[nerv_]

class Foo {
  constructor() {
    this.__x = 1;
    this.__y = 2;
  }
  get (key) {
    let k = `__${key}`;
    return k in this ? this[k] : null;
  }
  set (key, value) {
    let k = `__${key}`;
    this[k] = value;
  }
}

let object = new Foo();
alert(object.get('x'));
alert(object.get('z'));