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

A configurable mock file system. You know, for testing.

Purpose

Instead of testing against real file system resources, it should be easy to mock up a file system for tests.

Example

The code below creates a mock fs module that is configured to work with a few mock files and directories.

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

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

If you are testing a module that requires the real fs module, you can use rewire to inject a mock fs module for testing.

var rewire = require("rewire");
var moduleToTest = rewire('./path/to/module');

// inject the mock fs created above
moduleToTest.__set__('fs', fs);

// now functions in moduleToTest will use
// your mock fs instead of the real one

Status

The table below shows what is covered by the mock fs. Tests are currently run on Node 0.8 and 0.10. Eventually more versions will be supported.

function	status	notes
fs.Stats	90%	Incudes mode, size, atime, ctime, mtime, isFile(), and isDirectory()
fs.exists	100%	Complete
fs.existsSync	100%	Complete
fs.close	0%	Implement binding.close
fs.closeSync	0%	Implement binding.close
fs.open	0%	Implement binding.open
fs.openSync	0%	Implement binding.open
fs.read	0%	Implement binding.read
fs.readSync	0%	Implement binding.read
fs.write	0%	Implement binding.write
fs.writeSync	0%	Implement binding.write
fs.rename	0%	Implement binding.rename
fs.renameSync	0%	Implement binding.rename
fs.truncate	0%	Implement binding.ftruncate
fs.ftruncate	0%	Implement binding.ftruncate
fs.ftruncateSync	0%	Implement binding.ftruncate
fs.rmdir	0%	Implement binding.rmdir
fs.rmdirSync	0%	Implement binding.rmdir
fs.fdatasync	0%	Implement binding.fdatasync
fs.fdatasyncSync	0%	Implement binding.fdatasync
fs.fsync	0%	Implement binding.fsync
fs.fsyncSync	0%	Implement binding.fsync
fs.mkdir	0%	Implement binding.mkdir
fs.mkdirSync	0%	Implement binding.mkdir
fs.readdir	0%	Implement binding.readdir
fs.readdirSync	0%	Implement binding.readdir
fs.fstat	0%	Implement binding.fstat
fs.lstat	0%	Implement binding.lstat
fs.stat	90%	Provides stats object described above
fs.fstatSync	0%	Implement binding.fstat
fs.lstatSync	0%	Implement binding.lstat
fs.statSync	90%	Provides stats object described above
fs.readlink	0%	Implement binding.readlink
fs.readlinkSync	0%	Implement binding.readlink
fs.symlink	0%	Implement binding.symlink
fs.symlinkSync	0%	Implement binding.symlink
fs.link	0%	Implement binding.link
fs.linkSync	0%	Implement binding.link
fs.unlink	0%	Implement binding.unlink
fs.unlinkSync	0%	Implement binding.unlink
fs.fchmod	0%	Implement binding.fchmod
fs.fchmodSync	0%	Implement binding.fchmod
fs.chmod	0%	Implement binding.chmod
fs.chmodSync	0%	Implement binding.chmod
fs.fchown	0%	Implement binding.chown
fs.fchownSync	0%	Implement binding.chown
fs.utimes	0%	Implement binding.utimes
fs.utimesSync	0%	Implement binding.utimes
fs.futimes	0%	Implement binding.futimes
fs.futimesSync	0%	Implement binding.futimes