Accesssing ES6 class constructor function

James Treworgy jamietre at gmail.com
Thu Jan 5 17:31:51 UTC 2017


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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