What is the original file of a file with a extension of js with a suffix?

Many of my friends have just started to access the network and don't know what files are suffixed with js. In fact, this extension is. the js file is a javascript file. Why should I use <script src = "test. js "> </script> is used to call files. In fact, we often perform webpage operations, because if JavaScript files are written to webpages, It is not conducive to repeated modification, if we need to modify the js file in the future, we can directly modify the js file without modifying the entire webpage. Now, all websites generate static html, if you want to modify thousands of static pages, whether it is to replace or save website space, it is a good choice.
It should be noted that our js file can only contain JavaScript syntax, not simple text. We hope that we will be more involved in this field.