Introduction

⚠️ The Cypress Component Testing library is still in Alpha. We are rapidly developing and expect that the API may undergo breaking changes. Contribute to its development by submitting feature requests or issues here.

What is Component Testing?

Definition: Running tests on a component in isolation.

Typically component tests are run using a Node.js testing framework like jest or mocha. Components we want to test are rendered in a virtualized browser called jsdom.

With component testing in Cypress, you can achieve the same goal: test a component in isolation. Instead of having components render inside a terminal, Cypress renders components in a real browser. Since the components you are testing are visible in the browser, this makes it easier to test and debug when a test fails.

Component testing in Cypress is similar to end-to-end testing. The notable differences are:

  • There's no need to navigate to a URL. You don't need to call cy.visit() in your test.
  • Cypress provides a blank canvas where we can mount components in isolation.

Getting Started

A Cypress Component Test contains a mount function and assertions about the component it has rendered. A test may interact with component as a user would, using Cypress API commands like .click(), .type(), or many more.

With Cypress as the Test Runner and assertions framework, component tests in React and Vue look very similar. Here's an example, written in React:

import { mount } from '@cypress/react' // or @cypress/vue
import TodoList from './components/TodoList'

describe('TodoList', () => {
  it('renders the todo list', () => {
    mount(<TodoList />)
    cy.get('[data-testid=todo-list]').should('exist')
  })

  it('contains the correct number of todos', () => {
    const todos = [
      { text: 'Buy milk', id: 1 },
      { text: 'Learn Component Testing', id: 2 },
    ]

    mount(<TodoList todos={todos} />)

    cy.get('[data-testid=todos]').should('have.length', todos.length)
  })
})

If you are already familiar with Cypress, you'll notice it is almost exactly the same as a Cypress end-to-end testing - all your existing Cypress knowledge and experience is transferrable.

Project Setup

Let's go through the setup to start testing components. You can set it up with an existing React or Vue project, or start a new project from scratch. This guide assumes your project uses a Webpack based tool chain. For our experimental Vite based instructions, please see the information here.

Prerequisites

  • A project with a package.js file at the root that runs on Webpack 4 or 5.
  • A webpack.config.js file, or a way to access Webpack configuration. Refer to your framework's documentation.
  • Some components that you want to test that visually display in a browser. It could be a date picker, tabs, responsive images.
  • A basic knowledge of how to write tests in Cypress. (See the Getting Started guide.)

⚠️ Existing end-to-end users

If you are using Cypress Component Testing in a project that also has tests written with the Cypress End-to-End test runner, you may want to configure some Component Testing specific defaults.

You can configure or override Component Testing defaults in your configuration file (cypress.json by default) using the component key.

For example, if you would like to use a different viewport size or target different test files for Component Testing, your cypress.json might look like this:

{
  "testFiles": "cypress/integration/*.spec.js",
  "component": {
    "componentFolder": "src",
    "testFiles": ".*/__tests__/.*spec.tsx",
    "viewportHeight": 500,
    "viewportWidth": 700
  }
}

The Component Testing runner will use all the configuration at the root level of your configuration file and apply any Component Testing specific overrides.

Install

Start by running the command below to install dependencies. It will install both the latest version of Cypress and the tooling you need to run component testing.

npm install --save-dev cypress @cypress/react @cypress/webpack-dev-server webpack-dev-server
npm install --save-dev cypress @cypress/vue @cypress/webpack-dev-server webpack-dev-server

Once installed, you need to configure how Cypress will locate component spec files. In the following configuration file (cypress.json by default), all components test files contained within the src directory and match the glob given in the testFiles key.

{
  "component": {
    "componentFolder": "src",
    "testFiles": "**/*spec.{js,jsx,ts,tsx}"
  }
}

You will also need to configure the component testing framework of your choice by installing the corresponding component testing plugin. Read more about Cypress plugins in our plugins guide. For example, if you are using Create React App, you will need to use the react-scripts plugin as shown below in your cypress/plugins/index.js file.

