IsConstructor

[André Bargull]

> Quick note: that isConstructor isn't really viable unless you plan on using
> it with constructors that do not have side effects.
>
> Rick

The Proxy-based solution needs to be used in these cases. Now we just 
need to wait until Proxies are available everywhere! ;-)


>
>
> On Wed, Jun 11, 2014 at 10:58 AM, Rick Waldron <waldron.rick at gmail.com  <https://mail.mozilla.org/listinfo/es-discuss>>
> wrote:
>
> >/
> />/
> />/
> />/  On Wed, Jun 11, 2014 at 10:24 AM, Domenic Denicola <
> />/  domenic at domenicdenicola.com  <https://mail.mozilla.org/listinfo/es-discuss>> wrote:
> />/
> />>/  A variety of places in the spec use the new IsConstructor abstract
> />>/  operation. In ES5, this test (essentially, "does the object implement the
> />>/  [[Construct]] internal method") was restricted to the `new` operator. But
> />>/  in ES6, it is used in implementing a large variety of built-in functions:
> />>/
> />>/  - All Array methods
> />>/  - All %TypedArray% methods
> />>/  - All Promise methods (via NewPromiseCapability)
> />>/
> />>/  (Note that there are two uses: arrays and typed arrays do alternative
> />>/  logic for non-constructors; promises fail immediately. This inconsistency
> />>/  might be a bug?)
> />>/
> />>/  It seems to me that we should expose this primitive reflective operation
> />>/  to users, instead of having all of these methods be able to detect
> />>/  something that user code can't, and thus making them harder to explain or
> />>/  polyfill.
> />/
> />/
> />>/  Alternately, if we don't think users should be doing this kind of
> />>/  reflection, then we probably shouldn't be doing it ourselves. In which
> />>/  case, figuring out an alternate path for the above methods would be
> />>/  useful---perhaps they simply try to construct, and fail immediately if used
> />>/  with a non-constructible object, instead of falling back.
> />>/
> />/
> />/  I had similar questions a couple years ago and Allen advised that the
> />/  easiest polyfill for such a mechanism is:
> />/
> />/  function isConstructor(C) {
> />/    try {
> />/      new C();
> />/      return true;
> />/    } catch (e) {
> />/      return false;
> />/    }
> />/  }
> />/
> />/
> />/  Additionally, at the July 2012 tc39 meeting I proposed (over breakfast) an
> />/  ES7 "standard library module" that exported the abstract operations that
> />/  are now defined in chapter 7
> />/  http://people.mozilla.org/~jorendorff/es6-draft.html#sec-abstract-operations,  <http://people.mozilla.org/%7Ejorendorff/es6-draft.html#sec-abstract-operations,>
> />/  the response was positive but it was far too early to have a serious
> />/  discussion. Anyway, with that you'd just write:
> />/
> />/  import { isConstructor } from "es-abstract";
> />/
> />/
> />/  Rick
> />/
> />
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