On Mon, Oct 6, 2014 at 10:48 PM, Dmitry Soshnikov < [email protected]> wrote:
> On Mon, Oct 6, 2014 at 8:19 PM, Rick Waldron <[email protected]> > wrote: > >> >> >> On Mon, Oct 6, 2014 at 10:59 PM, Dmitry Soshnikov < >> [email protected]> wrote: >> >>> Hi, >>> >>> We're currently polyfilling `Array.from` in our codebase following the >>> latest draft [1]. And I was actually surprised by the `mapfn` being >>> involved into the algorithm. >>> >> >> Have you tried es6-shim? >> >> > > Yeah, I'm aware of it, though, we have own versions for better > performance, since may skip some internal checks, etc. > > >> >>> I mean, from the performance perspective it's probably .. ok-ish.. but >>> from the API design perspective it feels like combining the actual >>> transformation of an iterable (or of an array-like) with the mapping, is >>> the "too much of responsibility" anti-pattern. Why not filter then as well? >>> (or, as one of the comments on a diff review: "I'm actually >>> disappointed it won't do my laundry as well :P"). >>> >>> It probably should be just: >>> >>> ``` >>> Array.from(...).map(mapfn); >>> ``` >>> >>> Unless I'm missing something, and there was a special reason to include >>> the mapping. >>> >> >> Yes: >> https://github.com/rwaldron/tc39-notes/blob/c61f48cea5f2339a1ec65ca89827c8cff170779b/es6/2013-01/jan-30.md#revising-the-array-subclassing-kind-issue >> Any subclass of array will want to produce a list of its own "kind", but >> .map returns an Array. This thread is also a dup of: >> https://mail.mozilla.org/pipermail/es-discuss/2013-June/031500.html >> >> > I see. OK, thanks Rick, will re-read. > OK, have checked. And guess I still don't see a big issue, as long as the `Array.prototype.map` preserves the kind (by delegating to `new this.constructor(...)`). That specific example like: ``` NodeList.form(['div', 'span'], nodeName => document.createElement(nodeName)); ``` arguing that the `NodeList` can't create elements from the strings doesn't seems to me a good reason for introducing that anti-pattern, and it's really should be rewritten as: ``` NodeList.form(['div', 'span'].map(nodeName => document.createElement(nodeName))); ``` And other things are better be written: ``` <ArrayKind>.from(iterable).map(mapfn) ``` Am I still missing something? Dmitry
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