Mocha .then(done) doesn't work as expected - mocha.js

Question

This question is not about a problem which I can't solve, it is just a curiosity. I'm not very experienced with Mocha, but there's something interesting I've stumbled upon already.
What I want is to use done() to tell Mocha the promise has been resolved.
The following code DOESN'T work:
beforeEach((done) => {
user = new User({ name: 'Dummy' })
user.save()
.then(done)
})
I know I'm passing the result of the user.save() promise to done, but I think it shouldn't be a problem.
Instead this other code works:
beforeEach((done) => {
user = new User({ name: 'Dummy' })
user.save()
.then(() => done())
})
It seems to me that Mocha done() has some kind of control flow which leads to: Error: done() invoked with non-Error: {"_id":"5b65b9d2669f7b2ec0a3d503","name":"Dummy","__v":0}
Is it because done() wants strictly an error as its argument?
Why done() does even care about what I pass to it?
Can you make some example showing why done() argument to be an Error is useful?
Thanks in advance ;)

Answer 1

It is because done() in Mocha only accepts Error argument. In your case, your save() method returns json object not an Error ie new Error('failed save').
If we take a look at mocha test file, we can see that it won't accept other type of arguments.
// https://github.com/mochajs/mocha/blob/master/test/unit/runnable.spec.js#L358
describe('when done() is invoked with a string', function () {
it('should invoke the callback', function (done) {
var test = new Runnable('foo', function (done) {
done('Test error'); // specify done with string/text argument
});
test.run(function (err) {
assert(err.message === 'done() invoked with non-Error: Test error');
done();
});
});
});
But if we see the test when the argument is Error, it works
// https://github.com/mochajs/mocha/blob/master/test/unit/runnable.spec.js#L345
describe('when an error is passed', function () {
it('should invoke the callback', function (done) {
var test = new Runnable('foo', function (done) {
done(new Error('fail'));
});
test.run(function (err) {
assert(err.message === 'fail');
done();
});
});
});
Btw, I suggest that you avoid using done since mocha supports promise by specifying return statement. So, we change the code into
beforeEach(() => {
user = new User({ name: 'Dummy' })
return user.save().then(user => {
// antyhing todo with user
});
});
Hope it helps.