Test for visual regression with Jest-image-snapshot

Quick facts


sania dsouza

3 years ago | 1 min read

Jest has a feature called snapshot testing where a serializable value for the React tree is generated and then compared with a reference snapshot to check for differences.

However, this article focuses instead on the more visual screenshot comparison that is provided by the jest-image-snapshot package.

Quick facts:

  • It's a Jest matcher that performs image comparisons using pixelmatch
  • jest-image-snapshot will not work with anything below Jest 20.x.x
  • Could add a Gaussian blur for noise
  • Once the snapshot is taken, it works exactly the same as Jest snapshots

Sample test:

This test assumes you have Jest installed and have basic Jest know-how. The project was built using Create React App. After this initial set-up, follow the steps below for visual testing goodness.

  • Install the package :
npm i --save-dev jest-image-snapshot
  • Also install Puppeteer for user interaction
npm install puppeteer
  • Test script:
import { toMatchImageSnapshot } from 'jest-image-snapshot';
const puppeteer = require('puppeteer');
expect.extend({ toMatchImageSnapshot });

it('CreateReactApp home', async () => {
const browser = await puppeteer.launch();
const page = await browser.newPage();
await page.goto('http://localhost:3000');
const image = await page.screenshot();

  • Run the test the first time:
npm run test

This test opens the page running on the localhost, takes a snapshot and saves it in the folder _image_snapshots_.

The screenshot:

  • Make a change in the source code and re-run the test. The test fails this time.

Another sub-folder is created by the name _diff_output_.

The difference between the snapshot and the reference snapshot is shown marked:

This is a simple test. You could simulate more complicated user flows and take screenshots of sections of pages rather than full pages also.

The good stuff:

  • Easy setup once Jest is installed
  • Follows the Jest scaffolding of tests and can easily be integrated with existing functional tests
  • Multiple configuration options from the API
  • Could set image difference sensitivity percentage

Other things:

  • No support for Typescript
  • Outdated reference snapshots have to be removed manually and do not get cleared by using the -u flag of Jest. There is an environment variable that can be set up to remove the outdated snapshots but this utility must be used with caution.\


Created by

sania dsouza







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