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From: Darren Satkunas on 27 Nov 2009 09:54
I'm looking for a way to create a new instance of a Javascript Object
and assign it to an instance name that is determined at runtime. Is
there a way to do this without using the dreaded eval() function shown
below?

function MyObj(value){
this.value = value;
}
//use a static instance name
var Obj1 = new MyObj('abc');
console.log(Obj1.value);

//using a dynamic instance name
var runtimeInstanceName = 'Obj2'; //assume this string is determined
at runtime
eval(runtimeInstanceName+" = new MyObj('def');");
console.log(Obj2.value);
From: Dmitry A. Soshnikov on 27 Nov 2009 10:05
On Nov 27, 5:54 pm, Darren Satkunas <aluz...(a)gmail.com> wrote:
> I'm looking for a way to create a new instance of a Javascript Object
> and assign it to an instance name that is determined at runtime. Is
> there a way to do this without using the dreaded eval() function shown
> below?
>
> function MyObj(value){
>         this.value = value;}
>
> //use a static instance name
> var Obj1 = new MyObj('abc');
> console.log(Obj1.value);
>
> //using a dynamic instance name
> var runtimeInstanceName = 'Obj2'; //assume this string is determined
> at runtime
> eval(runtimeInstanceName+" = new MyObj('def');");
> console.log(Obj2.value);

If base object is known, in general case it looks like:

someObject[runtimeInstanceName] = new MyObj('def');

In case of global context, you can use `window` object to manages this
case (or, if exactly in global context - `this` keyword):

window[runtimeInstanceName] = new MyObj('def');
console.log(Obj2.value);

/ds
From: Darren Satkunas on 27 Nov 2009 10:14
On Nov 27, 10:05 am, "Dmitry A. Soshnikov"
<dmitry.soshni...(a)gmail.com> wrote:
> On Nov 27, 5:54 pm, Darren Satkunas <aluz...(a)gmail.com> wrote:
>
>
>
> > I'm looking for a way to create a new instance of a Javascript Object
> > and assign it to an instance name that is determined at runtime. Is
> > there a way to do this without using the dreaded eval() function shown
> > below?
>
> > function MyObj(value){
> >         this.value = value;}
>
> > //use a static instance name
> > var Obj1 = new MyObj('abc');
> > console.log(Obj1.value);
>
> > //using a dynamic instance name
> > var runtimeInstanceName = 'Obj2'; //assume this string is determined
> > at runtime
> > eval(runtimeInstanceName+" = new MyObj('def');");
> > console.log(Obj2.value);
>
> If base object is known, in general case it looks like:
>
> someObject[runtimeInstanceName] = new MyObj('def');
>
> In case of global context, you can use `window` object to manages this
> case (or, if exactly in global context - `this` keyword):
>
> window[runtimeInstanceName] = new MyObj('def');
> console.log(Obj2.value);
>
> /ds

Thanks Dmitry, works like a charm.

I didn't realize that `window` could be used as a reference to the
global scope.
From: Asen Bozhilov on 27 Nov 2009 10:15
Darren Satkunas wrote:
> I'm looking for a way to create a new instance of a Javascript Object
> and assign it to an instance name that is determined at runtime. Is
> there a way to do this without using the dreaded eval() function shown
> below?

> //using a dynamic instance name
> var runtimeInstanceName = 'Obj2'; //assume this string is determined
> at runtime
> eval(runtimeInstanceName+" = new MyObj('def');");

When been evaluate eval, will be defined property of Global Object.
See in ECMA 262-3 11.2.1 Property Accessors and from FAQ <URL:
http://www.jibbering.com/faq/#propertyAccessAgain >
From: Thomas 'PointedEars' Lahn on 27 Nov 2009 17:29
Darren Satkunas wrote:

> "Dmitry A. Soshnikov" wrote:
>> In case of global context, you can use `window` object to manages this
>> case (or, if exactly in global context - `this` keyword):
>>
>> window[runtimeInstanceName] = new MyObj('def');
>> console.log(Obj2.value);
>> [...]
>
> Thanks Dmitry, works like a charm.

It works by coincidence only; it tests positive only because of superficial
testing. It would not work so well if `runtimeInstanceName' happened to
store the name of a property of this Window instance that was implemented
read-only or had a significant meaning (e.g., `innerWidth' in Gecko-based
browsers; `window["innerWidth"] = new Object();' just managed to reduce the
width of my Iceweasel 3.5.5 browser window to about 20 pixels, while reading
it back yielded 474 [pixels]). DO NOT do this.

> I didn't realize that `window` could be used as a reference to the
> global scope.

First of all, you would not be referencing the/a scope (it cannot be
referenced), but the Variable Object of the global execution context, the
global object.

However, `window' fails in doing that, it refers to a Window instance (that
may have the ECMAScript Global Object in its prototype chain). As the first
page of the JavaScript 1.5 Reference says:

<https://developer.mozilla.org/en/Core_Javascript_1.5_Reference>
| The global object itself can be accessed by `this' in the global scope.

(whereas `scope' should read `execution context' there; objections?)

Assign it to a variable (I used `_global') or property (I am switching to
jsx.global) when needed in a local execution context.


PointedEars
--
Prototype.js was written by people who don't know javascript for people
who don't know javascript. People who don't know javascript are not
the best source of advice on designing systems that use javascript.
-- Richard Cornford, cljs, <f806at$ail$1$8300dec7(a)news.demon.co.uk>
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