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JavaScript: Object Definition Extension

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  JavaScript: Object Definition Extension Gene Wirchenko <genew@ocis.net> - 2011-11-10 20:02 -0800
    Re: JavaScript: Object Definition Extension "Evertjan." <exjxw.hannivoort@interxnl.net> - 2011-11-11 08:49 +0000
    Re: JavaScript: Object Definition Extension beegee <bgulian@gmail.com> - 2011-11-11 05:40 -0800
      Re: JavaScript: Object Definition Extension John G Harris <john@nospam.demon.co.uk> - 2011-11-11 20:11 +0000
        Re: JavaScript: Object Definition Extension Gene Wirchenko <genew@ocis.net> - 2011-11-11 21:02 -0800
          Re: JavaScript: Object Definition Extension John G Harris <john@nospam.demon.co.uk> - 2011-11-12 16:46 +0000
            Re: JavaScript: Object Definition Extension Gene Wirchenko <genew@ocis.net> - 2011-11-14 12:26 -0800
              Re: JavaScript: Object Definition Extension beegee <bgulian@gmail.com> - 2011-11-15 06:49 -0800
                Re: JavaScript: Object Definition Extension beegee <bgulian@gmail.com> - 2011-11-15 09:47 -0800
              Re: JavaScript: Object Definition Extension John G Harris <john@nospam.demon.co.uk> - 2011-11-15 15:43 +0000
                Re: JavaScript: Object Definition Extension Gene Wirchenko <genew@ocis.net> - 2011-11-15 10:31 -0800
                  Re: JavaScript: Object Definition Extension Jake Jarvis <pig_in_shoes@yahoo.com> - 2011-11-15 23:35 +0100
                  Re: JavaScript: Object Definition Extension John G Harris <john@nospam.demon.co.uk> - 2011-11-16 20:42 +0000
        Re: JavaScript: Object Definition Extension beegee <bgulian@gmail.com> - 2011-11-12 09:35 -0800
          Re: JavaScript: Object Definition Extension John G Harris <john@nospam.demon.co.uk> - 2011-11-13 11:40 +0000
            Re: JavaScript: Object Definition Extension beegee <bgulian@gmail.com> - 2011-11-13 11:53 -0800
              Re: JavaScript: Object Definition Extension John G Harris <john@nospam.demon.co.uk> - 2011-11-14 16:19 +0000
                Re: JavaScript: Object Definition Extension beegee <bgulian@gmail.com> - 2011-11-15 10:02 -0800
    Re: JavaScript: Object Definition Extension Elegie <elegie@anonymous.invalid> - 2011-11-12 12:53 +0100
      Re: JavaScript: Object Definition Extension Denis McMahon <denismfmcmahon@gmail.com> - 2011-11-12 16:04 +0000
        Re: JavaScript: Object Definition Extension John G Harris <john@nospam.demon.co.uk> - 2011-11-12 18:14 +0000
          Re: JavaScript: Object Definition Extension Denis McMahon <denismfmcmahon@gmail.com> - 2011-11-13 19:34 +0000
        Re: JavaScript: Object Definition Extension Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2011-11-12 19:31 +0100
        Re: JavaScript: Object Definition Extension Gene Wirchenko <genew@ocis.net> - 2011-11-13 20:33 -0800
      Re: JavaScript: Object Definition Extension John G Harris <john@nospam.demon.co.uk> - 2011-11-12 16:52 +0000
        Re: JavaScript: Object Definition Extension Elegie <elegie@anonymous.invalid> - 2011-11-12 20:49 +0100
          Re: JavaScript: Object Definition Extension Gene Wirchenko <genew@ocis.net> - 2011-11-13 20:21 -0800
            Re: JavaScript: Object Definition Extension Elegie <elegie@anonymous.invalid> - 2011-11-14 10:20 +0100

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#8265

On Sat, 12 Nov 2011 at 16:04:01, in comp.lang.javascript, Denis McMahon
wrote:

  <snip>
>I think the OP wants to be able to assign different locations and values
>to different commercial properties.
>
>eg: http://www.sined.co.uk/tmp/objects.htm
>
>I'm sure that someone will be along soon to tell us that I've done it
>wrong, although he won't actually go so far as to post "the right way",
>because he much prefers to criticise the coding of others than to expose
>his own code to critical comment.

Speak of the devil and up he pops. Thank you for your kind comments.

First off : there is no "the right way". There are many ways and several
of them are equally clean; the rest have various amounts of messiness
built in, and some are downright perverse.

In your example you build a Commercial object by doing :

 var factory = new Commercial();
    factory.setLocation("13 West Street");
    factory.setValue("750000");
    factory.setZoning("Light Industry");
    factory.setUse("Vehicle Bodyshop");

My view is that it's simpler and easier to read to do

 var factory = new Commercial
       (  "13 West Street",
          "750000",
          "Light Industry",
          "Vehicle Bodyshop" );

If you have many Commercial objects to create you have to remember to
type four  factory.setX(...)  every time. Forget one line once and
you've got a bug and a lot of code to look through to find it.

