Jest setup for unit tests in React Native - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is Jest in React Native testing?

Jest is a testing framework used to write and run unit tests for React Native apps. It helps check if components and functions work as expected.

beginner

Which command installs Jest and its React Native preset?

You run: npm install --save-dev jest @testing-library/react-native @testing-library/jest-native react-test-renderer to install Jest and tools for React Native testing.

intermediate

What is the purpose of jest.config.js in Jest setup?

The jest.config.js file tells Jest how to run tests, which files to look for, and how to handle React Native specifics.

intermediate

Why do we use react-test-renderer in React Native Jest tests?

It helps create a snapshot of a component’s UI so we can compare it later to catch unexpected changes.

beginner

How do you run Jest tests in a React Native project?

You run npm test or jest in the terminal to start Jest and run all tests.

What file usually contains Jest configuration in a React Native project?

A metro.config.js

B jest.config.js

C babel.config.js

D package.json

The jest.config.js file is the standard place to put Jest settings.

Which package helps to render React Native components for testing snapshots?

A react-native-snapshots

B jest-native

C react-test-renderer

D react-native-testing-library

react-test-renderer creates a serializable output of components for snapshot testing.

What command runs Jest tests in a React Native project?

A npm install

B npm run build

C npm start

D npm test

npm test runs the test script, which usually runs Jest.

Why do we mock native modules in Jest tests for React Native?

A To speed up tests and avoid errors from native code

B To make the app run faster on devices

C To add animations

D To improve UI design

Mocking native modules prevents errors and makes tests faster by simulating native behavior.

Which testing library is commonly used with Jest for React Native component testing?

A @testing-library/react-native

B Enzyme

C React Testing Library

D Mocha

@testing-library/react-native provides helpful tools to test React Native components with Jest.

Explain the steps to set up Jest for unit testing in a React Native project.

Think about installation, configuration, writing tests, and running them.

Describe how snapshot testing works with Jest in React Native.

Focus on rendering, saving, and comparing UI snapshots.

Practice

1. What is the primary purpose of setting up Jest in a React Native project?

easy

A. To run automated tests and catch bugs early

B. To improve app performance on devices

C. To style components with CSS

D. To deploy the app to app stores

Solution

1. Step 1: Understand Jest's role

   Jest is a testing framework used to run automated tests on code.

2. Step 2: Identify its main benefit

   Running tests helps find bugs early before releasing the app.

3. Final Answer:

   To run automated tests and catch bugs early -> Option A

4. Quick Check:

   Jest purpose = testing and bug catching [OK]

Hint: Jest is for testing code, not styling or deployment [OK]

Common Mistakes:

• Confusing Jest with styling tools
• Thinking Jest improves app speed
• Assuming Jest deploys apps

2. Which of the following is the correct way to import Jest functions in a React Native test file?

easy

A. import { test, expect } from '@testing-library/react-native';

B. No import needed; Jest globals are available automatically

C. const { test, expect } = require('jest');

D. import { test, expect } from 'jest';

Solution

1. Step 1: Recall Jest globals

   Jest provides test and expect as global functions by default.

2. Step 2: Understand import necessity

   Because they are globals, you do not need to import them explicitly.

3. Final Answer:

   No import needed; Jest globals are available automatically -> Option B

4. Quick Check:

   Jest globals = no import needed [OK]

Hint: Jest test and expect are global, no import required [OK]

Common Mistakes:

• Trying to import test and expect explicitly
• Confusing Jest with other libraries needing imports
• Using require instead of relying on globals

3. Given this test code snippet, what will be the test result?

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

medium

A. Test fails because 1 + 2 is not 3

B. Test fails with a syntax error

C. Test passes successfully

D. Test throws a runtime error

Solution

1. Step 1: Evaluate the expression inside expect

   1 + 2 equals 3, so expect(3).toBe(3) is true.

2. Step 2: Understand Jest test behavior

   Since the expectation matches, the test passes without errors.

3. Final Answer:

   Test passes successfully -> Option C

4. Quick Check:

   1 + 2 = 3, test passes [OK]

Hint: Check math inside expect; if true, test passes [OK]

Common Mistakes:

• Thinking 1 + 2 is not 3
• Confusing syntax errors with logic errors
• Assuming runtime errors without cause

4. You wrote this test but it fails with a syntax error:

test('check true value', () => {
  expect(true).toBeTrue();
});

What is the likely cause?

medium

A. toBeTrue() is not a Jest matcher; use toBe(true) instead

B. Missing import of toBeTrue from Jest library

C. Syntax error due to missing semicolon after expect

D. Jest does not support boolean tests

Solution

1. Step 1: Check Jest matchers for booleans

   Jest does not have a matcher called toBeTrue(); it uses toBe(true) instead.

2. Step 2: Identify cause of syntax error

   Using an unknown matcher causes Jest to throw an error.

3. Final Answer:

   toBeTrue() is not a Jest matcher; use toBe(true) instead -> Option A

4. Quick Check:

   Use toBe(true), not toBeTrue() [OK]

Hint: Use toBe(true) for boolean checks, not toBeTrue() [OK]

Common Mistakes:

• Trying to import non-existent matchers
• Assuming semicolon causes syntax error
• Believing Jest can't test booleans

5. You want to test a React Native component that renders a button with text 'Click me'. Which setup is correct to write a Jest test using @testing-library/react-native?

hard

A. import { render } from '@testing-library/react-native';
test('button text', () => {
const { getByText } = render();
expect(getByText('Click me')).toBeFalsy();
});

B. import { render } from 'react-native-testing-library';
test('button text', () => {
const { getByText } = render();
expect(getByText('Click me')).toBeNull();
});

C. import { render } from '@testing-library/react-native';
test('button text', () => {
const { getByText } = render();
expect(getByText('Click me')).toBeNull();
});

D. import { render } from '@testing-library/react-native';
test('button text', () => {
const { getByText } = render();
expect(getByText('Click me')).toBeTruthy();
});

Solution

1. Step 1: Use correct testing library import

   @testing-library/react-native is the recommended library for rendering components in Jest tests.

2. Step 2: Check assertion correctness

   getByText returns the element if found; expect(...).toBeTruthy() confirms it exists.

3. Final Answer:

   Correct import and expect(getByText('Click me')).toBeTruthy() -> Option D

4. Quick Check:

   Render + getByText + toBeTruthy() = correct test [OK]

Hint: Use getByText and expect(...).toBeTruthy() to check rendered text [OK]

Common Mistakes:

• Using wrong testing library import
• Expecting getByText to return null when text exists
• Using toBeFalsy() instead of toBeTruthy() for found elements