>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 <robjt...@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-requ...@mozilla.org wrote:
>
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> Today's Topics:
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> 1. Syntax operator for "default assignment if value doesn't
> exits" (Sultan)
>
> *From: *Sultan <thysul...@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@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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