You should include a JavaScript file in your pages. There are many ways to include javascript in your page.
This has several advantages:

1. By updating one file, you update your script on all pages.
2. The pages themselves become slightly lighter, because the include file is cached by the browser, so it doesn't have to be reloaded every time the user loads a new page.

1. Including javascript file at top of page before body section inside head tag:-