  John
-- 
John Harris

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#8290

On Sat, 12 Nov 2011 18:14:31 +0000, John G Harris wrote:

> Speak of the devil and up he pops. Thank you for your kind comments.

I wasn't actually talking about you. But if you recognise yourself in the 
description ;)

> In your example ....
> My view is that it's simpler and easier to read to do
> 
>  var factory = new Commercial
>        (  "13 West Street",
>           "750000",
>           "Light Industry",
>           "Vehicle Bodyshop" );

So you'd prefer http://www.sined.co.uk/tmp/objects2.htm then.

(I thought I posted this or something like it yesterday, but it hasn't 
appeared in my newsfeed yet)

Rgds

Denis McMahon

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#8267

Denis McMahon wrote:

> Elegie wrote:
>> function RealEstate(PropertyLocation, PropertyValue) {
>>    this.PropertyLocation=PropertyLocation;
>>    this.PropertyValue=PropertyValue;
>> }
>> 
>> function Commercial (PropertyZoning, PropertyUse) {
>>    this.PropertyZoning=PropertyZoning;
>>    this.PropertyUse=PropertyUse;
>>    }
>> 
>> Commercial.prototype=new RealEstate("Kamloops",150000);
>> 
>> var Prop1=new Commercial("C-1","light commercial"); ---
> 
> Wouldn't that mean that all Commercial properties have the same location
> and value?
> 
> I think the OP wants to be able to assign different locations and values
> to different commercial properties.
> 
> eg: http://www.sined.co.uk/tmp/objects.htm
> 
> I'm sure that someone will be along soon to tell us that I've done it
> wrong,

You have done it wrong.

> although he won't actually go so far as to post "the right way",

The right way is to _not_ have Commercial instances inherit from the same 
(empty Property) instance as both of your codes did.  So you have certainly 
not avoided the mistake that you mention above; instead, you have added 
other mistakes on top of it.  One, because there are no methods to refer to 
them, the variable declarations in `Property' are mystical incantations; the 
code will "work" exactly the same without them.

The intention of the original code obviously was to have Commercial 
instances inherit from RealEstate.prototype, i. e. Commercial was intended 
to be a specialization of RealEstate.  In your nonsense code, Commercial 
ends up to be a specialization of Property, and RealEstate is missing.

> because he much prefers to criticise the coding of others than to expose
> his own code to critical comment.

[I am sure someone has told you before that you are an obnoxious ignorant 
who does not have a first idea about these programming languages or the APIs 
they can be used with (see also your one dysfunctional, and the other 
unnecessarily inefficiant and incompatible checkbox toggle suggestions 
earlier).]

The first thing to make this work without changing too much of the original 
code is get the identifiers right.  Property names should be concise, and 
start lowercase:

  function RealEstate(location, value)
  {
    this.location = location;
    this.value = value;
  }

  function Commercial(zoning, use)
  {
    this.zoning = zoning;
    this.use = use;
  }

Then one needs to set up the prototype chain properly, i. e. let Commercial 
inherit from RealEstate.prototype, not from a single RealEstate instance.  
This is facilitated with a method that Lasse Reichstein Nielsen introduced 
here years ago as clone() [1], that I named inheritFrom() afterwards (as it 
does not really clone) [2], and that Douglas Crockford eventually came to 
name object() (at least in his interesting, but slightly erroneous 
JavaScript introduction at Yahoo! [3]).  In its non-optimized form (see 
jsx.object.inheritFrom() for the optimization [4]), it is:

  function inheritFrom(proto)
  {
    function Dummy() {}
    Dummy.prototype = proto;
    return new Dummy();
  }

  Commercial.prototype = inheritFrom(RealEstate.prototype);

With this approach, a Dummy instance is defined as the prototype of 
Commercial instances that does nothing else than to inherit from 
RealEstate.prototype, thus establishing the prototype chain link between 
Commercial instances and RealEstate.prototype:

  (new Commercial()) → (Commercial.prototype === new Dummy())
  → RealEstate.prototype

But as Commercial.prototype.constructor is now Dummy implicitly, it would be 
well that that be fixed (this part you got right):

  Commercial.prototype.constructor = Commercial;

Afterwards the Dummy instance that Commercial.prototype refers to can be 
refined, serving as the prototype of Commercial instances.

(Function.prototype.extend() does all that in JSX:object.js [4].  It allows 
this inheritance approach to be written

  Commercial.extend(RealEstate, extensions);

as you would in natural language.  That also adds a `_super' property to 
Commercial.prototype to refer to RealEstate.prototype, and it adds a 
`_super' property to Commercial to refer to RealEstate, among other things.)

The latter which then can be created with

  var prop1 = new Commercial("C-1", "light commercial");


PointedEars
______
[1] 
<https://groups.google.com/forum/#!msg/comp.lang.javascript/CT4_IGmUob8/Gi4CKqG7WYoJ> 
or <news:u0ybgfdr.fsf@hotpop.com>; 2004-05-20
[3] 
<https://groups.google.com/forum/#!topic/comp.lang.javascript/HMJosLXN9LQ> 
or <news:4750707D.9010907@PointedEars.de>; 2007-11-30
[2] 
<http://pointedears.de/websvn/filedetails.php?repname=JSX&path=%2Ftrunk%2Fobject.js>
[3] <http://www.youtube.com/watch?v=v2ifWcnQs6M>; 2007
-- 
When all you know is jQuery, every problem looks $(olvable).

