React Testing Library integration in NextJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does this test check for?

Given this React component and test, what behavior does the test verify?

NextJS

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

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

A The test checks if the button has a specific CSS class.

B The test checks if the button is disabled by default.

C The test checks if clicking the button triggers an alert.

D The test checks if the button renders with the label 'Click me'.

💡 Hint

Look at what the expect statement is verifying.

The test renders the Button component with label 'Click me' and verifies the button's text content matches that label.

❓ state_output

intermediate

What is the output of this test after clicking?

Consider this component and test. What will the test output be after simulating a click?

NextJS

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

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

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

A The test fails because the button text remains 'Count: 0' after click.

B The test passes because the button text updates to 'Count: 1' after click.

C The test throws an error because fireEvent.click is not defined.

D The test fails because the button is disabled and cannot be clicked.

💡 Hint

Check how the state changes on click and what the test expects.

The Counter component increments count state on click, so the button text updates to 'Count: 1'. The test expects this updated text and passes.

📝 Syntax

advanced

Which test code snippet correctly waits for async content?

You want to test a component that fetches data and shows it after loading. Which test snippet correctly waits for the data to appear?

A await screen.findByText('Data loaded');

B screen.getByText('Data loaded');

C await screen.queryByText('Data loaded');

D screen.findByText('Data loaded');

💡 Hint

Remember which query returns a promise and waits for the element.

findByText returns a promise and waits for the element to appear, so awaiting it correctly handles async content.

🔧 Debug

advanced

Why does this test fail with 'Unable to find role' error?

This test fails with an error saying it cannot find a button role. What is the cause?

NextJS

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

test('finds submit button', () => {
  render(<button type="submit">Send</button>);
  const btn = screen.getByRole('button', { name: 'Submit' });
  expect(btn).toBeInTheDocument();
});

A The button's visible text is 'Send', but the test looks for name 'Submit'.

B The button element is not rendered because render is missing a wrapper.

C getByRole cannot find buttons with type 'submit'.

D The test is missing an async wait for the button to appear.

💡 Hint

Check the button's label text and the name option in getByRole.

getByRole with name option matches accessible name, which is the button's visible text. The button text is 'Send', but the test looks for 'Submit', so it fails.

🧠 Conceptual

expert

What is the main benefit of using React Testing Library over Enzyme?

Choose the best explanation for why React Testing Library is preferred for testing React components compared to Enzyme.

A React Testing Library requires less setup because it mocks all network requests automatically.

B React Testing Library allows shallow rendering which Enzyme does not support.

C React Testing Library encourages testing components like users interact with them, focusing on accessibility and behavior rather than implementation details.

D React Testing Library uses snapshot testing exclusively, which Enzyme cannot do.

💡 Hint

Think about how tests should reflect user experience.

React Testing Library promotes testing from the user's perspective by querying DOM elements as users would, improving test reliability and accessibility focus. Enzyme focuses more on component internals.

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)