utm-enhance.js

var links = document.getElementsByTagName('a');
var params = new URLSearchParams(window.location.search);
var utmParams = {};

params.forEach(function (val, name) {
  if (name.startsWith('utm_')) {
    utmParams[name] = val;
  }
});

for(var i = 0; i < links.length; i++) {
  var link = links[i];
  var url = new URL(link.href, window.location);

  if (link.href && (url.protocol === 'http:' || url.protocol === 'https:')) {
    var changed = false;

    Object.keys(utmParams).forEach(function(param) {
      changed = true;
      url.searchParams.set(param, utmParams[param]);
    });

    if (changed) {
      link.href = url.toString();
    }
  }
}