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# `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 `fs.Stats` objects have the following properties: `dev`, `ino`, `nlink`, `mode`, `size`, `rdev`, `blksize`, `blocks`, `atime`, `ctime`, `mtime`, `birthtime`, `uid`, and `gid`. In addition, all of the `is*()` method are provided (e.g. `isDirectory()`, `isFile()`, et al.).
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 6 through 11. 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.