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#8303

On 12 Nov 2011 16:04:01 GMT, Denis McMahon <denismfmcmahon@gmail.com>
wrote:

[snip]

>I think the OP wants to be able to assign different locations and values 
>to different commercial properties.

     Yes.

[snip]

Sincerely,

Gene Wirchenko

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#8261

On Sat, 12 Nov 2011 at 12:53:57, in comp.lang.javascript, Elegie wrote:
>On 11/11/2011 05:02, Gene Wirchenko wrote :
>
>Hi,
>
>>    1) How should I be doing it?
>
>Using your example, what you probably are looking for is:
>
>---
>function RealEstate(PropertyLocation, PropertyValue) {
>  this.PropertyLocation=PropertyLocation;
>  this.PropertyValue=PropertyValue;
>}
>
>function Commercial (PropertyZoning, PropertyUse) {
>  this.PropertyZoning=PropertyZoning;
>  this.PropertyUse=PropertyUse;
>  }
>
>Commercial.prototype=new RealEstate("Kamloops",150000);
>
>var Prop1=new Commercial("C-1","light commercial");
>---
  <snip>

Your Commercial prototype is not a good idea. How, for instance, would a
new Commercial object get a property location that isn't Kamloops?

  John
-- 
John Harris

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#8269

On 12/11/2011 17:52, John G Harris wrote :

Hello John,

> Your Commercial prototype is not a good idea. How, for instance, would a
> new Commercial object get a property location that isn't Kamloops?

Actually I did not think about it at all, I simply wanted to introduce 
the OP to the concept of prototype chain, and have him reflect about it 
a bit...

Regards,
Elegie.

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#8302

On Sat, 12 Nov 2011 20:49:14 +0100, Elegie <elegie@anonymous.invalid>
wrote:

>On 12/11/2011 17:52, John G Harris wrote :
>
>Hello John,
>
>> Your Commercial prototype is not a good idea. How, for instance, would a
>> new Commercial object get a property location that isn't Kamloops?
>
>Actually I did not think about it at all, I simply wanted to introduce 
>the OP to the concept of prototype chain, and have him reflect about it 
>a bit...

     I already have been doing so.  That is why I wrote my test code
in the first place.

Sincerely,

Gene Wirchenko

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#8306

On 14/11/2011 05:21, Gene Wirchenko wrote :

Hi,

>> Actually I did not think about it at all, I simply wanted to introduce
>> the OP to the concept of prototype chain, and have him reflect about it
>> a bit...
>
>       I already have been doing so.  That is why I wrote my test code
> in the first place.

Reading you code in the first place induced me to think that while you 
were aware about inheritance (a way to structurally link objects 
together, so as to share properties and methods), you did not know 
anything about prototyped inheritance.

In short:
- you did not use any "prototype", but rather referred to some property 
named "template", about which I know nothing (my knowledge may be at 
fault though, because I only know the Third Edition of ECMAScript),
- you (apparently) tried to relate together two objects which shared 
common property identifiers, which could obviously only result in some 
properties shadowing the others,
- you did not know what the "new" keyword would do, i.e. the difference 
between the [[call]] and the [[construct]] behaviors of a function.

Consequently, I decided to give you an example that would let you 
discover the concept of prototype chain, and in addition I pointed you 
out to relevant parts of the specification, which could help you get a 
more solid knowledge about it.

The way that I participate in this group has many facets (as can be 
ascertained by reading my previous posts on this group):
- with beginners - and I put you in that category, as you have yourself 
qualified as such - I always do "tutoring javascript", i.e. point out to 
specifications and give working examples, but simplified to the point 
that the OP can understand it, and build on them to advance further on 
the javascript path,
- with proficient posters, I provide scripts solving problems, but not 
optimized (i.e. weak API, straightforward design), because I believe 
that context matters and that the OP has the skills to adjust the script 
to his / her needs,
- with experts, I usually do not provide scripts (because they would 
write them as easily as I do), but discuss aspects of the problem, 
giving my intuitions and reasonings. I do that very rarely though, 
because there are few "expert" topics which are of interest to me.

So, to sum it up, the example I provided was intentionally simple, since 
it was a product of "tutoring javascript". The problem that John and 
Denis have mentioned was probably the natural next step in understanding 
the prototype chain, and it is not a complicated one to solve, provided 
you really understand how the prototype chain works. The end-result, 
however, is not satisfying from a pattern point of view, and should lead 
you to ponder thoughtfully what should and should not be done with 
javascript inheritance (and what other alternate patterns you could use 
to better structure your code).

I am sorry that you do not appreciate my contributions. I will therefore 
put you at ease and refrain from posting them in your topics, so that 
you can concentrate on more useful answers. I wish you to enjoy your 
stay in that newsgroup, and to soon become proficient with the technology.

Regards,
Elegie.

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