Determine base class (last level before builtin)

getBaseClass.js

function getBaseClass(MaybeClass) {
  if (typeof MaybeClass !== 'function') {
    return null;
  }
  
  const Proto = Reflect.getPrototypeOf(MaybeClass);
  
  return !Proto.name
    ? MaybeClass
    : getBaseClass(Proto);
}