Prototype Object on Classes

[Yehuda Katz]

On 10/29/07, Lars T Hansen <lth at acm.org> wrote:
>
> On 10/29/07, Yehuda Katz <wycats at gmail.com> wrote:
> > I couldn't quite determine this from the white-paper:
> >
> > If I define a class using the new class constructs, and do *not* make
> it
> > dynamic, will I still be able to add new properties to the prototype
> object?
>
> Yes.
>
> > If so, doesn't that provide a simple way to end-run around the entire
> > lock-down that the new classical model provides?
>
> Depends on your point of view.  (My view is that non-dynamic objects
> are more useful to catch mistakes than to provide security.)
>
> You still can't:
>   - add properties to the object itself, since it's not dynamic -- you
> get a run-time error
>   - overwrite methods defined directly in the class (they're
> ReadOnly/DontDelete)



Even via the Prototype object? So this will fail?


class Foo { function bar() { } }
Foo.prototype.bar = function() { } #=> throw Error

Two more facts are relevant:
>   - Strict mode will report as illegal references to properties not
> defined in the class, if you've made the type of the object known
> through an annotation.



So this will fail?


class Foo { }
foo.prototype.bar = 1;
var x:Foo = new Foo;
y = x.bar; #=> throw Error


But this won't?


class Foo { }
foo.prototype.bar = 1
var x = new Foo;
y = x.bar #=> 1

  - When you define a class you get a prototype object that's
> basically empty, but whose [[Prototype]] is Object.prototype, so there
> are a couple of public properties there (toString etc).  You can add
> properties to your class's prototype object, or objects anywhere on
> the prototype chain, but basically anything defined in the class or
> any of its base classes take precedence over the properties in the
> prototype chain during lookup.



So the prototype object doesn't contain methods defined in a class?


class Foo { function bar() { } }
Foo.prototype.bar #=> undefined
Foo.prototype.toString #=> Object.prototype.toString()

I now realize a couple of these details were not mentioned in the
> paper but should have been.



Perhaps ;)

--lars
>
-- 
Yehuda Katz
Web Developer | Procore Technologies
(ph)  718.877.1325
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