bis - Array subclassing, .map and iterables (Re: Jan 30 TC39 Meeting Notes)

Aymeric Vitte vitteaymeric at
Tue Feb 12 04:10:41 PST 2013

and, I forgot and don't know exactly if this should fall here, a 
toString(encoding) method which supports streaming, see [3] using 

Le 12/02/2013 12:54, Aymeric Vitte a écrit :
> Le 10/02/2013 19:05, Allen Wirfs-Brock a écrit :
>> All the presentation decks from the meeting are publicly available at 
>> the short form:
>> In ES5, today you can use Array.prototype as the [[Prototype]] of 
>> non-Array objects or apply the Array.prototype methods to non-arrays. 
>>  The behaviors they get are non-necessarily what you would expect if 
>> you would really subclass Array.  We need to preserve the existing 
>> behaviors for these existing use cases but would like to provide the 
>> more reasonable behavior when class extends Array is used to create a 
>> subclass.
>> Array.prototype.concat is the most problematic of the existing 
>> methods in this regard.
> After reading this thread and the slides, what is the plan for Typed 
> Arrays (dedicated array like methods or Array subclassed like) ?
> I see the constraints for subclassing, but the solution a.from(b,map) 
> seems a little bit strange (even if I agree that it should return the 
> same instance as a) , and what about concat, slice, etc as you 
> mention? Couldn't we add something like a parameter to the methods : 
>,thisArg, true) or,true) returns example's 
> instance instead of Array (same processing as .of, .from), so you can 
> explicitely say that you want to return the same instance and don't 
> have backward compatilities issues (hopefully...) ?
> I have used a lot Typed Arrays for [1] [2] [3] and all along wondering 
> why Array like optimized methods were not available, especially 
> concat, and why it was speced entirely in TC39 while a spec already 
> exists (which specifies slice/subarray but surprisingly not concat), 
> but it will be extremely usefull to have the Array like methods in the 
> ES specs for Typed Arrays.
> Regards,
> [1]
> [2]
> [3]

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