What is a file with the extension js as suffix [Original]_Basic knowledge

Many friends have just started to contact the Internet and do not understand what files with the suffix js are. In fact, this file with the extension .js is a javascript file. Why do we need to use the
format to call the file? In fact, we often do this During the web page process, if the js file is written into the web page, it is not conducive to our repeated modifications. If we need to modify the js file in the future, we can directly change the js file without changing the entire web page. Now everyone's website is generated statically HTML, if you want to modify thousands of static pages, whether it is to replace or to save website space, it is a good choice. It should be noted that our js file can only contain js syntax, not simple text. I hope that brothers who have just started to contact this area will join more