I don't think it is possible to call the constructor as a function.
However, in JavaScript a class is just an object (unlike C# or Java,
where it's meta data), which means that you can pass it around and
dynamically extend it. E.g. you can intercept class creation like this:
function enhanceClass(c) {
return class EnhancedClass extends c {
constructor() {
//do your magic here
}
}
}
class MyClass {}
const EnhancedMyClass = enhanceClass(MyClass);
new EnhancedMyClass();
As for `toString` & not being able to call a function directly:
`toString` is already overwriteable, and in ES5 code it wasn't uncommon
to throw from a constructor if it wasn't called as one (for good
reasons). ES6 classes just do that for you.
On 01/05/2017 06:31 PM, James Treworgy wrote:
Hi - I am brand new to this list, I find myself here because of a
confounding issue related to ES6 classes vs. traditional constructors.
Forgive me if this is something that's been hashed out in times past.
I looked around for discussion online and couldn't find anything more
than the observation that the spec prohibits invoking it - not really
any discussion. Probably a failing of google more than anything else,
so if there's some discussion that I should read to catch up please
point me there.
Here's my issue. The ES6 spec prohibits invoking class constructors
without "new". This makes such functions a special case, e.g.
class Test() {}
// typeof Test === 'function' // yep
// Test.prototype.constructor === Test // yep
// Test() => nope ... TypeError: Class constructor Test cannot be
invoked without 'new'
// Test.call() ... nope
// Test.apply() ... nope
This has some interesting consequences. It means testing something for
typeof "function" no longer guarantees it can be invoked without
error. Also "function.toString()" can now return something that isn't
actually a legal function definiton (since it returns the whole class
as text). There seems to be no method, through various javascript
reflection/invocation techniques or otherwise, to invoke a class
constructor except by creating an instance of the class.
For tool-builders the consequences of this are significant. It's no
longer possible to create something that can extend/wrap/act on a
prototype by intercepting it's construction process, as it was before
with plain ES5 constructors. So classes are fundamentally different
than prototype contructors in how we can use them, far more than
syntactic sugar. This has come into play lately for me, as an DI
container we use that does exactly this doesn't work with ES6 classes
(and as far as I can tell, there's no way to make it work, other than
having devs no longer use class syntax).
This seems a strange design decision. Even conventional OO languages
like C# have the capability to reflect on classes and access the
constructor directly as a function. It seems to fly in the face of the
basic openness/dyanamic nature of JavaScript, and more signficantly,
creates a kind of backward incompatibility since a function is no
longer just a function.
I'm wondering whether I'm missing some mechanism for legally accessing
a class constructor as a function (other than parsing the output of
toString() and eval!) -- and generally thoughts on this aspect of the
ES6 specification.
Thank you!
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