How to determine whether jquery is loaded?

Method: 1. Use the "if (jQuery){//jQuery loaded} else{//not loaded}" statement to judge; 2. Use "if(typeof jQuery=='undefined'){ //jQuery is not loaded} else{//Loaded}" statement to judge.

The operating environment of this tutorial: windows7 system, jquery3.5 version. This method is suitable for all brands of computers.

Related recommendations: "jQuery Video Tutorial"

After the current webpage loads jQuery, the jQuery() or $() function will be defined, so check whether jQuery has been There are the following 2 methods for loading:

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, this is A relatively safe and reliable method, because after you load jQuery.js, you may load prototype.js or mootools.js, etc., in which the $() function may be redefined, and it will be inaccurate to detect whether the $() function exists. .

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>

One is from Baidu and the other is from Sina Yes, it is not recommended to use Google, Microsoft and other foreign ones. Domestic access is not as fast as domestic access because it is easy to be forced.

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