It's very easy to inject JavaScript into a page, but stop and think before you do it. Adding lots of different JavaScript libraries may affect the existing functionality of the site. You may overwrite the existing functions with the same name and break the core functionality.
If you are testing a site, it may make all of your tests invalid. Failures may arise because there is a clash between the scripts that you inject and the existing scripts used on the site. The flip side is also true—injecting a script may make the functionality that is broken, work.
If you are going to inject scripts into an existing site, be sure that you know what the consequences are.
If you are going to regularly inject a script, it may be a good idea to add some assertions to ensure that the functions that you are injecting do not already exist before...