# `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 18 through 22. 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.