QFrontend Quality · Interview preparation
When should tests use mocks?
Mock external systems when a test needs stable, isolated behavior, but keep realistic integration tests for important boundaries.
The idea to remember.
Mock external systems when a test needs stable, isolated behavior, but keep realistic integration tests for important boundaries. Excessive mocking can hide problems in how components work together.
01Learn by doing
From first example to real project.
Start with the smallest working idea, examine a more detailed example, then look at a real application pattern. Adapt dependencies, error handling and data models to your project.
Basic: test visible behavior
01 / BEGINNERPrefer what users can see to implementation details.
test('shows a heading', () => {
render(<Questions />);
expect(screen.getByRole('heading', { name: /questions/i })).toBeVisible();
});Intermediate: test asynchronous UI
02 / INTERMEDIATEWait for an accessible result instead of sleeping a fixed interval.
test('loads results', async () => {
render(<SearchPage />);
await user.click(screen.getByRole('button', { name: /search/i }));
expect(await screen.findByText('Results ready')).toBeVisible();
});Real scenario: protect an API route
03 / REAL SCENARIOApply validation and authorization server-side; client checks improve UX only.
app.post('/api/orders', authenticate, async (req, res) => {
if (!isValidOrder(req.body)) return res.status(400).json({ error: 'Invalid order' });
if (!canOrder(req.user, req.body)) return res.status(403).end();
res.json(await createOrder(req.user.id, req.body));
});02Check your understanding
Try it in your own words.
Explain when should tests use mocks without looking at the code. Then modify the intermediate example, describe one trade-off and identify when the real-world pattern fits.
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