create private properties in ES6 class using weakmaps

weakmaps.js

// taken from https://www.sitepoint.com/object-oriented-javascript-deep-dive-es6-classes/
let SimpleDate = (function() {
  let _years = new WeakMap();
  let _months = new WeakMap();
  let _days = new WeakMap();

  class SimpleDate {
    constructor(year, month, day) {
      // Check that (year, month, day) is a valid date
      // ...

      // If it is, use it to initialize "this" date
      _years.set(this, year);
      _months.set(this, month);
      _days.set(this, day);
    }

    addDays(nDays) {
      // Increase "this" date by n days
      // ...
    }

    getDay() {
      return _days.get(this);
    }
  }

  return SimpleDate;
}());