Rails spinoffs - Feb 2008 - Get class of object created via Prototype (Introspection)

Hi,
I have a question.
How is it possible in Prototype to determine the class of an object
(i.e. constructor).
Example:
I define a class:
  "Car=Class.create({})"
Then I instantiate it:
  "benz=new Car()"

So and now I want to know what class benz is. I know I can do "benz
instanceof Car" which returns true. But how can I get the class
directly (I have many classes and don''t want to compare every object
with every class)?

Thanks.

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You could cheat and simply add ...

 _className : ''Car'';

to the class.



On 06/02/2008, Herr Ernst
<herr.ernst-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
>
> Hi,
> I have a question.
> How is it possible in Prototype to determine the class of an object
> (i.e. constructor).
> Example:
> I define a class:
>   "Car=Class.create({})"
> Then I instantiate it:
>   "benz=new Car()"
>
> So and now I want to know what class benz is. I know I can do "benz
> instanceof Car" which returns true. But how can I get the class
> directly (I have many classes and don''t want to compare every
object
> with every class)?
>
> Thanks.
>
> >
>

-- 
-----
Richard Quadling
Zend Certified Engineer : http://zend.com/zce.php?c=ZEND002498&r=213474731
"Standing on the shoulders of some very clever giants!"

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You can''t, that''s a limitation of the language, unfortunately.

Tobie

On Feb 6, 9:51 am, Herr Ernst
<herr.er...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> Hi,
> I have a question.
> How is it possible in Prototype to determine the class of an object
> (i.e. constructor).
> Example:
> I define a class:
>   "Car=Class.create({})"
> Then I instantiate it:
>   "benz=new Car()"
>
> So and now I want to know what class benz is. I know I can do "benz
> instanceof Car" which returns true. But how can I get the class
> directly (I have many classes and don''t want to compare every
object
> with every class)?
>
> Thanks.
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This is bad news for me.
Maybe you can help me though. My users can create some classes in a
particular order which are stored at runtime in an array. But I want
to make this persistent, i.e. storing in a cookie. I thought about
building a serialized string which contains the class/constructor and
the constructor arguments for each object. I''ve helped myself with the
following hack:

Car=Class.create({
  initialize: function() {
    this._class="Car";
  }
});

var benz=new Car();

//recreated benz
var benzclone=eval("new "+benz._class+"()")

Pretty dirty, isn''t it. Or does someone know a better method?

On Feb 6, 10:39 am, Tobie Langel
<tobie.lan...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> You can''t, that''s a limitation of the language,
unfortunately.
>
> Tobie
>
> On Feb 6, 9:51 am, Herr Ernst
<herr.er...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>
> > Hi,
> > I have a question.
> > How is it possible in Prototype to determine the class of an object
> > (i.e. constructor).
> > Example:
> > I define a class:
> >   "Car=Class.create({})"
> > Then I instantiate it:
> >   "benz=new Car()"
>
> > So and now I want to know what class benz is. I know I can do
"benz
> > instanceof Car" which returns true. But how can I get the class
> > directly (I have many classes and don''t want to compare every
object
> > with every class)?
>
> > Thanks.
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var benzclone = new window[benz._class]()


Herr Ernst wrote:
> This is bad news for me.
> Maybe you can help me though. My users can create some classes in a
> particular order which are stored at runtime in an array. But I want
> to make this persistent, i.e. storing in a cookie. I thought about
> building a serialized string which contains the class/constructor and
> the constructor arguments for each object. I''ve helped myself with
the
> following hack:
>
> Car=Class.create({
>   initialize: function() {
>     this._class="Car";
>   }
> });
>
> var benz=new Car();
>
> //recreated benz
> var benzclone=eval("new "+benz._class+"()")
>
> Pretty dirty, isn''t it. Or does someone know a better method?
>
> On Feb 6, 10:39 am, Tobie Langel
<tobie.lan...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> > You can''t, that''s a limitation of the language,
unfortunately.
> >
> > Tobie
> >
> > On Feb 6, 9:51 am, Herr Ernst
<herr.er...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> >
> > > Hi,
> > > I have a question.
> > > How is it possible in Prototype to determine the class of an
object
> > > (i.e. constructor).
> > > Example:
> > > I define a class:
> > >   "Car=Class.create({})"
> > > Then I instantiate it:
> > >   "benz=new Car()"
> >
> > > So and now I want to know what class benz is. I know I can do
"benz
> > > instanceof Car" which returns true. But how can I get the
class
> > > directly (I have many classes and don''t want to compare
every object
> > > with every class)?
> >
> > > Thanks.
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If all of your classes are globals only (i.e. no namespacing), you can
get away with something like this:

function getName(obj) {
  for(var name in window)
    if(window[name] === obj) return name;
}

getName(benz.constructor)
// -> "Car"

