Subj : Re: Using 'apply' to construct an object
To   : Alexis Nikichine
From : Brendan Eich
Date : Wed Aug 31 2005 10:31 am

Alexis Nikichine wrote:
> Brendan Eich wrote:
> 
>> franck wrote:
>>
>>> In fact, I want to create an object ( new myClass... ) with a 
>>> variable argument list ( ...apply( xxx, myListOfArguments ) )
>>>
>>> An alternative way to achieve this is :
>>>
>>>   new myClass( args[0], args[1], args[2], args[3], args[4], ... 
>>> args[n] );
>>>
>>> But the constructor of the object I am trying to create failed if I 
>>> provide more than the expected number of arguments.
>>
>>
>>
>> You are quite right that it's not possible to combine new with apply, 
>> unfortunately.  We hit this working on Narcissus 
>> (http://lxr.mozilla.org/mozilla/source/js/narcissus/jsexec.js -- look 
>> for __construct__).
>>
>> This is something to work around for now, as you propose and as we did 
>> for Narcissus.  We'll fix it for Edition 4 / JS2.
> 
> 
> But let me point out that it is possible to work around it while staying 
>  in the current language:


Sure, but these are both workarounds.  The way Franck and Narcissus do 
it is limited to a certain number of arguments, but does a true 
constructor call.  The way you do it (cited below) wastes an object, but 
does not have the arguments limit.  Neither is ideal.

/be


> var constructWithArgs = (function() {
>     function Dummy(){ ; }// Scope-contained, "private",
>                          // dummy constructor. No other code
>                          // needs access to this constructor.
>     return (function(fun, args) {
>         Dummy.prototype = fun.prototype;
>         var tmp = new Dummy; // Internal [[Prototype]] of new
>                              // object is set to fun.prototype
>                              // but no other properties are
>                              // created/changed.
>         fun.apply(tmp, args); // Use constructor to create/apply
>                               // new properties to the Dummy instance,
>                               // creating an object indistinguishable
>                               // form one created with - fun -.
>         return tmp; // Return the new object
>     } )
> })
> 
> I won't claim credits for this, but leave them to Richard Cornford in 
> http://groups.google.fr/group/comp.lang.javascript/msg/6e06b7884b3db6ad
> 
> Or was some flaw overlooked in this code ?
> 
> Cheers,
> 
> Alexis
> 
> -- 
> Some domain is free

.