Add target="_blank" to external links with pure JavaScript.

target_blank.js

function targetBlank() {
  // remove subdomain of current site's url and setup regex
  var internal = location.host.replace(/^[^.]+\./g, '');
  internal = new RegExp(internal, "i");

  var a = document.getElementsByTagName('a'); // then, grab every link on the page
  for (var i = 0; i < a.length; i++) {
    var href = a[i].host; // set the host of each link
    if (!internal.test(href)) {
      // make sure the href doesn't contain current site's host
      a[i].setAttribute('target', '_blank'); // if it doesn't, set attributes
    }
  }
};