Cypress Img Snapshot

Based on cypress-image-snapshot.

Cypress Image Snapshot binds jest-image-snapshot image diffing logic to Cypress.io commands. The goal is to catch visual regressions during integration tests.

Installation

npm i cypress-img-snapshot --save

yarn add cypress-img-snapshot

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

import { addMatchImageSnapshotPlugin } from 'cypress-img-snapshot/dist/plugin'

module.exports = (on, config) => {
  addMatchImageSnapshotPlugin(on, config)
}

and in <rootDir>/cypress/support/commands.js add:

import { addMatchImageSnapshotCommand } from 'cypress-img-snapshot/dist/command'

addMatchImageSnapshotCommand()

Cypress GUI

When using cypress open, errors are displayed in the GUI

This isn't recommended as current client resolution and settings will influence the screenshots.

Cypress Run

For best results you should only take screenshot in headless mode. Otherwise, your client system (screen resolution) will influence the size of screenshot.

Disable Screenshots for `cypress open`

Edit support/commands.ts file and add following to disable screenshots for cypress open.

Cypress.Commands.overwrite('matchImageSnapshot', (originalFn, subject, name, options) => {
  if (Cypress.browser.isHeadless) {
    return originalFn(subject, name, options)
  }
  return cy.log('No screenshot taken when headed')
})

Increase Browser Size for Headless Browsers

Edit plugins/index.ts file and add following to start headless browsers with increased resolution.

module.exports = (on: PluginEvents, config: PluginConfigOptions) => {
  on('before:browser:launch', (browser, launchOptions) => {
    console.log('launching browser "%s" is headless? %s', browser.name, browser.isHeadless)

    const width = 1920
    const height = 1080

    console.log('setting the browser window size to %d x %d', width, height)

    if (browser.name === 'chrome' && browser.isHeadless) {
      launchOptions.args.push(`--window-size=${width},${height}`)

      // force screen to be non-retina and just use our given resolution
      launchOptions.args.push('--force-device-scale-factor=1')
      launchOptions.args.push('--cast-initial-screen-width=1600')
      launchOptions.args.push('--cast-initial-screen-height=900')
    }

    if (browser.name === 'electron' && browser.isHeadless) {
      // might not work on CI for some reason
      launchOptions.preferences.width = width
      launchOptions.preferences.height = height
    }

    if (browser.name === 'firefox' && browser.isHeadless) {
      launchOptions.args.push(`--width=${width}`)
      launchOptions.args.push(`--height=${height}`)
    }

    // IMPORTANT: return the updated browser launch options
    return launchOptions
  })
}

Composite Image Diff

When an image diff fails, a composite image is constructed.

Syntax

// addMatchImageSnapshotPlugin
addMatchImageSnapshotPlugin(on, config)

// addMatchImageSnapshotCommand
addMatchImageSnapshotCommand()
addMatchImageSnapshotCommand(options)

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

Usage

In your tests

describe('Login', () => {
  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 <rootDir>/cypress/snapshots.
customDiffDir: Path to the directory that diff images will be written to, defaults to a sibling __diff_output__ directory alongside each snapshot.
snapshotSizes: Sizes of screenshots to be generated. (default: [[375, 667], [1280, 800]])
clockDate: To set the clock to a specific date to get same screenshots each time. (default: new Date(Date.UTC(2019, 1, 1)))

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 <rootDir>/cypress/support/commands.js are:

addMatchImageSnapshotCommand({
  snapshotSizes: [[375, 667], [1280, 800]], // sizes of screenshots
  clockDate: new Date(Date.UTC(2019, 1, 1)), // setting cy.clock()
  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
});

Updating snapshots

Run Cypress with --env updateSnapshots=true in order to update the base image files for all of your tests.

Preventing failures

Run Cypress with --env failOnSnapshotDiff=false in order to prevent test failures when an image diff does not pass.

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:

Take screenshots with cy.screenshot() named according to the current test.
Check if saved snapshots exists in <rootDir>/cypress/snapshots and if so diff against that snapshots.
If there is a resulting diff, save it to <rootDir>/cypress/snapshots/__diff_output__.

Typescript

To support typing of cy.matchImageSnapshot() add the type to the tsconfig.json.

{
  "compilerOptions": {
    "types": [
      "cypress-img-snapshot"
    ]
  }
}

Troubleshooting

ResizeObserver loop limit exceeded

you can safely ignore it in Cypress with the following code in support/index.ts or commands.ts.

Cypress.on('uncaught:exception', (err) => {
    // returning false here prevents Cypress from failing the test
    return !err.message.includes('ResizeObserver')
})

wuifdesign/cypress-img-snapshot