mock-fs/readme.md

[![Build Status](https://github.com/tschaub/mock-fs/actions/workflows/test.yml/badge.svg?branch=main)](https://github.com/tschaub/mock-fs/actions?workflow=Test)

# `mock-fs`

The `mock-fs` module allows Node's built-in [`fs` module](http://nodejs.org/api/fs.html) 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.

```js
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()`](#mockrestore). Note that calling this may be **mandatory** in some cases. See [istanbuljs/nyc#324](https://github.com/istanbuljs/nyc/issues/324#issuecomment-234018654)

```js
// 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

### <a id='mockconfigoptions'>`mock(config, options)`</a>

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`](#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.*

### <a id='options'>`options`</a>

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

#### <a id='mappingoptions'>options</a>

| 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)`

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

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

### <a id='mockfileproperties'>`mock.file(properties)`</a>

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:
```js
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):
```js
// 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()`](mockdirectoryproperties) function described below.

### <a id='mockdirectoryproperties'>`mock.directory(properties)`</a>

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:
```js
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()`](#mocksymlinkproperties) function described below.

### <a id='mocksymlinkproperties'>`mock.symlink(properties)`</a>

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

### Restoring the file system

### <a id='mockrestore'>`mock.restore()`</a>

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:

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

### Bypassing the mock file system

#### <a id='mockbypass'>`mock.bypass(fn)`</a>

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

```js
// 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.

#### <a id='bypassasync'>Async Warning</a>

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.

```js
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 16 through 18.  Check the tickets for a list of [known issues](https://github.com/tschaub/mock-fs/issues).

### Using with Jest Snapshot Testing

`.toMatchSnapshot` in [Jest](https://jestjs.io/docs/en/snapshot-testing) 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:

```js
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.