Testing React components with Jest and Enzyme

October 2017: the article was updated to React 16 and Enzyme 3.

Some people say that testing React components is useless and in many cases it is, but there are a few cases when I think it’s useful:

• component libraries,
• open source projects,
• integration with 3rd party components,
• bugs, to prevent regressions.

I’ve tried many tools and finally have found a combination that I like enough to suggest to other developers:

• Jest, a test runner;
• Enzyme, a testing utility for React;
• enzyme-to-json to convert Enzyme wrappers for Jest snapshot matcher.

For the most of my tests I use shallow rendering with Jest snapshots.

Snapshot testing in Jest

Shallow rendering

Shallow rendering renders only component itself without its children. So if you change something in a child component it won’t change shallow output of your component. Or a bug, introduced to a child component, won’t break your component’s test. It also doesn’t require DOM.

For example this component:

const ButtonWithIcon = ({icon, children}) => (<button><Icon icon={icon} />{children}</button>);

Will be rendered by React like this:

<button><i class="icon icon_coffee"></i>Hello Jest!</button>

But like this with shallow rendering:

<button><Icon icon="coffee" />Hello Jest!</button>

Note that the Icon component was not rendered.

Snapshot testing

Jest snapshots are like those old text UIs with windows and buttons made of text characters: it’s a rendered output of your component stored in a text file.

You tell Jest that you want to be sure that output of this component should never change accidentally and Jest saves it to a file that looks like this:

exports[`test should render a label 1`] = `<labelclassName="isBlock">Hello Jest!</label>`;

exports[`test should render a small label 1`] = `<labelclassName="isBlock isSmall">Hello Jest!</label>`;

Every time you change your markup Jest will show you a diff and ask you to update a snapshot if the change was intended.

Jest stores snapshots besides your tests in files like __snapshots__/Label.spec.js.snap and you need to commit them with your code.

Why Jest

• Very fast.
• Snapshot testing.
• Awesome interactive watch mode that reruns only tests that are relevant to your changes.
• Helpful fail messages.
• Simple configuration.
• Mocks and spies.
• Coverage report with a single command line switch.
• Active development.
• Impossible to write silently wrong asserts like expect(foo).to.be.a.function instead of expect(foo).to.be.a(‘function’) in Chai because it’s the only natural thing to write after (correct) expect(foo).to.be.true.

Why Enzyme

• Convenient utilities to work with shallow rendering, static rendered markup or DOM rendering.
• jQuery-like API to find elements, read props, etc.

Setting up

First install all the dependencies including peer dependencies:

npm install --save-dev jest react-test-renderer enzyme enzyme-adapter-react-16 enzyme-to-json

You’ll also need babel-jest for Babel and ts-jest for TypeScript.

Update your package.json:

"scripts": {"test": "jest","test:watch": "jest --watch","test:coverage": "jest --coverage"},"jest": {"setupFiles": ["./test/jestsetup.js"],"snapshotSerializers": ["enzyme-to-json/serializer"]}

snapshotSerializers allows you to pass Enzyme wrappers directly to Jest’s snapshot matcher, without converting them manually by calling enzyme-to-json’s toJson function.

Create a test/jestsetup.js file to customize Jest environment (see setupFiles above):

import Enzyme, { shallow, render, mount } from 'enzyme';import Adapter from 'enzyme-adapter-react-16';

// React 16 Enzyme adapterEnzyme.configure({ adapter: new Adapter() });

// Make Enzyme functions available in all test files without importingglobal.shallow = shallow;global.render = render;global.mount = mount;

For CSS Modules also add to jest section in your package.json:

"jest": {"moduleNameMapper": {"^.+\\.(css|scss)$": "identity-obj-proxy"}}

And run:

npm install --save-dev identity-obj-proxy

Note that identity-obj-proxy requires node — harmony-proxies flag for Node 4 and 5.

