es6 hide methods from the class

Question

I am learning ES6 at the moment and I can't wrap my head around this. I call my plugin like this:

var fnLazy = new lazyloadImages();
fnLazy.initializeEvents();

If someone types "fnLazy" inside the webdeveloper toolbar I want to hide "this.lazyloadOnScroll" because it is not relevant. How can I add the function for the eventlisteners outside the class while "this" is still refering to the Class?

((root, factory) => {
    var plugin = "lazyloadImages";

    if (typeof define === "function" && define.amd) {
        define([], factory);
    } else if (typeof exports === "object") {
        module.exports = factory(plugin);
    } else {
        root[plugin] = factory(plugin);
    }
})(window, (plugin) => {

    'use strict';

  /* show lazy loadng images when appear in the viewport while scrolling */
  function fnDebounce() {
    console.log('debouncing');
    const that = this;
    this.removeIndex = [];

    this.lazyImages.forEach((item, i) => {
      if (pixieLib.inViewport(item, {percentVisible: that.config.threshold})) {
        if (item.hasAttribute('data-srcset')) {
          item.srcset = item.dataset.srcset;
        }
        item.src = item.dataset.src;
        item.classList.add('lazy-loaded');
        that.removeIndex.push(i);
      }
    });

    for (let i = this.removeIndex.length, item; i > 0; i--) {
      item = this.removeIndex[i - 1];
      this.lazyImages.splice(item, 1);
    }

    if (!this.lazyImages.length) {
      console.log('removeEventListeners');
      this.destroyEvents();
    }
  }

  function extend(src, props) {
    for (let prop in props) {
      if (props.hasOwnProperty(prop)) {
        let val = props[prop];
        if (val && Object.prototype.toString.call(val) === "[object Object]") {
          src[prop] = src[prop] || {};
          extend(src[prop], val);
        } else {
          src[prop] = val;
        }
      }
    }

    return src;
  }

  class lazyloadImages {
    constructor(config) {
      const defaultConfig = {
        className : '.js-lazyload-img',
        threshold : -100
      };
      this.config           = config ? extend(defaultConfig, config) : defaultConfig;
      this.lazyloadOnScroll = _.debounce(fnDebounce.apply(this), 100);
      this.lazyImages       = Array.prototype.slice.call(document.querySelectorAll(this.config.className));
      this.removeIndex;
    }

    destroyEvents() {
      window.removeEventListener('scroll', this.lazyloadOnScroll);
      window.removeEventListener('resize', this.lazyloadOnScroll);
      window.removeEventListener('load', this.lazyloadOnScroll);
    }

    initializeEvents() {
      window.addEventListener('scroll',  this.lazyloadOnScroll.bind(this));
      window.addEventListener('resize',  this.lazyloadOnScroll.bind(this));
      window.addEventListener('load',  this.lazyloadOnScroll.bind(this));
    }

    get reinit(){
      this.destroyEvents();
      this.lazyImages   = Array.prototype.slice.call(document.querySelectorAll(`${this.config.className}:not(.lazyloaded)`));
      this.initializeEvents();      
    }
  }

  return lazyloadImages;
});