mock-fs

What is mock-fs?

The mock-fs npm package allows you to mock the file system in Node.js, enabling you to test code that interacts with the file system without actually performing any real file system operations. This is particularly useful for unit testing and ensuring that your code behaves correctly in different file system scenarios.

What are mock-fs's main functionalities?

Mocking a Simple File System

This feature allows you to create a simple mock file system with directories and files. The `mock` function takes an object that represents the file system structure. After setting up the mock file system, you can run your code that interacts with the file system. Finally, you call `mock.restore()` to clean up and restore the real file system.

const mock = require('mock-fs');

mock({
  'path/to/fake/dir': {
    'some-file.txt': 'file content here',
    'empty-dir': {}
  }
});

// Your code that interacts with the file system

mock.restore();

Mocking File System Errors

This feature allows you to simulate file system errors, such as permission errors. In this example, the `mock.file` function is used to create a file with no permissions. When attempting to read the file, an error is thrown, which you can catch and handle in your code.

const mock = require('mock-fs');
const fs = require('fs');

mock({
  'path/to/fake/dir': {
    'some-file.txt': mock.file({
      content: 'file content here',
      mode: 0o000 // no permissions
    })
  }
});

try {
  fs.readFileSync('path/to/fake/dir/some-file.txt');
} catch (err) {
  console.error('Error reading file:', err);
}

mock.restore();

Mocking a Large File System

This feature allows you to create a large mock file system with many files. The `mock.directory` function is used to create a directory with a large number of files. This is useful for testing how your code handles large file systems.

const mock = require('mock-fs');

mock({
  'large-dir': mock.directory({
    items: Array.from({ length: 1000 }, (_, i) => ({
      [`file-${i}.txt`]: `content of file ${i}`
    })).reduce((acc, item) => Object.assign(acc, item), {})
  })
});

// Your code that interacts with the large file system

mock.restore();

Other packages similar to mock-fs

memfs

The memfs package provides an in-memory file system that can be used for testing and other purposes. Unlike mock-fs, which mocks the Node.js file system module, memfs provides a complete in-memory file system implementation that can be used as a drop-in replacement for the real file system. This can be useful for more complex scenarios where you need a fully functional file system.

fs-mock

The fs-mock package is another library for mocking the file system in Node.js. It provides a simpler API compared to mock-fs and is designed for basic use cases. While it may not have all the features of mock-fs, it can be a good choice for simpler scenarios where you just need to mock a few files and directories.

README

mock-fs

The mock-fs module allows Node's built-in fs module to be backed temporarily by an in-memory, mock file system. This lets you run tests against a set of mock files and directories instead of lugging around a bunch of test fixtures.

Example

The code below makes it so the fs module is temporarily backed by a mock file system with a few files and directories.

const mock = require('mock-fs');

mock({
  'path/to/fake/dir': {
    'some-file.txt': 'file content here',
    'empty-dir': {/** empty directory */}
  },
  'path/to/some.png': Buffer.from([8, 6, 7, 5, 3, 0, 9]),
  'some/other/path': {/** another empty directory */}
});

When you are ready to restore the fs module (so that it is backed by your real file system), call mock.restore(). Note that calling this may be mandatory in some cases. See istanbuljs/nyc#324

// after a test runs
mock.restore();

Upgrading to version 4

Instead of overriding all methods of the built-in fs module, the library now overrides process.binding('fs'). The purpose of this change is to avoid conflicts with other libraries that override fs methods (e.g. graceful-fs) and to make it possible to work with multiple Node releases without maintaining copied and slightly modified versions of Node's fs module.

Breaking changes:

• The mock.fs() function has been removed. This returned an object with fs-like methods without overriding the built-in fs module.
• The object created by fs.Stats is no longer an instance of fs.Stats (though it has all the same properties and methods).
• Lazy require() do not use the real filesystem.
• Tests are no longer run in Node < 4.

Some of these breaking changes may be restored in a future release.

