A quick example of how to test a function (make_ajax_request) that does some set-up, then calls an ajax request with some deferred handlers defined

test_deferred_spec.js

define(function(require) {
    'use strict';
    var $ = require('jquery'),
        Sinon = require('sinon');

    var always_count, done_count, fail_count;
    var make_ajax_request = function() {
        always_count = 0;
        done_count = 0;
        fail_count = 0;
        $.ajax({
            method: 'post',
            url: 'my_url',
            contentType: 'application/json'
        }).always(function(data, textStatus, jqXHR) {
            always_count++;
        }).done(function(data, textStatus, jqXHR) {
            done_count++;
        }).fail(function(data, textStatus, jqXHR) {
            fail_count++;
        });
    };

    describe('make_ajax_request', function(){
        beforeEach(function(){
            // reset counts
            always_count = done_count = fail_count = undefined;

            // stub out ajax to return a promise (we'll resolve it later in nested describes)
            this.d = $.Deferred();
            this.ajax_stub = Sinon.stub($, 'ajax').returns(this.d);
            // CALL
            make_ajax_request();
        });
        afterEach(function(){
            this.ajax_stub.restore();
        });

        // we haven't triggered the promise yet, just the initial code flow
        it('should reset the counts before ajaxing', function(){
            expect(always_count).toBe(0);
            expect(done_count).toBe(0);
            expect(fail_count).toBe(0);
        });

        describe('when ajax succeeds', function(){
            beforeEach(function(){
                // we've already called the function - it's finished executing, but the promise
                // response handlers haven't happened yet. Lets resolve the promise and watch them fly
                this.d.resolve('data', 'textStatus', 'jqXHR');
            });
            it('should increment always and done counts', function(){
                expect(always_count).toBe(1);
                expect(done_count).toBe(1);
                expect(fail_count).toBe(0);
            });
        });

        describe('when ajax fails', function(){
            beforeEach(function(){
                // reject will call fail instead of done
                this.d.reject('data', 'textStatus', 'jqXHR');
            });
            it('should increment always and fail counts', function(){
                expect(always_count).toBe(1);
                expect(done_count).toBe(0);
                expect(fail_count).toBe(1);
            });
        });
    });
});