See below:
I'm not sure who told Cameron that this is likely to change from the current
spec. but regardless I don't think it is a good idea.
Just to start, it means a method like:
Array.prototype.fill = function(value=null) {
this.forEach((v,i)=>this[i]=value)}
wouldn't do the obvious thing when called as:
someArray.fill(undefined)
Allen
Begin forwarded message:
> Resent-From: public-script-co...@w3.org
> From: Cameron McCormack <c...@mcc.id.au>
> Date: April 11, 2012 5:15:35 PM PDT
> To: "public-script-co...@w3.org" <public-script-co...@w3.org>
> Cc: Jonas Sicking <jo...@sicking.cc>, Brendan Eich <bren...@mozilla.com>,
> David Herman <dher...@mozilla.com>, Sam Tobin-Hochstadt <sa...@ccs.neu.edu>
> Subject: undefined being treated as a missing optional argument
>
> A few months ago we had some long discussions about what it means to pass
> undefined as the value of an optional argument, for example:
>
> void f(optional long x);
>
> called with:
>
> f(undefined);
>
> What we eventually settled on was that an explicit undefined is always
> treated as a specified argument, so for the above case undefined would be
> converted to the IDL long value 0. Here's where the original decision was
> announced:
>
> http://www.w3.org/mid/4ef3db9c.4080...@mcc.id.au
>
> Maybe the strongest reason for deciding this way was to align with how ES6
> was going to handle function argument default values. It's been brought to
> my attention though that this behaviour is not yet fixed for ES6 and there's
> a good possibility that ES6 will specify that passing undefined will cause
> the argument's default value to be used. If that happens, then it seems like
> we might again want Web IDL to change to be consistent with that.
>
> Unless there are any objections, I'm going to add a note to the spec to point
> out that undefined is currently not treated as a missing optional argument in
> alignment with ES6, but that ES6 may yet change and that we might revisit the
> decision at that point.
>
> (If we did revisit the decision, there are various knock-on changes we'd need
> to make (again) such as bringing dictionaries in line by having an undefined
> value specified for a dictionary member mean that it is not present, ensuring
> that the overload resolution algorithm can handle this, deciding what it
> means to pass undefined to an optional argument of IDL type any, and so on.)
>
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