Open all external links (different hostname than current page) in new tab/window with plain vanilla JavaScript.

target_blank--external.js

/**
* Open external links in new tab/window
*/

// All http:// links should open in new tab/window. Internal links are relative.
var anchors = document.querySelectorAll('a');

for (var i = 0; i < anchors.length; i++) {
	if (anchors[i].host !== window.location.hostname) {
		anchors[i].setAttribute('target', '_blank');
 	}
}

There might well be better solutions (and this one might have issues I'm not aware of yet), but this is a piece of code I use in my Google Analytics TYPO3 extension for detecting "same domain links":

var host                    = window.location.hostname.toLowerCase(),
regex                       = new RegExp('^(?:f|ht)tp(?:s)?\://(?:[^\@]\@)?([^:/]+)', 'im'),
match                       = url.match(regex),
domain                      = ((match ? match[1].toString() : ((url.indexOf(':') < 0) ? host : ''))).toLowerCase();

// Same domain
if (domain == host) {
    ...
}

where url is the href attribute value of the link. I remember I fiddled quite a while with this ... Btw. I just realize that I have no idea what would be the result with a link starting with "//", never tried this. ^^

Yeah, that's the point I recognized today. Having // links makes such RegEx things a complete mess. That's why I just used .host. Maybe I'll change that for testing. toLowerCase() is a good suggestion.

Give me some time please. I will do some tests with the regex later today, then let you know again ... Should be no problem to match the // properly as well ... I will also be able to improve my TYPO3 extension this way. ;)

Indeed, the regex didn't work with // links yet (and had a typo as well). I set up a codepen with updated regex, seems to work fine with quite a lot of different URL formats now. Did I miss a format?

http://codepen.io/jkphl/pen/dyqLu

I've tested the codepen version, but it's not working correctly. I'm using it in an XML template in the following form.

<script type='text/javascript'>
/*<![CDATA[*/
function makeExternal(link) {
  var url      = link.getAttribute('href'),
  host         = window.location.hostname.toLowerCase(),
  regex        = new RegExp('^(?:(?:f|ht)tp(?:s)?\:)?//(?:[^\@]+\@)?([^:/]+)', 'im'),
  match        = url.match(regex),
  domain       = ((match ? match[1].toString() : ((url.indexOf(':') < 0) ? host : ''))).toLowerCase();

  // Same domain
  if (domain != host) {    
    link.setAttribute('target', '_blank');    
  }
}

for (var l = 0, links = document.querySelectorAll('a'), ll = links.length; l < ll; ++l ){
   makeExternal(links[l]); 
} 
/*]]>*/
</script>