How to Stub Promise Function and Mock Resolved Output

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:

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. Testing nodejs Applications Book Cover

Show me the code

//Lab Pet
    return res; 
    return err;


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:

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


var sinon = require('sinon');
describe('#fetch()', 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
        assert(fetchStub.called, '#fetch() has been called');
        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);




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.


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