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Add note about snapshot testing #247

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Sep 11, 2018
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Add note about snapshot testing #247

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17 changes: 17 additions & 0 deletions readme.md
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Expand Up @@ -207,4 +207,21 @@ Mock file access is controlled based on file mode where `process.getuid()` and `

Tested on Linux, OSX, and Windows using Node 0.10 through 10.x. 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.

[![Current Status](https://secure.travis-ci.org/tschaub/mock-fs.png?branch=master)](https://travis-ci.org/tschaub/mock-fs)