Simple Engineering

Mocking and Stubbing walk hand in hand. In this blog, we document stubbing functions with promise constructs. The use cases are going to be based on Models. We keep in mind that there is a clear difference between mocking versus stub/spying and using fakes.

In this article we will talk about:

• Stub a promise construct by replacing it with a fake
• Stub a promising construct by using third-party tools
• Mocking database-bound input and output
  

Even though this blog post was designed to offer complementary materials to those who bought my Testing nodejs Applications book, the content can help any software developer to tuneup working environment. You use this link to buy the book.

Show me the code

//Lab Pet
window.fetch('/full/url/').then(function(res){ 
    service.doSyncWorkWith(res); 
    return res; 
}).catch(function(err){ 
    return err;
});

Example:

What can possibly go wrong?

When trying to figure out how to approach stub functions that return a promise, the following points may be a challenge:

• How to deal with the asynchronous nature of the promise.
• Making stubs drop-in replacements of some portion of the code block, and leave intact anything else.

The following sections will explore more on making points stated above work.

Content

• From Johnny Reeves Blog: Stub the services' Async function, then return mocked response

var sinon = require('sinon');
describe('#fetch()', function(){
    before(function(){ 
        //one way
        fetchStub = sinon.stub(window, 'fetch').returns(bakedPromise(mockedResponse));
        //other way
        fetchStub = sinon.stub(window, 'fetch', function(options){ 
            return bakedPromise(mockedResponse);
        });
        //other way
        fetchStub = sinon.stub(window, 'fetch').resolves(mockedResponse);

    });
    after(function(){ fetchStub.restore(); });
    it('works', function(){
        //use default function like nothing happened
        window.fetch('/url');
        assert(fetchStub.called, '#fetch() has been called');
        //or 
        assert(window.fetch.called, '#fetch() has been called');
    });
    it('fails', function(){
            //one way
        fetchStub = sinon.stub(window, 'fetch', function(options){ 
            return bakedFailurePromise(mockedResponse);
        });
        //another way using 'sinon-stub-promise's returnsPromise()
        //PS: You should install => npm install sinon-stub-promise
        fetchStub = sinon.stub(window, 'fetch').returnsPromise().rejects(reasonMessage);

    });
});

Example:

• bakedPromise() is any function that takes a Mocked(baked) Response and returns a promise
  
• This approach doesn't tell you if Service.doJob() has been expected. For That:
• source
• source

Conclusion

In this article, we established the difference between Promise versus regular callbacks and how to stub promise constructs, especially in database operations context, and replacing them with fakes. Testing tends to be more of art, than science, proactive makes perfect. There are additional complimentary materials in the “Testing nodejs applications” book.

References

• Testing nodejs Applications book
• Getting started with nodejs and mocha ~ SemaphoreCI Community Tutorials
• A TDD Approach to Building a Todo API Using nodejs and mongodb ~ SemaphoreCI Community Tutorials
• Evan Hahn in “How do I Jasmine“ – has pretty good examples that leverages spies along the way.

tags: #snippets #code #annotations #question #discuss