Initial Development

You can either hit a fake server, using [Sinon's FakeServer](http://sinonjs.org/docs/#fakeServer), or hit the real API, and use [VCR](https://github.com/vcr/vcr) to record the server's responses, so they can be re-played again when you run your tests.

I guess it depends on the complexity of the API to mock, sometimes it's easier to hit the real thing. If you do that, make sure to delete the recorded cassettes once in a while, to see if the API is still responding the same way (that they did not make breaking changes).