How to detect if jquery is loaded

Method to detect whether jquery has been loaded: 1. Determine whether [jQuery()] is defined, the code is [if (jQuery) { jQuery has been loaded}]; 2. Determine whether [$()] is defined Definition, the code is [if(typeof jQuery == 'undefined'].

The operating environment of this tutorial: windows7 system, jquery2.1.4 version, DELL G3 computer .

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Method to detect whether jquery has been loaded:

After the current webpage loads jQuery, the jQuery() or $() function will be defined, so there are the following two methods to detect whether jQuery has been loaded:

Method 1:

if (jQuery) { 
// jQuery 已加载 
} else { 
// jQuery 未加载 
}

Method 2:

if (typeof jQuery == 'undefined') { 
// jQuery 未加载 
} else { 
// jQuery 已加载 
}

Remarks:

Above we check whether the jQuery function has been defined, which is a relatively safe and reliable method method, because when you load jQuery.js, you may load prototype.js or mootools.js, etc., which may redefine the $() function, and detecting whether the $() function exists will not Accurate.

We usually load jquery. If the bandwidth and speed are not very good, you can consider citing a third-party jquery

<script src="//apps.bdimg.com/libs/jquery/2.1.4/jquery.min.js"></script>
<script type="text/javascript">
  if (typeof jQuery == &#39;undefined&#39;) {
    document.write(unescape("%3Cscript src=&#39;/skin/mobile/js/jquery.min.js&#39; type=&#39;text/javascript&#39;%3E%3C/script%3E"));
  }
</script>

or

<script src="http://lib.sinaapp.com/js/jquery11/1.8/jquery.min.js"></script>
<script>window.jQuery || document.write(unescape("%3Cscript src=&#39;/skin/mobile/js/jquery.min.js&#39; type=&#39;text/javascript&#39;%3E%3C/script%3E"))</script>

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