Docs

mock(config, options)

Configure the fs module so it is backed by an in-memory file system.

Calling mock sets up a mock file system with two directories by default: process.cwd() and os.tmpdir() (or os.tmpDir() for older Node). When called with no arguments, just these two directories are created. When called with a config object, additional files, directories, and symlinks are created. To avoid creating a directory for process.cwd() and os.tmpdir(), see the options below.

Property names of the config object are interpreted as relative paths to resources (relative from process.cwd()). Property values of the config object are interpreted as content or configuration for the generated resources.

Note that paths should always use forward slashes (/) - even on Windows.

options

The second (optional) argument may include the properties below.

• createCwd - boolean Create a directory for process.cwd(). This is true by default.
• createTmp - boolean Create a directory for os.tmpdir(). This is true by default.

Loading real files & directories

You can load real files and directories into the mock system using mock.load()

Notes

• All stat information is duplicated (dates, permissions, etc)
• By default, all files are lazy-loaded, unless you specify the {lazy: false} option

options

Option	Type	Default	Description
lazy	boolean	true	File content isn't loaded until explicitly read
recursive	boolean	true	Load all files and directories recursively

mock.load(path, options)

mock({
  // Lazy-load file
  'my-file.txt': mock.load(path.resolve(__dirname, 'assets/special-file.txt')),
  
  // Pre-load js file
  'ready.js': mock.load(path.resolve(__dirname, 'scripts/ready.js'), {lazy: false}),

  // Recursively loads all node_modules
  'node_modules': mock.load(path.resolve(__dirname, '../node_modules')),

  // Creates a directory named /tmp with only the files in /tmp/special_tmp_files (no subdirectories), pre-loading all content
  '/tmp': mock.load('/tmp/special_tmp_files', {recursive: false, lazy:false}),

  'fakefile.txt': 'content here'
});

Creating files

When config property values are a string or Buffer, a file is created with the provided content. For example, the following configuration creates a single file with string content (in addition to the two default directories).

mock({
  'path/to/file.txt': 'file content here'
});

To create a file with additional properties (owner, permissions, atime, etc.), use the mock.file() function described below.

mock.file(properties)

Create a factory for new files. Supported properties:

• content - string|Buffer File contents.
• mode - number File mode (permission and sticky bits). Defaults to 0666.
• uid - number The user id. Defaults to process.getuid().
• gid - number The group id. Defaults to process.getgid().
• atime - Date The last file access time. Defaults to new Date(). Updated when file contents are accessed.
• ctime - Date The last file change time. Defaults to new Date(). Updated when file owner or permissions change.
• mtime - Date The last file modification time. Defaults to new Date(). Updated when file contents change.
• birthtime - Date The time of file creation. Defaults to new Date().

To create a mock filesystem with a very old file named foo, you could do something like this:

mock({
  foo: mock.file({
    content: 'file content here',
    ctime: new Date(1),
    mtime: new Date(1)
  })
});

Note that if you want to create a file with the default properties, you can provide a string or Buffer directly instead of calling mock.file().

Creating directories

When config property values are an Object, a directory is created. The structure of the object is the same as the config object itself. So an empty directory can be created with a simple object literal ({}). The following configuration creates a directory containing two files (in addition to the two default directories):

// note that this could also be written as
// mock({'path/to/dir': { /** config */ }})
mock({
  path: {
    to: {
      dir: {
        file1: 'text content',
        file2: Buffer.from([1, 2, 3, 4])
      }
    }
  }
});

To create a directory with additional properties (owner, permissions, atime, etc.), use the mock.directory() function described below.

mock.directory(properties)

Create a factory for new directories. Supported properties:

• mode - number Directory mode (permission and sticky bits). Defaults to 0777.
• uid - number The user id. Defaults to process.getuid().
• gid - number The group id. Defaults to process.getgid().
• atime - Date The last directory access time. Defaults to new Date().
• ctime - Date The last directory change time. Defaults to new Date(). Updated when owner or permissions change.
• mtime - Date The last directory modification time. Defaults to new Date(). Updated when an item is added, removed, or renamed.
• birthtime - Date The time of directory creation. Defaults to new Date().
• items - Object Directory contents. Members will generate additional files, directories, or symlinks.

