As more and more developers adopt React as their front-end framework of choice, it becomes increasingly important to ensure that the code we write is of the highest quality. One important aspect of quality assurance is testing, and Jest is a popular testing framework that is often used with React.
Jest is an open-source testing framework that is built by Facebook, and it is designed to make testing your JavaScript code as easy and efficient as possible. It is fast, has an easy-to-use API, and comes with a number of built-in features that make testing a breeze.
In this article, we will guide you through the process of setting up Jest for React testing. We will cover everything you need to know to get started, from installing Jest to writing your first test case.
Jest For React Testing
Jest is a popular testing framework used for testing React applications. It is created by Facebook and is known for its ease of use and quick setup time. Jest is designed to work with React out of the box and provides many features to make testing easier.
Here are some of the features that make Jest a popular choice for testing React applications:
i) Snapshot testing: Jest provides a snapshot testing feature that allows you to compare the current output of a component with a previously stored snapshot. This makes it easy to check if any unintended changes have been made to the component’s output.
ii) Mocking: Jest provides built-in mocking capabilities that allow you to create mock functions and modules. This is useful when you need to simulate the behavior of a function or module that is not readily available or is difficult to test.
iii) Code coverage: Jest provides a code coverage report that shows the percentage of code that is covered by your tests. This is useful for identifying areas of your code that are not tested and may need additional attention.
iv) Asynchronous testing: Jest makes it easy to test asynchronous code by providing utilities for managing async code. This allows you to write tests that wait for promises or other async code to resolve before making assertions.
v) Easy setup: Jest is easy to set up and configure for your React application. It comes with a default configuration that works out of the box, but can be customized to fit your specific needs.
Overall, Jest is a powerful and flexible testing framework that makes it easy to test React applications. Its many features and easy setup make it a popular choice for developers who want to write reliable tests for their React components.
Setting Up Jest For React Testing
There are certain steps that you need to follow to set up Jest for React testing in 2023.
1) Install Jest
Jest can be installed in your project using npm or yarn. Here are the steps to install Jest using npm:
i) Open your terminal or command prompt and navigate to your project directory.
ii) Run the following command to install Jest as a development dependency in your project:
npm install –save-dev jest
This command will install Jest and add it to your project’s package.json file under the devDependencies section.
Once the installation is complete, you can start writing your tests using Jest. You can run your tests using the npm test command. By default, Jest looks for test files that are named with the .test.js or .spec.js extension. However, you can configure Jest to look for tests with other extensions by modifying the testMatch property in your Jest configuration file.
That’s it! You now have Jest installed in your project and can start writing tests for your application.
2) Create A Configuration File
To create a configuration file for Jest, you can create a file named jest.config.js in the root directory of your project. This file will contain the configuration options that Jest should use when running tests.
Here’s an example configuration file:
module.exports = {
// Indicates which file types should be considered as test files.
testMatch: [
‘**/__tests__/**/*.js?(x)’,
‘**/?(*.)+(spec|test).js?(x)’
],
// A list of paths to directories that Jest should use to search for files in.
roots: [‘<rootDir>/src’],
// A map from regular expressions to paths to transformers.
transform: {
‘^.+\\.(js|jsx|ts|tsx)$’: ‘babel-jest’
},
// The glob patterns Jest uses to detect test files.
testPathIgnorePatterns: [
‘<rootDir>/node_modules/’,
‘<rootDir>/build/’
],
// A list of reporter names that Jest uses when writing coverage reports.
coverageReporters: [‘json’, ‘text’, ‘html’],
// An array of file extensions your modules use.
moduleFileExtensions: [‘js’, ‘json’, ‘jsx’]
};
This example configuration file specifies the following options:
a) testMatch: Specifies the pattern used to identify test files.
b) roots: Specifies the directories in which Jest should look for files.
c) transform: Specifies the transformations Jest should apply to files before running tests.
d) testPathIgnorePatterns: Specifies the patterns to ignore when searching for test files.
e) coverageReporters: Specifies the reporters that Jest should use to generate coverage reports.
f) moduleFileExtensions: Specifies the file extensions that Jest should look for when searching for files.
You can modify this configuration file to suit the needs of your project. Once you’ve created your configuration file, Jest will automatically use it when running tests in your project.
3) Write Your Tests
Once you have Jest installed and your configuration file set up, you can start writing tests for your React application. Here are the steps to create a test file and write a test:
i) Create a new file with the extension .test.js or .spec.js. This will be your test file.
ii) In the test file, import the component or function that you want to test:
import { sum } from ‘./myFunctions’;
iii) Write your test. A test is a function that uses the Jest API to check that the component or function behaves as expected:
describe(‘sum’, () => {
test(‘adds 1 + 2 to equal 3’, () => {
expect(sum(1, 2)).toBe(3);
});
});
In this example, we’re testing a simple sum function that adds two numbers. The describe function groups related tests together, while the test function specifies a single test case. The expect function checks that the sum function returns the expected value.
iv) Run your tests by executing the npm test command in your project directory. Jest will search for test files in your project and execute any tests it finds.
You can add as many tests as you need to cover all the functionality of your component or function. Jest provides a wide range of matches and other utilities that you can use to create more complex tests.
By writing tests for your React application, you can ensure that your code works as expected and avoid introducing regressions as you make changes to your codebase.
4) Run Your Tests
To run your tests using Jest, you can use the npm test command in your project directory. Jest will automatically detect and run any test files in your project that match the pattern specified in your Jest configuration file.
When you run npm test, Jest will output the results of your tests in the console. If all tests pass, Jest will display a summary of the test results:
PASS src/components/Example.test.js
✓ renders without crashing (21ms)
Test Suites: 1 passed, 1 total
Tests: 1 passed, 1 total
Snapshots: 0 total
Time: 3.204s, estimated 4s
Ran all test suites.
If any tests fail, Jest will display an error message that describes the failure and the location of the failing test:
FAIL src/components/Example.test.js
- Example component › renders without crashing
TypeError: Cannot read property ‘map’ of undefined
10 | render() {
11 | const { items } = this.props;
> 12 | return items.map(item => <div key={item.id}>{item.name}</div>);
| ^
13 | }
14 | }
15 |
at Example.render (src/components/Example.js:12:17)
at ReactTestRenderer.render (node_modules/react-test-renderer/cjs/react-test-renderer.development.js:1052:32)
at Object.<anonymous> (src/components/Example.test.js:9:16)
Test Suites: 1 failed, 1 total
Tests: 1 failed, 1 total
Snapshots: 0 total
Time: 3.537s, estimated 4s
Ran all test suites.
Jest also provides a range of options that you can use to customize the behavior of your tests, such as running tests in watch mode, generating code coverage reports, and more. You can find more information about these options in the Jest documentation.
Wrapping Up
LambdaTest is a powerful cloud-based testing platform that can be used in conjunction with Jest to test React applications. By leveraging the scalability of LambdaTest’s cloud-based infrastructure, you can quickly and efficiently run your Jest tests on a wide range of browsers and devices, ensuring that your application works as expected across different environments.
In this blog, we covered the steps required to set up Jest for testing React applications. We also learned how to configure Jest to use LambdaTest as a test environment, write tests using Jest, and execute tests on LambdaTest’s cloud-based infrastructure.
Using LambdaTest and Jest together provides a robust testing solution for React developers that can help them catch regressions and bugs early in the development cycle, improving the overall quality of their applications.
So, if you’re looking to streamline your React testing workflow, consider using LambdaTest and Jest together to achieve comprehensive, scalable, and efficient testing for your React applications.