Tracking outbound links in Google Analytics (Universal/GA3) without adding onclick attributes to all your anchors

track_outbound_links.js

// Standardish tracking code here at the top.  
<script async src="https://www.googletagmanager.com/gtag/js?id=UA-XXXXXX-X"></script>
<script>
  window.dataLayer = window.dataLayer || [];
  function gtag(){dataLayer.push(arguments);}
  gtag('js', new Date());

  gtag('config', 'UA-XXXXXX-X');

  // Add this to track clicks on outbound links as events.
  document.addEventListener("click", function (e) {
    // On click, get the URL from the closest anchor.
    var url = e.target.closest('a').getAttribute('href');
    // If href does not contain our domain, it's an external link. Track it.
    if( url && url.indexOf( window.location.hostname ) < 0 ) {
      gtag('event', 'click', {
        'event_category': 'Outbound links',
        'event_label': url,
        'transport_type': 'beacon',
        'event_callback': function(){document.location = url;}
      });
    }
  });
</script>