Mocking modules and functions in Node.js - Step-by-Step Execution

Concept Flow - Mocking modules and functions

Test starts

↓

Import module

↓

Mock function or module

↓

Run code using mock

↓

Check mock calls or results

↓

Test ends

This flow shows how a test imports a module, replaces parts with mocks, runs code, and checks mock behavior.

Execution Sample

Node.js

import { jest } from '@jest/globals';
import fs from 'fs';

jest.mock('fs');

fs.readFileSync.mockReturnValue('mocked content');

const content = fs.readFileSync('file.txt', 'utf8');

This code mocks the fs module's readFileSync function to return 'mocked content' instead of reading a file.

Execution Table

Step	Action	Evaluation	Result
1	Import fs module	fs is real module	fs points to real fs module
2	jest.mock('fs')	Replace fs with mock	fs.readFileSync is now a mock function
3	fs.readFileSync.mockReturnValue('mocked content')	Set mock return	readFileSync returns 'mocked content' when called
4	Call fs.readFileSync('file.txt', 'utf8')	Call mock function	Returns 'mocked content'
5	Check content variable	content = 'mocked content'	content holds mocked string

💡 Test ends after verifying mock returns expected value

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
fs.readFileSync	real function	mock function	mock function with return 'mocked content'	called mock function	mock function
content	undefined	undefined	undefined	'mocked content'	'mocked content'

Key Moments - 2 Insights

Why does fs.readFileSync return 'mocked content' instead of reading a file?

Because jest.mock replaced fs.readFileSync with a mock function, and mockReturnValue set its return to 'mocked content' (see execution_table step 3 and 4).

Does jest.mock replace the whole module or just one function?

jest.mock replaces the whole module with a mock version. Individual functions like readFileSync become mock functions you can control (see execution_table step 2).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'content' after step 4?

A undefined

B 'mocked content'

C file contents from disk

D null

💡 Hint

Check the 'content' variable in variable_tracker after step 4

At which step does fs.readFileSync become a mock function?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

See execution_table step 2 where jest.mock replaces fs with a mock

If we remove jest.mock('fs'), what would fs.readFileSync return at step 4?

A 'mocked content'

B undefined

C Actual file content from disk

D An error

💡 Hint

Without mocking, fs.readFileSync calls the real function (see execution_table step 1)

Concept Snapshot

Mocking modules and functions in Node.js tests:
- Use jest.mock('module') to replace a module with mocks.
- Mock functions can have return values set with mockReturnValue.
- Calls to mocked functions return mocked data, not real data.
- This helps isolate tests from real dependencies.
- Always check mock calls and results in tests.

Full Transcript

In Node.js testing, mocking means replacing real modules or functions with fake versions that return controlled data. The test starts by importing the module, then jest.mock replaces it with a mock. You can set what the mock returns using mockReturnValue. When the code calls the mocked function, it returns the fake data instead of doing the real work. This lets tests run without depending on real files or network. The execution table shows each step: importing, mocking, setting return, calling, and checking results. Variables like fs.readFileSync change from real to mock functions. This approach helps write reliable tests that focus on the code logic, not external factors.