What is a CDN and How to load jQuery from CDN with Fallback?

What is a CDN ? 

CDN is an acronym for Content Delivery Network.
From wiki "A content delivery network or content distribution network (CDN) is a large distributed system of servers deployed in multiple data centers across the Internet. The goal of a CDN is to serve content to end-users with high availability and high performance."

As a developer  we mostly use CDNs for loading some JavaScript releated JS plugins and frame works like jQuery, Dojo, AngulaJS etc.. However CDN Not only used for downloading text/Script files but Its major use now a days is downloading objects like media files, software, documents etc.., applications like e-commerce, portals And also for live streaming media.

Which are the popular jQuery CDN ?

In this post we want to concentrate on jQuery related CDNs only so I ll be listing only jQuery related CDN sources mainly. You can use them in your application.

CDNs can offer a performance benefit by hosting jQuery on servers spread across the globe. This also offers an advantage that if the visitor to your webpage has already downloaded a copy of jQuery from the same CDN, it won't have to be re-downloaded.
Popular CDNs and their URLs

• jQuery's CDN provided by MaxCDN 
  Use

• Google CDN
  Use

• Microsoft CDN
  Use

• CDNJS CDN
  Use

• jsDelivr CDN
  Use

How to load jQuery locally when CDN fails?

There May be Situations where JavaScript file from CDN May not be loaded. There can be any reason for it as i mentioned it in my blog Here How to Check If JQuery Library Loaded?
WHat to do in this situation? Its simple just check If the jQuery library is loaded or not using This method and add the local JS file in Else condition as shown below.

That is it. Isn't it that simple???