Subj : Re: Enumeration of extended properties of Array and Object prototypes
To   : netscape.public.mozilla.jseng
From : Chris Zumbrunn <chris@deployzone.com>
Date : Wed Mar 24 2004 02:56 am

Brendan Eich writes:

> 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.

OK, I see the proposed workarounds/solutions and I now realize why 
you're asking for real-world examples.

It's true that the problem is limited to inspecting/debugging loops in 
the cases of user-defined string methods or methods for other kinds of 
objects that do not usually serve as collections. But everybody quickly 
learns to avoid adding user-defined methods for the Object or Array 
prototypes - because these objects are used as collections and are 
regularly enumerated in for/in loops.

> An easy way for scripts to filter is to use Object.prototype.hasOwnProperty:
....
> Besides hasOwnProperty, for Arrays you could enumerate from 0 to 
> length-1.

While this is an acceptable workaround for inspecting/debugging loops, 
it can't be the solution for user-defined methods for Object and Array 
because almost any existing Javascript code that has been written 
expects for/in loops to enumerate these objects without the need for 
such filtering. Adding a user-defined method for Object or Array would 
break virtually any existing script.

Georg Maass writes:

> Do not prototype them to create your own variation. Derivate them to 
> define your own variation.

I guess that would be the "correct" and "clean" solution. But I think 
there are legitimate benefits (convenience, streamlined code structure) 
that can be gained from adding user-defined methods to the standard 
prototypes. 

Brendan Eich writes:

> Ideally, there'd be a direct (no prototype enumeration) for..in variant, 
> but I kept the original language very simple, and no one has thought to 
> add such a construct in ECMA.

That wouldn't solve the problem because all the existing code that uses 
the standard for/in statement would still break as soon as a 
user-defined method is added to the Object or Array prototypes.

One real-world example that might serve well to illustrate what I mean 
would be getter/setter methods for data conversion of Javascript objects 
(for example to and from XML). 

Chris

.