Jest - The JavaScript Testing Framework - Introduction

What Is Jest

What Is Jest?

Jest is one of the most popular JavaScript testing frameworks. One of the strongest pillars of its foundation is providing an easy-to-use infrastructure for performing unit testing on react-native and react-based applications. The prime reasons behind its popularity are -

  • It provides immense simplicity in performing complex unit tests.
  • It makes the unit tests less time-consuming.
  • JavaScript is the most popular language used in web development. Jest allows you to validate everything that is developed by using JavaScript.
  • It comes with a complete package of a built-in mocking library, an assertion library, and a test runner.

Hence, if you are planning to efficiently perform unit tests on both the front and back end of your JavaScript-based applications, Jest can be the best-suited testing framework for you.

Evidently, there are a number of testing frameworks available for testing JavaScript-based applications, but why has Jest gained such immense popularity? That’s what you can watch below -

  • Jest was initially developed by Facebook to mainly test react applications. Continuous development and upgradation have currently made it a remarkably effective framework for testing web applications that are built using various JavaScript libraries such as Node JS, Angular JS, Vue JS, Babel, and Typescript.
  • Preciseness and accuracy are the strongest pillars of Jest’s capabilities. It allows you to write tests using an extremely useful & familiar API that lets you get the results quickly for the code written in it.
  • It makes you experience very fast and safe operations by running the tests in parallel after ensuring that they have a global state. Also, Jest decides the order of the tests by running the previously failed tests first and then reorganizing the next tests based on how long the test files are.
  • While using Jest, programmers get access to a very well-documented toolkit that enables them to parallely run the tests in their processes and achieve a remarkable pace.
  • Jest does not make you rely on any third-party tool to access its functionalities. It is significantly easy to install, set up, and run.
  • While Jest is mainly popular for the ease it provides in performing unit tests, it can easily be extended to include integration tests too.

Features or Advantages of Jest

  • Easy Installation and Setup: The first quality of any popular tool is always how easy it is to start using it. And, as we mentioned before, Jest has an extremely simple installation and setup process, which moves like the following -
  • It can be directly installed using the npm (Node Package Manager) or yarn library. You can use the ‘-g’ flag with an npm command line to install the Jest module and similarly to use the Jest command without requiring to set up the package files for various npm tests.

Jest allows you to use the numerous modules it has, and exempts you from the hassle of following the long traditional methods of creating projects.

  • High Performance and Speed: You already know that Jest is known for the pace and safety it provides in its operations. If your system properly performs, Jest can execute all your test cases efficiently without any delay. Jest is capable of saving a significant amount of time when your tests depend on the performance of your CPU.
    Jest follows the below strategy to achieve the high performance and speed that we are talking about.
  • The most time-consuming tests are executed first. It leads to the maximum utilization of all cores.
  • Parallel execution of test cases is a highly efficient step that enables you to perform multiple tests simultaneously against different configurations. Despite relying on automation, this method lets you have a bit of control over the execution of the test cases.
  • Jest is capable of catching babel transforms. (Babel is an open-source and free trans compiler of JavaScript)
  • Reducing The Number of Effective Tests: Reducing the number of effective tests can indeed help in improving the overall speed of executing the test suite. Jest offers pre-commit hooks that can truly reduce the number of tests performed by a test suite. These tests are widely known by the name of snapshot tests and are useful in reducing regression testing efforts.
  • Isolated Working of All Tests: One of the best parts about using Jest is that each jest test runs in its sandbox so that the working of one test does not interfere with the working of another one.
    Though we know that parallel execution of tests is extremely beneficial for maintaining high performance and speed of the tests, isolation of the test executions means providing a suitable environment for each test case so that it executes one test at a time, and the execution of one does not hamper another. This is another reason behind the fast operation of the Jest framework.
  • Supports TypeScript: TypeScript is an insanely popular superset of JavaScript. It uses plain JavaScript, and the best part is that it is used on both server-side and client-side.
    Here, our point of concern is that Jest supports TypeScript, and that becomes one of the most effective advantages of using it. You can easily implement it using the ts-jest package that acts as a typescript preprocessor along with the support of a source map for Jest. Also, it has an amazing agile community to support you at any point while using it.
  • No Dependence on Third-party Modules: Another major advantage of using Jest is that it comes with the complete package of all the features that are required in an efficient modern testing framework. The built-in modules provide such amazing support for plugins that there remains no need to download any other third-party executables like Mocha or Cucumber to perform certain activities.
    However, if you need to incorporate any other library with Jest, you can easily configure them without any compatibility issues.
  • Powerful Mocking Support: Jest provides you with support for powerful mocking. Mocking means replacing the internal dependencies with objects, which you can inspect and control. In Jest, replacing the dependencies means mostly replacing them with mock functions. All types of mocking such as timer mocking, functional Mocking, or mocking individual API calls, etc. are supported by Jest.
  • Supports Snapshot Testing: We already got to know the phrase “snapshot testing”, but we are yet to know what it is and why it adds to the positive side of Jest.
    Snapshot testing allows you to capture a string representing a given or provided component and store it in a file. Then you can compare the captured values in proper sequence with the values recorded in other frameworks to know how they match with the expected values. The snapshots are so detailed that if you want to change your UI (User Interface), you need to update your snapshot file too.
    Such amazing capability of validating components’ behavior using captured snapshots of them makes Jest a highly effective choice for performing snapshot testing on react native and react applications.

