Subj : Re: Using 'apply' to construct an object
To   : netscape.public.mozilla.jseng
From : Alexis Nikichine
Date : Wed Aug 31 2005 02:49 pm

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:

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

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Some domain is free

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