Array.prototype.toObjectByProperty( element=>element.property )

Thu Aug 10 07:47:37 UTC 2017

`fromEntries` is much less generic than `fromIterable` (since not every
iterable is an entries, but every entries is an iterable) and is much more
verbose to use in the use cases we have just discussed. I have faced such
cases often, but have NEVER faced a need for a `fromEntries`. Have you?
Even if so, I've shown how `fromEntries` functionality can be achieved via
`fromIterable` without any additional method calls, which is not the case
vice versa

On Thu, 10 Aug 2017, 3:26 a.m. Jordan Harband, <ljharb at gmail.com> wrote:

> Naveen:
> `Object.fromIterable(recentCountryNames, countryName=>countryName,
> countryName=>countriesByName[countryName])`
>
> could also be:
>
> `Object.fromEntries(Array.from(recentCountryNames, countryName =>
> ([countryName, countriesByName[countryName]])))`, without needing a
> potentially confusing "keyCallback, entryCallback" API nor without builtins
> needing to invoke user-supplied functions.
>
>
> TJ:
> I'm confused, can you provide a code sample?
>
> On Wed, Aug 9, 2017 at 2:08 AM, Naveen Chawla <naveen.chwl at gmail.com>
> wrote:
>
>> Hi Jordan, great question.
>>
>> An iterable of anything!
>>
>> This is the signature:
>>
>> ```
>> Object.fromIterable(iterable, keyFromElement[, valueFromElement])
>> ```
>>
>> Examples follow:
>>
>> Supposing you had an array:
>>
>> ```
>> [
>>     {
>>         countryName: 'UK',
>>         population: 65640000
>>     },
>>     {
>>         countryName: 'USA',
>>         population: 323100000
>>     },
>>     {
>>         countryName: 'Denmark',
>>         population: 5731000
>>     }
>> ]
>> ```
>> ...and you wanted to cache the items by country name for quick access, to
>> get:
>>
>> ```
>> {
>>     UK: {
>>         countryName: 'UK',
>>         population: 65640000
>>     },
>>     USA: {
>>         countryName: 'USA',
>>         population: 323100000
>>     },
>>     Denmark: {
>>         countryName: 'Denmark',
>>         population: 5731000
>>     }
>> }
>> ```
>>
>> ...you would simply do
>>
>> ```
>> const countriesByName = Object.fromIterable(countries,
>> country=>country.countryName);
>> ```
>>
>> to get that result. (the 2nd parameter defaults to return the iterated
>> value if not provided).
>>
>> As callbacks, the `keyFromElement` and `valueFromElement` parameters
>> allow you to supply anything you like to transform from the iterated
>> element (or from elsewhere) into the keys and values you want:
>>
>> e.g. if you had an array of strings called "recentCountryNames" with
>> `['UK', 'Denmark']`:
>>
>> ```
>> const recentCountryDetailsByName =
>> Object.fromIterable(recentCountryNames, countryName=>countryName,
>> countryName=>countriesByName[countryName])
>> ```
>>
>> would produce:
>>
>> ```
>> {
>>     UK: {
>>         countryName: 'UK',
>>         population: 65640000
>>     },
>>     Denmark: {
>>         countryName: 'Denmark',
>>         population: 5731000
>>     }
>> }
>> ```
>>
>> As an aside, it can easily transform entries as follows:
>> `Object.fromIterable(entries, entry=>entry[0], entry=>entry[1])` (if the
>> entry "keys" happen to be valid object keys, otherwise you can simply
>> provide a different transformation for the `keyFromElement` callback) but
>> works equally well with all types of iterables
>>
>> On Wed, 9 Aug 2017 at 13:55 Jordan Harband <ljharb at gmail.com> wrote:
