Writing tests

In this guide, we'll walk through writing test for our custom Strong component in Storybook.

HSDS uses Jest and Enzyme for testing.


All of HSDS's component test files are scoped in the same directory as the component, example:

└── components/
└── Button/
└── __tests__/
└── Button.test.js

Initial files

The first thing we'll need to do is create a dedicated __tests__ directory under Strong/:

└── components/
└── Strong/
└── __tests__/

Under __tests__/, we'll need to create the main Strong.test.js file:

└── components/
└── Strong/
└── __tests__/
└── Strong.test.js

Base test code

In our __tests__/Strong.test.js file, we'll need to add:

import React from 'react'
import { mount } from 'enzyme'
import Strong from '../Strong'
describe('classNames', () => {
test('Has default className', () => {
const wrapper = mount(<Strong />)
const el = wrapper.find('Strong')


HSDS favours mount rendering vs. shallow rendering for testing. The benefits are many. The only downside is mount is slightly slower compared to shallow.

Test development

For test development, open up your terminal and run the following command:

npm run dev

This fires up Jest in watch mode, and runs tests against modified files (and their associated files).

Code coverage

Coverage report, generated by Jest

To check code coverage, run the following command:

npm run test

This runs through the entire Jest test suite, and generates a coverage report under:

└── coverage/
└── lcov-report/
└── index.html

You can open the index.html in your browser to view the full report.

Since Aug 31, 2017, HSDS has maintained 100% code coverage for tests. Because HSDS is a component library (rather than an App), it's testing coverage is stricter. Everything must be tested. There are no exceptions... Except...


...the strange or untestable cases within the JSDOM environment (which Jest runs on). (lol). Many DOM APIs are not supported in JSDOM (e.g. height/width calculations, scrolling, positioning, etc...).

For anything that Jest/JSDOM cannot catch, use istanbul ignore to skip coverage checking.

Fabulous 🤩! Strong is now super reliable, thanks to tests!


Let's write some documentation to make sure other folks know how to use Strong.

See also