Use JavaScript as a scripting language in Ruby/Rails

I need to allow the users of my RoR (3) web application to customise parts of it with simple Javascript. This would involve allowing them to type in a single function which returns an object in JSON.

The RoR app th开发者_Go百科en has to run this Javascript function and use the result of it for further processes.

Does anyone have any experience, examples, frameworks or ideas as how to achieve this please?