entry point file, imports modules and appends functions to DOM

main.js

import { whichUserAgent, isFirefoxAgent, isWebKitAgent, isOldInternetExplorer } from './modules/userAgent';
import { doHttpRequest } from './modules/httpRequest';

document.addEventListener( 'DOMContentLoaded', () => {
  /**
   * checkHeightBody - Calls isFirefoxAgent, isWebKitAgent or isOldInternetExplorer
   *  dependig on userAgent
   *
   * @param  {Object} navigatorObject navigator Object
   * @return {Boolean} -
   */
  function checkHeightBody( navigatorObject ) {
    let checkHeight;
    const actualUserAgent = whichUserAgent( navigatorObject );

    switch ( actualUserAgent ) {

      // Firefox
      case 'Firefox' :
        checkHeight = isFirefoxAgent();
        break;

      // Chrome, Safari, IE Edge
      case 'AppleWebKit' :
        checkHeight = isWebKitAgent();
        break;

      // IE old versions
      case 'Trident' :
        checkHeight = isOldInternetExplorer();
        break;

      default:
        checkHeight = '';
    }

    return checkHeight;
  }

  /**
   * loadInfiniteScroll - callback passed to scroll event,
   * checks if scroll reaches the bottom
   */
  function loadInfiniteScroll() {
    const checkHeight = checkHeightBody( navigator );

    if ( checkHeight ) {
      doHttpRequest();
    }
  }

  document.addEventListener( 'scroll', loadInfiniteScroll );
});