Unconfigurable properties: why can I change [[Writable]] from true to false?

Allen Wirfs-Brock allen at wirfs-brock.com
Mon Jun 10 10:26:22 PDT 2013

The corresponding text describing [[Configurable]] in the ES6 draft has already been updated to clarify that [[Writable]] can always be set to false.


On Jun 10, 2013, at 4:57 AM, Axel Rauschmayer wrote:

> The ECMAScript 5.1 spec says (8.6.1):
> [[Configurable]]: If false, attempts to delete the property, change the property to be an accessor property, or change its attributes (other than [[Value]]) will fail.
> On the other hand [[DefineOwnProperty]] (8.12.9) allows one to change writability from true to false, even if a property is not configurable:
> 10 Else, if IsDataDescriptor(current) and IsDataDescriptor(Desc) are both true, then
> 	a. If the [[Configurable]] field of current is false, then
> 		i. Reject, if the [[Writable]] field of current is false and the [[Writable]] field of Desc is true.
> 		ii. If the [[Writable]] field of current is false, then
> 			1. Reject, if the [[Value]] field of Desc is present and SameValue(Desc.[[Value]], current.[[Value]]) is false.
> 	b. else, the [[Configurable]] field of current is true, so any change is acceptable.
> (10.a) seems to ignore the case of [[Writable]] of `current` being true. Is this deliberate?
> Should the description of [[Configurable]] be changed to better reflect the status quo?
> -- 
> Dr. Axel Rauschmayer
> axel at rauschma.de
> home: rauschma.de
> twitter: twitter.com/rauschma
> blog: 2ality.com
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