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().