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

Axel Rauschmayer axel at rauschma.de
Mon Jun 10 10:41:19 PDT 2013


Excellent.

On Jun 10, 2013, at 19:26 , Allen Wirfs-Brock <allen at wirfs-brock.com> wrote:

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