Subj : Re: undefined variables
To   : Stuart
From : Igor Bukanov
Date : Wed Aug 25 2004 11:50 am

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.

> 
> If I was creating a variable:
> 
> 	String name = null;
> 	ScriptableObject.defineProperty(myScope, "userName", name,
> ScriptableObject.READONLY);
> 
> Is it possible to set the 'value' of userName to undefined?

Sure, just use Context.getUndefinedValue() in place of the name. And 
then you may want to replace ScriptableObject.READONLY by 0: I guess you 
do not want read-only undefined value. BTW, when the script executes, it 
initialize all var names initially with:

ScriptableObject.defineProperty(scope, name, 
Context.getUndefinedValue(), ScriptableObject.PERMANENT);

where ScriptableObject.PERMANENT indicates that the property can not be 
deleted so "delete varName" would do nothing.
> 
> Also can you programatically test to see if the value of a variable is
> undefined?  I noticed that the getProperty can return Scriptable.NOT_FOUND -
> is this equivalent to undefined?

NO! Scriptable.NOT_FOUND is just a tag to indicate that there is no 
property in the object with the given name. It has nothing to do with 
undefined value. If the property exist and contain undefined value you 
will get Context.getUndefinedValue() as the result. And if it exists and 
contains null in JS sense you will get Java null.

Regards, Igor

.