Object.defineProperty with { get: undefined, set: undefined }

Mark S. Miller erights at google.com
Mon Aug 10 19:19:35 PDT 2009

On Mon, Aug 10, 2009 at 7:07 PM, Jeff Walden
<jwalden+es at mit.edu<jwalden%2Bes at mit.edu>
> wrote:

> On 3.8.09 18:39 , Allen Wirfs-Brock wrote:
>> Such a property would have pretty much the same effect as
>> Object.defineProperty(obj, propname, { value: undefined})
>> which creates a readonly, nonenumerable, nonconfigurable property whose
>> value is undefined. There isn't much (if any, I need to look a litter
>> deeper) real behavioral difference between such a data property and the
>> equivalent accessor property except for what can be observed using
>> Object.getOwnPropertyDescriptor.
> See the original email; in some contexts attempting to set the property
> will result in a TypeError being thrown.  If it were simply equivalent to a
> data descriptor with value undefined, I probably would be much less
> concerned.

It is equivalent (modulo reflection) to a *non-writable* property with value
undefined. Both throw when set from strict code. Neither throw when set from
non-strict code.

>  I suppose we could put more conditions into [[DefineOwnProperty]] to
>> reject
>> accessors where both [[Get]] and [[Put]] are undefined. Do you think this
>> is a significant issue?  Is it an error that is likely to be made?
>> [[DefineOwnPropoerty]] is already fairly complicated so unless there is
>> some real hazard to allowing such properties I'm inclined to leave it as
>> is.
> It's the throw-on-set behavior that I'm most worried about; I don't see how
> this particular flavor of property access is otherwise exposed, if this were
> to be forbidden.  As a consequence, I don't see much reason to require
> implementations to have to support the extra complexity (nor users to defend
> against it, as presumably they would to reduce confusion), not when it could
> be avoided by adding a single step to ToPropertyDescriptor:
> 9.b. If desc.[[Get]] is undefined and desc.[[Set]] is undefined, then throw
> a TypeError exception.
> Jeff
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