Testing client components in NextJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the testing library for React components.

NextJS

import { render } from '[1]';

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

Ajest-dom

Breact-testing-library

C@testing-library/react

Denzyme

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'enzyme' which is outdated for Next.js 14+

Using 'jest-dom' which is for assertions, not rendering

The @testing-library/react package is the standard for testing React components in Next.js.

2fill in blank

medium

Complete the code to simulate a user clicking a button in a client component test.

NextJS

import userEvent from '[1]';

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

Auser-event-library

Breact-user-event

Ctesting-library/user-event

D@testing-library/user-event

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect package names that don't exist

Confusing with 'react-testing-library' import

The @testing-library/user-event package provides utilities to simulate user interactions like clicks.

3fill in blank

hard

Fix the error in the test by completing the render call with the correct component import.

NextJS

import [1] from '@/components/SubmitButton';

render(<SubmitButton />);

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

ASubmitButton

BsubmitButton

Csubmit_button

DButtonSubmit

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase or snake_case names for components

Mismatching import name and usage

Component names must match exactly and start with a capital letter in React.

4fill in blank

hard

Fill both blanks to wait for the button to appear and then click it in the test.

NextJS

const button = await screen.[1]('button', { name: 'Submit' });
await userEvent.[2](button);

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

AfindByRole

Bclick

CgetByText

Dhover

Attempts:

3 left

💡 Hint

Common Mistakes

Using synchronous queries that fail if element is not immediately present

Using wrong userEvent methods like 'hover' instead of 'click'

Use findByRole to wait for the button asynchronously, then click to simulate the user click.

5fill in blank

hard

Fill all three blanks to mock a fetch call, render a component, and check the displayed text.

NextJS

global.fetch = jest.fn(() => Promise.resolve({
  json: () => Promise.resolve({ message: [1] })
}));

render(<MessageDisplay />);

expect(await screen.findByText([2])).toBeInTheDocument();
expect(global.fetch).toHaveBeenCalledWith([3]);

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

A'Hello World'

C'/api/message'

D'/api/data'

Attempts:

3 left

💡 Hint

Common Mistakes

Mismatching strings in mock and expect calls

Using wrong API endpoint in fetch call check

We mock fetch to return a message 'Hello World', check that text is displayed, and verify fetch was called with '/api/message'.

Practice

1. What is the main purpose of testing client components in Next.js?

easy

A. To catch UI bugs early by simulating user interactions

B. To improve server response time

C. To optimize database queries

D. To deploy the app faster

Solution

1. Step 1: Understand client component testing goal

   Testing client components focuses on the user interface and user actions to find bugs early.

2. Step 2: Compare options with testing goals

   Only To catch UI bugs early by simulating user interactions matches the goal of catching UI bugs by simulating user interactions.

3. Final Answer:

   To catch UI bugs early by simulating user interactions -> Option A

4. Quick Check:

   Testing client components = catch UI bugs early [OK]

Hint: Focus on UI and user actions for client component tests [OK]

Common Mistakes:

• Confusing client testing with server optimization
• Thinking testing speeds up deployment
• Mixing database concerns with UI testing

2. Which of the following is the correct way to import React Testing Library's render function in a Next.js client component test?

easy

A. import { render } from '@testing-library/react';

B. import render from 'react-testing-library';

C. import { render } from 'next/testing';

D. import { render } from 'react-dom';

Solution

1. Step 1: Identify correct package for render

   React Testing Library's render function is imported from '@testing-library/react'.

2. Step 2: Check import syntax

   import { render } from '@testing-library/react'; uses correct named import syntax and package name.

3. Final Answer:

   import { render } from '@testing-library/react'; -> Option A

4. Quick Check:

   Render import = '@testing-library/react' [OK]

Hint: Use '@testing-library/react' for render import [OK]

Common Mistakes:

• Using default import instead of named import
• Importing from wrong package like 'next/testing'
• Confusing 'react-dom' with testing library

3. Given this test code for a Next.js client component:

import { render, screen } from '@testing-library/react';
import Button from './Button';

test('renders button with label', () => {
  render(<Button label="Click me" />);
  const btn = screen.getByRole('button');
  expect(btn.textContent).toBe('Click me');
});

What will this test check?

medium

A. That the Button component does not render anything

B. That the Button component throws an error

C. That the Button component renders a button element with text 'Click me'

D. That the Button component renders a div with text 'Click me'

Solution

1. Step 1: Analyze render and query

   The test renders Button with label 'Click me' and queries for a button role element.

2. Step 2: Check assertion

   The test expects the button's text content to be exactly 'Click me'.

3. Final Answer:

   That the Button component renders a button element with text 'Click me' -> Option C

4. Quick Check:

   Button renders button with label 'Click me' [OK]

Hint: Look for role queries and text assertions to find tested output [OK]

Common Mistakes:

• Assuming error or no render without evidence
• Confusing element type (button vs div)
• Ignoring the text content check

4. Consider this test code snippet:

import { render, screen } from '@testing-library/react';
import Counter from './Counter';

test('increments count on button click', () => {
  render(<Counter />);
  const btn = screen.getByRole('button');
  btn.click();
  expect(screen.getByText('Count: 1')).toBeInTheDocument();
});

What is the main issue with this test?

medium

A. Using getByRole instead of queryByRole

B. Missing import of React in the test file

C. Expecting wrong text content 'Count: 1'

D. Using btn.click() instead of userEvent.click(btn) to simulate user action

Solution

1. Step 1: Identify how click is simulated

   The test uses btn.click(), which is a DOM method but does not simulate real user events fully.

2. Step 2: Recognize recommended practice

   React Testing Library recommends using userEvent.click(btn) for realistic user interaction simulation.

3. Final Answer:

   Using btn.click() instead of userEvent.click(btn) to simulate user action -> Option D

4. Quick Check:

   Use userEvent.click for user actions [OK]

Hint: Use userEvent.click, not btn.click, for user simulation [OK]

Common Mistakes:

• Forgetting to import userEvent
• Assuming btn.click triggers React events fully
• Confusing getByRole with queryByRole

5. You want to test a Next.js client component that conditionally renders a message only after a button click. Which approach correctly tests this behavior?

hard

A. Render the component and immediately check if the message is in the document

B. Render the component, use userEvent.click on the button, then check if the message appears

C. Use screen.getByText before clicking the button to find the message

D. Test only the button rendering without clicking it

Solution

1. Step 1: Understand conditional rendering on user action

   The message appears only after clicking the button, so the test must simulate that click.

2. Step 2: Verify test steps

   Render component, simulate userEvent.click on button, then check for message presence is correct.

3. Final Answer:

   Render the component, use userEvent.click on the button, then check if the message appears -> Option B

4. Quick Check:

   Simulate click before checking conditional content [OK]

Hint: Click button first, then check for conditional content [OK]

Common Mistakes:

• Checking message before user action
• Not simulating user click properly
• Ignoring conditional rendering logic