Cypress Image Diff

Cypress Image Diff binds jest-image-snapshot's image diffing logic to Cypress.io commands. The goal is to catch visual regressions during integration tests. Here's what it looks like when tests run with the Cypress GUI.

This then constructs/uploads an image diff for analysis.

Boom! Turns out you probably can't delete that intern's CSS from 6 months ago that somehow made its way to prod. :see_no_evil:

Installation

Install from npm

``bash
npm install --save-dev cypress-image-diff
`

then add the following in your project's /cypress/plugins/index.js:

`js
const {
addCleanupDiffOutputCommand,
addMatchImageSnapshotPlugin,
} = require('cypress-image-diff/plugin');

module.exports = (on) => {
addMatchImageSnapshotPlugin(on);
};
`

and in /cypress/support/commands.js add:

`js
import { addMatchImageSnapshotCommand } from 'cypress-image-diff/command';

addMatchImageSnapshotCommand(options);
addCleanupDiffOutputCommand();
`

Syntax

`js
// addMatchImageSnapshotPlugin
addMatchImageSnapshotPlugin(on);

// addMatchImageSnapshotCommand
addMatchImageSnapshotCommand();
addMatchImageSnapshotCommand(commandName);
addMatchImageSnapshotCommand(options);
addMatchImageSnapshotCommand(commandName, options);

// addCleanupDiffOutputCommand (WARNING: must be run __after__ addMatchImageSnapshotCommand)
addCleanupDiffOutputCommand();
addCleanupDiffOutputCommand(commandName);

// cleanupDiffOutput
.cleanupDiffOutput();

// ---or---

cy.cleanupDiffOutput();

// matchImageSnapshot
.matchImageSnapshot();
.matchImageSnapshot(name);
.matchImageSnapshot(options);
.matchImageSnapshot(name, options);

// ---or---

cy.matchImageSnapshot();
cy.matchImageSnapshot(name);
cy.matchImageSnapshot(options);
cy.matchImageSnapshot(name, options);
`

Usage

`js
describe('Login', () => {
before(function() {
cy.cleanupDiffOutput();
});

it('should be publicly accessible', () => {
cy.visit('/login');

// snapshot name will be the test title
cy.matchImageSnapshot();

// snapshot name will be the name passed in
cy.matchImageSnapshot('login');

// options object passed in
cy.matchImageSnapshot(options);

// match element snapshot
cy.get('#login').matchImageSnapshot();
});
});
`

Options

- customSnapshotsDir : Path to the directory that snapshot images will be written to, defaults to /cypress/snapshots.
- customDiffDir: Path to the directory that diff images will be written to, defaults to a sibling __diff_output__ directory alongside each snapshot.

Additionally, any options for cy.screenshot() and jest-image-snapshot can be passed in the options argument to addMatchImageSnapshotCommand and cy.matchImageSnapshot(). The local options in cy.matchImageSnapshot() will overwrite the default options set in addMatchImageSnapshot.

For example, the default options we use in /cypress/support/commands.js are:

`js
addMatchImageSnapshotCommand({
failureThreshold: 0.03, // threshold for entire image
failureThresholdType: 'percent', // percent of image or number of pixels
customDiffConfig: { threshold: 0.1 }, // threshold for each pixel
capture: 'viewport', // capture viewport in screenshot
});
`

How it works

We really enjoy the diffing workflow of jest-image-snapshot and wanted to have a similar workflow when using Cypress. Because of this, under the hood we use some of jest-image-snapshot's internals and simply bind them to Cypress's commands and plugins APIs.

The workflow of cy.matchImageSnapshot() when running Cypress is:

1. Take a screenshot with cy.screenshot() named according to the current test.
2. Check if a saved snapshot exists in /cypress/snapshots and if so diff against that snapshot.
3. If there is a resulting diff, save it to /cypress/snapshots/__diff_output__.
4. If the diff is intended, run Cypress again with --env updateSnapshots=true` to update the snapshots.