Syntax operator for "default assignment if value doesn't exits"

Rob Ede robjtede at icloud.com
Sat Jan 12 16:39:48 UTC 2019


Yeah I guess there exists a use case for that method still. But I don’t feel that use case merits special syntax.

> On 12 Jan 2019, at 16:25, Isiah Meadows <isiahmeadows at gmail.com> wrote:
> 
> I'd just prefer a way to detect the value `undefined` vs no such binding.
>> On Sat, Jan 12, 2019 at 10:26 Sultan <thysultan at gmail.com> wrote:
>> >const [variable = defaultValue] =  [maybeUndefinedValue]
>> 
>> This would fail throw for variables you don't know exist i.e in the following example polyfill will throw if Symbol doesn't exist.
>> 
>> const [Symbol = SymbolPolyfill] =  [Symbol]
>> 
>> This is specifically why convention is to use:
>> 
>> typeof maybeUndefinedValue === 'undefined'
>> 
>> >const fn = ({ key = defaultValue }) => {  console.log(key); }
>> 
>> The second example doesn't assign the key prop to the passed object.
>> 
>> 
>>> On Sat, Jan 12, 2019 at 5:54 PM Rob Ede <robjtede at icloud.com> wrote:
>>> pretty succinct with existing de-structuring syntax:
>>> 
>>> ```
>>> const [variable = defaultValue] =  [maybeUndefinedValue]
>>> 
>>> const fn = ({ key = defaultValue }) => {  console.log(key); }
>>> ```
>>> 
>>>> On 11 Jan 2019, at 12:00, es-discuss-request at mozilla.org wrote:
>>>> 
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>>>>   1. Syntax operator for "default assignment if value doesn't
>>>>      exits" (Sultan)
>>>> 
>>>> From: Sultan <thysultan at gmail.com>
>>>> Subject: Syntax operator for "default assignment if value doesn't exits"
>>>> Date: 11 January 2019 at 10:52:12 GMT
>>>> To: es-discuss <es-discuss at mozilla.org>
>> 
>>>> 
>>>> 
>>>> An operator syntax for the the "typeof" pattern used to detect if a environment/object has a value:
>>>> 
>>>> if (typeof variable === 'undefined') {...}
>>>> if (typeof object.key === 'undefined') {...}
>>>> 
>>>> This could manifest in destructuring as the following
>>>> 
>>>> var fn = ({ key ||= 1 }) => {
>>>> }
>>>> 
>>>> And with variables as:
>>>> 
>>>> var global ||= {}
>>>> 
>>>> Equivalent code:
>>>> 
>>>> (arg) => {
>>>>     if (typeof arg.key === 'undefined') {
>>>>         arg.key = 1
>>>>     }
>>>> }
>>>> 
>>>> if (typeof global === 'undefined') {
>>>> var global = {}
>>>> }
>>>> 
>>>> 
>>>> 
>>>> 
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