Destructuring `undefined` and `null`

[Allen Wirfs-Brock]

On Jan 19, 2015, at 12:06 PM, Brendan Eich wrote:

> Why should it throw if there's no [[Get]]?
> 
> Consider let {x} = o as short for let x = o.x (with o evaluated once and first, in the general case, of course). Then the empty object pattern does nothing and should not throw.
> 
> This seem best for generated code purposes, it gives a more general basis case.

`let { } = x; `

is probably something that should never be written or generated.  But consider:

`{x:{ }} = x;`  asserts: the RHS must be coercible to an object that has an 'x' property that is coercible to an object

compared to:

`{x: { }={}} = x;` asserts: the RHS must be coercible to an object and if it has an 'x' property its value must be coercible to an object

compared to:

`{ } =  x` asserts RHS must be coercible to an object

compared to:

`[ ] =  x` asserts RHS must be coercible to an Iterable object.

Allen











> 
> /be
> 
> 
> 
> Axel Rauschmayer wrote:
>> If I understand the spec correctly, destructuring works as follows:
>> 
>> ```js
>> let {} = undefined; // OK???
>> let {x} = undefined; // TypeError
>> 
>> let [] = undefined; // TypeError
>> let [y] = undefined; // TypeError
>> ```
>> 
>> Destructuring `undefined` (or `null`) via `{}` does not throw an exception (as per first rule inside [1]). Shouldn’t it throw one?
>> 
>> Thanks!
>> 
>> Axel
>> 
>> 
>> [1] https://people.mozilla.org/~jorendorff/es6-draft.html#sec-runtime-semantics-destructuringassignmentevaluation <https://people.mozilla.org/%7Ejorendorff/es6-draft.html#sec-runtime-semantics-destructuringassignmentevaluation>
>> 
>> -- 
>> Dr. Axel Rauschmayer
>> axel at rauschma.de <mailto:axel at rauschma.de>
>> rauschma.de
>> 
>> 
>> 
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asserts: the RHS must be coercible to an object that has an 'x' property that is coercible to an object