Testing App Router applications

What this means

Unit tests cover isolated logic, integration tests cover server boundaries, and browser tests cover real workflows.

In beginner terms, this topic answers one practical question: "What should I write, and why does React care about it?" Do not try to memorize the syntax first. First understand the idea, then connect the syntax to that idea.

Why it matters

Layered tests catch failures at the cheapest useful level.

When you build real React screens, this idea helps you decide where data should live, what the user should see, and what should happen after an interaction. That is why this lesson is part of the main path instead of being an optional detail.

Step by step

1. Notice the UI problem this topic solves. 2. Look at the smallest possible example. 3. Change one value and predict what should appear. 4. Run the example and compare the result with your prediction. 5. Use the practice task before moving on.

Small example

test('shows tracks', async ({ page }) => { await page.goto('/tracks'); });

Common mistake

Do not use testing app router applications only because it looks advanced. Start from the problem it solves.

Practice task

Change the example, predict the result, then explain the behavior in your own words.

Remember this

Test behavior and permissions, not implementation details.