---
title: "Comment by Serhiy Tymoshenko on \"Visual regression testing React Native apps with Detox and Jest\" | Hashnode"
description: "Thanks a lot for the idea and the article! Could you please elaborate more on Extend jest expect part? Didn't quite get how do you expose this jest extensio"
url: "https://hashnode.com/posts/visual-regression-testing-react-native-apps-with-detox-and-jest/62081c2ba2be760e35396a8c/comment/6333353cfa20fa4bc8e5044b"
---

[Discussion](https://hashnode.com/posts/visual-regression-testing-react-native-apps-with-detox-and-jest/62081c2ba2be760e35396a8c)

Comment [Visual regression testing React Native apps with Detox and Jest](https://hashnode.com/posts/visual-regression-testing-react-native-apps-with-detox-and-jest/62081c2ba2be760e35396a8c)

[Serhiy Tymoshenko](https://hashnode.com/@sergtimosh)

[Sep 27, 2022](https://hashnode.com/posts/visual-regression-testing-react-native-apps-with-detox-and-jest/62081c2ba2be760e35396a8c/comment/6333353cfa20fa4bc8e5044b)

Thanks a lot for the idea and the article! Could you please elaborate more on **Extend jest expect** part? Didn't quite get how do you expose this jest extension helper to the tests? Maybe you have some public repo to share the structure and configuration? Thanks in advance!

[Hugh McCamphill](https://hashnode.com/@hughleo)

[Sep 27, 2022](https://hashnode.com/posts/visual-regression-testing-react-native-apps-with-detox-and-jest/62081c2ba2be760e35396a8c/comment/6333353cfa20fa4bc8e5044b#reply-63334989e594fa4ba3b89207)

It's exposed globally because we do this:

**global.jestExpect = jestExpect**

This means you can just use jestExpect in the tests, like in the example.

[Serhiy Tymoshenko](https://hashnode.com/@sergtimosh)

[Sep 28, 2022](https://hashnode.com/posts/visual-regression-testing-react-native-apps-with-detox-and-jest/62081c2ba2be760e35396a8c/comment/6333353cfa20fa4bc8e5044b#reply-6333f81cfa20fa4bc8e51171)

[Hugh McCamphill](https://hashnode.com/@hughleo) Yes, I see that :) But it's still a question for me where and how do you expose jestExpect to already use in this block? I am lacking infrastructure understanding for that purpose To use **jestExpect** instead of the **expect** exposed by detox I have to import it explicitly.

Also I see the issue with **toMatchImage** not available in the methods scope after been extended to.

```
import { configureToMatchImageSnapshot } from 'jest-image-snapshot';
import fs from 'fs';
import path from 'path';
import kebabCase from 'lodash.kebabcase';
import jestExpect from 'expect'

const toMatchImage = configureToMatchImageSnapshot({
  comparisonMethod: 'ssim', failureThreshold: 0.002, failureThresholdType: 'percent'
});

jestExpect.extend({ toMatchImage });

jestExpect.extend({
  async toMatchImageSnapshot(screenName) {
    const platform =  device.getPlatform();
    const deviceName =  device.name.split(' ').slice(1).join('');
    const deviceType = JSON.parse(deviceName).type.replace(',','');

    const SNAPSHOTS_DIR = `__image_snapshots__/${platform}/${deviceType}`;

    const { testPath, currentTestName } = this;

    const customSnapshotsDir = path.join(path.dirname(testPath), SNAPSHOTS_DIR);
    const customSnapshotIdentifier = kebabCase(`${path.basename(testPath)}-${currentTestName}-${screenName}`)

    const tempPath = await device.takeScreenshot(screenName);
    const image = fs.readFileSync(tempPath);
    jestExpect(image).toMatchImage({ customSnapshotIdentifier, customSnapshotsDir });
                       ~~~~~~~~~~~
    return { pass: true }
  },
});

global.jestExpect = jestExpect
```

[Hugh McCamphill](https://hashnode.com/@hughleo)

[Dec 9, 2022](https://hashnode.com/posts/visual-regression-testing-react-native-apps-with-detox-and-jest/62081c2ba2be760e35396a8c/comment/6333353cfa20fa4bc8e5044b#reply-63934f82344783061f8c55fb)

[Serhiy Tymoshenko](https://hashnode.com/@sergtimosh) - sorry I don't quite follow

Yes - in our tests we import jestExpect when needed.

for the scoping issue you see - is that not just a linting issue?