Return value of forEach

[Kris Kowal]

forEach should return undefined. Array-like methods for iterators solve the
problem of building unnecessary memory pressure for intermediate maps.

The return value of forEach is special. True, for arrays it is always
undefined. However, the forEach method on an iterator returned by a
generator function should be the return value of the generator. Consider
this range generator that returns the number of visited values.

```js
function *range(start, stop, step) {
    for (var index = start; index < stop; stop += step) {
        yield index;
    }
    return (stop - start) / step;
}
var iterator = range(0, 10, 1);
var iterations = iterator.forEach((x) => console.log(x));
console.log(iterations);
```

Kris Kowal

On Fri, Oct 16, 2015 at 7:30 AM, Andrea Giammarchi <
andrea.giammarchi at gmail.com> wrote:

> That's usually a made-up issue ... the example code is using two maps
> instead of computing the entire thing once in a composed function **and**
> it already creates multiple copies of the initial Array.
>
> Anyway, having `var log = DEBUG ? x => console.log(x) || x : x => x` in
> means you could do this:
>
> ```js
> const a = log([1, 2, 3]
>   .map(square))
>   .map(plus1)
>   .reduce(add);
> ```
>
> but again, we are using multiple maps and a reduce so if we want to be
> realistic, and it's for debugging purpose only, I think using map would be
> just fine.
>
> Regards
>
>
> On Fri, Oct 16, 2015 at 3:17 PM, Eric Devine <devineej at gmail.com> wrote:
>
>> Array#map does the non-trivial operation of copying the entire array. In
>> the example for loging to the console, this behavior is unintended. Perhaps
>> an underscore-like tap method should be considered to be added to the Array
>> prototype for these cases.
>>
>>
>> On Fri, Oct 16, 2015 at 9:32 AM, Andrea Giammarchi <
>> andrea.giammarchi at gmail.com> wrote:
>>
>>> Like I've written before, if you need to return an array you can use map
>>> instead of forEach
>>>
>>> `arr.map(x => console.log(x) || x)`
>>>
>>> forEach has been like this since ever so while you wonder what kind of
>>> code would look like based on the fact forEach doesn't return  anything, I
>>> actually wonder how come there are still people expecting forEach to return
>>> something.
>>>
>>> So, it would be a breaking change and actually it's not needed since you
>>> have map.
>>>
>>> If the new Array, and for debugging purpose, is not desired, use `var
>>> log = DEBUG ? x => console.log(x) || x : x => x` and abuse it as much as
>>> you like.
>>>
>>> Would any of this work?
>>>
>>> Regards
>>>
>>>
>>>
>>> On Fri, Oct 16, 2015 at 2:23 PM, Niloy Mondal <niloy.mondal84 at gmail.com>
>>> wrote:
>>>
>>>> > That'd be a compatibility break.
>>>>
>>>> Ugh... you mean people actually depend on `forEach` returning
>>>> `undefined` (which it always does) to do further task?
>>>>
>>>> I wonder what that kinda code would look like >.<
>>>>
>>>> On Fri, Oct 16, 2015 at 6:08 PM, Frankie Bagnardi <f.bagnardi at gmail.com
>>>> > wrote:
>>>>
>>>>> That'd be a compatibility break.
>>>>>
>>>>> If we end up getting :: though:
>>>>>
>>>>> ```js
>>>>> function logEach(){
>>>>>   this.forEach((x) => console.log(x));
>>>>>   return this;
>>>>> }
>>>>>
>>>>>
>>>>> const a = [1, 2, 3]
>>>>>   .map(square)
>>>>>   ::logEach()
>>>>>   .map(plus1)
>>>>>   .reduce(add);
>>>>> ```
>>>>>
>>>>> You could make that a global variable so you can sprinkle it around
>>>>> your code in development.
>>>>>
>>>>> Having some developer tools in the language would be nice though. I
>>>>> don't even think console.log is in the spec. A global like Debug.logThis
>>>>> for example would be a more general ::-able.
>>>>>
>>>>>
>>>>> On Fri, Oct 16, 2015 at 5:32 AM, Andrea Giammarchi <
>>>>> andrea.giammarchi at gmail.com> wrote:
>>>>>
>>>>>> ```js
>>>>>> const a = [1, 2, 3]
>>>>>>   .map(square)
>>>>>>   .map(x => console.log(x) || x )
>>>>>>   .map(plus1)
>>>>>>   .reduce(add);
>>>>>> ```
>>>>>>
>>>>>> Regards
>>>>>>
>>>>>>
>>>>>> On Fri, Oct 16, 2015 at 10:13 AM, Niloy Mondal <
>>>>>> niloy.mondal84 at gmail.com> wrote:
>>>>>>
>>>>>>> Currently, `Array.prototype.forEach` returns `undefined`. It would
>>>>>>> be more
>>>>>>> useful if it returns the array itself.
>>>>>>>
>>>>>>> Say I have written some code like this...
>>>>>>>
>>>>>>> ```js
>>>>>>> const a = [1, 2, 3]
>>>>>>>   .map(square)
>>>>>>>   .map(plus1)
>>>>>>>   .reduce(add);
>>>>>>> ```
>>>>>>>
>>>>>>> For some reason, I am not getting the expected output. For
>>>>>>> debugging, I would
>>>>>>> like the print the values after each step. My first initial reaction
>>>>>>> is to
>>>>>>> put a `forEach` step in between and print the values like so...
>>>>>>>
>>>>>>> ```js
>>>>>>> const a = [1, 2, 3]
>>>>>>>   .map(square)
>>>>>>>   .forEach(x => console.log(x))
>>>>>>>   .map(plus1)
>>>>>>>   .reduce(add);
>>>>>>> ```
>>>>>>>
>>>>>>> Unfortunately, this does not work as `forEach` returns `undefined`.
>>>>>>> I now have
>>>>>>> to comment out all the code below it. Having the _plug and play_
>>>>>>> behaviour for
>>>>>>> `forEach` would be very convenient.
>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> es-discuss mailing list
>>>>>>> es-discuss at mozilla.org
>>>>>>> https://mail.mozilla.org/listinfo/es-discuss
>>>>>>>
>>>>>>>
>>>>>>
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>>>>>>
>>>>>
>>>>
>>>
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>
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