Mocha and generators

Lately I have been writing a dependency injection module for node.js which works with coroutines. I'll probably write more about it later since I find this topic super interesting but for now, one can look at this introduction to generators. I am using the excellent bluebird promises library for the coroutine part.

So let's try to write a mocha test with coroutines, there are a few gotcha.

Here's our basic generator:

(notice the * at the end). Here's how to use it (node >= 0.11.4 with --harmony flag):

A basic mocha test would be:

And this fails... Because the second call to g.next() returns the promise object. A fix would be:

This pass, but is not very pretty, and suddendly, we have to get back the asynchronous callback back :(

A better solution is to use a coroutine.

Much nicer !

Be careful to one gotcha though, everything running inside the coroutine is actually wrapped inside a promise. So if there is any exception inside, it won't by default be thrown outside. For example:

This test will pass. You can add logging with

I haven't find yet a good enough solution to this problem, where I can have a failing test if an unexpected excepction is thrown.

If anyone has an idea ?