Small Proposal "!in"

[Tobias Buschor]

There are valid use-cases.
As an example, some browsers have "onfocus" as an own property of "window",
some as an inherited.

```js
if ('onfocus' !in window) {
    // polyfill onfocus...
}
```


Am Mi., 18. Juli 2018 um 18:32 Uhr schrieb Mike Samuel <mikesamuel at gmail.com
>:

>
>
> On Wed, Jul 18, 2018 at 12:21 PM Michael Theriot <
> michael.lee.theriot at gmail.com> wrote:
>
>> I think it is irrelevant; the operator already exists and I would assume
>> if you want the negation of it you are using it correctly in the first
>> place. Otherwise are you not just arguing for its removal altogether? But
>> to answer your question one case that comes to mind is trapping get/has in
>> a proxy handler.
>
>
> Why should we assume that only people who consistently use `in` correctly
> would want the negation?  It seems that people who use it incorrectly
> because they are confused about the precise semantics or don't care might
> want the negation too.  If there are more of the latter then we should not
> assume what you assume.
>
> Proxy handler code is important, but very few developers will ever write a
> proxy handler over their careers, so this seems like a marginal use case.
> Besides, Reflect.has is probably a better choice in a proxy handler.
>
> I am not arguing for removing `in`.  That would break the web.  I am just
> arguing for prioritizing changes that provide features that more closely
> match the semantics developers typically want over making it more
> convenient to write code that seems to work in casual testing but has
> subtly wrong semantics.
>
>
>
>
>
>
>
>> On Wednesday, July 18, 2018, Mike Samuel <mikesamuel at gmail.com> wrote:
>>
>>>
>>>
>>> On Wed, Jul 18, 2018 at 11:05 AM Michael Theriot <
>>> michael.lee.theriot at gmail.com> wrote:
>>>
>>>> I think `in` and `instanceof` could both benefit from having negated
>>>> versions.
>>>>
>>>> Assuming the developer is using `in` correctly, hasOwnProperty concerns
>>>> are irrelevant. Either way they would attempt to use !(a in b), not
>>>> !hasOwnProperty.
>>>>
>>>
>>> Why should we assume the developer is using `in` correctly?
>>> Apologies if I buried my question at the end.  It was, what are the use
>>> cases for `in` that would not be better served by an ergonomic, infix
>>> hasOwnProperty?
>>>
>>>
>>> Same reason we don't use...
>>>> !(a == b) // a != b
>>>> !(a === b) // a !== b
>>>>
>>>
>>>
>>>> !(a > b) // a <= b
>>>> (!(a > b) && !(a == b)) // a < b
>>>>
>>>
>>> I'm not sure this is relevant to your larger point, and I've already
>>> conceded ergonomics, but
>>> these last two are not equivalent because NaN is weird.
>>>
>>> a = NaN, b = 0
>>> [!(a > b), a <= b]  // [true, false]
>>> [!(a > b) && !(a == b), a < b]  // [true, false]
>>>
>>>
>>>
>>>
>>>
>>>> On Thursday, June 28, 2018, 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()
>>>>> }
>>>>>
>>>>> What about a !in operator to write it like this?
>>>>> if ('x' !in obj  &&  'y' !in obj) {
>>>>>      doit()
>>>>> }
>>>>>
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>>>>
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-- 
Freundliche Grüsse
Tobias Buschor

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