typeof extensibility, building on my Value Objects slides from Thursday's TC39 meeting

David Bruant bruant.d at gmail.com
Mon Aug 5 01:53:44 PDT 2013


Le 05/08/2013 05:54, Brendan Eich a écrit :
> David Bruant wrote:
>> Le 04/08/2013 00:10, Brandon Benvie a écrit :
>>> On 8/3/2013 12:30 PM, David Bruant wrote:
>>>> That said, I recently worked on a project and I reviewed a pull 
>>>> request with "typeof x === 'object'" to ask to replace to 
>>>> 'Object(x) === x'.
>>> On a side note, I think your version of isObject is problematic 
>>> because it requires allocating an object every time the function 
>>> encounters a primitive.
>> It's not required. That's the raw reading of the spec and that's 
>> probably what the implementations do, but it's not required. When 
>> reading "if(Object(x) === x)", the JS engine, at the time of calling 
>> "Object(x)" knows that the outcome won't be used except for its 
>> identity. It also knows that what matters isn't even the identity, 
>> but whether the identity is equal to x. In essence, the allocation 
>> isn't required per se.
>
> Does any engine actually optimize away the wrapper?
Is this pattern used in a benchmark engines optimize against?
Filed https://bugzilla.mozilla.org/show_bug.cgi?id=901442 on the 
SpiderMonkey side.

David


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