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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.

Hugh McCamphill 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