On Jul 5, 2012, at 10:54 AM, Brendan Eich wrote:
> Allen privately observed that Array forEach skips holes, matching for-in.
> That counts for a lot with me -- we have only a blind
> for(i=0;i<a.length;i++)... loop not skipping holes, but of course it
> wouldn't. That is weak precedent on which to build for-of.
Specifically, for consistency, I think
array.forEach((v)=>console.log(v));
and
for (let v of array) console.log(v);
should yield the same results.
[].forEach skips "holes" so the default iterator for arrays should too. All
the other "Array extras" also have the skipping behavior.
>
> 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);
Allen
>
> /be
>
> Jason Orendorff wrote:
>> On Thu, Jul 5, 2012 at 6:11 AM, teramako<[email protected]> wrote:
>>> Firefox 15 or later can use for-of statement.
>>> I have a question about this.
>>>
>>> var array = new Array(3);
>>> for (var i in array) {
>>> console.log(i);
>>> }
>>> for (var v of array){
>>> console.log(v);
>>> }
>>>
>>> This for-in code logs nothing of course but for-of code logs 3 of undefined.
>> [...]
>>> Are these behaviors correct ?
>>
>> Well, they're certainly intentional. There's no final specification
>> yet, so it could change.
>>
>> The rationale for this is that for-of should act like the canonical
>> for-loop over an array:
>>
>> var array = new Array(3);
>> for (var i = 0; i< array.length; i++)
>> console.log(array[i]); // "undefined" 3 times
>>
>> for (var v of array)
>> console.log(v); // "undefined" 3 times
>>
>> The corresponding for-in loop is much rarer, with good reason. Note
>> that "for-in" on an Array is underspecified (in terms of the order in
>> which the property names are visited and what happens when properties
>> are added or removed during iteration). The behavior we implemented
>> for Array iterators is straightforward and easy to use, and it's
>> easier to specify fully. See
>> <http://wiki.ecmascript.org/doku.php?id=harmony:iterators>.
>>
>> I think Map will end up being better than Array for sparse collections.
>>
>> -j
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