An array destructing specification choice

[Allen Wirfs-Brock]

On Nov 5, 2011, at 12:38 PM, Erik Arvidsson wrote:

> On Sat, Nov 5, 2011 at 11:55, Brendan Eich <brendan at mozilla.com> wrote:
>> The issue with ... in an array destructuring pattern is different from the case without. We have a choice, as you say. It's not obvious that doing a "get" of 'length' on the RHS (once per ...) is the right answer. It's plausible in my view that ... captures all indexed properties (index as defined by ECMA-262).
> 
> // A
> var [x, y, z] = {0: 0, 1: 1, 2: 2, length: 0};
> `$x $y $z`  // '0 1 2'
> 
> // B
> var [x, ...xs] = {0: 0, 1: 1, 2: 2, length: 2};
> `$x [$xs]`  // '0 [1]'
> 
> I think A is clear. It is just syntactic sugar for x = $tmp[0] etc. No
> need to check length. In this case the lhs drives this decision.
> 
> B IMO must check the length or it would have to iterate over all
> own(?) properties in rhs instead of just iterating over startIndex to
> length - 1. It is also more consistent with other operations that work
> on array like objects (slice, apply, splice...).
> 
> In Traceur we do the following:
> 
> var x, xs;
> (function($0) {
>  x = $0[0];
>  xs = Array.prototype.slice.call($0, 1);
>  return $0;
> }).call(this, {
>  0: 0,
>  1: 1,
>  2: 2,
>  length: 2
> });
> 
> I believe this is simpler to understand than to say that all indexed
> properties are used.

Just to be explicit about this point, the above use of slice is dependent upon length in exactly the way I'm suggesting...

Allen