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Created September 21, 2016 16:15
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Using mock-fs with jest
// __tests__/example.js
it('should serve as a nice example', () => {
const fs = require('fs')
// fs can be set up at any point by calling __configureFs.
'/test': {
'': '----',
// mock-fs' other exports are also available on the fs object.
'': fs.__mockFile({
content: 'file content here',
ctime: new Date(1),
mtime: new Date(1)
const results = fs.readdirSync('/test')
expect(results).toEqual(['', ''])
// __mocks__/fs.js
const mockfs = require('mock-fs');
const container = {
__internalFs: null,
__configureFs: (conf) => {
container.__internalFs = mockfs.fs(conf);
__mockFile: mockfs.file,
__mockSymlink: mockfs.symlink,
const proxyfs = new Proxy(container, {
get: function (target, property, receiver) {
if (target.hasOwnProperty(property)) {
return target[property];
} else {
if (target.__internalFs === null) {
target.__internalFs = mockfs.fs();
return target.__internalFs[property];
module.exports = proxyfs;
// mock-fs hooks the native fs on first require, so unfortunately just using
// the __mocks__ file isn't enough; jest will mock fs itself and stuff gets
// complicated. I'm actually not completely sure _what_ happens, but the end
// result is that you need to require mock fs before anything else or it
// doesn't work.
"devDependencies": {
"jest": "^15.1.1",
"mock-fs": "^3.11.0",
"scripts": {
"test": "jest"
"jest": {
"setupFiles": ["<rootDir>/jestsetup.js"]
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I think this can be made in a much more simpler manner. Here is the code for a custom function that saves and loads a file to disk:

import * as mock from 'mock-fs'
// This is a library that promisifies fs, you can use fs as well
import * as fs from 'mz/fs'
import * as fileHandler from './../../utils/fileHandler'

beforeEach(async () => {
  // Creates an in-memory file system 
    '/test': {
      '': 'hello world!'

afterEach(async () => {

it('saves a file given it\'s contents', async () => {
  await fileHandler.saveFile('/test/', 'ciao world!')
  return fs.exists('/test/')
  .then(exist => expect(exist).toBeTruthy())

it('loads a file from disk', async () => {
  return fileHandler.loadFile('/test/')
  .then(data => expect(data).toBe('hello world!'))

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@cesalberca did you tested it with4.x from mock-fs i have the same problem like @heyimalex
it looks to complex to me, but your version wont work neither...

did you both find a better solution today?

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@digitalkaoz For what it's worth, @cesalberca's solution works exactly as expected. In my case I use sync access and it works like a charm.

Here are my dependencies :

"dependencies": {
    "jest": "^23.4.1",
    "mock-fs": "^4.5.0"

NB : I create an in-memory mock beforeEach assertion, but I call manually mock.restore() in my test, since it needs to happen at different times (before file access to assert the Error thrown, after file access to assert the success).

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