Summary of jQuery errors and solutions

Have you ever encountered the problem of jQuery is not define? When testing a program today, jQuery has obviously been loaded but it always prompts that jQuery is not defined. After many tests, the problem was finally discovered. Here is a brief summary. Friends who need it can refer to it. I hope it can help everyone.

Usually there are several solutions to this situation:

1: Check whether the jquery file is imported

Even if the file is introduced, is it passed? Problems may occur when downloading software such as full-site downloaders. It is recommended to use Thunder to download from the official website.

2: To check whether the path is wrong, you can click the js file path in the page source code.

Sometimes some paths will be automatically added to js, ​​causing path errors

3: JS is an interpreted language, which is executed sequentially based on tag references. $ is an object generated in jQuery. If it needs to be used, the jquery.js file must be placed in the JS that uses it. Front.

Place the jQuery library before the JavaScript scripts that depend on jQuery, and put these codes into document.ready to ensure that the DOM is loaded.

<script type="text/javascript" src="path/to/jquery.min.js"></script>
<script type="text/javascript">
 $(document).ready(function() {
  //依赖于jQuery的代码
 });
</script>

4. Sometimes we use a third-party cdn’s js to load the CDN’s jQuery and it fails or times out.

When provided There is a problem with jQuery's CDN that causes jQuery to fail to load, or the browser times out when loading the jQuery file due to network problems, and a jQuery undefined error occurs.

Solution: Mount the jQuery file on your own website as a backup , if the CDN fails to load jQuery, use your own website to store jQuery. In this case, most users can still speed up access through CDN, and can avoid errors if there is a problem with the CDN.

<script src="https://cdn.staticfile.org/jquery/3.1.1/jquery.min.js"></script>
<script> 
window.jQuery || document.write(&#39;<script src="/js/jquery.min.js"><\/script>&#39;))
</script>

There will be problems whether it is downloaded through some software such as a whole-site downloader. It is recommended to use Thunder to download from the official website.

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