Subj : Re: for/in loops
To   : netscape.public.mozilla.jseng
From : Alex Fritze
Date : Wed May 12 2004 03:12 pm

Brendan Eich wrote:
> Alex Fritze wrote:
> 
>> Hi,
>>
>> A couple of questions about for/in:
>>
>> 1.
>> Getters & setters defined with __defineGetter__/__defineSetter__ are 
>> currently not enumerated in for/in loops. Is there a reason for that?
> 
> 
> 
> Looks like a bug to me, especially in light of the prior getter= / 
> setter= operators implemented by yours truly, on which __dG__ and __dS__ 
> were based.  getter= and setter= were deprecated due to JS2 / ECMA-262 
> Edition 4 forward compatibility concerns.
> 
> I'll fix in the 1.8a trunk.
> 

I see the fix is already on the trunk. That was quick. Thanks!

> 
>> 2.
>> The code
>>   function f() { var x,y,z; }
>>   f.foo = "bar";
>>   for(var p in f) dump(p);
>> yields
>>   xyzfoo
>>
>> Why are f's private variables enumerated?
> 
> 
> 
> Compatibility with the pre-ECMA Netscape JS implementations.  We could 
> ditch this, but I'm not sure I wouldn't break something I care about. 
> Does this break you?

No, it doesn't really break me, it just struck me as odd. I'm wanting to 
  store meta information about a function (e.g. a help string) as 
function properties. I guess, while it might cause some jumping through 
hoops in SetFunctionSlot(), that is ok by ECMA and Spidermonkey and not 
asking for trouble, right?

> ECMA allows extensions, although extending function objects with 
> properties such as these makes for trouble.  See 
> http://lxr.mozilla.org/mozilla/source/js/src/jsinterp.c#443 and 
> surrounding.

Thanks for pointing me to that comment. I think I understand now how 
args & vars end up as properties, although I still don't get how 
enumerating them is useful - especially now that they are distinct from 
function properties from a user perspective.

Regards
Alex

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