When it comes to selecting a unit testing framework for JavaScript, Jest stands out as a leading contender for the top position. Originally created by Facebook with a specific emphasis on testing React applications, Jest has gained immense popularity and widespread usage in the development community.
Initially designed to cater to the testing needs of React components, Jest quickly became the go-to choice for testing React-based projects. Its seamless integration with React’s testing ecosystem, including features like snapshot testing and shallow rendering, made it highly appealing to developers working on React applications.
As Jest continued to evolve and improve, its capabilities extended beyond React and found utility in testing other JavaScript applications as well. Developers recognized the simplicity, reliability, and powerful features offered by Jest, leading to its adoption for testing front-end JavaScript applications across various frameworks and libraries.
What is the Jest Framework?
It offers a simplified approach to front-end unit testing, reducing the complexity and configuration typically required for such tests.
One of Jest’s key strengths is its ability to validate various aspects of JavaScript, including the rendering of web applications in browsers. This makes it a popular choice for automated browser testing, positioning it as one of the leading testing frameworks in the JavaScript ecosystem.
Jest provides a comprehensive package that includes an assertion library, a test runner, and a built-in mocking library. This combination of features simplifies the testing process and enhances productivity for developers.
The framework’s simplicity is particularly notable, making it well-suited for testing JavaScript library projects such as AngularJS, Vue.js, Node.js, Babel, and TypeScript. Jest’s ease of use and broad compatibility contribute to its widespread adoption and popularity among JavaScript developers.
Why should you use Jest?
- Speed and Performance: Jest is known for its fast execution of test cases. Its optimized design and efficient test runner enable speedy testing, allowing developers to run their tests quickly and obtain results in a timely manner. This can significantly improve the overall testing process and save valuable time during development.
- Pre-commit Hooks: Jest provides the convenience of pre-commit hooks, which allow developers to run specific sets of test cases that are relevant to the current changes or have been modified since the last commit. This feature ensures that only the necessary tests are executed, reducing unnecessary testing overhead and enhancing productivity.
- Easy Migration: Jest offers a code mods module that simplifies the process of migrating existing test projects to the Jest framework. This module enables seamless migration and switching of test projects to Jest programmatically, without any compatibility issues or extensive code modifications. This ease of migration makes it convenient for teams to adopt Jest without significant disruptions to their existing testing workflows.
- Feature-rich and Configurable: Jest is not only a reliable test runner but also provides a range of advanced features and configurations. It includes capabilities such as automatic module mocking, coverage threshold setup, and module mappers. These features allow developers to customize their testing environment according to their specific requirements. Jest also supports running multiple projects within the same runner and offers custom resolvers like Babel and Webpack, enhancing its flexibility and adaptability.
Prerequisites and installations
To begin writing Selenium test automation scripts with Jest, certain prerequisites and installations need to be completed. The following libraries and packages must be installed on the system to execute Jest test scripts:
- NodeJS and Node Package Manager (npm): NodeJS can be installed either through the npm manager or by directly downloading the Windows Installer binary from the official nodejs.org website. This provides the runtime environment for executing JavaScript code.
- Selenium WebDriver: Selenium WebDriver is a dependency for the Jest module and is required for interacting with web browsers.
- Jest Module: The Jest module needs to be installed using npm as a development dependency. This module provides the testing framework for writing and executing tests.
- Java Development Kit (JDK): Since Jest is a Selenium test framework and Selenium is built on Java, the Java Development Kit must be installed on the system. This involves installing the JDK and configuring the system with the JAVA environment.
- Browser Driver: To interact with web browsers, a suitable browser driver needs to be installed. For example, to use Chrome as the browser, install the Chromedriver by running the following npm command.
Once these installations and configurations are complete, testers can execute commands in the command line using the “jest” keyword along with the Java SDK.
Jest utilizes Selenium as a test framework, and Selenium is based on Java. With the necessary setup in place, testers can proceed to write and run Selenium test scripts using Jest for efficient web automation testing.
Setting up Jest
To begin with, there are several steps involved in installing and configuring Jest. Follow the instructions below:
- Create a new directory and navigate to that directory using the command line:
bash
- Set up the NPM environment by initializing a new package.json file:
- Install Jest as a development dependency using npm
- Modify the package.json file to configure the NPM environment to use Jest. Add the following code snippet to the “scripts” section:
This modification allows you to run the tests using the command npm test.
Identifying Desired Actions
Before writing tests, it is crucial to define the expected behavior and outcomes for the functions you will be testing.
Writing Tests
In general, tests consist of the following components:
- A describe function that takes two arguments:
- A string describing the test block, which appears in the terminal when the tests are run.
