When I was doing the project today, I encountered a super strange problem. My program in HTML runs well. Instead of running JSP in the project, I have been reporting $ is not a function. After looking for a long time, I tried a lot and did not solve it. Later, I suddenly thought that the order of the js file I introduced was changed, and the jQuery's JS file was introduced first, and other JS files were placed behind it. Really solve the problem! Intersection Intersection It turns out that when the page is loaded, the sequentially loaded file. If jQuery's js file is not the first to be introduced, then the $. The method name in other js will report an error. Only by introducing jQuery's js files first can the $. Method correctly. You hereby backup!