React Testing Library integration in NextJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the render function from React Testing Library.

NextJS

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

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

Amount

BfireEvent

Cshallow

Drender

Attempts:

3 left

💡 Hint

Common Mistakes

Using mount or shallow which are from Enzyme, not React Testing Library.

Importing fireEvent instead of render.

The render function is used to render React components in tests.

2fill in blank

medium

Complete the code to query a button element by its role.

NextJS

const { getByRole } = render(<button>Click me</button>);
const button = getByRole([1]);

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

A"link"

B"button"

C"textbox"

D"checkbox"

Attempts:

3 left

💡 Hint

Common Mistakes

Using roles like "link" or "textbox" which do not match a button.

Forgetting to put the role string in quotes.

The role for a button element is "button". This helps find the button in the test.

3fill in blank

hard

Fix the error in the test by completing the assertion to check if the button is in the document.

NextJS

expect(button).[1]();

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

AtoHaveTextContent

BtoBeNull

CtoBeInTheDocument

DtoBeVisible

Attempts:

3 left

💡 Hint

Common Mistakes

Using toBeNull() which checks for absence.

Using toHaveTextContent() which checks text, not presence.

The matcher toBeInTheDocument() checks if the element exists in the rendered output.

4fill in blank

hard

Fill both blanks to simulate a user click event on the button.

NextJS

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

const { getByRole } = render(<button>Click me</button>);
[2].fireEvent.click(getByRole('button'));

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

AfireEvent

BuserEvent

Cclick

Dsimulate

Attempts:

3 left

💡 Hint

Common Mistakes

Using userEvent without importing it properly.

Using simulate which is from Enzyme, not React Testing Library.

fireEvent is imported and used to simulate events like clicks in React Testing Library.

5fill in blank

hard

Fill all three blanks to write a test that renders a component, finds a button by role, and checks if it is enabled.

NextJS

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

const { [2] } = [3](<button>Submit</button>);
expect(getByRole('button')).toBeEnabled();

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

Arender

BgetByRole

Dscreen

Attempts:

3 left

💡 Hint

Common Mistakes

Using screen without importing or destructuring properly.

Confusing getByRole as a function to import directly.

We import render, destructure getByRole from the render result, and call render to render the component.

Practice

1. What is the main purpose of React Testing Library in a Next.js project?

easy

A. To manage global state in React applications

B. To style React components with CSS-in-JS

C. To test React components by simulating user interactions and checking visible output

D. To optimize React components for faster rendering

Solution

1. Step 1: Understand React Testing Library's role

   It focuses on testing components by simulating how users interact with them and what they see.

2. Step 2: Compare options with this role

   Only To test React components by simulating user interactions and checking visible output describes testing user interactions and visible output, which matches React Testing Library's purpose.

3. Final Answer:

   To test React components by simulating user interactions and checking visible output -> Option C

4. Quick Check:

   Testing user behavior = D [OK]

Hint: React Testing Library tests user actions and visible UI [OK]

Common Mistakes:

• Confusing testing library with styling or state management tools
• Thinking it optimizes rendering performance
• Assuming it manages app state

2. Which of the following is the correct way to import the render function from React Testing Library?

easy

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

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

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

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

Solution

1. Step 1: Identify the official package for React Testing Library

   The correct package is '@testing-library/react' for React Testing Library functions.

2. Step 2: Check import syntax

   React Testing Library exports render as a named export, so use curly braces around render.

3. Final Answer:

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

4. Quick Check:

   Named import from '@testing-library/react' = B [OK]

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

Common Mistakes:

• Using default import instead of named import
• Importing from wrong package like 'react-dom' or 'next/testing'
• Missing curly braces around render

3. Given this test code snippet, what will screen.getByText('Hello World') return?

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

function Greeting() {
  return <h1>Hello World</h1>;
}

test('renders greeting text', () => {
  render(<Greeting />);
  const element = screen.getByText('Hello World');
  return element;
});

medium

A. An error because screen is not defined

B. The <h1> element containing 'Hello World'

C. null because 'Hello World' is not found

D. An array of all elements with text 'Hello World'

Solution

1. Step 1: Understand what render and screen.getByText do

   render displays the component in a virtual DOM; screen.getByText finds the first element with matching text.

2. Step 2: Check the component output

   Greeting returns <h1>Hello World</h1>, so screen.getByText('Hello World') finds this <h1> element.

3. Final Answer:

   The <h1> element containing 'Hello World' -> Option B

4. Quick Check:

   getByText finds matching element = C [OK]

Hint: getByText returns the matching DOM element, not null or array [OK]

Common Mistakes:

• Thinking getByText returns null if found
• Confusing screen with undefined variable
• Assuming getByText returns multiple elements

4. Identify the error in this test code snippet:

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

function Button() {
  const [count, setCount] = React.useState(0);
  return <button onClick={() => setCount(count + 1)}>Clicked {count} times</button>;
}

test('increments count on click', () => {
  render(<Button />);
  const button = screen.getByRole('button');
  fireEvent.click(button);
  expect(button.textContent).toBe('Clicked 1 times');
});

medium

A. Expect statement should check button.innerHTML

B. Using fireEvent instead of userEvent

C. Incorrect role used in getByRole

D. Missing import of React from 'react'

Solution

1. Step 1: Check imports for React usage

   The component uses React.useState but React is not imported, causing a runtime error.

2. Step 2: Verify other parts

   fireEvent.click and getByRole('button') are correct usage; expect checks textContent correctly.

3. Final Answer:

   Missing import of React from 'react' -> Option D

4. Quick Check:

   React must be imported to use React.useState = A [OK]

Hint: Always import React when using React APIs like useState [OK]

Common Mistakes:

• Assuming fireEvent is wrong instead of userEvent
• Thinking getByRole('button') is incorrect
• Checking innerHTML instead of textContent

5. You want to test a Next.js component that fetches data on mount and displays it. Which approach correctly integrates React Testing Library with async data fetching?

import { render, screen, waitFor } from '@testing-library/react';
import React from 'react';

function DataDisplay() {
  const [data, setData] = React.useState(null);
  React.useEffect(() => {
    fetch('/api/data').then(res => res.json()).then(setData);
  }, []);
  if (!data) return <div>Loading...</div>;
  return <div>Data: {data.value}</div>;
}

test('shows fetched data', async () => {
  global.fetch = jest.fn(() => Promise.resolve({
    json: () => Promise.resolve({ value: 'Hello' })
  }));
  render(<DataDisplay />);
  await waitFor(() => screen.getByText('Data: Hello'));
  expect(screen.getByText('Data: Hello')).toBeInTheDocument();
});

hard

A. Correctly mocks fetch and waits for async update with waitFor

B. Should use fireEvent to trigger fetch manually

C. Missing cleanup of fetch mock after test

D. Should use getByText without waitFor for async content

Solution

1. Step 1: Check mocking of fetch

   The test mocks global.fetch to return a resolved promise with expected data, which is correct for simulating API calls.

2. Step 2: Verify waiting for async UI update

   Using waitFor to wait until the text 'Data: Hello' appears is the right way to handle async rendering in tests.

3. Final Answer:

   Correctly mocks fetch and waits for async update with waitFor -> Option A

4. Quick Check:

   Mock fetch + waitFor async UI = A [OK]

Hint: Mock fetch and use waitFor to handle async UI updates [OK]

Common Mistakes:

• Trying to trigger fetch with fireEvent instead of mocking
• Not waiting for async content before assertions
• Forgetting to cleanup mocks (not critical here)