External Link Prompter

externalChecker.js

(function() {

  /**
   * Wrapper Element
   * @type {Element}
   */
  const wrapperElement = document.querySelector('body');

  /**
   * Root Domain
   * @type {string} rootDomain
   */
  const rootDomain = window.location.hostname;


  const checkExternalLink = (event) => {

    /**
     * Event Target
     * @type {Node}
     */
    let target = event.target;

    let external, parent;

    if (target.nodeName !== 'A') {

      /**
       * @type {HTMLElement} HTML Element
       */
      parent = target.parentNode;

      /**
       * Check to make sure that Parent does not equal wrapperElement,
       * does not equal parent element (this means it was found) and is not null
       * (a catch all to make sure that we don't get into an infinite loop)
       */
      while (parent !== wrapperElement && target !== parent && parent !== null) {
        if (parent.nodeName === 'A') {
          target = parent;
        } else {
          parent = parent.parentNode;
        }
      }
    }

    /**
     * Ensure target is a link
     */
    if (target.nodeName === 'A') {

      /**
       * Check that href exists, links is not relative and root is not in domain; if all pass, link is external.
       * @type {boolean}
       */
      external = target.href !== '' && target.href.indexOf('/') !== 0 && target.href.indexOf(rootDomain) < 0;

      if (external) {
        if (!window.confirm("You are about to leave the website. Do you wish to continue?")) {

          /**
           * User clicked "Cancel"; Prevent event action but allow propagation to continue in
           * case other listeners exist!
           */
          event.preventDefault();
        }
      }
    }
  };


  /**
   * Bind click event to wrapper element.
   */
  if (wrapperElement) {
    wrapperElement.addEventListener('click', checkExternalLink);
  }
}());