Table of contents

1. More libraries
2. Experiment Subject
3. Experiment Environment
4. Snapshot Testing
   1. Little bit of terminology
   2. Getting Started
   3. Running tests
5. Useful Links

More libraries

To append additional layer of abstraction and make our as developer life easier few extra libraries should be added to project:

• react-test-renderer
• @testing-library/react
• @testing-library/jest-dom
• @testing-library/user-event

Open root of project in your terminal and invoke following command

npm:

npm install -D react-test-renderer @testing-li
brary/react @testing-library/jest-dom @testing-library/user-event

yarn:

yarn add -D react-test-renderer @testing-li
brary/react @testing-library/jest-dom @testing-library/user-event

Experiment Subject

Before starting performing tests component is needed. To save time contents of mock component is provided below:

component.jsx

import React from 'react';

export default function TestComponent() {
    return (
        <div>
            Test Component
        </div>
    );
};

Experiment Environment

In the same folder create component.test.jsx. This file will contain all tests written in this post

import React from 'react';
import TestComponent from './component';

describe('TestComponent', () => {

});

Snapshot Testing

Little bit of terminology

Jest Documentation on Snapshot testing

The first time this test is run, Jest creates a snapshot file. On subsequent test runs, Jest will compare the rendered output with the previous snapshot. If they match, the test will pass. If they don't match, either the test runner found a bug in your code that should be fixed, or the implementation has changed and the snapshot needs to be updated

Getting Started

Exactly for purpose of snapshot testing react-test-renderer was installed

1 - Import function create from react-test-renderer into component.test.jsx

import { create } from 'react-test-renderer';

2 - Create test suite

it('should match snapshot', () => {

    });

3 - Populate test suite

expect(
            create(<TestComponent />)
                .toJSON()
        )
            .toMatchSnapshot();

What happens in test suite we created:

1. Rendering component using function create and passing JSX component as argument
2. On result of calling create calling method toJSON to serialise output
3. Passing serialised value to expect function and calling matcher toMatchSnapshot

Jest will process snapshot itself, no additional input from developer is needed

Running tests

Now try to invoke script test

If you did everything correctly output should be nearly the same. In short, since snapshot didn't exist Jest saved it inside of __snapshots__ directory next to test file and passed test. Run test again to see difference

Jest used existing snapshot for comparison. Now make some changes in component, e.g. change text in it and run script test again

Since content that is being rendered has been changed snapshot test had failed. That is exactly the purpose of snapshot tests: track down changes/errors in render process/result

Now, if we know that received change is correct, we need to update snapshot accordingly. To do that just call script test with argument -u

npm:

npm run test -u

yarn:

yarn test -u

Here is the result

That is everything you need to know to start with snapshot testing and apply it ещ your components

Useful Links

• Jest Documentation on Snapshot testing
• React Test Renderer Documentation

Next time will share on how to perform more granular unit testing of components, difference between testing functional and class components and how to test react hooks