Testing is one of the processes to make sure that the functionality created in any application is working in accordance with the business logic and coding specification. React recommends React testing library to test React components and jest test runner to run the test. The react-testing-library allows the components to be checked in isolation.

It can be installed in the application using below command −

npm install --save @testing-library/react @testing-library/jest-dom

Create React app

Create React app configures React testing library and jest test runner by default. So, testing a React application created using Create React App is just a command away.

cd /go/to/react/application 
npm test

The npm test command is similar to npm build command. Both re-compiles as and when the developer changes the code. Once the command is executed in the command prompt, it emits below questions.

No tests found related to files changed since last commit.
Press `a` to run all tests, or run Jest with `--watchAll`.

Watch Usage
   › Press a to run all tests.
   › Press f to run only failed tests.
   › Press q to quit watch mode.
   › Press p to filter by a filename regex pattern.
   › Press t to filter by a test name regex pattern.
   › Press Enter to trigger a test run.  

Pressing a will try to run all the test script and finally summaries the result as shown below −

Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        4.312 s, estimated 12 s
Ran all test suites.

Watch Usage: Press w to show more. 

Testing in a custom application

Let us write a custom React application using Rollup bundler and test it using React testing library and jest test runner in this chapter.

First, create a new react application, react-test-app using Rollup bundler by following instruction in Creating a React application chapter.

Next, install the testing library.

cd /go/to/react-test-app 
npm install --save @testing-library/react @testing-library/jest-dom

Next, open the application in your favorite editor.

Next, create a file, HelloWorld.test.js under src/components folder to write test for HelloWorld component and start editing.

Next, import react library.

import React from ''react'';

Next, import the testing library.

import { render, screen } from ''@testing-library/react''; import ''@testing-library/jest-dom'';

Next, import our HelloWorld component.

import HelloWorld from ''./HelloWorld'';

Next, write a test to check the existence of Hello World text in the document.

test(''test scenario 1'', () => {
   render(<HelloWorld />);
   const element = screen.getByText(/Hello World/i);
   expect(element).toBeInTheDocument();
});

The complete source code of the test code is given below −

import React from ''react'';
import { render, screen } from ''@testing-library/react'';
import ''@testing-library/jest-dom'';
import HelloWorld from ''./HelloWorld'';

test(''test scenario 1'', () => {
   render(<HelloWorld />);
   const element = screen.getByText(/Hello World/i);
   expect(element).toBeInTheDocument();
});

Next, install jest test runner, if it is not installed already in the system.

npm install jest -g

Next, run jest command in the root folder of the application.

jest

Next, run jest command in the root folder of the application.

PASS  src/components/HelloWorld.test.js
   √ test scenario 1 (29 ms)

Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        5.148 s
Ran all test suites.