What is the correct procedure to add a javascript file to SharePoint ?

I want to add some scipt to my pages. I would like to refer this javascript
file, in a Content Editor (CE) web part and add CE on my pages. The problem is
javascript file absolute path, I cannot refer it if I place it in Layouts.

what I will NOT do is copy and paste the javascipt in content editor web part
directly. The reason is that very instance of CE will store its own copy of
javascript code which will prevent me from centralize update.