Next.js 15 Fullstack (App Router + Server Actions) · Lesson

Integration Testing Pages

Perform integration tests for Next.js pages and API routes to ensure different parts of your application work together.

Integration Testing Pages is a free Next.js 15 Fullstack (App Router + Server Actions) lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Next.js 15 Fullstack (App Router + Server Actions) learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Intro to Integration Tests

Welcome! In this lesson, we'll explore integration testing for your Next.js applications.

Integration tests verify that different parts of your application work correctly together. Instead of isolated units, we check the flow between components, pages, and API routes.

Why Test Pages & Routes?

For Next.js, integration testing is crucial because it ensures:

Pages render correctly with their data.
User interactions (like form submissions) trigger the right backend logic.
API routes respond as expected when called from the frontend or other services.

It helps catch issues that unit tests might miss when components interact.

Tools: Jest, RTL, next/test-utils

We'll primarily use these tools for integration testing:

Jest: A popular JavaScript testing framework.
React Testing Library (RTL): For rendering React components (like your Next.js pages) and simulating user interactions.
@next/test-utils: Provides helpers specifically for testing Next.js features, like API routes.

Testing a Page Component

To test a Next.js page component, we can render it using React Testing Library. This allows us to assert if the correct content is displayed and interact with elements on the page.

You'll typically import the page component directly and render it in your test file.

Code: Simple Page Test

Here's how you might test a simple About page to ensure its heading is rendered.

// pages/about.js
export default function About() {
  return (
    <div>
      <h1>About Us</h1>
      <p>This is the about page.</p>
    </div>
  );
}

// __tests__/about.test.js
import { render, screen } from '@testing-library/react';
import About from '../pages/about';

describe('About Page', () => {
  it('renders the about page heading', () => {
    render(<About />);
    const heading = screen.getByRole('heading', { name: /About Us/i });
    expect(heading).toBeInTheDocument();
  });
});

User Interaction in Tests

Integration tests often involve simulating user behavior. React Testing Library provides utilities like fireEvent (or userEvent for more realistic interactions) to click buttons, type into inputs, and more.

This lets you verify that your page responds correctly to user input, just like a real user would.

// components/Counter.js
import { useState } from 'react';

export default function Counter() {
  const [count, setCount] = useState(0);
  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </div>
  );
}

// __tests__/counter.test.js
import { render, screen, fireEvent } from '@testing-library/react';
import Counter from '../components/Counter';

describe('Counter Component', () => {
  it('increments count on button click', () => {
    render(<Counter />);
    const button = screen.getByRole('button', { name: /Increment/i });
    fireEvent.click(button);
    expect(screen.getByText(/Count: 1/i)).toBeInTheDocument();
  });
});

Testing Next.js API Routes

Next.js API routes are server-side functions. You can test them by importing their handler function and using createNextApiHandler from @next/test-utils to simulate requests.

This allows you to send mock requests and assert the status code and JSON response.

// pages/api/hello.js
export default function handler(req, res) {
  res.status(200).json({ name: 'John Doe' });
}

// __tests__/api/hello.test.js
import { createNextApiHandler } from '@next/test-utils';
import handler from '../../pages/api/hello';

describe('/api/hello', () => {
  it('returns a name', async () => {
    const testHandler = createNextApiHandler(handler);
    const res = await testHandler({ method: 'GET' });
    expect(res.statusCode).toBe(200);
    expect(await res.json()).toEqual({ name: 'John Doe' });
  });
});

Mocking External Calls in API

API routes often interact with databases or other external services. In integration tests, you should mock these external dependencies to ensure your tests are fast and predictable.

Jest's mocking capabilities are perfect for this, allowing you to control the return values of these external calls.

// lib/db.js (simplified)
export const getUser = async (id) => ({ id, name: 'Mock User' });

// pages/api/user/[id].js
import { getUser } from '../../../lib/db';

export default async function handler(req, res) {
  const { id } = req.query;
  const user = await getUser(id);
  res.status(200).json(user);
}

// __tests__/api/user.test.js
import { createNextApiHandler } from '@next/test-utils';
import handler from '../../pages/api/user/[id]';
import * as db from '../../lib/db';

describe('/api/user/[id]', () => {
  it('returns a user with mocked DB', async () => {
    jest.spyOn(db, 'getUser').mockResolvedValue({ id: '1', name: 'Test User' });

    const testHandler = createNextApiHandler(handler);
    const res = await testHandler({ method: 'GET', query: { id: '1' } });
    expect(res.statusCode).toBe(200);
    expect(await res.json()).toEqual({ id: '1', name: 'Test User' });

    jest.restoreAllMocks();
  });
});

Pages & Server Actions Integration

Testing pages that use Server Actions involves rendering the page, simulating a form submission, and asserting that the Server Action was called with the correct data.

You can mock the Server Action function itself to control its return value and verify it was invoked.

// app/actions.js
'use server';
export async function submitForm(formData) {
  const name = formData.get('name');
  return { success: true, message: `Hello, ${name}!` };
}

// app/page.js
import { submitForm } from './actions';

export default function Page() {
  return (
    <form action={submitForm}>
      <input type="text" name="name" defaultValue="Guest" />
      <button type="submit">Say Hello</button>
    </form>
  );
}

// __tests__/page.test.js
import { render, screen, fireEvent } from '@testing-library/react';
import Page from '../app/page';
import * as actions from '../app/actions';

describe('Home Page with Server Action', () => {
  it('submits form and calls server action', async () => {
    const mockSubmitForm = jest.spyOn(actions, 'submitForm')
      .mockResolvedValue({ success: true, message: 'Hello, Test User!' });

    render(<Page />);

    const input = screen.getByRole('textbox', { name: /name/i });
    fireEvent.change(input, { target: { value: 'Test User' } });

    const button = screen.getByRole('button', { name: /Say Hello/i });
    fireEvent.click(button);

    expect(mockSubmitForm).toHaveBeenCalledWith(
      expect.any(FormData)
    );
    mockSubmitForm.mockRestore();
  });
});

Test Your Knowledge

Time for a quick check on integration testing!

Recap: Building Robust Apps

Great job! You've learned the fundamentals of integration testing in Next.js.

We covered how to test pages and API routes.
You saw how to simulate user interactions and mock external dependencies.
Tools like Jest, React Testing Library, and @next/test-utils are essential.

By writing integration tests, you build more robust applications and catch bugs earlier in the development process!

Frequently asked questions

Is the “Integration Testing Pages” lesson free?

Yes — the full text of “Integration Testing Pages” is free to read here on the web, and the Next.js 15 Fullstack (App Router + Server Actions) course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Next.js 15 Fullstack (App Router + Server Actions) course, upgrade to CoddyKit PRO.

What will I learn in “Integration Testing Pages”?

Perform integration tests for Next.js pages and API routes to ensure different parts of your application work together. You practise Next.js 15 Fullstack (App Router + Server Actions) with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Next.js 15 Fullstack (App Router + Server Actions)?

No prior experience is required. Next.js 15 Fullstack (App Router + Server Actions) on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Integration Testing Pages” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Next.js 15 Fullstack (App Router + Server Actions) lesson?

Yes. Every Next.js 15 Fullstack (App Router + Server Actions) lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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