function getInternalSlots(x) {
      var internalSlots = internalSlotMap.get(x);
      if (!internalSlots) {
          internalSlots = Object.create(null);
          internalSlotMap.set(x, internalSlots);
      }
      return internalSlots;
  }
https://tc39.es/ecma402/#sec-intl-numberformat-constructor