bootstrap cdn or local

1<!-- CSS -->
2<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta.2/css/bootstrap.min.css" integrity="sha384-PsH8R72JQ3SOdhVi3uxftmaW6Vc51MKb0q5P2rRUpPvrszuE4W1povHYgTpBfshb" crossorigin="anonymous">
3
4<!-- JavaScript -->
5<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta.2/js/bootstrap.min.js" integrity="sha384-alpBpkh1PFOepccYVYDB4do5UnbKysX5WZXm3XxPqe5iKTfUKjNkCk9SaVuEZflJ" crossorigin="anonymous"></script>
6
7<!-- Local fallback -->
8<script>
9  if (! $.fn.modal) {
10    document.write('<script src="/js/bootstrap.min.js"></script>');
11    document.write('<link rel="stylesheet" href="/css/bootstrap.min.css">');
12  }
13</script>
14