Unit Testing Asynchronous Service Function with Angular/Karma/Jasmine

The test in this code isn't working as expected. Testing the return value of an asynchronous function seems to be a challenge.

describe('mocking services', function () {

    var someService, deferred;

    beforeEach(function () {

        module(function($provide){
            $provide.factory('someService', function($q){
                return{
                    trySynch: function(){
                        return 33;
                    },
                    tryAsynch: function(){
                        deferred = $q.defer();
                        return deferred.promise;
                    }
                };
            });
        });

        inject(function (_someService_) {
            someService = _someService_;
        });
    });

    it('should be able to validate values from both functions', function () {
        expect(someService.trySynch()).toEqual(33);

        var retVal;
        someService.tryAsynch().then(function(r){
            retVal = r;
        });
        deferred.resolve(44);
        expect(retVal).toEqual(44);
    });

});

Upon running the test, I encountered the following error:

Chrome 36.0.1985 (Mac OS X 10.9.4) mocking services should be able to validate values from both functions FAILED
        Expected undefined to equal 44.
        Error: Expected undefined to equal 44.
            at null.<anonymous> (/Users/selah/WebstormProjects/macrosim-angular/test/spec/services/usersAndRoles-service-test.js:34:24)

Any suggestions on how to get this test to pass?

Answer №1

When simulating asynchronous calls with $q, it's important to remember to utilize $rootScope.$apply() due to the way $q is implemented.

Specifically, the .then method doesn't execute synchronously; it's purposely designed to always be asynchronous, regardless of how it's invoked - whether synchronously or asynchronously.

In order to achieve this behavior, $q is closely tied to $rootScope. Therefore, in your unit tests, you must inform $rootScope that a change has occurred (such as triggering a digest cycle). This can be accomplished by calling $rootScope.$apply().

For more information, refer to this resource (especially the "Differences between Kris Kowal's Q and $q section")

A functional example appears below:

describe('mocking services', function () {

    var someService, deferred, rootScope;

    beforeEach(function () {

        module(function($provide){
            $provide.factory('someService', function($q){
                return{
                    trySynch: function(){
                        return 33;
                    },
                    tryAsynch: function(){
                        deferred = $q.defer();
                        return deferred.promise;
                    }
                };
            });
        });

        inject(function ($injector) {
            someService = $injector.get('someService');
            rootScope = $injector.get('$rootScope');
        });
    });

    it('should be able to test values from both functions', function () {
        expect(someService.trySynch()).toEqual(33);

        var retVal;
        someService.tryAsynch().then(function(r){
            retVal = r;
        });
        deferred.resolve(44);
        rootScope.$apply();
        expect(retVal).toEqual(44);
    });

});

Answer №2

$q's promise is still being resolved asynchronously.

Here's a quick test, although it's using an older version of Angular: http://plnkr.co/edit/Pk8Af23IKLzYQgzWJvTb

This particular test should function properly:

it('should successfully test values from both functions', function (done) {

    expect(someService.trySynch()).toEqual(33);

    someService.tryAsynch().then(function(result){
        expect(result).toEqual(44);
        done();
    });

    deferred.resolve(44);
});

Answer №3

Executing rootScope.$apply() prior to the expect clause that tests my asynchronous function ensures its success. Conversely, providing an incorrect value results in expected failure.

Although my test functions properly, I remain puzzled by the significance of using rootScope.$apply(). If someone could analyze and explain my code, I would gratefully recognize your response as the correct answer!

The functional test code I have implemented is as follows:

describe('simulating services', function () {

    var someService, deferred, rootScope;

    beforeEach(function () {

        module(function($provide){
            $provide.factory('someService', function($q){
                return{
                    trySynch: function(){
                        return 33;
                    },
                    tryAsynch: function(){
                        deferred = $q.defer();
                        return deferred.promise;
                    }
                };
            });
        });

        inject(function ($injector) {
            someService = $injector.get('someService');
            rootScope = $injector.get('$rootScope');
        });
    });

    it('should successfully compare values from both functions', function () {
        expect(someService.trySynch()).toEqual(33);

        var resultValue;
        someService.tryAsynch().then(function(r){
            resultValue = r;
        });
        deferred.resolve(44);
        rootScope.$apply();
        expect(resultValue).toEqual(44);
    });

});

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