On Apr 13, 2012, at 11:35 AM, Oliver Hunt wrote:
> What happens if i have:
>
> function foo(a=1, b=2) { log(a, b, arguments.length); }
>
> foo();
1, 2, 2
> foo(undefined);
1, 2, 1
> foo(3);
3, 2, 1
> foo(undefined, undefined);
1, 2, 2
> foo(undefined, 3);
1, 3, 2
> foo(3, undefined);
3, 2, 2
> Default values are for when arguments are not passed,
Stated without evidence!
> it does not make logical sense to say that they're the value given just
> because someone has passed undefined.
It makes perfect sense. But hey, I had the same reaction as you at first, so I
sympathize. Default values conceptually represent "no value provided," and the
natural way to think of this from a language designer or implementer's
perspective is to think about whether the syntax of the call included an
argument at the particular position. But this is not the *programmer's*
perspective. From the programmer's perspective, it's "do I have a value to
provide for this argument?" And in reality, the answer to that question is
often a *dynamic* one, not a static one. You often have to make the decision at
runtime whether to provide a value for a given argument. When you do, the
semantics based on arguments.length has terrible consequences: you either have
to write a combinatorial explosion of separate calls (code bloat --
unacceptable), or you just end up reimplementing the default logic by
conditionally producing the default value (abstraction violation --
unacceptable).
So after thinking more about it, I came to the conclusion that:
- default values are for when no value *of the expected input type* was provided
- undefined is almost universally used in JS as a sentinel value for "I have no
value of the expected input type," because it's not statically typed so it's
not possible to express option types
- whenever you need to make conditional decisions about whether you have a
value for various arguments, the arguments.length semantics has unacceptable
engineering consequences
- in the rare cases where you want to treat undefined as an acceptable value of
the expected input type, just don't use default values
> It also makes the behavioural definition more complex.
The behavioral definition is trivial. If the value of an argument with a given
default is undefined, you use the default.
Dave
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