有条件地加载jQuery

Posted 2021-02-25

if window.jQuery is undefined, there must have been a problem downloading the script from the CDN. In that case, proceed to the right side of the && operator, and insert a script linking to a local version of jQuery.

<!-- Grab Google CDN jQuery. fall back to local if necessary -->  
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js"></script>  
<script>!window.jQuery && document.write('<script src="js/jquery-1.4.2.min.js"></script>')</script>