Are function arguments with the value `undefined` "present"?

David Bruant bruant.d at
Fri May 31 07:23:03 PDT 2013

Hi Till,

I believe Boris Zbarski touched on this recently at 
(I haven't read the full thread, so can't tell what the conclusion were, 
but it's probably worth checking out)


Le 31/05/2013 04:33, Till Schneidereit a écrit :
> I couldn't find any precise definition for the spec term "present", so 
> this is not entirely clear to me.
> Example:
> In step 7 of Array.prototype.reduce, a TypeError must be 
> thrown if the optional argument `initialValue` isn't present. If a 
> value of `undefined` does *not* cause the argument to be present, that 
> function can be implemented with the following signature:
> `function ArrayReduce(callbackfn, initialValue = undefined)`
> If, however, the value `undefined` means that the argument is present, 
> the signature should probably be:
> `function ArrayReduce(callbackfn,`
> The question isn't entirely academic, as I'm trying to get the 
> self-hosted implementations of builtins in SpiderMonkey to be as close 
> to the spec as possible.
> thanks,
> till
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