Analog to Object.getPropertyDescriptor() for *changing* a property value?

Brendan Eich brendan at
Thu Jun 23 11:08:20 PDT 2011

On Jun 23, 2011, at 4:54 AM, Axel Rauschmayer wrote:

>> How about this? You have a bunch of local variables in various functions referencing different objects. But by some obscure yet legal means you use one such reference to mutate the prototype object they share in common, so all the others appear to be mutated too.
>> This is not inherently "evil", but it's usually a bug. Usually the other different objects do not wish to be mutated so indirectly, and the users of their properties (own and delegated) in those various functions may not be prepared for such a change.
> So you are mainly worried about mutating methods?

No, any data property.

> My main concern is encapsulation: If you have class methods, I would want to give them the option of having class state. I want the same rules that hold for the object level (state + behavior) to hold for the meta-object level.

The prototype is not the class.

>> Consider monkey-patching. You want to add value to a shared prototype, but you might break for-in loops. Ok, that can be solved with ES5 Object.defineProperty -- but not if the mutation breaks some other invariant.
> I would probably never use for-in loops in

That does not address the general problem. Please read the whole sentence including the part I wrote after the "--".


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