@emotion/jest

What is @emotion/jest?

@emotion/jest is a package that provides custom Jest matchers for testing Emotion styles. It allows you to write tests that can check if your components have the correct styles applied, making it easier to ensure that your styled components behave as expected.

What are @emotion/jest's main functionalities?

toHaveStyleRule

The `toHaveStyleRule` matcher allows you to check if a component has a specific style rule applied. This is useful for ensuring that your styled components have the correct styles.

const { toHaveStyleRule } = require('@emotion/jest');
expect.extend({ toHaveStyleRule });

// Example usage
import styled from '@emotion/styled';

const Button = styled.button`
  color: hotpink;
`;

// In your test file
expect(Button).toHaveStyleRule('color', 'hotpink');

Other packages similar to @emotion/jest

jest-styled-components

jest-styled-components provides similar functionality for styled-components. It offers custom Jest matchers to test styled-components, allowing you to check if your components have the correct styles. While @emotion/jest is tailored for Emotion, jest-styled-components is specifically designed for styled-components.

jest-emotion

jest-emotion is another package that provides Jest matchers for Emotion. It is similar to @emotion/jest but may offer different or additional matchers. Both packages serve the same purpose of testing Emotion styles, but they might have slight differences in their APIs or features.

README

@emotion/jest

Jest testing utilities for emotion

Installation

npm install --save-dev @emotion/jest

Snapshot Serializer

The easiest way to test React components with emotion is with the snapshot serializer. You can register the serializer via the snapshotSerializers configuration property in your jest configuration like so:

// jest.config.js
module.exports = {
  // ... other config
  snapshotSerializers: [
    '@emotion/jest/serializer' /* if needed other snapshotSerializers should go here */
  ]
}

To assist with shallow rendering, there's a custom enzyme snapshot serializer, that includes the enzyme-to-json serializer, which is available by importing @emotion/jest/enzyme-serializer. If you already have the enzyme-to-json serializer added to snapshotSerializers, it will need to be removed to allow this to work.

// jest.config.js
module.exports = {
  // ... other config
  snapshotSerializers: ['@emotion/jest/enzyme-serializer']
}

Or you can add the serializer via the expect.addSnapshotSerializer method like so: (the example below is with react-test-renderer but @emotion/jest also works with enzyme and react-testing-library)

import React from 'react'
import renderer from 'react-test-renderer'
import { createSerializer } from '@emotion/jest'
import styled from '@emotion/styled'

expect.addSnapshotSerializer(createSerializer())

test('renders with correct styles', () => {
  const H1 = styled.h1`
    float: left;
  `

  const tree = renderer.create(<H1>hello world</H1>).toJSON()

  expect(tree).toMatchSnapshot()
})

Refer to the testing doc for more information about snapshot testing with emotion.

Options

classNameReplacer

@emotion/jest's snapshot serializer replaces the hashes in class names with an index so that things like whitespace changes won't break snapshots. It optionally accepts a custom class name replacer, it defaults to the below.

function classNameReplacer(className, index) {
  return `emotion-${index}`
}

import { createSerializer } from '@emotion/jest'

expect.addSnapshotSerializer(
  createSerializer({
    classNameReplacer(className, index) {
      return `my-new-class-name-${index}`
    }
  })
)

DOMElements

@emotion/jest's snapshot serializer inserts styles and replaces class names in both React and DOM elements. If you would like to disable this behavior for DOM elements, you can do so by passing { DOMElements: false }. For example:

import { createSerializer } from '@emotion/jest'

// configures @emotion/jest to ignore DOM elements
expect.addSnapshotSerializer(createSerializer({ DOMElements: false }))

Custom matchers

toHaveStyleRule

To make more explicit assertions when testing your styled components you can use the toHaveStyleRule matcher.

import React from 'react'
import renderer from 'react-test-renderer'
import { matchers } from '@emotion/jest'
import styled from '@emotion/styled'

// Add the custom matchers provided by '@emotion/jest'
expect.extend(matchers)

test('renders with correct styles', () => {
  const Svg = styled('svg')`
    width: 100%;
  `

  const Div = styled('div')`
    float: left;
    height: 80%;
    &:hover {
      width: 50px;
    }
    ${Svg} {
      fill: green;
    }
    span {
      color: yellow;
    }
    @media screen and (max-width: 1200px) {
      font-size: 14px;
    }
  `

  const tree = renderer
    .create(
      <Div>
        <Svg />
        <span>Test</span>
      </Div>
    )
    .toJSON()

  expect(tree).toHaveStyleRule('float', 'left')
  expect(tree).not.toHaveStyleRule('height', '100%')
})

You can provide additional options for toHaveStyleRule matcher. target - helps to specify css selector or other component where style rule should be found.

expect(tree).toHaveStyleRule('width', '50px', { target: ':hover' })

expect(tree).toHaveStyleRule('color', 'yellow', { target: 'span' })

expect(tree).toHaveStyleRule('fill', 'green', { target: `${Svg}` })

media - specifies the media rule where the matcher should look for the style property.

expect(tree).toHaveStyleRule('font-size', '14px', {
  media: 'screen and (max-width: 1200px)'
})

Use media and target options to assert on rules within media queries and to target nested components, pseudo-classes, and pseudo-elements.

import React from 'react'
import renderer from 'react-test-renderer'
import { matchers } from '@emotion/jest'
import styled from '@emotion/styled'

// Add the custom matchers provided by '@emotion/jest'
expect.extend(matchers)

test('renders with correct link styles', () => {
  const Container = styled.div`
    font-size: 14px;

    a {
      color: yellow;
    }

    a:hover {
      color: black;
    }

    @media (min-width: 768px) {
      font-size: 16px;
    }
  `

  const tree = renderer.create(<Container>hello world</Container>).toJSON()

  expect(tree).toHaveStyleRule('color', 'yellow', { target: /a$/ })
  expect(tree).toHaveStyleRule('color', 'black', { target: 'a:hover' })
  expect(tree).toHaveStyleRule('font-size', '16px', {
    media: '(min-width: 768px)'
  })
})

Thanks

Thanks to Kent C. Dodds who wrote jest-glamor-react which this library is largely based on. ❤️