Also see https://jamie.build/javascripts-new-private-class-fields.html

private-variables.js

function _displayModel (_private) {
  console.log(_private.model);
}

function bind (that, func) {
  return function(private) {
    return func.bind(that, private);
  };
}

class Car {
  constructor (model) {
    this._private = {
      model: model
    };
    
    return {
      displayModel: bind(this, _displayModel)(this._private)
    };
  }
}

const car = new Car('CX-9');
car.displayModel();

console.log(car.displayModel.toString()); // prints function () { [native code] }
console.log(car._private) // undefined

Following the Single Responsibility Principle in SOLID, this shouldn’t be too big of a deal to maintain.