- A callback function that includes the individual tests.
- One or more test functions that take two arguments:
- A string describing the specific test action.
- A callback function that consists of an expect function and a matcher function.
- Chained to the matcher, the expect function takes the function invocation being tested and describes the expected results.
Running the Tests
When running the tests, they will initially fail because the functions have not been implemented yet. To execute the tests, run the command npm test, which matches the script defined in the package.json file.
Writing the Functions
To implement the logic for our functions, follow these steps:
- Create a new file in the /jest-example directory called exampleFunctions.js. The filename should match the name used in the test file, without the .test extension.
- Open the exampleFunctions.js file and define the two functions you want to test. Make sure to export these functions so that the test file can access them.
- Save the file and run the tests again. You will notice that all four tests still fail, but now Jest provides a more informative message about what went wrong.
Advantages of Jest
Jest is a highly advantageous framework for JavaScript automation testing, offering numerous benefits that have made it a preferred choice among developers.
- Zero Configuration
One major advantage of Jest is its zero-configuration setup. It comes pre-configured with essential features for unit testing, such as code coverage and test execution reports. This eliminates the need for extensive configuration and allows developers to quickly start testing their code.
- Snapshots
Jest’s snapshot testing feature enables easy comparison of current test results with previously saved snapshots. This simplifies the detection of unexpected changes and reduces the effort required for manual testing, saving valuable time during the development process.
- Fast and Reliable
Jest excels in speed and reliability. It supports parallel test execution, significantly reducing the overall test execution time. Failed tests are prioritized, ensuring they are addressed promptly, while the remaining tests are intelligently queued based on their execution time. This efficient approach enhances testing productivity and reliability.
- Comprehensive Documentation and Support
Jest offers comprehensive documentation with practical examples, making it easy for developers to understand and utilize the framework effectively. Moreover, the Jest community is vibrant and actively provides support through platforms like Discord. This active community ensures developers can quickly troubleshoot issues and find solutions.
- Effortless Mocking
Jest simplifies the process of mocking objects that are out of scope, enabling developers to perform more effective testing. It provides a Mock Functions API that allows for spying on function calls, enabling efficient testing and verification of function behavior.
- Code Coverage Analysis
Jest provides an effortless way to analyze code coverage for tests. With the simple “–coverage” command, developers can obtain code coverage reports without any additional configuration. This enables them to identify untested cases and areas that require further testing, ensuring comprehensive test coverage.
Limitations
Jest, like any other testing framework, has certain limitations that developers should be aware of. These limitations include:
Learning Curve
For developers who are accustomed to using other testing libraries, learning Jest may pose a challenge. Jest has its own set of features, functions, and syntax, which may require time and effort to become familiar with. This can result in a slower learning curve and initial difficulty in writing Jest tests.
IDE Compatibility
Jest may not be fully compatible with all integrated development environments (IDEs). This means that developers using unsupported IDEs may not have access to powerful features like IntelliSense and auto-import. This can be a disadvantage for developers who rely heavily on these features to enhance their workflow.
Library and Tooling Support
Compared to other testing frameworks, Jest may have limited support for libraries and tools. Developers who require a wide range of libraries and tools for testing their code may find that Jest does not offer the same level of support as other frameworks. This can restrict the options available for testing and hinder the flexibility of the testing process.
Auto-Mocking Performance
Jest’s auto-mocking feature, while useful in certain scenarios, can impact the performance of the testing process. Jest automatically creates mock versions of dependencies, which can be time-consuming to set up and run. This can result in slower test execution compared to other testing libraries.
To overcome some of these limitations, Jest cloud grids like LambdaTest provide the ability to scale Jest testing. LambdaTest offers the option to automate cross-browser testing on a browser farm with over 40 browsers and operating systems. This scalability improves test coverage and product quality.
By visiting the LambdaTest platform and signing up, developers can run their Jest automation testing scripts online. LambdaTest provides free 100 automation testing minutes upon sign-up, along with additional benefits such as manual cross-browser testing and responsive testing.
Conclusion
In this article, we have covered the fundamental concepts of the Jest testing framework. We began by explaining the process of installing Jest and demonstrated its usage for testing JavaScript files, focusing on simplicity. We emphasized the significance of testing in web application development and how Jest aids developers in accomplishing this objective.
Jest aims to simplify the process of writing and running tests for JavaScript code. Serving as a test runner, Jest provides developers with a robust set of features that make test creation, execution, and organization seamless and user-friendly.
Also, Read Best Practices for Writing Reliable and Maintainable Selenium-Mocha Tests.
