private_function.js

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

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