Default operator strawman - ||| rather than ??

[Thaddee Tyl]

On Tue, Jun 12, 2012 at 6:11 PM, Tab Atkins Jr. <jackalmage at gmail.com> wrote:
> On Tue, Jun 12, 2012 at 2:52 PM, Thaddee Tyl <thaddee.tyl at gmail.com> wrote:
>> On the other hand, non-existent properties are a use-case.
>> CoffeeScript provides a similar feature, but for null properties.
>>
>> For example, the following:
>>
>>    o =
>>     a: 1
>>     b: 2
>>
>>    alert o.c?.d
>>
>> compiles to:
>>
>>    var o, _ref;
>>
>>    o = {
>>      a: 1,
>>      b: 2
>>    };
>>
>>    alert((_ref = o.c) != null ? _ref.d : void 0);
>>
>> Special-casing undefined makes as much sense as special-casing null,
>> like CoffeeScript does.
>>
>> My point is that it is not obvious whether the non-existent properties
>> use-case should be "undefined"-specific or "null"-specific (or both).
>
> That's... pretty wrong.  In the example above, o.c returns undefined,
> not null.  If CoffeeScript *actually* compiles to the code you posted,
> then it only works because they're using != instead of !==, and null
> and undefined are both falsey.  However, it'll break if o.c is defined
> and is a falsey value like 0 or false.
>
> Try it.  Put the following in your nearest console and see what it returns:
>
> ({a:1, b:2}).c === undefined

CoffeeScript's property checking returns "undefined" if the property
is either undefined or null, and returns the property itself
otherwise.
If the property is false or 0, it returns the property (either false or 0).
I don't understand why you say it breaks if o.c is a falsey value like
0 or false.

The following code:

    Boolean.prototype.d = 'Property'

    o =
     a: 1
     b: 2
     c: false

    alert o.c?.d

will show "Property", as we'd expect.

My point still stands. Being "undefined"-specific is arbitrary.
CoffeeScript could have been "undefined"-specific; they were
"undefined" + "null"-specific, which I believe makes more sense.