Mocking modules and functions in Node.js - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to mock a function using Jest.

Node.js

const myFunc = jest.[1]();

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

Afn

BmockFn

CmockFunction

DspyOn

Attempts:

3 left

💡 Hint

Common Mistakes

Using jest.mockFn() which does not exist.

Using jest.spyOn() which is for spying, not creating a mock function.

The jest.mock() function is used to mock modules, but to mock a function directly, jest.fn() is the correct method.

2fill in blank

medium

Complete the code to mock the entire 'fs' module in Jest.

Node.js

jest.[1]('fs');

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

Amock

BspyOn

Cfn

DrequireMock

Attempts:

3 left

💡 Hint

Common Mistakes

Using jest.spyOn() which is for spying on functions, not mocking modules.

Using jest.fn() which mocks functions, not modules.

To mock an entire module in Jest, you use jest.mock('moduleName').

3fill in blank

hard

Fix the error in spying on a method 'readFile' of the 'fs' module.

Node.js

const fs = require('fs');

jest.[1](fs, 'readFile').mockImplementation((path, cb) => cb(null, 'data'));

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

AmockFunction

Bmock

Cfn

DspyOn

Attempts:

3 left

💡 Hint

Common Mistakes

Using jest.mock() which replaces the whole module, not spying on a method.

Using jest.fn() which creates a new mock function but does not spy on existing ones.

To spy on a method of an existing module, jest.spyOn(object, 'method') is used.

4fill in blank

hard

Fill both blanks to mock a function and set its return value.

Node.js

const mockFunc = jest.[1]();
mockFunc.[2](() => 42);

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

Afn

BmockReturnValue

CmockImplementation

DspyOn

Attempts:

3 left

💡 Hint

Common Mistakes

Using spyOn for creating a new mock function.

Using mockReturnValue instead of mockImplementation when a function is needed.

You create a mock function with jest.fn() and set its behavior with mockImplementation().

5fill in blank

hard

Fill all three blanks to mock a module, spy on a method, and restore the original after test.

Node.js

jest.[1]('axios');
const axios = require('axios');
const spy = jest.[2](axios, 'get');

// After test
spy.[3]();

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

Amock

BspyOn

CmockRestore

Dfn

Attempts:

3 left

💡 Hint

Common Mistakes

Using jest.fn() to mock the whole module.

Forgetting to restore the original method after spying.

You mock the module with jest.mock(), spy on a method with jest.spyOn(), and restore the original method with mockRestore().

Practice

1. What is the main purpose of mocking modules or functions in Node.js testing?

easy

A. To replace real modules or functions with fake ones for isolated testing

B. To speed up the production code execution

C. To permanently change the behavior of modules in the app

D. To automatically generate documentation for modules

Solution

1. Step 1: Understand mocking concept

   Mocking means replacing real code with fake versions to test parts independently.

2. Step 2: Identify purpose in testing

   This helps avoid slow or unreliable dependencies and focuses tests on the code itself.

3. Final Answer:

   To replace real modules or functions with fake ones for isolated testing -> Option A

4. Quick Check:

   Mocking = isolated testing [OK]

Hint: Mocking means faking parts to test code alone [OK]

Common Mistakes:

• Thinking mocking speeds up production code
• Believing mocking changes app behavior permanently
• Confusing mocking with documentation generation

2. Which of the following is the correct syntax to mock an entire module named utils using Jest?

easy

A. jest.spyOn('utils')

B. jest.fake('utils')

C. jest.mock('utils')

D. mock('utils')

Solution

1. Step 1: Recall Jest mocking syntax

   Jest uses jest.mock('moduleName') to mock entire modules.

2. Step 2: Match syntax to options

   Only jest.mock('utils') uses the correct jest.mock() function with the module name.

3. Final Answer:

   jest.mock('utils') -> Option C

4. Quick Check:

   jest.mock() mocks modules [OK]

Hint: Use jest.mock('moduleName') to mock whole modules [OK]

Common Mistakes:

• Using jest.spyOn for whole modules instead of functions
• Trying mock() or jest.fake() which don't exist
• Passing module name incorrectly as a variable

3. Given the code below, what will be the output of console.log(fetchData()) after mocking?

import * as api from './api';

jest.mock('./api', () => ({
  fetchData: () => 'mocked data'
}));

console.log(api.fetchData());

medium

A. 'real data'

B. 'mocked data'

C. undefined

D. Error: fetchData is not a function

Solution

1. Step 1: Understand jest.mock replacement

   The jest.mock call replaces the entire './api' module with the provided object.

2. Step 2: Check the mocked function return

   The mocked fetchData returns the string 'mocked data'.

3. Final Answer:

   'mocked data' -> Option B

4. Quick Check:

   Mocked function returns mocked data [OK]

Hint: Mocked functions return what you define in jest.mock [OK]

Common Mistakes:

• Expecting original function output
• Assuming undefined because of mocking
• Confusing import and mock timing

4. What is wrong with this Jest mocking code snippet?

import * as math from './math';

jest.spyOn(math, 'add');

math.add.mockReturnValue(10);

console.log(math.add(2, 3));

medium

A. No error, the code works and logs 10

B. mockReturnValue cannot be used on spy without mockImplementation

C. The spy is created correctly, but math.add is not mocked properly

D. jest.spyOn should be called after calling math.add

Solution

1. Step 1: Understand jest.spyOn usage

   jest.spyOn creates a mock function that can be controlled.

2. Step 2: Check mockReturnValue usage

   Calling mockReturnValue(10) on the spy sets the return value to 10.

3. Final Answer:

   No error, the code works and logs 10 -> Option A

4. Quick Check:

   spyOn + mockReturnValue works [OK]

Hint: spyOn returns a mock you can control directly [OK]

Common Mistakes:

• Thinking spyOn needs mockImplementation first
• Believing spyOn must be after function call
• Assuming spyOn does not mock return values

5. You want to mock only the sendEmail function from the notifications module to return true during tests, but keep other functions real. Which is the best approach?

hard

A. Use jest.mock('notifications') and redefine all functions manually

B. Replace the entire module with a fake object returning true for sendEmail

C. Import sendEmail separately and overwrite it directly

D. Use jest.spyOn(notifications, 'sendEmail').mockReturnValue(true)

Solution

1. Step 1: Understand partial mocking need

   We want to mock only one function, not the whole module.

2. Step 2: Use jest.spyOn for specific function

   jest.spyOn allows mocking a single function while keeping others real.

3. Final Answer:

   Use jest.spyOn(notifications, 'sendEmail').mockReturnValue(true) -> Option D

4. Quick Check:

   spyOn mocks single function only [OK]

Hint: Use jest.spyOn to mock one function, not whole module [OK]

Common Mistakes:

• Mocking entire module unnecessarily
• Trying to overwrite imported functions directly
• Not using spyOn for partial mocks