Figuring out the behavior of WindowProxy in the face of non-configurable properties

Mark S. Miller erights at
Tue Jan 27 22:02:49 PST 2015

On Tue, Jan 27, 2015 at 7:22 PM, Brendan Eich <brendan at> wrote:

> Mark S. Miller wrote:
>> The reason why the intent is unwarranted is that the descriptor omits
>> "configurable:" rather than explicitly saying "configurable: true". If the
>> owner object already has a configurable own property of the same name, then
>> a defineProperty where the "configurable:" is omitted defines an own
>> property preserving the configurability of the original own property.
> Wild, and genius.


> How many more narrow escapes can we make and keep both web compat and
> integrity? :-P

How many will we need? ;)

> Is there any downside? What is the bad case that observably changes
> behavior, if any (not involving proxies)?

You get the following non-intuitive but allowed behavior.

if (!hasOwnProperty(W, P)) {
  defineProperty(W, P, { value: V })
  console.log(getOwnPropertyDescriptor(W, P).configurable); // true

However, you could also get this behavior if W is a proxy, so it doesn't
introduce any new cases beyond what's already possible. It is only

That's not much of a downside, and I can't think of any other downside.

I'm too tired to search the state space right now, throwing this out as a
> challenge.

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