Plain Javascript to normalize target attribute for External Links

gistfile1.js

<script>
// I needed to trigger a non-interactive Google Analytics Event on external link click in 
// Google Tag Manager v2. This is a hacky workaround for targeting external links on using
// a trigger like this:
//    Event Type: Link Click	[element: target] [starts with] ["_"]

// This is used as a HTML tag in Google Tag Manager v2 to normalize all external <A> links
// to have a "target" attribute.  Priority 100, Triggered on All Pages and Event = gtm.dom

(function(){
  // Find external links and fix target (target not set, href isn't relative, current domain isn't in href)
  var fixlinks = document.querySelectorAll('a:not([target]):not([href*="'+window.location.host+'"]):not([href^="/"])');
  for (var i=0; i<fixlinks.length; i++){
    fixlinks[i].setAttribute('target', '_blank');  // or, more semantically rel=external
  }
}());
</script>