在html文件中调用Javascript文件

I have code that has a ball bouncing around on a canvas I create hte canvas in HTML and the ball code is in a .js file. When I run the html file the ball code does not work but when I put the ball code between tags in the html file it does work. Anyone see what reason it doesn't work whe its in a .js file?I have code that has a ball bouncing around on