Jest setup for Next.js - Step-by-Step Execution

Concept Flow - Jest setup for Next.js

Install Jest and Testing Libraries

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Create jest.config.js

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Add Babel or SWC config for Jest

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Write test files in __tests__ folder

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Run jest command

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View test results

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Fix errors or add tests

This flow shows the steps to set up Jest in a Next.js project, from installing packages to running tests and fixing errors.

Execution Sample

NextJS

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

test('renders welcome message', () => {
  render(<Home />);
  expect(screen.getByText('Welcome to Next.js!')).toBeInTheDocument();
});

This test renders the Next.js Home page and checks if the welcome message is shown.

Execution Table

Step	Action	Evaluation	Result
1	Import render and screen from testing-library	Imports ready	Functions available
2	Import Home component from pages/index	Component loaded	Home component ready
3	Call render(<Home />)	Home component rendered	DOM tree created in virtual DOM
4	Call screen.getByText('Welcome to Next.js!')	Search DOM for text	Element found
5	Call expect(...).toBeInTheDocument()	Check element presence	Test passes
6	Test completes	No errors	Success

💡 Test ends after checking the welcome message is in the document

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
renderedDOM	null	Virtual DOM with Home component	Same	Same
foundElement	undefined	undefined	Element with text 'Welcome to Next.js!'	Same

Key Moments - 3 Insights

Why do we need to import render and screen from @testing-library/react?

render creates a virtual DOM for the component to test, and screen lets us query that DOM. See execution_table steps 1 and 3.

What happens if the text 'Welcome to Next.js!' is not found?

screen.getByText throws an error, causing the test to fail at step 4, showing the text is missing.

Why do we write tests inside __tests__ or with .test.js extension?

Jest looks for these files to run tests automatically, so placing tests there ensures they run when jest is executed.

Visual Quiz - 3 Questions

Look at the execution_table, what is the state of renderedDOM after step 3?

A Virtual DOM with Home component rendered

B Null, nothing rendered yet

C An error occurred

D Text 'Welcome to Next.js!' found

💡 Hint

Check the 'renderedDOM' variable in variable_tracker after Step 3

At which step does the test check if the welcome message is present?

A Step 2

B Step 5

C Step 4

D Step 6

💡 Hint

Look at execution_table where expect(...).toBeInTheDocument() is called

If the Home component did not render, what would happen at step 4?

A screen.getByText would find the element anyway

B Test would pass without checking

C screen.getByText would throw an error

D render would fix the problem automatically

💡 Hint

Refer to key_moments about what happens if text is not found

Concept Snapshot

Jest setup for Next.js:
1. Install jest, @testing-library/react, and jest-environment-jsdom.
2. Create jest.config.js with testEnvironment set to 'jsdom'.
3. Configure Babel or SWC for Jest to handle JSX.
4. Write tests in __tests__ folder or *.test.js files.
5. Run tests with 'jest' command.
6. Use render() and screen from testing-library to test components.

Full Transcript

To set up Jest in Next.js, first install Jest and React Testing Library. Then create a jest.config.js file specifying the jsdom environment. Configure Babel or SWC to transform JSX for Jest. Write test files in a __tests__ folder or with .test.js extensions. Use render() to render components in a virtual DOM and screen to query elements. Run jest to execute tests and see results. If tests fail, fix errors or add more tests. This process helps ensure your Next.js components work as expected.