Why testing Next.js matters - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the testing library for React in Next.js.

NextJS

import { render } from '[1]';

Drag options to blanks, or click blank then click option'

Areact-test-renderer

Bnext/testing

C@testing-library/react

Djest

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'jest' directly for rendering

Importing from 'next/testing' which doesn't exist

We use @testing-library/react to render components for testing in Next.js.

2fill in blank

medium

Complete the code to write a simple test that checks if a Next.js component renders text.

NextJS

test('renders welcome message', () => {
  const { getByText } = render(<HomePage />);
  expect(getByText('[1]')).toBeInTheDocument();
});

Drag options to blanks, or click blank then click option'

AWelcome to Next.js

BHello World

CClick me

DSubmit

Attempts:

3 left

💡 Hint

Common Mistakes

Using unrelated button text

Using generic greetings not in the component

The test checks if the text 'Welcome to Next.js' appears in the component.

3fill in blank

hard

Fix the error in the test by completing the missing import for the Next.js component.

NextJS

import [1] from '../pages/index';

Drag options to blanks, or click blank then click option'

AHomePage

BMain

CIndexPage

DApp

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'App' which is for _app.js

Using 'Main' or 'IndexPage' which are not standard names

The default export from ../pages/index is usually named HomePage in examples.

4fill in blank

hard

Fill both blanks to mock a Next.js router method in a test.

NextJS

jest.mock('next/router', () => ({
  useRouter: () => ({
    [1]: jest.fn(),
    [2]: jest.fn()
  })
}));

Drag options to blanks, or click blank then click option'

Apush

Bpop

Creplace

DgoBack

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'pop' or 'goBack' which are not Next.js router methods

We mock push and replace methods of Next.js router to test navigation.

5fill in blank

hard

Fill all three blanks to write a test that waits for data to load in a Next.js component.

NextJS

import { render, screen, [1] } from '@testing-library/react';

render(<DataComponent />);

const item = await [2](() => screen.getByText('[3]'));
expect(item).toBeInTheDocument();

Drag options to blanks, or click blank then click option'

AwaitFor

BfindByText

CLoading complete

Dwait

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'wait' which is not a function in testing-library

Using 'getByText' which is not async

We import waitFor to wait for async changes, use waitFor with screen.getByText to find text, and check for 'Loading complete' text.

Practice

1. Why is testing important in a Next.js application?

easy

A. It reduces the size of the app bundle.

B. It makes the app load faster for users.

C. It automatically writes code for new features.

D. It helps catch bugs early and ensures the app works correctly.

Solution

1. Step 1: Understand the purpose of testing

   Testing is done to find bugs and verify that the app behaves as expected.

2. Step 2: Relate testing to Next.js app quality

   By testing, developers catch problems early, preventing bugs from reaching users.

3. Final Answer:

   It helps catch bugs early and ensures the app works correctly. -> Option D

4. Quick Check:

   Testing improves app reliability = D [OK]

Hint: Testing finds bugs early to keep apps working well [OK]

Common Mistakes:

• Thinking testing makes apps faster
• Believing tests write code automatically
• Confusing testing with app size optimization

2. Which of the following is the correct way to write a simple test in Next.js using Jest?

easy

A. check('adds numbers', () => { expect(1 + 2).toEqual(3); });

B. test('adds numbers', () => { expect(1 + 2).toBe(3); });

C. run('adds numbers', () => { assert(1 + 2 === 3); });

D. verify('adds numbers', () => { expect(1 + 2).toBe(4); });

Solution

1. Step 1: Identify the correct Jest test syntax

   Jest uses the test function with a description and a callback containing assertions.

2. Step 2: Check the assertion method

   expect(value).toBe(expected) is the correct way to check equality in Jest.

3. Final Answer:

   test('adds numbers', () => { expect(1 + 2).toBe(3); }); -> Option B

4. Quick Check:

   Jest test syntax = A [OK]

Hint: Jest tests use test() and expect().toBe() [OK]

Common Mistakes:

• Using non-existent functions like check or run
• Wrong assertion methods like assert or toEqual incorrectly
• Expecting wrong results in assertions

3. Consider this test code snippet in a Next.js app:

test('renders welcome message', () => {
  const message = renderWelcome();
  expect(message).toBe('Welcome to Next.js!');
});

What will happen if renderWelcome() returns 'Welcome to Next.js!'?

medium

A. The test will pass successfully.

B. The test will fail with a syntax error.

C. The test will fail because of a runtime error.

D. The test will be skipped automatically.

Solution

1. Step 1: Understand the test assertion

   The test expects the value returned by renderWelcome() to be exactly 'Welcome to Next.js!'.

2. Step 2: Match the returned value with expected

   Since renderWelcome() returns 'Welcome to Next.js!', the expectation matches perfectly.

3. Final Answer:

   The test will pass successfully. -> Option A

4. Quick Check:

   Matching output means test passes = C [OK]

Hint: If output matches expected, test passes [OK]

Common Mistakes:

• Assuming test fails on correct output
• Confusing syntax errors with test failures
• Thinking tests skip without reason

4. You wrote this test in your Next.js app:

test('check sum', () => {
  expect(sum(2, 3)).toBe(6);
});

But the function sum adds two numbers normally. What is the problem here?

medium

A. The sum function is missing from the test file.

B. The test syntax is wrong; toBe cannot be used here.

C. The expected value is incorrect; sum(2, 3) should be 5, not 6.

D. The test will pass because 6 is close to 5.

Solution

1. Step 1: Understand the sum function behavior

   The sum function adds two numbers, so sum(2, 3) returns 5.

2. Step 2: Compare expected and actual values in test

   The test expects 6, which does not match the actual 5, causing the test to fail.

3. Final Answer:

   The expected value is incorrect; sum(2, 3) should be 5, not 6. -> Option C

4. Quick Check:

   Expected value must match actual = A [OK]

Hint: Check if expected matches actual function output [OK]

Common Mistakes:

• Assuming test syntax is wrong when it's correct
• Thinking test passes with wrong expected value
• Ignoring function implementation details

5. You want to add a new feature to your Next.js app safely. How does having tests help you do this?

hard

A. Tests let you check that existing features still work after adding new code.

B. Tests automatically add the new feature for you.

C. Tests make the app run faster with new features.

D. Tests remove the need to write documentation for new features.

Solution

1. Step 1: Understand the role of tests in development

   Tests verify that current app behavior remains correct even after changes.

2. Step 2: Connect tests to safe feature addition

   By running tests after adding features, developers ensure no old parts break.

3. Final Answer:

   Tests let you check that existing features still work after adding new code. -> Option A

4. Quick Check:

   Tests protect existing features during updates = B [OK]

Hint: Tests confirm old features work after changes [OK]

Common Mistakes:

• Thinking tests write code automatically
• Believing tests improve speed directly
• Assuming tests replace documentation