utm parse and add to links

utm_parse.js

function getRefQueryParam(name) {
  name = name.replace(/[\[]/, '\\[').replace(/[\]]/, '\\]');
  var regex = new RegExp('[\\?&]' + name + '=([^&#]*)');
  var results = regex.exec(location.search);
  return results === null ? '' : decodeURIComponent(results[1].replace(/\+/g, ' '));
};

let utmParamQueryString = '';
let utmParamQueryStringTrimmed = '';

(function () {
  let utm_source = getRefQueryParam('utm_source');
  let utm_medium = getRefQueryParam('utm_medium');
  let utm_content = getRefQueryParam('utm_content');
  let utm_campaign = getRefQueryParam('utm_campaign');
  let utm_term = getRefQueryParam('utm_term');
  let roistat = getRefQueryParam('roistat');
  let roistat_referrer = getRefQueryParam('roistat_referrer');
  let roistat_pos = getRefQueryParam('roistat_pos');

  if (utm_source) {
    utmParamQueryString += '&utm_source=' + utm_source;
  }
  if (utm_medium) {
    utmParamQueryString += '&utm_medium=' + utm_medium;
  }
  if (utm_content) {
    utmParamQueryString += '&utm_content=' + utm_content;
  }
  if (utm_campaign) {
    utmParamQueryString += '&utm_campaign=' + utm_campaign;
  }
  if (utm_term) {
    utmParamQueryString += '&utm_term=' + utm_term;
  }
  if (roistat) {
    utmParamQueryString += '&roistat=' + roistat;
  }
  if (roistat_referrer) {
    utmParamQueryString += '&roistat_referrer=' + roistat_referrer;
  }
  if (roistat_pos) {
    utmParamQueryString += '&roistat_pos=' + roistat_pos;
  }

  if (utmParamQueryString.length > 0) {
    utmParamQueryString = utmParamQueryString.substring(1);
    utmParamQueryStringTrimmed = utmParamQueryString;
    utmParamQueryString = '?' + utmParamQueryString;
  }
})();

var navLinks = document.querySelectorAll('body a');

navLinks.forEach(function (item) {
  item.href += utmParamQueryString;
});