我找到了解决方案，它也解释了为什么它在 JSFiddle 中有效.
问题是我如何导入 jQuery:我做到了

I found the solution, it explains as well why it does work in JSFiddle.
The problem was how I imported jQuery: I did

<script type="text/javascript" src="jQuerylink"/>

它必须是

<script type="text/javascript" src="jQuerylink" ></script>

我不知道为什么会这样，但现在可以了.感谢所有的努力.

I don't know why this is so, but now it works. Thanks for all effort.