Just a super-simple HTML + JS app for calling an API asynchronously and displaying the results.
(almost*) No frameworks / libraries used; just plain ol' JS.

*Uses Prism for syntax highlighting, just so it's prettier.

You can set up a pre-existing server with the required API (/spaces?format={format}) to serve the public directory, and browse to 'index.html'.

Alternatively, you can use the provided simple express server using node main.js from the server directory.
*note that you need express installed for that (npm install express).

1. Clear separation of concerns makes the code more maintainable and testable (see below on how to extend for testing).
   The largest module is the controller, which coordinates the UI with the backend;
   It's separated from any business logic or data manipulation, so those can be changed or replaced without changing code relating to the UI (for example- it's easy to change the data retrieval method, or the supported formats, without changes to the controller itself).
2. Internationalization is easy using translation modules.
3. Tiny- the whole thing is 15KB non-minified.

1. Display the full info about the response:
   The dataService module can be extended to return more info about the response (i.e headers).
   It's then very easy to display it with Prism, using language-http.
2. Unit tests:
   It's fairly easy to write a test page that'd test the functionality of the mainController module;
   Simply create an HTML file with the required elements (textbox, button, dropdown), and the dependencies can be easily mocked.
3. Cross-browser minor issues
   So obviously this works great with FF and Chrome, but IE is a different story, as usual.
   The app is fully functional on IE10 and 9, however, for IE9 on a very small screen, the UI looks significantly less attractive (since IE9 doesn't implement flex).
   Had I had more time, and IE9 was a critical issue, I would've tried some workarounds (for example- http://stackoverflow.com/a/28709421)