To create a mock filesystem with a directory with the relative path some/dir that has a mode of 0755 and two child files, you could do something like this:

mock({
  'some/dir': mock.directory({
    mode: 0755,
    items: {
      file1: 'file one content',
      file2: Buffer.from([8, 6, 7, 5, 3, 0, 9])
    }
  })
});

Note that if you want to create a directory with the default properties, you can provide an Object directly instead of calling mock.directory().

Creating symlinks

Using a string or a Buffer is a shortcut for creating files with default properties. Using an Object is a shortcut for creating a directory with default properties. There is no shortcut for creating symlinks. To create a symlink, you need to call the mock.symlink() function described below.

mock.symlink(properties)

Create a factory for new symlinks. Supported properties:

• path - string Path to the source (required).
• mode - number Symlink mode (permission and sticky bits). Defaults to 0666.
• uid - number The user id. Defaults to process.getuid().
• gid - number The group id. Defaults to process.getgid().
• atime - Date The last symlink access time. Defaults to new Date().
• ctime - Date The last symlink change time. Defaults to new Date().
• mtime - Date The last symlink modification time. Defaults to new Date().
• birthtime - Date The time of symlink creation. Defaults to new Date().

To create a mock filesystem with a file and a symlink, you could do something like this:

mock({
  'some/dir': {
    'regular-file': 'file contents',
    'a-symlink': mock.symlink({
      path: 'regular-file'
    })
  }
});

Restoring the file system

mock.restore()

Restore the fs binding to the real file system. This undoes the effect of calling mock(). Typically, you would set up a mock file system before running a test and restore the original after. Using a test runner with beforeEach and afterEach hooks, this might look like the following:

beforeEach(function() {
  mock({
    'fake-file': 'file contents'
  });
});
afterEach(mock.restore);

Bypassing the mock file system

mock.bypass(fn)

Execute calls to the real filesystem with mock.bypass()

// This file exists only on the real FS, not on the mocked FS
const realFilePath = '/path/to/real/file.txt';
const myData = mock.bypass(() => fs.readFileSync(realFilePath, 'utf-8'));

If you pass an asynchronous function or a promise-returning function to bypass(), a promise will be returned.

Async Warning

Asynchronous calls are supported, however, they are not recommended as they could produce unintended consequences if anything else tries to access the mocked filesystem before they've completed.

async function getFileInfo(fileName) {
  return await mock.bypass(async () => {
    const stats = await fs.promises.stat(fileName);
    const data = await fs.promises.readFile(fileName);
    return {stats, data};
  });
}

Install

Using npm:

npm install mock-fs --save-dev

Caveats

When you require mock-fs, Node's own fs module is patched to allow the binding to the underlying file system to be swapped out. If you require mock-fs before any other modules that modify fs (e.g. graceful-fs), the mock should behave as expected.

Note mock-fs is not compatible with graceful-fs@3.x but works with graceful-fs@4.x.

Mock file access is controlled based on file mode where process.getuid() and process.getgid() are available (POSIX systems). On other systems (e.g. Windows) the file mode has no effect.

Tested on Linux, OSX, and Windows using Node 12 through 16. Check the tickets for a list of known issues.

Using with Jest Snapshot Testing

.toMatchSnapshot in Jest uses fs to load existing snapshots. If mockFs is active, Jest isn't able to load existing snapshots. In such case it accepts all snapshots without diffing the old ones, which breaks the concept of snapshot testing.

Calling mock.restore() in afterEach is too late and it's necessary to call it before snapshot matching:

const actual = testedFunction()
mock.restore()
expect(actual).toMatchSnapshot()

Note: it's safe to call mock.restore multiple times, so it can still be called in afterEach and then manually in test cases which use snapshot testing.