Unfortunately, the only practical way to know that without risking side effects is to read the human-produced documentation on the function, or to have a human read the code.
On Mon, Jan 23, 2017 at 8:19 AM, Isiah Meadows <[email protected]> wrote: > I'll point out that all I need is some way to detect anything that is > callable except for classes. Basically, anything that has a `[[Call]]` that > doesn't throw unconditionally without entering a different scope (like > classes, but not %ThrowTypeError%). > > And to clarify, these would both be included per above, since it does > enter a different scope: > > ```js > // New language scope > function foo() { > throw new TypeError() > } > > // New native scope (%ThrowTypeError%) > "use strict" > var foo = Object.getOwnPropertyDescriptor( > arguments, "callee" > ).get > ``` > > `Reflect.isConstructor` isn't sufficient for my needs, since it would > still consider the first `foo` above to be a constructor. > > On Sat, Jan 21, 2017, 01:14 Jordan Harband <[email protected]> wrote: > >> The reality here is that we're all playing around what the definition of >> "constructor" and "callable" mean. >> >> In the spec, "constructors" are defined as having a `[[Construct]]` >> internal slot; something is "callable" if it has a `[[Call]]` internal >> slot. By that definition (https://tc39.github.io/ >> ecma262/#sec-isconstructor and https://tc39.github.io/ >> ecma262/#sec-iscallable), a `class` constructor is indeed callable >> (because it's defined to have `[[Call]]` throw a `TypeError`). Similarly, >> as defined in the spec, `typeof class Foo {}` is "function" only because it >> has a `[[Call]]` internal slot (https://tc39.github.io/ecma262/#table-35); >> if constructors lacked a `[[Call]]` slot (such that `IsConstructor` >> returned `true` for them), then `typeof someConstructor` would return >> "object". >> >> Obviously the committee could have chosen to define these things >> differently; but this is how things are defined. >> >> >> >> I'm going to claim that conceptually - eg, to the majority of users - >> something is a constructor when it's *intended* to be used with `new`. In >> other words, "what it returns" isn't actually the point - if you're "meant" >> to use it with `new`, it's a constructor, otherwise, it's a function - and >> functions that work with both can be thought of as a dual >> constructor/factory (the factory calls the constructor for you). However, >> that conceptual definition is not one that can be programmatically >> determined, because "programmer intention" in this regard simply isn't >> inherently enshrined in an observable way, whether choosing `class` or >> `function`. >> >> Personally I think it would be wonderful to expose `IsConstructor` >> (`IsCallable` is already exposed via `typeof` returning "function"), but I >> feel quite confident that neither actually address any of the use cases in >> this thread. >> >> In addition, "safe to call" or "safe to use with `new`" all depends on >> your definition of "safe", and it's clear from this thread that that does >> not have a universally agreed-upon definition either. >> >> It might be useful to distill things down to 1) concrete use cases for >> calling or instantiating a function where you don't already know what it >> does, and can't assert in your human documentation what you expect; 2) a >> definition of "constructor", either a different one if you don't agree with >> mine above, or if you do, a form of mine that can be programmatically >> determined, and 3) a definition of "safe" which means more than "doesn't >> throw an exception". >> >> On Fri, Jan 20, 2017 at 6:52 PM, Scott Sauyet <[email protected]> wrote: >> >> Bradley Meck wrote: >> >> 1. every call to `new Foo(...args)` creates a distinct object not >> >> reference-equal to any other such call. >> > >> > Would this mean >> > >> > ``` >> > function factory() { return []; } >> > ``` >> > >> > would qualify as a constructor? >> >> I'm thinking of that as a necessary, but not a sufficient condition. >> >> If it fails that test it cannot be a constructor. It's clear that >> there are other tests. It's not clear to me what they are. >> >> -- Scott >> _______________________________________________ >> es-discuss mailing list >> [email protected] >> https://mail.mozilla.org/listinfo/es-discuss >> >> >> _______________________________________________ >> es-discuss mailing list >> [email protected] >> https://mail.mozilla.org/listinfo/es-discuss >> >
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