Small Proposal "!in"

[kai zhu]

-1

this feature mostly cross-cuts "!obj.hasOwnProperty('foo')".  the only
semantic-difference is "in" will check prototype-chain as jordan
pointed out (and widely regarded as a bad design-pattern), while
"hasOwnProperty" [correctly] will not.  in every real-world use-case
i've encountered, hasOwnProperty is what you actually want instead of
in.  and for looping, "Object.keys(obj).forEach(...)" is usually
faster than "for (key in obj) {...}" (partly because of again, the
bad-design prototype-chain lookup inherent with "in").

also, imagine the cognitive-load placed on yourself when debugging
integration-javascript with a mix of these cross-cutting
design-patterns:

```js
// can you easily spot where the subtle integration-bug is
// when dealing with code has a mashup of
// in and hasOwnProperty?

if ('foo' !in obj) {
    ...
}
...
if (!obj.hasOwnProperty('bar') {
    ....
}
```

in summary for those who like to adhere to the python/jslint philosopy
of "there should be one and preferably only one way to do any given
task":
1. for property-checks, "hasOwnProperty" is generally the one-and-only
correct-way to do it.
2. for looping over properties, "Object.keys(obj).forEach(...)" is
generally the one-and-only correct-way to do it.

following the above 2 guidelines will save you alot of needless
cognitive-load when doing integration-debugging of web-projects.

-kai

On 7/10/18, Naveen Chawla <naveen.chwl at gmail.com> wrote:
> Good point. I've never needed falsey valid values in my obj/maps yet, so
> it'll continue to be very rare that I will need to use `in` (and I'll
> continue to prefer !obj.x when I don't).
>
> However, for this proposal I would prefer a new keyword like `notin`
> instead of `!in`, because the `!` followed by a word normally indicates the
> inverse boolean of a value in other cases, whereas a clean break like
> `notin` I think seems clearer. Reserved keyword would probably be needed
> though?
>
> On Tue, 10 Jul 2018 at 09:44 Jordan Harband <ljharb at gmail.com> wrote:
>
>> `!obj.x` will return true for any falsy property - null, undefined,
>> positive or negative zero, NaN, and the empty string. `!(x in obj)` will
>> return true only if `x` is not an own property on `obj` nor anything in
>> its
>> prototype chain. They are decidedly different tests and they check for
>> decidedly different things.
>>
>> On Mon, Jul 9, 2018 at 9:08 PM, Naveen Chawla <naveen.chwl at gmail.com>
>> wrote:
>>
>>> I don't use `in`.
>>>
>>> What do I get with `'x' !in obj` or `!(x in obj)` that I don't get with
>>> !obj['x'] ?
>>>
>>> Genuinely asking - I don't know what I am missing.
>>>
>>> I use obj[x] because I believe it's a more familiar syntax and I believe
>>> I get the same outcome..(?)..
>>>
>>> On Mon, 9 Jul 2018 at 22:41 Steve Fink <sphink at gmail.com> wrote:
>>>
>>>> +1 from me for !in. It's a surprisingly common nuisance.
>>>>
>>>> And I don't care for the !obj.x workaround at all -- even if you can
>>>> survive the difference in semantics, from a code reading point of view
>>>> this
>>>> is saying something entirely different.
>>>>
>>>> And it is very different semantically. 'x' in obj does [[HasProperty]];
>>>> obj.x does [[GetProperty]]. With
>>>>
>>>>   obj = { get x() { print("getter"); return 3; } };
>>>>
>>>> then |"x" in obj| does not print "getter" while |obj.x| does.
>>>>
>>>>
>>>> On 06/29/2018 12:26 AM, Cyril Auburtin wrote:
>>>>
>>>>
>>>> ```js
>>>> if (!obj.x && !obj.y) {
>>>>      doit()
>>>> }
>>>> ```
>>>> The cases where they are equal to 0, '', null, undefined shouldn't
>>>> matter imo, if for example those x and y are numbers, they would be
>>>> defined, defaulted to 0, and you would test for `!== 0` rather if
>>>> needed
>>>>
>>>> Le jeu. 28 juin 2018 à 18:31, Guylian Cox <guyliancox at gmail.com> a
>>>> écrit :
>>>>
>>>>> I agree, it's very annoying to have to write it !(x in y). I've been
>>>>> wanting this operator for a very, very long time.
>>>>>
>>>>> If there is interest for !in, I think !instanceof deserves to be
>>>>> included too.
>>>>>
>>>>> Le jeu. 28 juin 2018 à 18:19, T.J. Crowder <
>>>>> tj.crowder at farsightsoftware.com> a écrit :
>>>>>
>>>>>> On Thu, Jun 28, 2018 at 5:14 PM, Tobias Buschor <
>>>>>> tobias.buschor at shwups.ch> wrote:
>>>>>> > I dont like to write:
>>>>>> > if ( !('x' in obj) &&  !('y' in obj) ) {
>>>>>> >      doit()
>>>>>> > }
>>>>>> >
>>>>>> > I was even tempted to write it that way:
>>>>>> > if ('x' in obj  ||  'y' in obj) { } else {
>>>>>> >      doit()
>>>>>> > }
>>>>>>
>>>>>> There's
>>>>>>
>>>>>> ```js
>>>>>> if (!('x' in obj  ||  'y' in obj)) {
>>>>>>      doit()
>>>>>> }
>>>>>> ```
>>>>>>
>>>>>> That said, I've wanted !in many a time, in a minor sort of way...
>>>>>>
>>>>>> -- T.J. Crowder
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