Subj : Re: undefined values
To   : Matthias Thiel <matthias@atip.de>
From : Brendan Eich <brendan@meer.net>
Date : Mon Aug 04 2003 09:54 pm

Matthias Thiel wrote:

> Hi,
> 
> is there a possibility to check, whether a variable is defined or not 
> (using SpiderMonkey) ?
> I tried out scripts like "x==undefined", but I got an error instead of true or false.
> With Rhino, such scripts are working and returning true or false. Is it a feature of Rhino ?
> 

I have no idea why that would work in Rhino -- perhaps you are not 
checking for an error exception?

You can test whether x identifies a property in an object o (or in an 
object on its prototype chain) via the 'in' operator:

     if ('x' in o)
         print('x identifies a property in o with value ' + uneval(o.x));

If you want to test whether a variable property identifier names a 
property in o, you would generalize the above like so:

     var v = 'x';
     if (v in o) {
         printf(v + ' identifies a property in o with value ' +
                uneval(o[v]));
     }

/be

.