Open external links in a new tab without jQuery

The links property returns a collection of all <area> and <a> elements in a document with a value for the href attribute.

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

https://developer.mozilla.org/en-US/docs/Web/API/Document/links

Pure JS:

function externalLinks() {
  for(var c = document.getElementsByTagName("a"), a = 0;a < c.length;a++) {
    var b = c[a];
    b.getAttribute("href") && b.hostname !== location.hostname && (b.target = "_blank")
  }
}
;
externalLinks();