Javascript code cannot immediately changed in Python web application when debuging

I use Flask to build a web application. I run the .py file and then change the code for debugging in real time. I found if I write Javascript in HTML file, the change will be committed immediately. But if I put the Javascript in independent .js file, the change can not be committed immediately. I need to create a "private window" in Firefox, so there is no cache, no cookies, and I can see the change committed. What is the problem? What is the correct way to debug such python-javascript web application?