> On Nov 30, 2015, at 11:57 AM, Geoffrey Garen <gga...@apple.com> wrote:
> 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.

Thanks for all the answers, everyone.

I like Geoff’s proposal.

Seems like we should not have to wait long for the “long term”. It seems that 
the built-in compiler could start magically transforming “@undefined” instead 
of magically transforming “undefined” any time we like; likely a simple find 
and replace job. Or it could do both during a transition period. It could even 
treat a bare “undefined” as an error if we are worried that we will make that 

Then perhaps later we could relieve the built-in compiler of those 
responsibilities if we think that’s important.

— Darin
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