How to “Detect” If Your CDN is Down And Load a Script Locally As Backup

I’m sure this has been around for a while now, but I just stumbled upon it and found it interesting:

<script src="//"></script>
window.jQuery || document.write('<script src="js/vendor/jquery-1.9.1.min.js"><\/script>')</script>

Basically, this does a check to see if jQuery has loaded properly from the CDN and, if it hasn’t, it tries to load it locally.