On Wed, Jun 11, 2014 at 10:24 AM, Domenic Denicola <
domenic at domenicdenicola.com> 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,
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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I had similar questions a couple years ago and Allen advised that the
easiest polyfill for such a mechanism is:

```js
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,
the response was positive but it was far too early to have a serious
discussion. Anyway, with that you'd just write:

```js
import { isConstructor } from "es-abstract";
```