||= is much needed?

[Rick Waldron]

On Thu, Jun 14, 2012 at 2:39 PM, Allen Wirfs-Brock <allen at wirfs-brock.com>wrote:

>
> snip
>



> I guess my concern is that there are significant existing subsystems where
> null is distinguished from undefined or where null has a specifically
> defined meaning that does not apply to undefined.  For example:
>
>     [[Prototype]] manipulation (Object.create, Object.getPrototypeOf,
> __proto__)
>     RegExp exec and derived APIs
>     JSON stringify/parse
>     the DOM (WebIDL, provides lots of options vis [TreatUndefinedAs]] to
> explicitly control the mapping of undefined)
>

More examples... event objects created by DOM APIs use null as the initial
value of any data property whose value won't be known until an actual event
has occurred.

See:
http://www.w3.org/TR/webmessaging/
http://www.w3.org/TR/2012/WD-DOM-Level-3-Events-20120614/



> snip
>



> One way to have it both ways is to have multiple syntactic forms for
> default value initializers.  EG:
>
> function f(a = 1, b ??= 2, c ||= 3) { }  //assuming ??= is undefined or
> null defaulting guard and ||= is falsy
>
> I'm not particularly convinced that the additional complexity is warranted
> but it would place the choice into ES programmers hands rather us trying to
> anticipate the typical intent and disadvantaging the untypical.
>

I agree with this entirely.

||= is complementary to || and makes sense - developers will embrace this
as is.


?? and ??= seem like "something is unknown" and unknown things can
otherwise be described as "undefined". Definitively, |null| is intentional
-- which implies something "known" and therefore cannot qualify as
"undefined". I think sticking to undefined will help to fix the abused "==
null" patterns in extant code (I'm thinking in the long term of course)

If null testing is needed:

a = a != null ? a : default;


I realize that the strawman currently spells it "?:", consider this message
an opposition to that.


Rick


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