Mocking native modules in React Native - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the native module mock.

React Native

import { [1] } from 'react-native';

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

ANativeModules

BNativeModule

CNativeModulesMock

DNativeModuleMock

Attempts:

3 left

💡 Hint

Common Mistakes

Using singular NativeModule instead of NativeModules.

Trying to import a mock directly without NativeModules.

The correct import for accessing native modules in React Native is NativeModules.

2fill in blank

medium

Complete the code to mock the native module method 'getDeviceName'.

React Native

NativeModules.DeviceInfo = {
  getDeviceName: jest.fn().mockReturnValue([1])
};

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

APixel 5

Bjest.fn()

C'getDeviceName'

D'Pixel 5'

Attempts:

3 left

💡 Hint

Common Mistakes

Not using quotes around the string value.

Passing jest.fn() instead of a string to mockReturnValue.

The mockReturnValue should be a string representing the device name, so it must be in quotes.

3fill in blank

hard

Fix the error in the mock setup by completing the code.

React Native

jest.mock('react-native', () => ({
  NativeModules: {
    DeviceInfo: {
      getDeviceName: jest.fn().mockResolvedValue([1])
    }
  }
}));

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

Ajest.fn()

B'Pixel 5'

C'getDeviceName'

DPixel 5

Attempts:

3 left

💡 Hint

Common Mistakes

Passing an unquoted string causing a ReferenceError.

Using jest.fn() inside mockResolvedValue.

mockResolvedValue expects the resolved value, which should be a string in quotes.

4fill in blank

hard

Fill both blanks to mock two native module methods correctly.

React Native

NativeModules.DeviceInfo = {
  getDeviceName: jest.fn().mockReturnValue([1]),
  getSystemVersion: jest.fn().mockReturnValue([2])
};

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

A'iPhone 13'

B'15.4'

C15.4

DiPhone 13

Attempts:

3 left

💡 Hint

Common Mistakes

Not quoting the version number as a string.

Using unquoted values causing runtime errors.

Both mocked return values should be strings, so they need to be quoted.

5fill in blank

hard

Fill all three blanks to mock a native module with async methods.

React Native

jest.mock('react-native', () => ({
  NativeModules: {
    DeviceInfo: {
      getDeviceName: jest.fn().mockResolvedValue([1]),
      getSystemVersion: jest.fn().mockResolvedValue([2]),
      isEmulator: jest.fn().mockResolvedValue([3])
    }
  }
}));

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

A'Galaxy S21'

B'12.0'

Cfalse

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Using unquoted strings causing errors.

Using strings instead of booleans for isEmulator.

Async mocks should resolve to the correct types: strings for names and versions, boolean for isEmulator.

Practice

1. What is the main purpose of mocking native modules in React Native testing?

easy

A. To replace native code with fake implementations for easier testing

B. To speed up the app in production

C. To add new native features automatically

D. To convert JavaScript code into native code

Solution

1. Step 1: Understand mocking concept

   Mocking replaces real native code with fake versions to isolate tests from native dependencies.

2. Step 2: Identify purpose in testing

   This helps test JavaScript logic without needing actual native modules running.

3. Final Answer:

   To replace native code with fake implementations for easier testing -> Option A

4. Quick Check:

   Mocking = Replace native code for testing [OK]

Hint: Mocking means faking native code to test JS easily [OK]

Common Mistakes:

• Thinking mocking speeds up production
• Confusing mocking with adding features
• Believing mocking converts JS to native

2. Which syntax correctly mocks a native module named NativeModuleExample using Jest?

easy

A. mock('NativeModuleExample', () => ({ method: jest.fn() }));

B. jest.mock('NativeModuleExample', { method: jest.fn });

C. jest.mock NativeModuleExample { method: jest.fn() };

D. jest.mock('NativeModuleExample', () => ({ method: jest.fn() }));

Solution

1. Step 1: Recall Jest mock syntax

   Jest uses jest.mock(moduleName, factoryFunction) to mock modules.