Writing tests

Testing basic component rendering

That’s enough for most non-interactive components:

test('render a label', () => {const wrapper = shallow(<Label>Hello Jest!</Label>);expect(wrapper).toMatchSnapshot();});

test('render a small label', () => {const wrapper = shallow(<Label small>Hello Jest!</Label>);expect(wrapper).toMatchSnapshot();});

test('render a grayish label', () => {const wrapper = shallow(<Label light>Hello Jest!</Label>);expect(wrapper).toMatchSnapshot();});

Testing props

Sometimes you want to be more explicit and see real values in tests. In that case use Enzyme API with regular Jest assertions:

test('render a document title', () => {const wrapper = shallow(<DocumentTitle title="Events" />);expect(wrapper.prop('title')).toEqual('Events');});

test('render a document title and a parent title', () => {const wrapper = shallow(<DocumentTitle title="Events" parent="Event Radar" />);expect(wrapper.prop('title')).toEqual('Events — Event Radar');});

In some cases you just can’t use snapshots. For example if you have random IDs or something like that:

test('render a popover with a random ID', () => {const wrapper = shallow(<Popover>Hello Jest!</Popover>);expect(wrapper.prop('id')).toMatch(/Popover\d+/);});

Testing events

You can simulate an event like click or change and then compare component to a snapshot:

test('render Markdown in preview mode', () => {const wrapper = shallow(<MarkdownEditor value="*Hello* Jest!" />);

expect(wrapper).toMatchSnapshot();  

wrapper.find('\[name="toggle-preview"\]').simulate('click');  

expect(wrapper).toMatchSnapshot();  

});

Sometimes you want to interact with an element in a child component to test effect in your component. For that you need a proper DOM rendering with Enzyme’s mount method:

test('open a code editor', () => {const wrapper = mount(<Playground code={code} />);

expect(wrapper.find('.ReactCodeMirror')).toHaveLength(0);  

wrapper.find('button').simulate('click');  

expect(wrapper.find('.ReactCodeMirror')).toHaveLength(1);  

});

Testing event handlers

Similar to events testing but instead of testing component’s rendered output with a snapshot use Jest’s mock function to test an event handler itself:

test('pass a selected value to the onChange handler', () => {const value = '2';const onChange = jest.fn();const wrapper = shallow(<Select items={ITEMS} onChange={onChange} />);

expect(wrapper).toMatchSnapshot();  

    wrapper.find('select').simulate('change', {  
    target: { value },  
});  

expect(onChange).toBeCalledWith(value);  

});

Not only JSX

Jest snapshots work with JSON so you can test any function that returns JSON the same way you test your components:

test('accept custom properties', () => {const wrapper = shallow(<LayoutflexBasis={0}flexGrow={1}flexShrink={1}flexWrap="wrap"justifyContent="flex-end"alignContent="center"alignItems="center"/>);expect(wrapper.prop('style')).toMatchSnapshot();});

Debugging and troubleshooting

Debugging shallow renderer output

Use Enzyme’s debug method to print shallow renderer’s output:

const wrapper = shallow(/*~*/);console.log(wrapper.debug());

Failing tests with enabled coverage

When your tests fail with — coverage flag with diff like this:

-<Button+<Component

Try to replace arrow function component with regular function:

- export default const Button = ({ children }) => {+ export default function Button({ children }) {

requestAnimationFrame error

You may see an error like this when you run your tests:

console.error node_modules/fbjs/lib/warning.js:42  Warning: React depends on requestAnimationFrame. Make sure that you load a polyfill in older browsers. http://fb.me/react-polyfills

React 16 depends on [requestAnimationFrame](https://reactjs.org/docs/javascript-environment-requirements.html), so you need to add a polyfill to your tests:

// test/jestsetup.jsimport 'raf/polyfill';

Resources

• Jest cheat sheet
• by Max Stoiber
• Migrating to Jest by Kent C. Dodds
• Migrating Ava to Jest by Jason Brown

Thanks to Chris Pojer, Max Stoiber and Anna Gerus for proofreading and comments.

P. S. Check out my open source project: React Styleguidist, a component style guide generator with hot reloaded dev server.

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