
2. Via `new`: new C(...)
3. Via `call`: C.call(this, ...)
4. Via `super`, in a sub-instance, as a method (similar to #3):
super.constructor(...)
C[@@create] is only invoked during #2 (by the `new` operator). Thus, the
constructor’s role is always: set up an uninitialized instance. You
could add a check against an instance being initialized twice, but that
doesn’t seem to be what the slides are about.
The slides mention one use case: What if you want to have a function Foo
that can be either called as a function or as a constructor? You can’t
detect the difference if Foo is inside a namespace object. That is, you
want to distinguish #1 from all others, but #1 could happen to a
namespaced C. Observations:
– Does recognizing whether `this` is initialized really help here? Isn’t
it more about checking whether `this` is an instance of C?
– I’ve always considered this to be more of an anti-pattern for
non-builtin code. Thus, it seems useful to support a check, but only so
that namespaced constructors can avoid accidentally polluting the
namespace object.
– Won’t namespace objects go away with modules?
Thanks!
Axel
--
Dr. Axel Rauschmayer
a...@rauschma.de <mailto:a...@rauschma.de>
home: rauschma.de <http://rauschma.de>
twitter: twitter.com/rauschma <http://twitter.com/rauschma>
blog: 2ality.com <http://2ality.com>
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