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.

Example

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

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

var restore = mock({
  '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 */}
});

Note that the mock function returns a restore function. When you are ready to restore the fs module (so that it is backed by your real file system), call restore().

/// after a test runs
restore();

Status

The table below shows what is covered by the mock fs. Tests are run on Node 0.8, 0.10, and 0.11.

function	status	notes
fs.ReadStream	100%	Complete
fs.Stats	100%	Incudes dev, ino, nlink, mode, size, rdev, blksize, blocks, atime, ctime, mtime, uid, gid, and is*() methods
fs.WriteStream	100%	Complete
fs.appendFile	100%	Complete
fs.appendFileSync	100%	Complete
fs.chmod	100%	Complete
fs.chmodSync	100%	Complete
fs.chown	100%	Complete
fs.chownSync	100%	Complete
fs.close	100%	Complete
fs.closeSync	100%	Complete
fs.createReadStream	100%	Complete
fs.createWriteStream	100%	Complete
fs.exists	100%	Complete
fs.existsSync	100%	Complete
fs.fchmod	100%	Complete
fs.fchmodSync	100%	Complete
fs.fchown	100%	Complete
fs.fchownSync	100%	Complete
fs.fdatasync	100%	Complete
fs.fdatasyncSync	100%	Complete
fs.fstat	100%	Provides a stats object
fs.fstatSync	100%	Provides a stats object
fs.fsync	100%	Complete
fs.fsyncSync	100%	Complete
fs.ftruncate	100%	Complete
fs.ftruncateSync	100%	Complete
fs.futimes	100%	Complete
fs.futimesSync	100%	Complete
fs.lchmod	100%	Complete
fs.lchmodSync	100%	Complete
fs.lchown	100%	Complete
fs.lchownSync	100%	Complete
fs.link	100%	Complete
fs.linkSync	100%	Complete
fs.lstatSync	100%	Complete
fs.lstat	100%	Complete
fs.mkdir	100%	Complete
fs.mkdirSync	100%	Complete
fs.open	100%	Complete
fs.openSync	100%	Complete
fs.read	100%	Complete
fs.readSync	100%	Complete
fs.readFile	100%	Complete
fs.readFileSync	100%	Complete
fs.readdir	100%	Complete
fs.readdirSync	100%	Complete
fs.readlink	100%	Complete
fs.readlinkSync	100%	Complete
fs.realpath	100%	Complete
fs.realpathSync	100%	Complete
fs.rename	100%	Complete
fs.renameSync	100%	Complete
fs.rmdir	100%	Complete
fs.rmdirSync	100%	Complete
fs.stat	100%	Provides a stats object
fs.statSync	100%	Provides a stats object
fs.symlink	100%	Complete
fs.symlinkSync	100%	Complete
fs.truncate	100%	Complete
fs.truncateSync	100%	Complete
fs.unlink	100%	Complete
fs.unlinkSync	100%	Complete
fs.utimes	100%	Complete
fs.utimesSync	100%	Complete
fs.write	100%	Complete
fs.writeSync	100%	Complete
fs.writeFile	100%	Complete
fs.writeFileSync	100%	Complete
fs.FSWatcher	0%	Implement fs.FSWatcher
fs.unwatchFile	0%	Implement binding.StatWatcher
fs.watch	0%	Implement fs.FSWatcher
fs.watchFile	0%	Implement binding.StatWatcher