Checking if a URL is broken in Javascript

This question has been posted on Stack before, but none so specific as to what I'm trying to understand.

The simplest way to check if a URL is corrrect to send a http Head request. But how do you use that to specify the URL ?

I found this in a previous post :

function UrlExists(url) {
  var http = new XMLHttpRequest();
  http.open('HEAD', url, false);
  http.send();
  return http.status!=404;
}

But it doesn't seem to run in firebug for a few reasons.

My apologies ahead of time for being daft.