Code to return link rel=dnsprefetch

prefetchbuilder.js

(function() {
  var requests = window.performance.getEntries();
  var hosts = {};
  var output = "";
  
  for(var requestIdx = 0; requestIdx < requests.length; requestIdx++) {
    var request = requests[requestIdx];
    var origin = new URL(request.name).origin;
      
    hosts[origin] = 1;
  }
  
  for(var key in hosts) {
    output += "<link rel=\"dns-prefetch\" href=\"" + key + "\">\n";
  }
    
  alert(output);
})();