005 - Testing

The scaffolded plugin comes with tests already in place. Let's walk through how they work and how to extend them.

Unit tests

Backstage frontend plugins use Jest as the test runner, React Testing Library for rendering components, and MSW (Mock Service Worker) for intercepting HTTP requests.

TodoList tests

Open plugins/todo/src/components/TodoList/TodoList.test.tsx. The TodoList component is presentational, so testing it is straightforward — pass in data and verify it renders:

await renderInTestApp(<TodoList todos={todos} />);

expect(screen.getByText('First task')).toBeInTheDocument();

renderInTestApp from @backstage/frontend-test-utils sets up a minimal Backstage app context around the component, providing all the standard APIs that components might depend on.

TodoPage tests

Open plugins/todo/src/components/TodoPage/TodoPage.test.tsx. The TodoPage component fetches data from the backend, so the tests use MSW to intercept HTTP requests and return test data:

const server = setupServer();
registerMswTestHooks(server);

it('renders todos from the backend', async () => {
  server.use(
    http.get('*/api/todo/todos', () =>
      HttpResponse.json({
        items: [{ id: '1', title: 'Mocked task' /* ... */ }],
      }),
    ),
  );

  await renderInTestApp(<TodoPage />);

  expect(await screen.findByText('Mocked task')).toBeInTheDocument();
});

A few things to note:

• registerMswTestHooks sets up and tears down the MSW server around each test, so handlers do not leak between tests.
• screen.findByText returns a promise that waits for the element to appear. This handles the asynchronous nature of data fetching without needing explicit waitFor calls.
• The URL pattern */api/todo/todos matches regardless of the host, which keeps the test independent of the discovery API's resolved base URL.

The tests also cover the error case — when the backend returns a 500 status, the component falls back to rendering example todo items instead of displaying an error panel.

Running the tests

From the repository root:

yarn test plugins/todo

Writing additional tests

As you add features to your plugin, follow the same patterns:

• For presentational components, pass props and assert on rendered output.
• For components that fetch data, use MSW to mock the HTTP responses.
• Prefer screen.findByText over waitFor for async assertions.
• Test both the success and error paths.

Integration tests

For end-to-end validation, you can use Playwright to test your plugin in a real browser against a running Backstage instance. Integration tests catch issues that unit tests cannot, such as routing problems, CSS regressions, or issues with the full API round-trip.

A basic Playwright test for the todo page might look like this:

import { test, expect } from '@playwright/test';

test('todo page shows the todo list', async ({ page }) => {
  await page.goto('/todo');
  await expect(page.getByText('Welcome to todo!')).toBeVisible();
  await expect(page.getByRole('table')).toBeVisible();
});

Integration tests are most valuable for critical user flows. For most component-level validation, the unit testing approach described above provides faster feedback with less setup.