MSW is a tool that allows you to intercept API calls and modify their responses. This helps you test your app in more realistic scenarios, with different responses.

From a high-level view, MSW lets us intercept API calls in two scenarios:

1. In the browser: This allows you to mock API responses while you're developing the app, making it easier to test different responses as you write your code.
2. In a Node.js environment: This is typically the case when running tests, such as with Vitest.

In this section, we'll configure MSW for the second scenario, to mock API calls in a Node.js environment during testing.

1. Step 1

Install MSW as a development dependency and initialize it:

npm install msw --save-dev
npx msw init ./public

1. Setup Mock Server

Next, under you /tests/mock folder create a mock server that runs in a Node environment for use in your tests:

5. Update the Test Setup

In your test setup file, configure the mock server to start and stop during the testing lifecycle:

This configuration ensures that MSW is properly set up, cleaned up after each test, and the handlers are reset to maintain test isolation.

4. Setting up Handlers

Next, under your /tests/mocks folder setup your handlers. MSW provides handlers for different HTTP methods, like GET, POST, etc. Here's how you can mock the GET request for our API:

and then you set those handlers in your server:

6. Writing Tests

Here’s how you can write tests that use the mocked server and handlers. In ItemList.test.tsx:

Check out that to query the list container I set a data-testid="list"in the container element of that component.

In this example:

• We mock the API's GET response using MSW.
• We handle both successful and failed API requests.
• We test that the UI displays the correct information or error message based on the response.

Let me know if you need any further improvements or explanations!