Subj : Re: Undefined Data State?
To   : netscape.public.mozilla.jseng
From : Brian Genisio <BrianGenisio@yahoo.com>
Date : Mon Jan 26 2004 07:41 am

Brendan Eich wrote:
> Brian Genisio wrote:
> 
>> Hello all,
>>
>> Lets say in the browser, you use an object that is undefined, such as :
>>
>> alert(window.opener); // assuming window.opener is undefined
>>
>> In this case, the browser will print "undefined".
>>
>> My question:
>>
>> Can the JSObject have a value of undefined?
> 
> 
> 
> An object is a type of value, so it can't have two values, itself and 
> undefined.  If you look at the ECMA-262 Edition 3 spec, you can see that 
> the [[DefaultValue]] internal method tries toString, then valueOf.  So 
> you could define a valueOf method on an object that returned undefined 
> (the undefined value).  But what would be the point?
> 
>>   Or does the object's private data need to keep that state... for 
>> instance, is the alert method checking a valid private object?
> 
> 
> 
> The private data slot of some native objects has nothing to do with any 
> of this.  I'm not sure why you think it does -- is there some glaring 
> misstatement in the docs that needs to be fixed?
> 
>>   Or is it checking the JSObject to see if it undefined?
> 
> 
> 
> What JSObject?  For window.opener, window must denote an object, or be 
> of a type able to be converted to an object (boolean => Boolean, number 
> => Number, string => String).  If window referred to null or undefined, 
> you would get a "window has no properties" ReferenceError on evaluating 
> window.opener.  If window is or can be converted to an object, then if 
> it has a property named 'opener', that property's value will be fetched 
> (the prototype chain will be searched as usual).  If it has no such 
> property (in window or any prototype), then undefined is the result of 
> evaluating window.opener.  In this case, there is no second object.
> 
> If window.opener denotes a property whose value is another object (the 
> opening window), then that second object will be the result.  But 
> there's no overloading of object and undefined values on a single 
> property, in any event.
> 
>> In other words, does the JS engine maintain undefined state?  Or does 
>> the programmer maintain undefined state with a valid object? 
> 
> 
> 
> I still don't know what you mean.  The Undefined type has one value, 
> undefined (also generated via void 0 and many other forms).  It's not an 
> object.  An object value can't coexist in a property with any other 
> value.  You can make an object convert to undefined via valueOf(), but 
> only in certain situations: when being converted to a number, in which 
> case the undefined further converts to NaN; or to a string, in which 
> case you get "undefined":
> 
> js> o = {valueOf:function(){return undefined;}};
> [object Object]
> js> o - 2;
> NaN
> js> o + 'hi';
> undefinedhi
> 
> (Per ECMA-262, an object never converts to a boolean value other than 
> true, so you can write 'if (window.opener)' to test for a non-null, 
> non-undefined, non-false, non-zero, not-empty-string reference without 
> having to say != null or otherwise constrain the expression to have 
> object or null type.)
> 
> /be

Thanks for the clarification.  I am a new spidermonkey user, who got 
confuesed between the multiple levels of abstraction; jsvals represent 
JSObjects, which have private data that points to an interface to the 
object I am representing... it gets confusing quickly.

Basically, I was writing a getProperty function for my object, and I was 
confused to what I needed to return in order to return undefined.

As I now understand (I could still be wrong), that I dont need to set 
anything to the return jsval for it to be undefined... if I leave it 
alone and return true from the getProperty function, then undefined is 
returned to the javascript code by default.

Thanks,
Brian

.