How does jsFiddle allow and execute user-defined JavaScript without being dangerous?

I’ve been working on a JS library and would like to setup a demo page on Github that allows, for example, users to define their own callbacks and execute commands.

I know “eval() is evil” and I can see how blind eval() of scripts could lead to XSS and other security issues. I’m trying to cook up some alternative schemes.

I really enjoy the interactivity of jsFiddle. I’ve taken a look at their source but was hoping someone could lay out here how jsFiddle allows and executes user-defined JavaScript without being dangerous. So long as it doesn’t involve a 3rd party echo server, I’m hoping I can emulate the approach.

jsFiddle executes user scripts on a separate domain, (try it and see).
Therefore, it can’t interact with the parent frame and it can’t steal cookies.

You can actually do this without a separate server by placing a static page in a separate domain that reads from its querystring in Javascript.
You can communicate back using the page title (and so can the enemy).

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