Why declare jQuery twice? [duplicate]

Question

What is the purpose of the following code?

Note that before the second script code below, jquery.min.js is already included with googleapis.

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.4/jquery.min.js"></script> <script>window.jQuery || document.write('<script src="/assets/js/vendor/jquery.min.js"><\/script>')</script> 

Answer 1

It's to fallback to the jquery.min.js file stored on the same server when the CDN is down or cannot be reached.

It's essentially saying:

// 1. Try to download and run https://ajax.googleapis.com/ajax/libs/jquery/2.1.4/jquery.min.js  // 2. Was jQuery successfully loaded from the CDN? If so, it will be defined: if (window.jQuery) {     // Yes, it's defined. Good. Nothing more needed. } else {     // No, it's not defined. Use my copy:     document.write('<script src="/assets/js/vendor/jquery.min.js"><\/script>') } 

Read more here and you can find the original code here.

Regarding window.jQuery || document.write(...) that is essentially shorthand for the code above. When defined window.jQuery will be truthy and so the statement on the right hand side of || will not be executed; however, if it's not defined it will be falsy and the statement on the right hand side of || will be executed.