Example of WeakMap holding private data for module-defined and exported class

timecard.js

module TimeCard {
  let wm = new WeakMap();

  export class TimeCard {
    constructor( time ) {
      wm.set(this, { time });
    }
    time( value = undefined ) {
      if ( value === undefined ) {
        return wm.get(this).time;
      }
      wm.get(this).time = value;
      return this;
    }
  }
}

import { TimeCard } from TimeCard;

var card = new TimeCard(Date.now());

console.log(card.time());

setTimeout(function() {
   console.log(
     card.time(Date.now()).time()
   );
}, 10);