未捕获的ReferenceError：当jquery.js位于文档末尾时，未定义$

I have a project where all the JS files are referenced in the footer, as is recommended for speed of page loading, including the link to the Jquery file. This code produces an "Uncaught ReferenceError", I assume because Jquery has not been defined before my script is called:I have a project where all the JS files are ref