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.
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.
<shamelessplug>If you want to use any of these inside emacs check out my js-comint mode: http://js-comint-el.sourceforge.net/ Makes it really easy to do things like write code in one buffer and execute it in another. </shamelessplug>
/System/Library/Frameworks/JavaScriptCore.framework/Versions/A/Resources/jsc
You can just do:
sudo ln /System/Library/Frameworks/JavaScriptCore.framework/Versions/A/Resources/jsc /bin/jsc
To be able to invoke it directly from the command line.