The MDN docs for private fields mention:

It is a syntax error to refer to # names from out of scope. It is also a syntax error to refer to private fields that were not declared before they were called, or to attempt to remove declared fields with delete.

Outside of private fields, I can define a function which returns a constructor which can then be invoked as a class:

function CtorCreator(...) {
   const CtorBeingCreated = function Whatever(...) {...}
   // programmatically manipulate CtorBeingCreated
   return CtorBeingCreated
}

const Ctor = CtorCreator(...)
const instance = new Ctor();

In this I can programmatically define fields and methods based on arguments passed to the CtorCreator function. For example

...
   CtorBeingCreated.prototype[...] = ...
...

Can I also define private fields and methods that way? E.g. some way to allow me to do

   ...
   addPrivateFieldToCtor(CtorBeingCreated, <field-name>, <initial-value>)
   // or 
   addPrivateMethodToCtor(CtorBeingCreated, <method-name>, <method-body>) // method body can access <field-name> previously defined  
   ...