Inject

Injects script into web page and iframes.

document.body.innerHTML = "Hello world";

inject action creates <script> tag in the context of the page and executes the script. inject action doesn't provide you access to the extensions api, for that reason you might want to use bg-inject. Also you can use cs-inject to inject code into the content script, it allows you modifing page using with some sugar(i.e. JQuery) and in general is more safe as it doesn't conflicting with the page or other extensions content Scripts.