Determine if a value is Callable/Constructible

Tom Schuster tom at
Mon Mar 30 13:48:02 UTC 2015

Thanks Caitlin for actually putting this onto github! I wasn't aware of
that process when I posted about this to the mailinglist. Asking again from
last time: Should we have Type == Object check like Reflect.isExtensible?


On Sun, Mar 29, 2015 at 11:51 PM, Caitlin Potter <caitpotter88 at>

> **disclaimer** I know this has been brought up before, but bump :>
> People are experimenting with polyfilled class implementations, which
> don’t all correctly throw when called as a function (no `new`). Eventually,
> they’re likely to be disappointed that this isn’t legal, and might have to
> undergo some serious pains to fix their applications.
> I notice that this is particularly problematic for AngularJS, because
> classes are registered with an injector, which doesn’t know if it can
> `[[Call]]` them or not. It will later on try to `[[Call]]` (depending on
> how the class was registered with DI). It would be really great if we had a
> way to determine if this was going to throw or not, other than looking at
> the
> stringified value of a function, so that these libraries could be updated
> to accomodate new class behaviour without pains (try/catch or processing
> Function.toString())
> Some ideas:
> Reflect.isConstructor(fn) -> true if Class constructor, generator, or
> legacy (and non-builtin) function syntactic form
> Reflect.isCallable(fn) -> true for pretty much any function, except for
> class constructors and a few builtins
> I know it’s way too late for ES6, but maybe some kind of fast-tracked
> extension is in order? it should be pretty simple to implement these (and
> SM and v8 have variations of these in the runtime anyways)
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