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Custom Jest matcher for axe for testing accessibility

⚠️✋ This project does not guarantee that what you build is accessible.

The GDS Accessibility team found that around ~30% of access barriers are missed by automated testing.

Tools like axe are similar to code linters such as eslint or stylelint: they can find common issues but cannot guarantee that what you build works for users.

You'll also need to:

Checks that do not work in jest-axe

Color contrast checks do not work in JSDOM so are turned off in jest-axe.

Installation:

npm install --save-dev jest jest-axe jest-environment-jsdom

TypeScript users can install the community maintained types package:

npm install --save-dev @types/jest-axe

Usage:

/**
 * @jest-environment jsdom
 */
const { axe, toHaveNoViolations } = require('jest-axe')

expect.extend(toHaveNoViolations)

it('should demonstrate this matcher`s usage', async () => {
  const render = () => '<img src="#"/>'

  // pass anything that outputs html to axe
  const html = render()

  expect(await axe(html)).toHaveNoViolations()
})

Screenshot of the resulting output from the usage example

Note, you can also require 'jest-axe/extend-expect' which will call expect.extend for you. This is especially helpful when using the jest setupFilesAfterEnv configuration.

Testing React

const React = require('react')
const { render } =  require('react-dom')
const App = require('./app')

const { axe, toHaveNoViolations } = require('jest-axe')
expect.extend(toHaveNoViolations)

it('should demonstrate this matcher`s usage with react', async () => {
  render(<App/>, document.body)
  const results = await axe(document.body)
  expect(results).toHaveNoViolations()
})

Testing React with React Testing Library

const React = require('react')
const App = require('./app')

const { render } = require('@testing-library/react')
const { axe, toHaveNoViolations } = require('jest-axe')
expect.extend(toHaveNoViolations)

it('should demonstrate this matcher`s usage with react testing library', async () => {
  const { container } = render(<App/>)
  const results = await axe(container)
  
  expect(results).toHaveNoViolations()
})

Note: If you're using react testing library <9.0.0 you should be using the cleanup method. This method removes the rendered application from the DOM and ensures a clean HTML Document for further testing.

If you're using React Portals, use the baseElement instead of container:

it('should work with React Portals as well', async () => {
  const { baseElement } = render(<App/>)
  const results = await axe(baseElement)
  
  expect(results).toHaveNoViolations()
})

Testing Vue with Vue Test Utils

const App = require('./App.vue')

const { mount } = require('@vue/test-utils')
const { axe, toHaveNoViolations } = require('jest-axe')
expect.extend(toHaveNoViolations)

it('should demonstrate this matcher`s usage with vue test utils', async () => {
  const wrapper = mount(Image)
  const results = await axe(wrapper.element)

  expect(results).toHaveNoViolations()
})

Testing Vue with Vue Testing Library

const App = require('./app')

const { render } = require('@testing-library/vue')
const { axe, toHaveNoViolations } = require('jest-axe')
expect.extend(toHaveNoViolations)

it('should demonstrate this matcher`s usage with react testing library', async () => {
  const { container } = render(<App/>)
  const results = await axe(container)
  
  expect(results).toHaveNoViolations()
})

Note: If you're using vue testing library <3.0.0 you should be using the cleanup method. This method removes the rendered application from the DOM and ensures a clean HTML Document for further testing.

Testing Angular with Nx

import { ComponentFixture, TestBed } from "@angular/core/testing";
import { axe } from "jest-axe";

import { SomeComponent } from "./some.component";

describe("SomeComponent", () => {
  let fixture: ComponentFixture<SomeComponent>;

  beforeEach(() => {
    TestBed.configureTestingModule({
      declarations: [SomeComponent],
    });

    fixture = TestBed.createComponent(SomeComponent);
  });

  it("should create", async () => {
    const results = await axe(fixture.nativeElement);
    expect(results).toHaveNoViolations();
  });
});

Note: You may need to extend jest by importing jest-axe/extend-expect at test-setup.ts

Usage with jest.useFakeTimers() or mocking setTimeout

thrown: "Exceeded timeout of 5000 ms for a test. Use jest.setTimeout(newTimeout) to increase the timeout value, if this is a long-running test."

aXe core does not work when timers (setTimeout) are mocked. When using jest.useFakeTimers() aXe core will timeout often causing failing tests.

We recommend renabling the timers temporarily for aXe:

jest.useRealTimers();
const results = await axe(wrapper.element);
jest.useFakeTimers();

Axe configuration

The axe function allows options to be set with the same options as documented in axe-core:

const { axe, toHaveNoViolations } = require('jest-axe')

expect.extend(toHaveNoViolations)

it('should demonstrate this matcher`s usage with a custom config', async () => {
  const render = () => `
    <div>
      <img src="#"/>
    </div>
  `

  // pass anything that outputs html to axe
  const html = render()

  const results = await axe(html, {
    rules: {
      // for demonstration only, don't disable rules that need fixing.
      'image-alt': { enabled: false }
    }
  })

  expect(results).toHaveNoViolations()
})

Testing isolated components

All page content must be contained by landmarks (region)

When testing with aXe sometimes it assumes you are testing a page. This then results in unexpected violations for landmarks for testing isolation components.

You can disable this behaviour with the region rule:

const { configureAxe } = require('jest-axe')

const axe = configureAxe({
  rules: {
    // disable landmark rules when testing isolated components.
    'region': { enabled: false }
  }
})

Setting global configuration

If you find yourself repeating the same options multiple times, you can export a version of the axe function with defaults set.

Note: You can still pass additional options to this new instance; they will be merged with the defaults.

This could be done in Jest's setup step

// Global helper file (axe-helper.js)
const { configureAxe } = require('jest-axe')

const axe = configureAxe({
  rules: {
    // for demonstration only, don't disable rules that need fixing.
    'image-alt': { enabled: false }
  }
})

module.exports = axe

// Individual test file (test.js)
const { toHaveNoViolations } = require('jest-axe')
const axe = require('./axe-helper.js')

expect.extend(toHaveNoViolations)

it('should demonstrate this matcher`s usage with a default config', async () => {
  const render = () => `
    <div>
      <img src="#"/>
    </div>
  `

  // pass anything that outputs html to axe
  const html = render()

  expect(await axe(html)).toHaveNoViolations()
})

Setting custom rules and checks.

The configuration object passed to configureAxe, accepts a globalOptions property to configure the format of the data used by axe and to add custom checks and rules. The property value is the same as the parameter passed to axe.configure.

// Global helper file (axe-helper.js)
const { configureAxe } = require('jest-axe')

const axe = configureAxe({
  globalOptions: {
    checks: [/* custom checks definitions */]
  },
  // ...
})

module.exports = axe

Setting the level of user impact.

An array which defines which impact level should be considered. This ensures that only violations with a specific impact on the user are considered. The level of impact can be "minor", "moderate", "serious", or "critical".

// Global helper file (axe-helper.js)
const { configureAxe } = require('jest-axe')

const axe = configureAxe({
  impactLevels: ['critical'],
  // ...
})

module.exports = axe

Refer to Developing Axe-core Rules for instructions on how to develop custom rules and checks.

Thanks