That is extremely useful. Do you know if any testing libraries or other useful functionality have been built around this?

I suppose if you are a WebKit-only developer, this utility could have many useful applications.

There's also mozrepl, which lets you evaluate code in Firefox from the command-line or Emacs.

Thanks, I'll look how to get this working on linux.

At least Arch has `js`package which gives you js command to obtain JS shell; in fact it is just Mozilla's SpiderMonkey. Anyway, the best option for "standalone JS" is node, mainly because it has a sane way of importing code from other files.

Alternatively, you could use Rhino Shell (https://developer.mozilla.org/en/Rhino_Shell) - it even supports tab completion!

My Chrome install on Windows has the same thing. I push ctrl+shift+c to load the developer tools, and click on console and it's a very similar tool to what he was using.