On Apr 13, 2012, at 12:05 PM, Oliver Hunt wrote:
> On Apr 13, 2012, at 11:48 AM, David Herman <dher...@mozilla.com> wrote:
>
>> On Apr 13, 2012, at 11:35 AM, Oliver Hunt wrote:
>>
>>> foo(undefined);
>>
>> 1, 2, 1
>
> Uh what? I pass no arguments and arguments.length is 2, and i pass one
> argument and arguments.length is 1?
See my follow-up, answer to the first one was wrong; arguments.length is of
course 0 in the first case.
>>> foo(undefined, undefined);
>>
>> 1, 2, 2
>
> So i've passed 2 arguments, but they both get replaced?
Yes.
>>> foo(undefined, 3);
>>
>> 1, 3, 2
>
> So if you have default values we have to emit code to check for undefined in
> each case?
>
> is foo(a=5, b) valid? The above examples imply that it should be.
I'm somewhat ambivalent about it. Sure, it's true that "undefined always gets
replaced by default" means it's possible to get a defaulted argument followed
by a non-defaulted argument. But we also don't have to encourage writing
positional arguments like that; they can use options objects for that.
>>> foo(3, undefined);
>>
>> 3, 2, 2
>>
>>> Default values are for when arguments are not passed,
>>
>> Stated without evidence!
>
> C++, python, ...
That's precedent, but not an actual argument about *why* it should be done one
way or another. (Also, the decision is utterly different in a statically typed
language, so C++ isn't relevant.)
> This makes it impossible for me to distinguish between passing undefined as
> an argument and not passing an argument.
That's right. In that case, you should not use the x=expr default syntax. But
this is an uncommon case. The semantics you advocate is optimizing for the
wrong cases.
> If i've written:
>
> foo(undefined)
>
> There's a very clear expectation that i have selected undefined as the value
> i want to be passed.
You can just as easily say that if you've written:
foo(getOptionalValue())
there's a very clear expectation that if getOptionalValue() returns undefined,
you don't want the value to be passed.
> Yet you're saying this should be interpreted as foo() (only with a shorter
> arguments.length).
Longer argument.length, but yes.
Dave
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