This article shows a brief example on how to properly mock and resolve a $q promise from within a Jasmine unit test.

This article is part of my "Learning NG" series, presenting some of my adventures while learning Angular. Check out the series intro and other articles. Something not quite as you think? I'm more than happy for any kind of feedback and improvement suggestions.

Problem

I have a service, let’s call it consumerService which uses another service personService to make an asynchronous call. In the case of a successful response, it calls a method on a 3rd service, called helloWorldService. What I’d like to test is the fact that the helloWorldService gets called upon a successful call of the personService.

Roughly, the code looks like this:

app.factory('consumerService', function(personService, helloWorldService){
  var service = {
    doSomething: doSomething
  };
  return service;
  
  function doSomething(){
    personService.get()
      .then(function(){
        // I want to test this one here
        helloWorldService.sayHello();
      })
  }
  
});

To test it, I need to stub the personService.get() function s.t. it positively resolves the promise it returns.

Solution

For stubbing, we can use Sinon.js. The first param is the object on which to operate upon, then the name of the function to stub and as the 3rd param I pass in the function that gets called instead of the original one.

sandbox.stub(personService, 'get', function(){
    return $q.when();
  });

Note, return $q.when() automatically creates and returns a resolved promise. This article goes a bit deeper on promises, totally worth checking out.

Then we also stub the helloWorldService.sayHello function to be able to listen to the number of invocations.

...
sandbox.stub(helloWorldService, 'sayHello');

Then invoke the SUT (subject under test):

...
consumerService.doSomething();

Finally, the expectations:

$scope.$apply();
expect(helloWorldService.sayHello.calledOnce).toBeTruthy();

What does that $scope.$apply() do there?? That’s the important piece actually. If you don’t call it, your promise won’t get resolved. The Angular documentation emphasizes this: https://docs.angularjs.org/api/ng/service/$q.

So, all wrapped up (highlighting the important pieces):

var sandbox;
var $scope;
...

describe('using the person service', function() {

  beforeEach(inject(function($rootScope, _consumerService_, _personService_, _helloWorldService_) {
      sandbox = sinon.sandbox.create();
      $scope = $rootScope.$new();
      ...
    }));
    
    afterEach(function(){
      sandbox.restore();
    });

    ...
    
    it('should properly resolve the promise', inject(function($q) {
      sandbox.stub(personService, 'get', function(){
        return $q.when();
      });
      sandbox.stub(helloWorldService, 'sayHello');
      
      consumerService.doSomething();
      
      // THIS IS THE IMPORTANT CALL HERE
      $scope.$apply();
      
      expect(helloWorldService.sayHello.calledOnce).toBeTruthy();
    }));
});

You can try it live in the Plunkr below.


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