[javascript] How do I mock a service that returns promise in AngularJS Jasmine unit test?

I have myService that uses myOtherService, which makes a remote call, returning promise:

angular.module('app.myService', ['app.myOtherService'])
  .factory('myService', [
    myOtherService,
    function(myOtherService) {
      function makeRemoteCall() {
        return myOtherService.makeRemoteCallReturningPromise();
      }

      return {
        makeRemoteCall: makeRemoteCall
      };      
    }
  ])

To make a unit test for myService I need to mock myOtherService, such that its makeRemoteCallReturningPromise method returns a promise. This is how I do it:

describe('Testing remote call returning promise', function() {
  var myService;
  var myOtherServiceMock = {};

  beforeEach(module('app.myService'));

  // I have to inject mock when calling module(),
  // and module() should come before any inject()
  beforeEach(module(function ($provide) {
    $provide.value('myOtherService', myOtherServiceMock);
  }));

  // However, in order to properly construct my mock
  // I need $q, which can give me a promise
  beforeEach(inject(function(_myService_, $q){
    myService = _myService_;
    myOtherServiceMock = {
      makeRemoteCallReturningPromise: function() {
        var deferred = $q.defer();

        deferred.resolve('Remote call result');

        return deferred.promise;
      }    
    };
  }

  // Here the value of myOtherServiceMock is not
  // updated, and it is still {}
  it('can do remote call', inject(function() {
    myService.makeRemoteCall() // Error: makeRemoteCall() is not defined on {}
      .then(function() {
        console.log('Success');
      });    
  }));  

As you can see from the above, the definition of my mock depends on $q, which I have to load using inject(). Furthermore, injecting the mock should be happening in module(), which should be coming before inject(). However, the value for the mock is not updated once I change it.

What is the proper way to do this?

Honestly.. you are going about this the wrong way by relying on inject to mock a service instead of module. Also, calling inject in a beforeEach is an anti-pattern as it makes mocking difficult on a per test basis.

Here is how I would do this...

module(function ($provide) {
  // By using a decorator we can access $q and stub our method with a promise.
  $provide.decorator('myOtherService', function ($delegate, $q) {

    $delegate.makeRemoteCallReturningPromise = function () {
      var dfd = $q.defer();
      dfd.resolve('some value');
      return dfd.promise;
    };
  });
});

Now when you inject your service it will have a properly mocked method for usage.

We can also write jasmine's implementation of returning promise directly by spy.

spyOn(myOtherService, "makeRemoteCallReturningPromise").andReturn($q.when({}));

For Jasmine 2:

spyOn(myOtherService, "makeRemoteCallReturningPromise").and.returnValue($q.when({}));

(copied from comments, thanks to ccnokes)

using sinon :

const mockAction = sinon.stub(MyService.prototype,'actionBeingCalled')
                     .returns(httpPromise(200));

Known that, httpPromise can be :

const httpPromise = (code) => new Promise((resolve, reject) =>
  (code >= 200 && code <= 299) ? resolve({ code }) : reject({ code, error:true })
);

describe('testing a method() on a service', function () {    

    var mock, service

    function init(){
         return angular.mock.inject(function ($injector,, _serviceUnderTest_) {
                mock = $injector.get('service_that_is_being_mocked');;                    
                service = __serviceUnderTest_;
            });
    }

    beforeEach(module('yourApp'));
    beforeEach(init());

    it('that has a then', function () {
       //arrange                   
        var spy= spyOn(mock, 'actionBeingCalled').and.callFake(function () {
            return {
                then: function (callback) {
                    return callback({'foo' : "bar"});
                }
            };
        });

        //act                
        var result = service.actionUnderTest(); // does cleverness

        //assert 
        expect(spy).toHaveBeenCalled();  
    });
});

I found that useful, stabbing service function as sinon.stub().returns($q.when({})):

this.myService = {
   myFunction: sinon.stub().returns( $q.when( {} ) )
};

this.scope = $rootScope.$new();
this.angularStubs = {
    myService: this.myService,
    $scope: this.scope
};
this.ctrl = $controller( require( 'app/bla/bla.controller' ), this.angularStubs );

controller:

this.someMethod = function(someObj) {
   myService.myFunction( someObj ).then( function() {
        someObj.loaded = 'bla-bla';
   }, function() {
        // failure
   } );   
};

and test

const obj = {
    field: 'value'
};
this.ctrl.someMethod( obj );

this.scope.$digest();

expect( this.myService.myFunction ).toHaveBeenCalled();
expect( obj.loaded ).toEqual( 'bla-bla' );

The code snippet:

spyOn(myOtherService, "makeRemoteCallReturningPromise").and.callFake(function() {
    var deferred = $q.defer();
    deferred.resolve('Remote call result');
    return deferred.promise;
});

Can be written in a more concise form:

spyOn(myOtherService, "makeRemoteCallReturningPromise").and.returnValue(function() {
    return $q.resolve('Remote call result');
});

You can use a stubbing library like sinon to mock your service. You can then return $q.when() as your promise. If your scope object's value comes from the promise result, you will need to call scope.$root.$digest().

var scope, controller, datacontextMock, customer;
  beforeEach(function () {
        module('app');
        inject(function ($rootScope, $controller,common, datacontext) {
            scope = $rootScope.$new();
            var $q = common.$q;
            datacontextMock = sinon.stub(datacontext);
            customer = {id:1};
           datacontextMock.customer.returns($q.when(customer));

            controller = $controller('Index', { $scope: scope });

        })
    });


    it('customer id to be 1.', function () {


            scope.$root.$digest();
            expect(controller.customer.id).toBe(1);


    });