Most developers know they should write tests. Almost nobody does it first.

Here's the problem: when you write tests after the code, you're testing what you built. When you write tests before the code, you're testing what it should do. That's a fundamentally different mindset — and it produces fundamentally better software.

The Next.js Testing Problem

Next.js has blurred the line between frontend and backend. Server Components fetch data, API routes handle business logic, middleware manages auth, SSR renders pages on the fly. One untested edge case in any of those layers can cascade into production bugs that are nearly impossible to trace.

TDD solves this by making you think about every edge case before you write the implementation.

What TDD Actually Looks Like in Next.js

// Step 1: Write the test (RED)
describe("GET /api/tasks", () => {
  it("returns 401 without auth", async () => {
    const request = new NextRequest("http://localhost/api/tasks");
    const response = await GET(request);
    expect(response.status).toBe(401);
  });
});

// Step 2: Run it — it fails. Good.
// Step 3: Write the minimum code to pass (GREEN)
// Step 4: Refactor. Repeat.

This course covers everything: SSR testing, API route testing, mocking databases and external APIs, integration testing with MSW, React Server Components, and CI/CD pipelines with quality gates.

If you're serious about shipping Next.js apps that don't break in production, this is the system.