For the time being, I like “x === undefined”.

Long term, I’d like us to switch to “x === @undefined”.

We use @ to indicate reserved words in built-ins. Currently, “@undefined" does 
not exist, but the built-in compiler magically transforms “undefined” a safe 
reserved word.

The typeof and void 0 code should be fast, but I find it obtuse.

Geoff

> On Nov 30, 2015, at 11:39 AM, Filip Pizlo <fpi...@apple.com> wrote:
> 
> I’ve also been guilty of:
> 
>       if (xxx === void 0)
> 
> This is slightly better than saying “undefined”, since that’s not actually a 
> reserved word.
> 
> I believe that all of these should perform the same.  We should pick one 
> based on what looks nicest and what has the most clear semantics.
> 
> -Filip
> 
> 
> 
>> On Nov 30, 2015, at 11:37 AM, Darin Adler <da...@apple.com> wrote:
>> 
>> I see the following in some code:
>> 
>>   if (xxx === undefined)
>> 
>> And I see the following in some other code:
>> 
>>   if (typeof xxx == “undefined”)
>> 
>>   or
>> 
>>   if (typeof xxx === “undefined”)
>> 
>> Is one preferred over the other, style-wise? Is one more efficient than the 
>> other?
>> 
>> — Darin
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