Mocking modules and functions in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when mocking a module function with Jest?

Consider the following Node.js module and test code using Jest. What will be the output of the test?

Node.js

// math.js
export function add(a, b) {
  return a + b;
}

// math.test.js
import * as math from './math';

test('mock add function', () => {
  jest.spyOn(math, 'add').mockImplementation(() => 42);
  const result = math.add(1, 2);
  console.log(result);
});

A 42

B 3

C undefined

D TypeError: math.add is not a function

💡 Hint

Think about what jest.spyOn with mockImplementation does to the original function.

The jest.spyOn replaces the original add function with a mock that returns 42 regardless of input. So calling math.add(1, 2) returns 42.

📝 Syntax

intermediate

Which option correctly mocks a default export function in Jest?

Given a module with a default export function, which Jest mock syntax correctly mocks it?

Node.js

// greet.js
export default function greet(name) {
  return `Hello, ${name}!`;
}

// greet.test.js
import greet from './greet';

jest.mock('./greet', () => {
  // Fill in the mock implementation
});

A return { greet: jest.fn(() => 'Hi!') };

B return jest.fn(() => 'Hi!');

C return { default: jest.fn(() => 'Hi!') };

D ;)'!iH' >= )((nf.tsej nruter

💡 Hint

Default exports are properties named 'default' in the mock object.

When mocking a default export, the mock must return an object with a default property that is the mocked function.

🔧 Debug

advanced

Why does this Jest mock not replace the imported function?

Look at this test code. Why does the mock not replace the original function when called?

Node.js

// utils.js
export function fetchData() {
  return 'real data';
}

// app.js
import { fetchData } from './utils';
export function getData() {
  return fetchData();
}

// app.test.js
import { getData } from './app';
import * as utils from './utils';

jest.mock('./utils');
test('mock fetchData', () => {
  utils.fetchData.mockReturnValue('mocked data');
  const result = getData();
  console.log(result);
});

A Because utils.fetchData is not imported correctly.

B Because fetchData is not a mock function and cannot use mockReturnValue.

C Because jest.mock must be called at the top level, not inside the test.

D Because getData imports fetchData before the mock is applied, so it uses the original.

💡 Hint

Consider when the module is imported and when the mock is applied.

The getData function imports fetchData before the mock is applied, so it uses the original function. Jest mocks must be set up before importing the module that uses the function.

❓ state_output

advanced

What is the output after resetting mocks in Jest?

Given this test code, what will be the output of the console.log statements?

Node.js

const utils = {
  getNumber: () => 5
};

jest.mock('./utils', () => ({
  getNumber: jest.fn(() => 10)
}));

import { getNumber } from './utils';

console.log(getNumber());

getNumber.mockReset();

console.log(getNumber());

A

10
5

B

10
undefined

C

10
10

D

5
undefined

💡 Hint

What does mockReset do to the mock function's implementation?

mockReset clears the mock implementation, so the function returns undefined after reset.

🧠 Conceptual

expert

Which statement about mocking ES modules with Jest is true?

Select the correct statement about mocking ES modules and their functions in Jest.

A You can mock named exports by replacing the entire module with jest.mock and returning an object with the named exports as jest.fn().

B jest.spyOn cannot be used on ES module functions because they are read-only.

C To mock a default export, you must always use jest.fn() directly without wrapping in an object.

D Jest automatically mocks all ES module functions without any configuration.

💡 Hint

Think about how jest.mock replaces modules and how named exports are mocked.

jest.mock can replace the entire module by returning an object with named exports mocked as jest.fn(). This is the standard way to mock named exports.

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