Feature proposal

[Michael Luder-Rosefield]

Is strikes me that every single Array method that takes an iteratee
function (signature (value, index, array)) should be able to iterate
through the array in reverse, in a standardised way.

At the moment, we have:

   - every
   - filter
   - find
   - findIndex
   - forEach
   - indexOf / lastIndexOf
   - map
   - reduce / reduceRight
   - some

which is not very consistent at all. Perhaps we could add an `iterOrder`
method that changes the way the array is iterated through?

I propose it could take 1 parameter:

   - If -1 is passed in, the array is iterated through in reverse.
   - If 1,0 or undefined is passed through, the array is iterated through
   normally.
   - If an array of numbers is passed through, these represent the indexes
   in the order they will be processed.
   - Any illegible indexes in the passed-in array will be ignored
      - Any eligible indexes not given will be ignored
   - If a generator function is passed in, it should take the array (or a
   length value) and spit out indexes.
   - If a non-generator function is passed in, it should spit out an array
   of indexes, which will be used as-per arrays being passed in

Whether that iterOrder is reset after an iteration could go either way. I'd
suggest it probably should, with an option to persist.

Example:

```
let arr = [ 8,4,7,3 ];

arr.iterOrder().map(x => x) === arr
arr.iterOrder(-1).map(x => x) === arr.reverse() === [ 3,7,4,8 ]
arr.iterOrder([ 2,1 ]).map(x => x) === [ 7,4 ]
// haven't got time to give function arg examples, but randomisers would be
fairly trivial

// original indexes are preserved during iteration:
arr.iterOrder(-1).forEach((val, i) => console.log(val, i))
// > 3, 3
// > 7, 2
// > 4, 1
// > 8, 0
```

This is a messy first-pass at the problem, and I can already see potential
issues, but it would at least standardise and generalise the problem



On Wed, 18 Jul 2018 at 17:44 Cyril Auburtin <cyril.auburtin at gmail.com>
wrote:

> sorry you get 1, and need again to subtract the array length, `arr.length
> -1 - 1` to get the final result 3
>
> Le mer. 18 juil. 2018 à 18:41, Cyril Auburtin <cyril.auburtin at gmail.com>
> a écrit :
>
>> there you go
>> ```
>> console.log([7, 4, 6, 7, 12].findIndex((_, i, a) =>
>> isPrime(a[a.length-1-i]))); // 3
>> ```
>>
>> Le mer. 18 juil. 2018 à 11:17, Dmitry Shulgin <shulhindvst at gmail.com> a
>> écrit :
>>
>>>
>>> ---------- Forwarded message ----------
>>> From: Dmitry Shulgin <shulhindvst at gmail.com>
>>> Date: 2018-07-05 10:36 GMT+03:00
>>> Subject: Feature proposal
>>> To: es-discuss at mozilla.org
>>>
>>>
>>> I came across a task of finding an index of the last element in array
>>> that satisfies the condition, so i reversed array and found it by *findIndex
>>> *function.
>>> I was thinking:
>>> Why do we have *findIndex*` method, but no *findLastIndex*?
>>> It seems logical to follow *[index|lastIndex]Of* pair, doesn't it?
>>> Reversing array in this case seems too complicated to me.
>>>
>>> So, i would like to suggest new method like
>>> *Array.prototype.findLastIndex()*
>>>
>>> Example:
>>>
>>> function isPrime(element, index, array) {
>>>   var start = 2;
>>>   while (start <= Math.sqrt(element)) {
>>>     if (element % start++ < 1) {
>>>       return false;
>>>     }
>>>   }
>>>   return element > 1;
>>> }
>>>
>>> console.log([4, 6, 8, 12].findIndex(isPrime)); // -1, not found
>>> console.log([7, 4, 6, 7, 12].findIndexLast(isPrime)); // 3
>>>
>>>
>>> Would be glad to implement, if it makes sense.
>>>
>>> Thx for replies.
>>>
>>> P.S. Small issue on GitHub was closed due to offtop (not an issue, as i
>>> understand).
>>> https://github.com/tc39/ecma262/issues/1253
>>>
>>> _______________________________________________
>>> es-discuss mailing list
>>> es-discuss at mozilla.org
>>> https://mail.mozilla.org/listinfo/es-discuss
>>>
>> _______________________________________________
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>
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