On Feb 6, 11:33 am, Herr Ernst
<herr.er...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> This is bad news for me.
> Maybe you can help me though. My users can create some classes in a
> particular order which are stored at runtime in an array. But I want
> to make this persistent, i.e. storing in a cookie. I thought about
> building a serialized string which contains the class/constructor and
> the constructor arguments for each object. I''ve helped myself with
the
> following hack:
>
> Car=Class.create({
>   initialize: function() {
>     this._class="Car";
>   }
>
> });
>
> var benz=new Car();
>
> //recreated benz
> var benzclone=eval("new "+benz._class+"()")
>
> Pretty dirty, isn''t it. Or does someone know a better method?
>
> On Feb 6, 10:39 am, Tobie Langel
<tobie.lan...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>
> > You can''t, that''s a limitation of the language,
unfortunately.
>
> > Tobie
>
> > On Feb 6, 9:51 am, Herr Ernst
<herr.er...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>
> > > Hi,
> > > I have a question.
> > > How is it possible in Prototype to determine the class of an
object
> > > (i.e. constructor).
> > > Example:
> > > I define a class:
> > >   "Car=Class.create({})"
> > > Then I instantiate it:
> > >   "benz=new Car()"
>
> > > So and now I want to know what class benz is. I know I can do
"benz
> > > instanceof Car" which returns true. But how can I get the
class
> > > directly (I have many classes and don''t want to compare
every object
> > > with every class)?
>
> > > Thanks.
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Thanks very much.
Classes are not global, so I use the above method.
Bye

On Feb 6, 12:00 pm, Tobie Langel
<tobie.lan...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> If all of your classes are globals only (i.e. no namespacing), you can
> get away with something like this:
>
> function getName(obj) {
>   for(var name in window)
>     if(window[name] === obj) return name;
>
> }
>
> getName(benz.constructor)
> // -> "Car"
>
> On Feb 6, 11:33 am, Herr Ernst
<herr.er...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>
> > This is bad news for me.
> > Maybe you can help me though. My users can create some classes in a
> > particular order which are stored at runtime in an array. But I want
> > to make this persistent, i.e. storing in a cookie. I thought about
> > building a serialized string which contains the class/constructor and
> > the constructor arguments for each object. I''ve helped myself
with the
> > following hack:
>
> > Car=Class.create({
> >   initialize: function() {
> >     this._class="Car";
> >   }
>
> > });
>
> > var benz=new Car();
>
> > //recreated benz
> > var benzclone=eval("new "+benz._class+"()")
>
> > Pretty dirty, isn''t it. Or does someone know a better method?
>
> > On Feb 6, 10:39 am, Tobie Langel
<tobie.lan...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>
> > > You can''t, that''s a limitation of the language,
unfortunately.
>
> > > Tobie
>
> > > On Feb 6, 9:51 am, Herr Ernst
<herr.er...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>
> > > > Hi,
> > > > I have a question.
> > > > How is it possible in Prototype to determine the class of an
object
> > > > (i.e. constructor).
> > > > Example:
> > > > I define a class:
> > > >   "Car=Class.create({})"
> > > > Then I instantiate it:
> > > >   "benz=new Car()"
>
> > > > So and now I want to know what class benz is. I know I can
do "benz
> > > > instanceof Car" which returns true. But how can I get
the class
> > > > directly (I have many classes and don''t want to
compare every object
> > > > with every class)?
>
> > > > Thanks.
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On Feb 6, 8:33 pm, Herr Ernst
<herr.er...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> This is bad news for me.
> Maybe you can help me though. My users can create some classes in a
> particular order which are stored at runtime in an array. But I want
> to make this persistent, i.e. storing in a cookie. I thought about
> building a serialized string which contains the class/constructor and
> the constructor arguments for each object. I''ve helped myself with
the
> following hack:
>
> Car=Class.create({
>   initialize: function() {
>     this._class="Car";
>   }
It would be better to reference the object itself rather than it''s
name:

      this._class = Car;

As far as I know, using an underscore for a property name is intended
to indicate a private variable, but _class is public and "class" is a
reserved word.  Why not just use the public constructor property?

