How to make assertions inside a promise when any errors thrown don't bubble up?

Question

Running this with mocha results in timing out, rather than letting mocha catch the error so it could fail immediately..

var when = require('when');
var should = require('should');

describe('', function() {
    it('', function(done) {
        var d = when.defer();
        d.resolve();
        d.promise.then(function() {
            true.should.be.false;
            false.should.be.true;
            throw new Error('Promise');
            done();
}); }); });

^{http://runnable.com/me/U7VmuQurokZCvomD}

Is there another way to make assertions inside the promise, such that when they fail they are caught by mocha causing it to fail immediately?

---

As per chai recommendation, I looked into it and it seems I have to have a direct access to the promise object, right? The problem is that I'm not using promise directly.. My bad if I simplified but This would be a more closer to reality example

function core_library_function(callback){
    do_something_async(function which_returns_a(promise){
        promise.then(function(){
            callback(thing);
}); }); }

describe('', function() {
    it('', function(done) {
        core_library_function(function(thing){
            ...
            done();                         
}); }); });

So I really have no control over the promise directly, it's abstracted far far away.

Answer 1

When using promises with Mocha, you'll have to return the promise in the test and will want to remove the done parameter since the callback isn't being used.

it('', function() {
    var d = when.defer();
    d.resolve();
    return d.promise.then(function() {
        throw new Error('Promise');
    });
});

This is described in the docs under Working with Promises:

Alternately, instead of using the done() callback, you can return a promise.