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

Isiah Meadows isiahmeadows at gmail.com
Sat Jan 12 16:25:51 UTC 2019


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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