es6-privateClassVar.js

let Foo = (function () {
  let __secrets = new WeakMap();
  class Foo {
    constructor () {
      __secrets.set(this, Math.floor(Math.random()*100));
    }
  
    get bar() {
      return __secrets.get(this);
    }

  }
  
  return Foo;// (() => new Foo())()
}());

console.log(new Foo().bar);
console.log(new Foo().bar);

Based on http://sravi-kiran.blogspot.com/2015/01/PrivateMembersInES6Classes.html