Based on my proof of concept for live JavaScript execution, I've built a slightly more functional version and released it at https://github.com/robertdmunn/livesandbox. It runs by opening the local file so you don't need a web server running to access it.

This app is still very much a prototype, but it allows you to write JavaScript in the browser using the open source CodeMirror plugin, the same plugin used in dev tools in Chrome, Firefox, and Safari, among other places. I am planning on putting the functionality into a web app to enable more advanced functionality such as importing external libraries.

Meanwhile, if you would like a scratch pad for executing JavaScript, including writing output to a browser window, take a look at it. It is released under the MIT license, so do with it as you please.