private

var log = console.log;
var assign = Object.assign;
var MyClass = (function () {
  var state = new Map();
  function makeDefaultState() {
    return { a: 0 };
  }
  function getState() {
    return state.get(this);
  }
  function MyClass(init) {
    if (!(this instanceof MyClass)) return new MyClass(init);
    state.set(this, assign(makeDefaultState(), init || {}));
  }
  function privateFunc(a) {
    return getState.call(this).a += a;
  }
  assign(MyClass.prototype, {
    publicFunc(a) {
      return privateFunc.call(this, a);
    }
  });
  return MyClass;
})();

var instance = MyClass({ a: 5 });
log(instance.publicFunc(5));