Object.prototype, getters and setters

Greetings,

I find the JavaScript's Object.prototype and getter/setter mechanism
very nice. However, I need some help with extending an object with
getters/setters in the derived class. For example:

A : function () {}

A.prototype =
{
a : null,

get a : function () { return a++; }
};


B : function () {}

B.prototype = new A;

Now, I would like to define a getter/setter in B. How do I do that?
There is no way to use the same syntax as in "A.prototype = ..." above.

Thanks for your help.

Regards,
--
Wei

re: Object.prototype, getters and setters

Wei Wang wrote:

[color=blue]
> I find the JavaScript's Object.prototype and getter/setter mechanism
> very nice. However, I need some help with extending an object with
> getters/setters in the derived class. For example:
>
> A : function () {}
>
> A.prototype =
> {
> a : null,
>
> get a : function () { return a++; }
> };
>
>
> B : function () {}
>
> B.prototype = new A;
>
> Now, I would like to define a getter/setter in B. How do I do that?
> There is no way to use the same syntax as in "A.prototype = ..." above.[/color]

What you have looks like some pseudo syntax to me, here is how it works
in pratice:

function A () {
this._a = 0;
}
A.prototype.__defineGetter__('a',
function () {
return this._a++;
}
);


function B () {
A.apply(this);
this._b = 0;
}
B.prototype = new A();
B.prototype.__defineGetter__('b',
function () {
return this._b--;
}
);

var b = new B();
alert(b.a);
alert(b.a);
alert(b.b);
alert(b.b);

And note that getters/setters are an extension to the ECMAScript edition
3 standard that is only implemented in the Mozilla JavaScript engine.


--

Martin Honnen
http://JavaScript.FAQTs.com/

re: Object.prototype, getters and setters

Martin Honnen wrote:
[color=blue]
> Wei Wang wrote:[color=green]
>> [...]
>> A : function () {}
>>
>> A.prototype =
>> {
>> a : null,
>>
>> get a : function () { return a++; }
>> };
>>
>>
>> B : function () {}
>>
>> B.prototype = new A;
>>
>> Now, I would like to define a getter/setter in B. How do I do that?
>> There is no way to use the same syntax as in "A.prototype = ..." above.[/color]
>
> What you have looks like some pseudo syntax to me,[/color]

But it is not entirely. Prototypes of objects inheriting directly
from Object can be defined using an Object literal and getters may
be defined with the `get' keyword before the property identifier:

<http://developer-test.mozilla.org/docs/Core_JavaScript_1.5_Guide:Creating_New_Objects:Def ining_Getters_and_Setters>

Whether the latter really *works* in an implementation is another issue.
It does work in JavaScript 1.5 as implemented in Mozilla/5.0 (tested
successfully with `Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.7.8)
Gecko/20050513 Firefox/1.0.4 (Debian package 1.0.4-1) Mnenhy/0.7.2.0'):

var A = function () {}; // although I'd prefer: function A() {}

A.prototype = {
a: null,
get b() { return this.a++; }
};

var x = new A;
alert(x.b); // 0
alert(x.b); // 1
[color=blue]
> here is how it works in pratice: [...][/color]

Since the above works in practice as well, I don't see a need to call
the __defineGetter__() method explicitely. Since method calls are
less efficient than variable instantiation, one could probably increase
efficiency with my approach.

What of course does not work is having `a' as identifier for both the
non-function property and the getter; identifiers have to be unique
within a scope.


PointedEars