Subj : Re: Enumeration of extended properties of Array and Object prototypes
To   : Chris Zumbrunn <chris@deployzone.com>
From : Brendan Eich <brendan@meer.net>
Date : Sun Mar 21 2004 11:12 am

Chris Zumbrunn wrote:
>
> OK, so there will still be no way to make a user-defined dynamic or 
> class property non-enumerable.

Not unless something has changed in the ECMA working draft.

>>Why do you want this, exactly?  For what purpose?
> 
> Hmm, I'm surpised you're asking. What's the point of being able to add 
> custom methods to the Object or Array prototypes if doing so will break 
> any application that uses for/in loops?

I'm asking for a real-world example where such a loop breaks because of 
the user-defined prototype property.  Almost all such loops that I've 
seen are inspecting/debugging loops, where the added prototype 
properties can be filtered out, by script or by the reader.  It's not 
ideal, but it's not a fatal ("will break") situation.

An easy way for scripts to filter is to use Object.prototype.hasOwnProperty:

     for (var i in o) {
         if (o.hasOwnProperty(i))
             print(i, o[i]);
     }

It's easy to imagine for..in loops that are critical to a script, and 
that would fail in light of user-defined prototype properties. But I 
didn't want to imagine, I was looking for reality ;-).

/be

.