Oliver Hunt wrote:
On Oct 10, 2012, at 9:12 AM, Allen Wirfs-Brock <[email protected]
<mailto:[email protected]>> wrote:
On Oct 10, 2012, at 6:39 AM, David Bruant wrote:
2012/10/10 Keith Cirkel <[email protected]
<mailto:[email protected]>>
You /are/ actually passing in a second argument though, your
second argument is `undefined`. Default arguments aren't meant
to replace `undefined` values, they're meant to
be permissible for omission.
No, that isn't what the draft specification now says. It was
originally that way, but TC39 based upon discussions here, decided
that an explicit undefined argument value triggers default value
initialization.
I still disagree with this decision, but from an implementation
standpoint the cost difference is fairly minimal (there is of course
an additional runtime performance cost, but i suspect it would be
negligible).
The reason for treating undefined (and only undefined) as the sentinel
value that triggers defaulting may not be clear to everyone. It is so
that delegation layers do not have reproduce all the defaulting logic
just to call the target function with the right number of parameters:
function foo(a, b, c) {
log(stuff, a, b, c);
return bar(a, b, c);
}
function bar(a = da, b = db, c = dc) {
...
}
Without this, foo(1) and foo(2,3) calls would need to be special-cased
in foo's body to call bar with only 1 or 2 arguments.
I guess it depends on whether the committee feels that we should be
encouraging the use of null over undefined in new syntactic constructs.
What has null to do with your disagreement with there being a default
sentinel? Or did you mean that you disagreed with only undefined being
that sentinel?
/be
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