Using MSW (Mock Service Worker) in an Angular project

Tim Deschryver

During the past weeks, I've seen posts around the library MSW (MockServiceWorker).


Seamless REST/GraphQL API mocking library for browser and Node.

MSW seems to gain some popularity fast, and most of it is coming from the React community. It even became the recommended approach to mock HTTP requests with React Testing Library. Kent C. Dodds wrote a blog post "Stop mocking fetch" in which he explains what problems MSW can resolve. If you're not familiar with MSW, I would suggest skimming through the documentation or to read Kent's blog before continuing.

Because it receives a lot of positive feedback I wanted to give this a shot with Angular. In this post, we'll create a proof of concept to search a GitHub user for a username.

The real implementation of the GitHub service, we search on "timdeschryver" and receive the GitHub user "timdeschryver".

The implementation of the Angular service, looks as this:

If you want to test a component using the GitHubService, typically a mocked service instance is provided.

Setup link

The first step is to install MSW with the following command. This will create the ./src/mockServiceWorker.js file which includes the Service Worker.

To be able to serve the service worker, add it to the assets inside the angular.json file. With this step, the mockServiceWorker.js file will be copied over to the build output.

That's all for the configuration, now it's time to create the mocked server.

The last part is to import the mock. We're adding the mock to the environment file so it won't be included in a production build.

This gives us the following result.

The mock implementation of the GitHub service, we search on "timdeschryver" and receive the mocked user "mocked-timdeschryver".

Karma/Jasmine Tests link

Because Angular uses a different builder during tests, we also need to add the mockServiceWorker to the test builder.

Just like when the application is served, the mock server needs to be imported to register the service worker. We import the mocks in the ./src/test.ts file where the test environment is created, so it's available for all tests.

Now, we can simply write the test without having to provide a mocked instance of the service.

Jest Tests link

For Jest, we can't use the current setup. Don't worry, the mocks are reused but because Jest runs in a Node environment, the worker can't be.

To reuse the mocks, move it to another file so it's possible to share the setup between a browser environment and a Node environment.

Next, import the mock handlers while setting up the Node server.

Lastly, start the server before each test. For this, we can use the before hooks.

The test itself remains the same.

Cypress Tests link

Because Cypress is running tests against the served application, no action is required here. We just have to write the spec.

Conclusion link

While Angular already provides a way to mock our services via its dependency injection, I think we can still benefit from MSW in an Angular project.

It doesn't just help to mock services during tests, but as the example setup shows, it can also be used during the development cycle. Especially, when multiple teams (frontend and backend) are working on the same application or when the backend isn't ready yet.

MSW also provides a way to share a server across multiple layers of the testing pyramid. The server can be used during unit tests, integration tests, and end-to-end tests. This can help with the maintenance of your tests.

The code from this post can be found on GitHub.


Edit 2020-08-11: there's now an official MSW Angular REST example

Incoming links

Feel free to update this blog post on GitHub, thanks in advance!

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