Persist UTM Parameters in a URL by decorating on-page <a> elements. Required on all pages.

persist_utm_params.js

// Script to persists the listed querystring parameters across the length of the session.
(function() {
  var domainsToDecorate = [
          'exampledomain.com' //add or remove domains (without https or trailing slash)
      ],
      queryParams = [
          'utm_medium', //add or remove query parameters you want to transfer
          'utm_source',
          'utm_campaign',
          'utm_adgroup',
          'utm_content',
          'utm_term',
          'gclid',
          'fbclid'
      ]
 //decorate all links on the page
  var links = document.querySelectorAll('a');
// check if links contain domain from the domainsToDecorate array and then decorates
  for (var linkIndex = 0; linkIndex < links.length; linkIndex++) {
      for (var domainIndex = 0; domainIndex < domainsToDecorate.length; domainIndex++) {
          if (links[linkIndex].href.indexOf(domainsToDecorate[domainIndex]) > -1 && links[linkIndex].href.indexOf("#") === -1) {
              links[linkIndex].href = decorateUrl(links[linkIndex].href);
          }
      }
  }
// decorates the URL with query params
  function decorateUrl(urlToDecorate) {
    //urlToDecorate uncessary ?
    //check if there are any parameters existing
      var collectedQueryParams = [];
      for (var queryIndex = 0; queryIndex < queryParams.length; queryIndex++) {
          if (getQueryParam(queryParams[queryIndex])) {
              collectedQueryParams.push(queryParams[queryIndex] + '=' + getQueryParam(queryParams[queryIndex]))
          }
      }
   //YL: added as a fix to ensure url is not modified when UTMs are not in place
      if(collectedQueryParams.length < 1){ 
        urlToDecorate = urlToDecorate
      }
      else{
        urlToDecorate = (urlToDecorate.indexOf('?') === -1) ? urlToDecorate + '?' : urlToDecorate + '&';
      }
      return urlToDecorate + collectedQueryParams.join('&');
  }
  // borrowed from https://stackoverflow.com/questions/831030/
  // a function that retrieves the value of a query parameter
  function getQueryParam(name) {
      if (name = (new RegExp('[?&]' + encodeURIComponent(name) + '=([^&]*)')).exec(window.location.search))
          return decodeURIComponent(name[1]);
  }
})();