Board Thread:Support Requests - Designing Your Wikia/@comment-25597400-20160504040214/@comment-25508186-20160504082950

Common.js exists both for personal as well as site-wide use.

You'd want to place the script in your wiki's MediaWiki:Common.js if you want the script to work for everyone. Personal JS (located at User:Name/common.js) is loaded only for that specific user.

For more info on various CSS and JS pages and what skins/users they affect, see this.