Private function example based on raganwald's "Classes are Expressions" http://raganwald.com/2015/06/04/classes-are-expressions.html Discussion: https://news.ycombinator.com/item?id=9660908

private_function.js

let Person = (() = > {
  let firstNameProperty = Symbol('firstName'),
      lastNameProperty  = Symbol('lastName'),
      renameProperty    = Symbol('rename');

  let rename = function(first, last) {
    this[firstNameProperty] = first;
    this[lastNameProperty] = last;
    return this;
  }

  return class Person {
    constructor (first, last) {
      this[renameProperty] = rename.bind(this);
      this[renameProperty](first, last);
    }
    fullName () {
      return this[firstNameProperty] + " " + this[lastNameProperty];
    }
  };
)();