[![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 ### `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`](#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)` ```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. ### `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: ```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. ### `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: ```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. ### `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: ```js 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: ```js 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() ```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. #### 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. ```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 18 through 22. 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.

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