js.constructorOf[SomeJSClass] to retrieve the JS constructor

[sjrd]

Add the following primitive method in the package object js:

def constructorOf[C <: js.Any]: js.Dynamic = sys.error("stub")

The compiler will check at compile-time that C refers to a class (not a trait nor module class, nor any other complex type), similarly to classOf[C]. C can be native or not.

The method returns the JS constructor of the class C. In IR terms, this is JSLoadConstructor(ClassType(C)).

This is particularly important for Scala.js-defined JS classes, as it allows to obtain the JS constructor of a JS class without exporting it (which allows dce) nor instantiating it.

Implementation note: since C will be erased by erasure, PrepJSInterop must rewrite calls to this method to some runtime method runtime.constructorOf(classOf[C]). The backend will then see the ClassConstant(C) as a parameter of runtime.constructorOf; it is of course invalid to call runtime.constructorOf(cls) with a cls which is not a ClassConstant.

[chandu0101]

This is particularly important for Scala.js-defined JS classes, as it allows to obtain the JS constructor of a JS class without exporting it (which allows dce) nor instantiating it.

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