Can javascript code tell if the browser is in the background?

Question

I would like to tell whether my browser's tab is in focus, and whether the browser itself is in the foreground or in the background.

Is there a javascript interface that queries the foreground/background status of the browser?

Update Following Palpatim's answer below, I found this practical HTML5Rocks tutorial about the page visibility API.

Answer 1

Your browser support requirements will determine whether this is viable, but the Page Visibility API may get you part of the way there.

It doesn't determine foreground/background status; to my knowledge there is no API available to JavaScript that will give you that level of access, so you'd have to rely on a browser plugin to report it. However, it will report whether a page is visible or not, which for many use cases is all that's required.