privateClass.js

const Test = (_ => {
  const weakMap = new WeakMap();
  return class {
    constructor() {
        weakMap.set(this, _=> console.log('test'))
    }
    doMethod() { 
      weakMap.get(this)(); 
    }
  };
})();

new Test().doMethod(); // test
Reflect.ownKeys(new Test()) // [];