Using new foo where foo is a new Foo object.

[blackholebutterfly]

Why isn't this allowed?

function Foo(){
this.param1 = 'Constructor Function Foo';
}

var foo = new Foo;
var newfoo = new foo;

The last line above gives an error. I thought all objects could be
used as constructors. But it seems the foo object above cannot be used
with the 'new' operator to create more objects. Does anyone know why
that is?
Thanks.

[slebetman]

On Dec 22, 2:18 am, [e-mail] wrote:

Why isn't this allowed?

function Foo(){
this.param1 = 'Constructor Function Foo';

}

var foo = new Foo;
var newfoo = new foo;

The last line above gives an error. I thought all objects could be
used as constructors. But it seems the foo object above cannot be used
with the 'new' operator to create more objects. Does anyone know why
that is?
Thanks.

Javascript's object inheritence model is prototypal. But for a bunch
of historical reasons the designers of the language decided to
implement a model which tries to emulate classical inheritence (hence
the new keyword). This is why as you've found out "new" works for
"Constructor" objects but not for regular objects.

Other prototypal languages have an "object" operator instead of "new".
Traditionally, the difference between "object" and "new" is that while
"new" creates an object instance from a class (or constructor
functions in javascript) "object" creates an object instance directly
from a live object. Unfortunately the designers of javascript, while
trying to bend it to emulate classical inheritence, neglected to
provide an object operator/function.

Fortunately javascript's prototype model is complete enough that we
can write our own object method:

function object(o) {
function F() {}
F.prototype = o;
return new F();
}

So now you can write:

function Foo(){
this.param1 = 'Constructor Function Foo';
}

var foo = new Foo;
var newfoo = object(foo);

See http://javascript.crockford.com for more on this topic.

[blackholebutterfly]

You thought wrong. Constructors have to be objects that implement the
internal [[Construct]] method which applies to native Function objects and
certain host objects. See the ECMAScript Specification, Edition 3 Final,
sections 11.2.2 and 13.2.

Section 13.2 mentions a [[Class]] property, and elsewhere it says it
is "a string value indicating the kind of this object." Does that mean
that if I use the typeof operator only objects of type "Function" can
be used as constructors? How to tell if a given object implements the
internal [[Construct]] method?