Track outbound links with Google's gtag.js

trackoutboundlink-gtag.htm

<script>
var trackOutboundLink = function(url) {
  gtag('event', 'click', {
    'event_category': 'outbound',
    'event_label': url,
    'transport_type': 'beacon'
  });
}
function onclickCallback(e) {
  var e = window.e || e;
  if ( (e.target.tagName !== 'A') || (e.target.host == window.location.host) ) return;
  trackOutboundLink(e.target); // GA event tracker
}
if (document.addEventListener)
  document.addEventListener('click', onclickCallback, false);
else
  document.attachEvent('onclick', onclickCallback);
</script>