Quick note: that isConstructor isn't really viable unless you plan on using it with constructors that do not have side effects.
Rick On Wed, Jun 11, 2014 at 10:58 AM, Rick Waldron <[email protected]> wrote: > > > > On Wed, Jun 11, 2014 at 10:24 AM, Domenic Denicola < > [email protected]> 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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