MediaWiki:Gadget-extlinks.js

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 // Make external links open in a new tab or window. Why someone would want this i have no idea. Copied from wikibooks page with same name.
 function external_links() {
   var links = document.getElementsByTagName('a');
 
   for (i = 0, length = links.length; i < length; i++)
   {
     if ((hasClass(links[i], "external") || hasClass(links[i], "extiw")) && links[i].href.indexOf(wgServer) != 0) {
       if (links[i].target)
         links[i].target = "_blank";
       else if (links[i].setAttribute)
         links[i].setAttribute("target", "_blank");
     }
   }
 }
 
addOnloadHook(external_links);