Subj : Re: "named" vs. anonymous function
To   : Brendan Eich
From : Brendan Eich
Date : Fri Dec 10 2004 02:26 pm

Brendan Eich wrote:

> This is so that each function has the proper scope parent, yet we don't 
> have to compile each time to get a new function for each dynamic scope. 
>   Each dynamic scope, when activated, gets a clone of the 
> compiler-created function object, whose __proto__ is that 
> compiler-created funobj.
> 
> ECMA does not specify __proto__, so there's no way for a script to 
> notice this difference.  It could notice that function object identity 
> (== or ===) differs, but that's allowed by Edition 3 (search for "joined 
> function object").
> 
> /be
> 
>> spidermonkey shell:
>>
>> js> x=function foo(){}
>> function foo() {
>> }
>>
>> js> y=function (){}
>> function () {
>> }


To clarify further, a named function object, per ECMA-262 Edition 3 
11.2.5 and 13, the production:

FunctionExpression :
     function Identifier ( FormalParameterListopt ) { FunctionBody }

A named function expression is scoped by an Object created to hold its 
name only while it is active.  That is its activation's scope parent, so 
that cannot match the compiler-visible static scope parent (the global 
object in your js shell example).  Hence the clone.

Named and anonymous function objects do have different scope chains, per 
ECMA-262 Edition 3, but that does not map directly onto the __proto__ 
difference you discovered.  The latter is really an implementation 
detail allowed by the spec.

/be

>>
>> js> x.__proto__===Function.prototype
>> false
>> js> x.__proto__.__proto__===Function.prototype
>> true
>> js> y.__proto__===Function.prototype
>> true
>>
>> a graph perhaps better visualizing it:
>> <http://egachine.berlios.de/named-vs-anon.png> (13k)
>>
>> rhino does not do this:
>>
>> js> x=function foo(){}
>> function foo() {
>> }
>>
>> js> y=function (){}
>> function () {
>> }
>>
>> js> x.__proto__===Function.prototype
>> true
>> js> y.__proto__===Function.prototype
>> true
>>
>>
>> Jens

.