Jest setup for unit tests in React Native

Introduction

Jest helps you check if your React Native code works right. Setting it up means you can find mistakes early and fix them fast.

When you want to check if a button click works as expected.

When you add a new feature and want to make sure it doesn't break old code.

When you want to test functions that handle data or calculations.

When you want to run tests automatically before sharing your app.

When you want to catch bugs before users see them.

Syntax

React Native

npm install --save-dev jest @testing-library/react-native @testing-library/jest-native react-test-renderer

// Add to package.json
"scripts": {
  "test": "jest"
}

// Create jest.config.js
module.exports = {
  preset: 'react-native',
  setupFilesAfterEnv: ['@testing-library/jest-native/extend-expect'],
  transformIgnorePatterns: [
    'node_modules/(?!(react-native|@react-native|@react-navigation)/)'
  ]
};

Use npm install --save-dev to add testing tools only for development.

The jest.config.js file tells Jest how to work with React Native.

Examples

This command installs Jest and tools to test React Native components.

React Native

npm install --save-dev jest @testing-library/react-native @testing-library/jest-native react-test-renderer

This script lets you run tests by typing npm test in the terminal.

React Native

"scripts": {
  "test": "jest"
}

This config file sets up Jest to understand React Native code and adds helpful matchers for testing.

React Native

module.exports = {
  preset: 'react-native',
  setupFilesAfterEnv: ['@testing-library/jest-native/extend-expect'],
  transformIgnorePatterns: [
    'node_modules/(?!(react-native|@react-native|@react-navigation)/)'
  ]
};

Sample App

This test checks if the button calls the function when pressed. It uses Jest and React Native Testing Library.

React Native

import React from 'react';
import { Text, TouchableOpacity } from 'react-native';
import { render, fireEvent } from '@testing-library/react-native';

function Button({ onPress, title }) {
  return (
    <TouchableOpacity onPress={onPress} accessibilityLabel="button">
      <Text>{title}</Text>
    </TouchableOpacity>
  );
}

test('Button calls onPress when pressed', () => {
  const onPressMock = jest.fn();
  const { getByA11yLabel } = render(<Button onPress={onPressMock} title="Click me" />);
  const button = getByA11yLabel('button');
  fireEvent.press(button);
  expect(onPressMock).toHaveBeenCalledTimes(1);
});

Important Notes

Always add accessibilityLabel to components to find them easily in tests.

Run npm test to start Jest and see test results.

Keep tests small and focused on one thing at a time.

Summary

Jest helps catch bugs early by running tests on your React Native code.

Install Jest and testing libraries, then add a config file to set it up.

Write simple tests to check if components behave as expected.

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