How do i make external scripts using the script tag?

Question

I understand that the script tag can point to an external script file through the src attribute. But, how do i create an external script file that can actually be opened and read?

Answer 1

If it were php script, you would type in in a plain text editor like notepad and save it with the .php file extension.. I imagine it's the same for java script just that the extension is .js in this case

Answer 2

How do i save it with the j.s file extension. How exactly is that done?