About the classes thing, most encapsulation needs can be satisfied with the namespace concept that David sketched above, however if you are looking for any kind of inheritance or even polymorphism you're out of luck (to be honest, I don't know ECMA 6 well enough to know what's going on there).

Could you describe what kind of thing you want to achieve using classes? I am positive there's a solution in the current HISE implementation.

So to create a class I would do this

namespace myClass
{
    reg _privateVariable; //Use underscore for variables that should be private
    reg publicVariable;

    inline function createInstanceOfClass(id, someParameter)
    {
        local obj = {};
        
        obj.id = id;
        obj.someParameter = someParameter;
        return obj;
    }

    inline function changeParameter(obj, parameter, value) //Pass in the instance of the object
    {
        obj[parameter] = value;
    }
}

This setup is very efficient because each object doesn't need to contain all of the class functions it is just a standalone object that can make use of the functions within the namespace. It provides all the flexibility of true classes without the overhead.