Question regarding duplicate computed __proto__ properties

Allen Wirfs-Brock allen at
Sun Jan 4 21:13:17 PST 2015

On Jan 4, 2015 9:06 PM, Fred Schott <fkschott at> wrote:
> In Section B.3.1 on "__proto__ Property Names in Object Initializers" there is a paragraph explaining when duplicate properties will result in a syntax error. It says:
>> 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 form PropertyDefinition : PropertyName : AssignmentExpression .
> Where PropertyName is defined as: 
>> PropertyName[Yield,GeneratorParameter] :
>>   LiteralPropertyName
>>   [+GeneratorParameter] ComputedPropertyName
>>   [~GeneratorParameter] ComputedPropertyName[?Yield]
>> LiteralPropertyName :
>>   IdentifierName
>>   StringLiteral
>>   NumericLiteral
> That paragraph (using the definitions provided) seems to assert that it is a syntax error if there are any duplicate uses of __proto__ with an IdentifierName, StringLiteral, or ComputedPropertyName. To translate this into an example, it seems to assert that in ES6 this is not valid:
>> var obj = {
>>   __proto__: somePrototype,
>>   ["__proto__"]: somePrototype
>> // Error: SyntaxError

No, this is not supposed to be a syntax error.

> Is that correct? Step 6 of Section B.3.1 explicitly states that the computed ["__proto__"] does not have the same special behavior as the literal property. But from my understanding of other es-discuss topics & resources, the restriction on duplicate "__proto__" properties also does not apply to computed properties. If that is true, then the quoted paragraph above seems to be incorrect or misleading.
> Can anyone clarify this? I may just be misunderstanding the docs and/or the recent discussions. Or it could be that the definition has changed around that quoted paragraph and it needs to be updated.

The latter. Please file a bug at

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