2. Step 2: Check correct usage

   jest.mock('NativeModuleExample', () => ({ method: jest.fn() })); correctly uses jest.mock with a factory returning an object with mocked methods.

3. Final Answer:

   jest.mock('NativeModuleExample', () => ({ method: jest.fn() })); -> Option D

4. Quick Check:

   jest.mock with factory function = jest.mock('NativeModuleExample', () => ({ method: jest.fn() })); [OK]

Hint: Use jest.mock('module', () => ({})) syntax [OK]

Common Mistakes:

• Omitting jest. before mock
• Passing object directly instead of factory function
• Using wrong parentheses or braces

3. Given this mock setup:

jest.mock('NativeModule', () => ({ getValue: () => Promise.resolve('mocked') }));

NativeModule.getValue().then(result => console.log(result));

What will be logged to the console?

medium

A. Promise { 'mocked' }

B. undefined

C. mocked

D. Error: getValue is not a function

Solution

1. Step 1: Understand the mock function

   The mocked getValue returns a resolved Promise with string 'mocked'.

2. Step 2: Analyze the then callback

   The then callback logs the resolved value, which is 'mocked'.

3. Final Answer:

   mocked -> Option C

4. Quick Check:

   Promise resolves to 'mocked' logged [OK]

Hint: Promises resolve before then logs value [OK]

Common Mistakes:

• Expecting the Promise object to be logged
• Forgetting to return a Promise in mock
• Confusing undefined with resolved value

4. You wrote this mock:

jest.mock('NativeModule', () => ({ getData: jest.fn }));

But NativeModule.getData.mockReturnValue('test') throws an error. What is the problem?

medium

A. jest.mock cannot mock functions

B. jest.fn is missing parentheses, so getData is not a mock function instance

C. NativeModule is not imported correctly

D. getData should return a Promise, not jest.fn

Solution

1. Step 1: Check jest.fn usage

   jest.fn is a function that must be called as jest.fn() to create a mock function.

2. Step 2: Identify error cause

   Without parentheses, getData is assigned the function jest.fn itself, not a mock function instance, causing error when using mock methods like mockReturnValue.

3. Final Answer:

   jest.fn is missing parentheses, so getData is not a mock function instance -> Option B

4. Quick Check:

   Use jest.fn() to create mock function [OK]

Hint: Always call jest.fn() with parentheses [OK]

Common Mistakes:

• Assigning jest.fn without ()
• Assuming jest.mock auto-mocks functions
• Ignoring import errors

5. You want to mock a native module method fetchData that normally returns a Promise resolving to an object. Which mock implementation correctly simulates this behavior?

hard

A. jest.mock('NativeModule', () => ({ fetchData: () => Promise.resolve({ id: 1, name: 'Test' }) }));

B. jest.mock('NativeModule', () => ({ fetchData: () => ({ id: 1, name: 'Test' }) }));

C. jest.mock('NativeModule', () => ({ fetchData: jest.fn(() => 'Test') }));

D. jest.mock('NativeModule', () => ({ fetchData: jest.fn(() => Promise.reject('Error')) }));

Solution

1. Step 1: Understand original method behavior

   fetchData returns a Promise resolving to an object, so mock must do the same.

2. Step 2: Evaluate options

   jest.mock('NativeModule', () => ({ fetchData: () => Promise.resolve({ id: 1, name: 'Test' }) })); returns Promise.resolve with correct object, matching original behavior.

3. Final Answer:

   jest.mock('NativeModule', () => ({ fetchData: () => Promise.resolve({ id: 1, name: 'Test' }) })); -> Option A

4. Quick Check:

   Mock Promise resolve with object = jest.mock('NativeModule', () => ({ fetchData: () => Promise.resolve({ id: 1, name: 'Test' }) })); [OK]

Hint: Mock async methods with Promise.resolve returning expected data [OK]

Common Mistakes:

• Returning object directly instead of Promise
• Mocking with rejected Promise when success expected
• Returning string instead of object