Unit testing Node.js fs with mock-fs

04 March 2021

If you're using the fs module to do things like write to files, or modify file names, you might have wondered - how do I unit test this?

In this post I'll be showing you how you can use mock-fs to easily unit test your Node.js scripts.

If you want to learn more about Node.js, check out my posts on automating file renaming with Node.js and writing to files with Node.js.

Set up your Node.js script to be tested

To begin with, we'll be using an example Node.js script that uses fs to replace the string "Hello" with "Goodbye".

This example is fully synchronous, and only uses fs readFileSync and writeFileSync:

const { readFileSync, writeFileSync } = require('fs');

const modifyFile = () => {
    const file = `${process.cwd()}/folderName/index.md`

    const content = readFileSync(file, 'utf8');    const newContent = content.replace('Hello', 'Goodbye');

    writeFileSync(file, newContent);};

If your script is fully synchronous, you'll have no problems and you can keep scrolling down to the mock-fs part below.

However if you're using async functions like fs readFile or writeFile, you'll need to make sure that your script has finished before beginning the unit tests.

We can do this using the fs Promises API.

Using the fs Promises API

Instead of using readFile, use promises.readFile, and you'll be returning a Promise:

const { promises } = require('fs');

const modifyFile = async () => {
    const file = `${process.cwd()}/folderName/index.md`

    return promises.readFile(file, 'utf8').then(content => {        const newContent = content.replace('Hello', 'Goodbye')
        return promises.writeFile(file, newContent);    });

This means that in your unit test, you can now use await and make sure your script has completed before testing it:

test('should replace Hello with Goodbye', async () => {
    await modifyFile();
    // ...

Before we make any assertions though, we’ll also need to add some mocks.

Mock your files and folders using mock-fs

We want to be able to mock out some files, because otherwise you would need to have dummy test files that live in your test folder, and you would also need to reset them to their original state at the end of each unit test.

With mock-fs, we can mock out folder structures and the content of files.

Make sure you have it installed first:

npm i mock-fs -D 
# or
yarn add mock-fs -D

Then, add it to the beforeAll hook in your test:

import mock from 'mock-fs';
import { main } from './modifyFile';

describe('modifyFile script', () => {
    beforeAll(() => {
            'folderName': {
                'index.md': '# Hello world!',

    afterAll(() => {

These folder names are relative to the root of your repository. Here we’re mocking a folder/file structure like this:

    index.md // <- contains "# Hello world!"

Write a unit test on file modification with mock-fs

Now we can continue with our unit test. We can assert on the file's contents:

test('should replace hello with goodbye', async () => {
    const file = `${process.cwd()}/folderName/index.md`
    const expectedResult = `# Goodbye world`;

    await modifyFile();

    const result = readFileSync(file, 'utf8');

When we call modifyFile, we'll be modifying the mocked file. We can then confirm that the file was successfully modified by using readFileSync to read it.

Write a unit test on file renaming with mock-fs

In the case where we want to unit test that files were renamed, we can do the following:

import glob from 'glob';

test('should successfully move and rename files', async () => {
    const expectedFiles = [

    await modifyFile();

    const files = glob.sync(`${process.cwd()}/folderName/*.md`);


Since we have used mock-fs, your script can also rename mocked files. Then we can use glob to verify that our files were renamed as expected.


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