// cypress/plugins/index.js

module.exports = (on, config) => {
  if (config.testingType === 'component') {
    require('@cypress/react/plugins/react-scripts')(on, config)
  }

  return config
}
// cypress/plugins/index.js

module.exports = (on, config) => {
  if (config.testingType === 'component') {
    const { startDevServer } = require('@cypress/webpack-dev-server')

    // Vue's Webpack configuration
    const webpackConfig = require('@vue/cli-service/webpack.config.js')

    on('dev-server:start', (options) =>
      startDevServer({ options, webpackConfig })
    )
  }

  return config
}
// cypress/plugins/index.js

module.exports = (on, config) => {
  if (config.testingType === 'component') {
    const { startDevServer } = require('@cypress/webpack-dev-server')

    // Your project's Webpack configuration
    const webpackConfig = require('../../webpack.config.js')

    on('dev-server:start', (options) =>
      startDevServer({ options, webpackConfig })
    )
  }

  return config
}

Note we have a conditional check against config.testingType. This is useful if your project is already using existing plugins for the End-to-end runner, and you don't want them to conflict.

If you have a different React development environment from Create React App, such as Next.js, or use a Vue template other than vue-cli, you will need to import the appropriate plugin. See a list of officially maintained plugins here. Each of these plugins perform the same tasks under the hood. Alternatively, if you have your own Webpack configuration, you can just provide it (without need for a specific plugin) as specified above.

Writing Component Tests

This example assumes a project with a <Button /> component.

We recommend locating your component tests next to the components you are testing. If you are using our recommended testFiles glob (**/*spec.{js,jsx,ts,tsx}) as described above:

  • Navigate to where this component exists in your code.
  • Create a spec file in the same folder called Button.spec.jsx.

Otherwise create Button.spec.jsx in the relevant directory based on your configuration.

Once the test file exists, we can begin to write a test called Button.spec.jsx:

import * as React from 'react'
import { mount } from '@cypress/react'
import Button from './Button'

it('Button', () => {
  mount(<Button>Test button</Button>)
  cy.get('button').contains('Test button').click()
})
import { mount } from '@cypress/vue'
import Button from './Button'

it('Button', () => {
  // with JSX
  mount(() => <Button>Test button</Button>)

  // ... or ...
  mount(Button, {
    slots: {
      default: 'Test button',
    },
  })

  cy.get('button').contains('Test button').click()
})
  • Open Cypress in Component Testing mode:
npx cypress open-ct
  • Select the spec file in the sidebar. You should see the following:
Single Spec file with single test run
  • Try to modify the test in your editor, make the test fail, etc. The tests will re-run instantly with immediate visual feedback.

Set up CI

Sometimes we want to run tests without interactivity and see all results in the terminal, like when we run our tests in continuous integration.

To run all component tests in the terminal, run the command below:

npx cypress run-ct

In the project we just built, this command will show the following results.

Result of headless test run

To make the component tests part of your continuous integration pipeline, add a script to run npx cypress run-ct to your CI configuration.

Experimental

The tools listed in this section are actively being developed. We will support early adoption of these projects in order to get community feedback. Please report issues against these projects in Github or contact us on Discord for additional support.

Vite

For a quick-start, please take a look at the boilerplate repositories below. To setup a project from scratch please check out the below plugins file.

From Boilerplate

From Scratch

To get started with Vite, please follow the installation instructions for Webpack, but replace @cypress/webpack-dev-server with @cypress/vite-dev-server. Here is a sample plugins.js file where the dev-server:start event is registered with Vite.

import { startDevServer } from '@cypress/vite-dev-server'

export default function (on, config) {
  on('dev-server:start', (options) => {
    const viteConfig = {
      // import or inline your vite configuration from vite.config.js
    }
    return startDevServer({ options, viteConfig })
  })
  return config
}

Exactly like Webpack, you should start Cypress with yarn cypress open-ct. Writing component tests when using Vite is exactly the same as when using Webpack. Minor differences may occur depending on the

Known issues

  • Missing Vitesse support
  • May flake in resource-constrained CI machines
  • Issues with certain PWA plugins

Debugging Strategies

Issues will often arise during the initial compilation and start of your project. Please collect logs and focus on stripping down your viteConfig to work around any issues. Please log any issues in Github.

Collecting logs

To debug any Vite issues, run Cypress using DEBUG=cypress:* yarn cypress open-ct.

Compiling your project

Remove all plugins and pass in an empty viteConfig into the startDevServer options. Add plugins in one-at-a-time