Limitations of Jest

Like every other innovation of technology, Jest also has some limitations that you must know to build your strategies for avoiding them. Hence, let’s check them out.

  • Despite being the most feature-rich testing framework for JavaScript applications, Jest still has a lot to achieve like other available libraries like Mocha, etc.
    As a consequence of that fact, Jest still has comparatively less tooling and library support available, and it becomes difficult for developers to use it to run and debug their tests in an IDE like WebStorm.
  • Using auto-mocking features can cause a delay in your test suite. That’s because if a module has more dependencies, Jest needs to work to fake them out and make them behave like real dependencies to test your code.
  • Snapshot testing with Jest is not suitable for projects that produce large snapshot files with thousands of lines.

Though Jest comes with these limitations, with proper strategies, these can be avoided, and you can achieve amazing results by using this framework.

Jest vs. Mocha - A Detailed Comparison

While Jest is gaining significant popularity as a testing framework for JavaScript applications, Mocha also holds its legacy in that field. Now, a detailed comparison between these two testing giants can help you choose the best one for you.



Jest was designed to be used as a single, standalone testing framework with the objective of having no external dependencies.

Mocha is designed to work with a selection of other libraries, and that makes the developers bound to specify which library they want to use.

Jest requires strict requirements for setting up libraries.

Mocha does not require strict requirements for setting up libraries.

Jest is easier to learn.

Mocha is comparatively more difficult to learn.

Jest has less support for logical conjunctions or disjunctions.

Mocha remarkably supports logical conjunctions or disjunctions.

Jest has built-in support for both asynchronous and snapshot testing.

Mocha does not have built-in support for both asynchronous and snapshot testing.

Jest supports only unit testing.

Mocha supports unit testing, integration testing, and end-to-end testing.

Though this comparison shows that Mocha has some remarkable qualities that are helping it hold its legacy, Jest is a more futuristic option that has significant potential to keep offering more advantages in testing JavaScript applications.

Browsers Supported By Jest

Another important quality of Jest is that it is compatible with the most popular browsers, exempting you from the worry about cross-browser compatibility. The most renowned browsers that are supported by Jest are -


This article clearly shows that Jest is an extremely useful and convenient testing framework that can solve all your testing needs for JavaScript applications. All the in-depth information mentioned above can help you understand why it has gained such immense popularity and how you can make the best out of using it to fulfill your testing requirements. The overall beneficial consequences of using Jest overpower the traditional advantages of using legacy libraries like Mocha.

However, JavaScript-based applications are not the only ones that you need to test. There are a number of important test types that require you to use highly efficient test automation tools to achieve good test coverage. And, you must know that Preflight is one of the most renowned low-code test automation tools that can solve all your testing problems without involving you in a little bit of coding. The best part is that you can get a glimpse of such an amazing testing experience for free.

To get more information about all our products, you are always welcome on our website, and for any query, feel free to reach out to us anytime. Also, if you love to read technical articles, do consider checking out our blog page.