Guide on simulating a promise in a Karma unit test that is invoked within a function

Within my Angular controller, I have the following function that retrieves data from a promise and I need to test if it returns the expected result.

    function getName() {
      var name = "";
      nameService.getName().then(function (data) {
        name = data.name;
      });
      return name;
    }

I am unsure of how to mock the promise call with fake data. Can I use $httpBackend or $provide? I attempted the following but it did not work:

it("function getName should get the name from the nameService.getNameInfo function", function () {
        var name = { name: "name1"};
        spyOn(mockNameService, 'getNameInfo').and.callFake(function() {
            return {
                then: function(callback) {return callback(name);}
            };
        });
        var result = myCtrl.getName();
        expect(result).toEqual("name1");
    });

Answer №1

Consider using the following code snippet:

spyOn(fakeService, 'getFakeInfo').and.returnValue($q.resolve('mockedData'));

This will help you mock your data effectively. Next step is to make sure it resolves correctly. Add this verification:

expect(myController.getFake().then(function(result){
    expect(result).toBe('mockedData');
}).toBeResolved();

Answer №2

The issue lies not in your unit testing code, but rather in the incorrect implementation within the application code itself. For instance:

function retrieveName() {
      var name = "";
      nameService.getName().then(function (data) {
        name = data.name;
      });
      return name; //it will return "" since the promise has not yet been resolved
    }

Within this snippet, the function retrieveName will consistently return an empty string "", failing to reflect the values assigned upon resolution of the promise name = data.name. This discrepancy stems from the asynchronous nature of promises, where the function retrieveName returns the empty string before the promise is fully resolved, leading to inaccuracies in the output. Therefore, the initial code should be refactored and rectified to address this dilemma first and foremost!

Ultimately, your unit test scripts have effectively pinpointed a flaw in your code, fulfilling their fundamental purpose. Not every failed test necessitates immediate correction within the tests themselves; instead, when encountering discrepancies, it's crucial to analyze the actual code structure to discern the root cause. That's the primary objective behind composing unit tests.

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