Ana Badescu writes:

The Javascript client is actually a node.js module, and users will be able to use it as part of a node.js application which just as Postorious, is server-side.

Oops, that's right, the REST API normally isn't exposed except to localhost. My bad.

However, we can set up proper test cases for each API, in Javascript as well. There is a large number of testing strategy modules that offer everything to set up a testing environment, stubs, mocks.

My point is that no, you can't. If you don't have real use cases, you will write tests that don't matter, and you will fail to cover things that some real use case depends on. Often enough, those things can be really basic, and embarrass both your programmer's pride and your schedule. Both strategies, a priori (here's my API definition, make sure it works) and use-case-based (here's my use case, make sure the API can implement it) are necessary.