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Visual Testing

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What you'll learn​

  • How visual testing complements functional testing
  • How to implement visual diffing yourself or using 3rd party service
  • How to ensure the application is in consistent state before capturing an image

Functional vs. visual testing​

Cypress is a functional test runner. It drives the web application the way a user would, and checks if the app functions as expected: if the expected message appears, an element is removed, or a CSS class is added after the appropriate user action. A typical Cypress test, for example, can check if a toggled "Todo" item gets a class of "completed" after the .toggle is checked:

it('completes todo', () => {
  cy.visit('/') // opens TodoMVC running at "baseUrl"
  cy.get('.new-todo').type('write tests{enter}')
  cy.contains('.todo-list li', 'write tests').find('.toggle').check()

  cy.contains('.todo-list li', 'write tests').should('have.class', 'completed')
})
Passing Cypress functional test

Cypress does NOT see how the page actually looks though. For example, Cypress will not see if the CSS class completed grays out the label element and adds a strike-through line.

Completed item style

You could technically write a functional test asserting the CSS properties using the have.css assertion, but these may quickly become cumbersome to write and maintain, especially when visual styles rely on a lot of CSS styles.

cy.get('.completed').should('have.css', 'text-decoration', 'line-through')
cy.get('.completed').should('have.css', 'color', 'rgb(217,217,217)')

Your visual styles may also rely on more than CSS, perhaps you want to ensure an SVG or image has rendered correctly or shapes were correctly drawn to a canvas.

Luckily, Cypress gives a stable platform for writing plugins that can perform visual testing.

Typically such plugins take an image snapshot of the entire application under test or a specific element, and then compare the image to a previously approved baseline image. If the images are the same (within a set pixel tolerance), it is determined that the web application looks the same to the user. If there are differences, then there has been some change to the DOM layout, fonts, colors or other visual properties that needs to be investigated.

For example, one can use the cypress-plugin-snapshots plugin and catch the following visual regression:

.todo-list li.completed label {
  color: #d9d9d9;
  /* removed the line-through */
}
it('completes todo', () => {
  cy.visit('/')
  cy.get('.new-todo').type('write tests{enter}')
  cy.contains('.todo-list li', 'write tests').find('.toggle').check()

  cy.contains('.todo-list li', 'write tests').should('have.class', 'completed')

  // run 'npm install cypress-plugin-snapshots --save'
  // capture the element screenshot and
  // compare to the baseline image
  cy.get('.todoapp').toMatchImageSnapshot({
    imageConfig: {
      threshold: 0.001,
    },
  })
})

This open source plugin compares the baseline and the current images side by side if pixel difference is above the threshold; notice how the baseline image (Expected result) has the label text with the line through, while the new image (Actual result) does not have it.

Baseline vs current image

Like most image comparison tools, the plugin also shows a difference view on mouse hover:

Highlighted changes

Tooling​

There are several published, open source plugins, listed in the

Visual Testing Plugins section, and several commercial companies have developed visual testing solutions on top of Cypress listed below.

Open source​

Listed in the Visual Testing Plugins section.

Applitools​

First joint webinar with Applitools

Second joint webinar with Applitools with a focus on Component Testing

https://applitools.com/
ResourceDescription
Official docsApplitools' Cypress documentation
TutorialApplitools' Cypress tutorial
WebinarCreating a Flawless User Experience, End-to-End, Functional to Visual – Practical Hands-on Session, a webinar recorded together with Cypress and Applitools
BlogTesting a chart with Cypress and Applitools

Percy​

https://percy.io/
info

Real World Example New​

The Cypress Real World App (RWA) uses the cy.percySnapshot() command provided by the Cypress Percy plugin to take visual snapshots throughout the user journey end-to-end tests

Check out the Real World App test suites to see these Percy and Cypress in action.

ResourceDescription
Official docsPercy's Cypress documentation
TutorialPercy's Cypress tutorial
WebinarCypress + Percy = End-to-end functional and visual testing for the web, a webinar recorded together with Cypress and Percy.io
BlogThe companion blog for the Cypress + Percy webinar
SlidesThe companion slides for the Cypress + Percy webinar
BlogTesting how an application renders a drawing with Cypress and Percy

Happo​

https://happo.io/
ResourceDescription
Official docsHappo's Cypress documentation
WebinarKeep your UI Sharp: Ensuring Functional and Visual Quality with Cypress.io + Happo.io, a webinar recorded together with Cypress and Happo
BlogThe companion blog for the Cypress + Happo webinar

Chromatic​

Chromatic leverages your existing Cypress setup—configuration, mocking, and tests—to enable visual testing of your application's UI. With the Chromatic plugin installed, Chromatic captures an archive of your UI while your Cypress tests are running.

https://chromatic.com
ResourceDescription
Official docsChromatic's Cypress documentation
BlogChromatic’s Cypress integration announcement and tutorial

Best practices​

As a general rule there are some best practices when visual testing.

Recognize the need for visual testing​

Assertions that verify style properties**

cy.get('.completed').should('have.css', 'text-decoration', 'line-through')
  .and('have.css', 'color', 'rgb(217,217,217)')
cy.get('.user-info').should('have.css', 'display', 'none')
...

If your end-to-end tests become full of assertions checking visibility, color and other style properties, it might be time to start using visual diffing to verify the page appearance.

DOM state​

tip

Best Practice: Take a snapshot after you confirm the page is done changing.

For example, if the snapshot command is cy.mySnapshotCommand:

Incorrect Usage

// the web application takes time to add the new item,
// sometimes it takes the snapshot BEFORE the new item appears
cy.get('.new-todo').type('write tests{enter}')
cy.mySnapshotCommand()

Correct Usage

// use a functional assertion to ensure
// the web application has re-rendered the page
cy.get('.new-todo').type('write tests{enter}')
cy.contains('.todo-list li', 'write tests')
// great, the new item is displayed,
// now we can take the snapshot
cy.mySnapshotCommand()

Timestamps​

tip

Best Practice: Control the timestamp inside the application under test.

Below we freeze the operating system's time to Jan 1, 2018 using cy.clock() to ensure all images displaying dates and times match.

const now = new Date(2018, 1, 1)

cy.clock(now)
// ... test
cy.mySnapshotCommand()

Application state​

tip

Best Practice: Use cy.fixture() and network mocking to set the application state.

Below we stub network calls using cy.intercept() to return the same response data for each XHR request. This ensures that the data displayed in our application images does not change.

cy.intercept('/api/items', { fixture: 'items' }).as('getItems')
// ... action
cy.wait('@getItems')
cy.mySnapshotCommand()

Visual diff elements​

tip

Best Practice: Use visual diffing to check individual DOM elements rather than the entire page.

Targeting specific DOM element will help avoid visual changes from component "X" breaking tests in other unrelated components.

Component testing​

tip

Best Practice: Use Component Testing to test the individual components functionality in addition to end-to-end and visual tests.

If you are working on React components, read Visual testing for React components using open source tools, browse slides, and watch the companion videos.

See also​

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