Neo.mjs uses standard modular JavaScript, so you're free to use other class 
features, like private members.
<pre class="neo">
class Human extends Mammal {
    static config = {
        className: 'Simple.example.Human',
        name: 'J. Doe',
        married: false,
    }
    static #privateStaticField = 'foo'
    #privateInstanceField = 'bar'
    #privateInstanceMethod(){
        console.log(`Psst. Don't tell anyone, but ${this.#privateInstanceField} and ${Human.#privateStaticField}`);
    }
    speak(tellSecret){
        console.log(`Hello! My name is ${this.name}. I am ${this.married?'':'not'} married.`);
        if (tellSecret) this.#privateInstanceMethod();
    }
    yodel(){
        console.log('Yodelay hee hoo!');
    }}

const myPerson = Neo.create(Human, {
    name: 'Herbert'
});
myPerson.speak(true);

Neo.setupClass(Human);
</pre>
