Question regarding duplicate __proto__ properties
allen at wirfs-brock.com
Sun Nov 30 09:53:08 PST 2014
On Nov 29, 2014, at 3:38 PM, Nicholas C. Zakas wrote:
> In Rev 28, B.3.1 it states:
> It is a Syntax Error if PropertyNameList of PropertyDefinitionList contains any duplicate entries for"__proto__" and at least two of those entries were obtained from productions of the formPropertyDefinition : PropertyName : AssignmentExpression .
> I noted that the duplicate name restriction was eliminated back in July and was just wondering if __proto__ is indeed a special case?
>  https://github.com/rwaldron/tc39-notes/blob/46d2396e02fd73121b5985d5a0fafbcdbf9c9072/es6/2014-07/jul-29.md#41-review-latest-draft
Yes, `__proto__ : <expr>` in an object literal is a special form with different semantics than the other PropertyDefinition forms. The relaxation of the duplicate entry was explicitly not applied to it.
It's worth noting that:
["__proto__"] : <expr>
do not mean the same thing.
The first form does a [[SetPrototypeOf]] and is the form that the above early error applies to.
The second form does a [[DefineProperty]] and does not place any restrictions of duplicates.
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