Consider http://jsbin.com. It's already equipped with various versions of the most prominent JavaScript frameworks for testing. Furthermore, it stores your code online and creates a short-URL so you can pass examples around, and quickly refer back to previous items of interest. You'll see many of us here using it frequently to help each other.
If you really want to work on your own, you can fork jsbin on GitHub: http://github.com/remy/jsbin