>
> });
>
> var benz=new Car();
>
> //recreated benz
> var benzclone=eval("new "+benz._class+"()")
  var benzclone = new benz.constructor();

> Pretty dirty, isn''t it. Or does someone know a better method?
See above.  Alternatively, consider using the features built into the
language.  Use a normal constructor and the constructor property of
objects constructed from it:

  function Car(make) {
    this.make = make;
  }

  var benz = new Car(''Benz'');
  var fiat = new benz.constructor(''Fiat'');

  alert(fiat.make);


--
Rob

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On Feb 7, 4:07 am, RobG <rg...-AFFH1GffN5hPR4JQBCEnsQ@public.gmane.org>
wrote:
> On Feb 6, 8:33 pm, Herr Ernst
<herr.er...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>
> > This is bad news for me.
> > Maybe you can help me though. My users can create some classes in a
> > particular order which are stored at runtime in an array. But I want
> > to make this persistent, i.e. storing in a cookie. I thought about
> > building a serialized string which contains the class/constructor and
> > the constructor arguments for each object. I''ve helped myself
with the
> > following hack:
>
> > Car=Class.create({
> >   initialize: function() {
> >     this._class="Car";
> >   }
>
> It would be better to reference the object itself rather than it''s
> name:
>
>       this._class = Car;
>
> As far as I know, using an underscore for a property name is intended
> to indicate a private variable, but _class is public and "class"
is a
> reserved word.  Why not just use the public constructor property?
>
>
>
> > });
>
> > var benz=new Car();
>
> > //recreated benz
> > var benzclone=eval("new "+benz._class+"()")
>
>   var benzclone = new benz.constructor();
>
> > Pretty dirty, isn''t it. Or does someone know a better method?
>
> See above.  Alternatively, consider using the features built into the
> language.  Use a normal constructor and the constructor property of
> objects constructed from it:
>
>   function Car(make) {
>     this.make = make;
>   }
>
>   var benz = new Car(''Benz'');
>   var fiat = new benz.constructor(''Fiat'');
>
>   alert(fiat.make);
>
> --
> Rob
this is an elaborate example of how you could implement something like
this...

var Car = Class.create({

  initialize: function(make) {
    this.constructor = ''Car'';
    this.make = make;
  }

});

var Fiat = Class.create(Car, {

  initialize: function($super, type) {
    this.constructor = ''Fiat'';
    $super(''Fiat'');
    this.type = type;
  }

});

var panda = new Fiat(''Panda'');
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On Feb 7, 9:12 pm, Wizz
<woutaw...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> On Feb 7, 4:07 am, RobG
<rg...-AFFH1GffN5hPR4JQBCEnsQ@public.gmane.org> wrote:
> > On Feb 6, 8:33 pm, Herr Ernst
<herr.er...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
[...]
> > > Car=Class.create({
> > >   initialize: function() {
> > >     this._class="Car";
> > >   }
>
> > It would be better to reference the object itself rather than
it''s
> > name:
>
> >       this._class = Car;
>
> > As far as I know, using an underscore for a property name is intended
> > to indicate a private variable, but _class is public and
"class" is a
> > reserved word.  Why not just use the public constructor property?
>
> > > });
>
> > > var benz=new Car();
>
> > > //recreated benz
> > > var benzclone=eval("new "+benz._class+"()")
>
> >   var benzclone = new benz.constructor();
>
> > > Pretty dirty, isn''t it. Or does someone know a better
method?
>
> > See above.  Alternatively, consider using the features built into the
> > language.  Use a normal constructor and the constructor property of
> > objects constructed from it:
>
> >   function Car(make) {
> >     this.make = make;
> >   }
>
> >   var benz = new Car(''Benz'');
> >   var fiat = new benz.constructor(''Fiat'');
>
> >   alert(fiat.make);
>
> > --
> > Rob
>
> this is an elaborate example of how you could implement something like
> this...
I think you meant to reply to the OP.
> var Car = Class.create({
>   initialize: function(make) {
>     this.constructor = ''Car'';
See above.

>     this.make = make;
>   }
>
> });
>
> var Fiat = Class.create(Car, {
>
>   initialize: function($super, type) {
>     this.constructor = ''Fiat'';
>     $super(''Fiat'');
>     this.type = type;
>   }
>
> });
>
> var panda = new Fiat(''Panda'');
The question (if I understood it correctly) was how to create an
object from the same constructor as some other object when you don''t
know what the original constructor was - i.e. something like:

function makeAnother( obj, args ) {

  return another = new obj.constructor( args );

}


--
Rob

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