for-of statement of sparse array

Brendan Eich brendan at
Thu Jul 5 18:15:50 PDT 2012

Jason Orendorff wrote:
> On Thu, Jul 5, 2012 at 2:09 PM, Allen Wirfs-Brock<allen at>  wrote:
>>> Map may win at some point, who knows? It's not winning if one wants an array, numeric indexing, .length, the usual prototype methods.
>> We could consider also have "dense" interators available:
>>       for (let v of array.denseValues) console.log(v);
> This makes sense to me, but most arrays are meant to be dense in
> practice. So perhaps it makes more sense to add methods specifically
> for sparse arrays, making developers type more characters in the rare
> case rather than the common case:
>      for (let v of array.sparseValues()) ...;
>      for (let [i, v] of array.sparseItems()) ...;

No, this is human-hostile. Programmers don't generally make holes on 
purpose. You're right that accidental holes may mean bugs, but not 
necessarily -- especially if skipped. It's unclear and (I agree) messy, 
but given holes and 1JS compatibility, we can't really change the main 
iteration facilities used predominantly on arrays, and also used 
generically on all objects including arrays, to skip holes.

The JSFixed request for Object.prototype.forEach wants nothing like 
for(i=0;i<a.length;i++), when used on an Array instance. It must skip 
holes. This upholes the Array forEach (and all other extras) 
hole-skipping. The deck is stacked against for(;;) iteration in my view.

But to get back to your suggestion, that does not mean programmers think 
about sparse arrays usefully or specifically. They want generic 
iteration; they do not want holes to be iterated. At least, based on my 
experience and what I see others writing.


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