>>
>>> I think you're misunderstanding; the function would of course take an
>>> iterable. However, an iterable of what?
>>>
>>> If it's an iterable of objects, then what's the key and what's the
>>> value? What if it's an iterable of strings?
>>>
>>> The only thing that makes sense is if it's an iterable that provides
>>> both a key and a value - and "entries" is the idiomatic structure in the
>>> language to respect a list of key/value pairs (besides "an object", of
>>> course).
>>>
>>> What would you suggest?
>>>
>>> On Wed, Aug 9, 2017 at 1:08 AM, Naveen Chawla <naveen.chwl at gmail.com>
>>> wrote:
>>>
>>>> It is more generic than `fromEntries`
>>>>
>>>> On Wed, 9 Aug 2017 at 13:32 Naveen Chawla <naveen.chwl at gmail.com>
>>>> wrote:
>>>>
>>>>> Iterable to object via `Object.fromIterable`
>>>>>
>>>>> On Wed, 9 Aug 2017 at 13:31 Jordan Harband <ljharb at gmail.com> wrote:
>>>>>
>>>>>> JS doesn't have interfaces (yet, tho there's a proposal) and
>>>>>> regardless, the "interface" for "iterable" is "it has Symbol.iterator,
>>>>>> nothing more".
>>>>>>
>>>>>> The only place a method like this - that produces an object - could
>>>>>> possibly exist, is a static method on Object.
>>>>>>
>>>>>> I've already outlined two existing methods to copy one object's
>>>>>> entries to another; the only new functionality would be "creating an object
>>>>>> from entries", hence Object.fromEntries or similar.
>>>>>>
>>>>>> I still haven't seen any use cases that aren't covered by the
>>>>>> existing "copy one object to another", or by a possible "entries to object"
>>>>>> - does anyone have any?
>>>>>>
>>>>>> On Wed, Aug 9, 2017 at 12:56 AM, Naveen Chawla <naveen.chwl at gmail.com
>>>>>> > wrote:
>>>>>>
>>>>>>> But I accept that this a very tall order for ES
>>>>>>>
>>>>>>> On Wed, 9 Aug 2017 at 13:22 Naveen Chawla <naveen.chwl at gmail.com>
>>>>>>> wrote:
>>>>>>>
>>>>>>>> Java has a great example of such a construct: default interface
>>>>>>>> methods
>>>>>>>>
>>>>>>>> On Wed, 9 Aug 2017 at 13:21 Naveen Chawla <naveen.chwl at gmail.com>
>>>>>>>> wrote:
>>>>>>>>
>>>>>>>>> The `toObject` behaviour doesn't need to be "implemented" on a
>>>>>>>>> per-iterable class basis. It has a constant behaviour: iterate and on each
>>>>>>>>> next(), pass the value to the `toKeyFromElement` and `toValueFromElement`
>>>>>>>>> callbacks to generate and return an object. There must be some construct by
>>>>>>>>> which that can be achieved. I wouldn't call it "better" to put it on Object
>>>>>>>>> (for the reasons stated), but rather a compromise in the absence of any
>>>>>>>>> such construct
>>>>>>>>>
>>>>>>>>> On Wed, 9 Aug 2017 at 13:12 T.J. Crowder <
>>>>>>>>> tj.crowder at farsightsoftware.com> wrote:
>>>>>>>>>
>>>>>>>>>> On Wed, Aug 9, 2017 at 8:35 AM, Naveen Chawla <
>>>>>>>>>> naveen.chwl at gmail.com> wrote:
>>>>>>>>>> >
>>>>>>>>>> > It would be in the `iteratable` `protocol` (interface)
>>>>>>>>>>
>>>>>>>>>> As Jordan said, that's likely to be a nonstarter. The Iterable
>>>>>>>>>> protocol is *very* lean (exactly one required property) for a reason: So it
>>>>>>>>>> can be supported with minimum investment. Much better, IMHO, to put
>>>>>>>>>> functions on `Object` and `Map` (which is why that's what I suggested).
>>>>>>>>>>
>>>>>>>>>> -- T.J. Crowder
>>>>>>>>>>
>>>>>>>>>
>>>>>>
>>>
>
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