Javascript: App boilerplate (ES2015)

app.js

/**
 * App JS
 *
 * This is the main scripts file.
 *
 * Author: PubliOne S.r.l.
 * Author URI: http://publione.it
 *
 * @version  0.1.0
 */
require( './libs/modernizr.js' );
const debounce = require( 'lodash/debounce' );

( function( window, document, undefined ) {
  'use strict';

  const win = window;
  const doc = document;
  const htmlEl = doc.getElementsByTagName( 'html' )[0];
  const headEl = doc.getElementsByTagName( 'head' )[0];
  const bodyEl = doc.getElementsByTagName( 'body' )[0];

  const transEndEventNames = {
    'WebkitTransition': 'webkitTransitionEnd',
    'MozTransition': 'transitionend',
    'OTransition': 'oTransitionEnd',
    'msTransition': 'MSTransitionEnd',
    'transition': 'transitionend'
  }
  const transitionEnd = transEndEventNames[ Modernizr.prefixed( 'transition' ) ];

  /**
   * Website config.
   */
  const config = win.CONFIG || { domain: '', lang: 'it' };

  /**
   * Track window size.
   */
  let winSize = {
    width:  win.innerWidth,
    height: win.innerHeight
  }

  /**
   * Merge two objects.
   *
   * @param  {object}  a  First object
   * @param  {object}  b  Second object
   * @return {object}     Merged object
   */
  function extend( a, b ) {
    for( var key in b ) {
      if( b.hasOwnProperty( key ) ) {
        a[ key ] = b[ key ];
      }
    }
    return a;
  }

  /**
   * Load asynchronous script.
   *
   * @param  {string}  src  The script uri
   */
  function loadScript( src ) {
    let script = doc.createElement('script');
    script.type = 'text/javascript';
    script.async = 'true';
    script.src = src;
    headEl.appendChild( script );
  }

  /**
   * Update var which holds window dimensions.
   *
   * @param  {integer}  width   New width of window
   * @param  {integer}  height  New height of window
   * @return {object}           Updated window size
   */
  function updateWinSize( width, height ) {
    return winSize = {
      width:  width,
      height: height
    }
  }

  /**
   * Add a scrolled class to the body when page has scrolled.
   *
   * @param  {Integer}  scrollOffset   Number of pixels scrolled before class is applied
   * @param  {String}   scrolledClass  Class applied to body element
   */
  function toggleScrolledClass( scrollOffset = 0, scrolledClass = 'is-scrolled' ) {
    let bodyClassList = bodyEl.classList;
    let scrollTop     = win.pageYOffset;

    ( scrollTop >= scrollOffset )
      ? bodyClassList.add( scrolledClass )
      : bodyClassList.remove( scrolledClass );
  }

  /**
   * Initiate all the events.
   */
  function init() {

    // Update size vars on window resize
    win.addEventListener( 'resize', debounce( function() {
      updateWinSize( win.innerWidth, win.innerHeight );
    }, 250, { trailing: true } ) );

    // Add a scrolled class to the body when page has scrolled
    win.addEventListener( 'scroll', debounce( function() {
      toggleScrolledClass( 150 );
    }, 250, { trailing: true } ) );

    win.addEventListener( 'load', function() {
      // remove loader, ect.
    });

  }

  /**
   * Constructor.
   */
  return { init: init() };

} )( window, window.document );