Subj : Re: undefined variables
To   : netscape.public.mozilla.jseng
From : Brendan Eich
Date : Wed Aug 25 2004 08:18 am

Igor Bukanov wrote:
> Stuart wrote:
> 
>> Hi,
>>
>> I am a little confused as to the use of undefined in javascript and 
>> Rhino.
>>
>> I think my fundamental problem is that I do not understand the purpose of
>> having both an Undefined and Null type/value in ECMAScript.
> 
> 
> I suggest to post a generic question about that topic to the newsgroup 
> so perhaps SpiderMonkey folks could tell the story.


The undefined value is the initial value of properties created without 
any other value, and the value returned when you access a property by a 
qualified expression (e.g., obj.prop, not prop) and no such property 
exists in the object (meaning directly in the object, or indirectly in 
an object on its prototype chain).

The null value is different, and connotes "no object" or "null 
reference".  It's occasionally useful to have a distinct value for these 
connotations from the undefined value, but note that null == undefined 
(and vice versa; == is reflexive and symmetric but not transitive; of 
course, as you would hope, null !== undefined).

/be

.