Understanding basics of JEST Framework

Posted By :Rajat Soni |27th February 2023


Jest is a prominent JavaScript testing framework created by Facebook. It's known for its simplicity and speed, making it a top choice for testing React applications. Jest provides a comprehensive set of features that make testing easy and efficient. In this blog, we will discuss some of the key features of Jest and how to use them.




Jest can be installed using npm or yarn. To install Jest using npm, run the following command:

npm install --save-dev jest

Once installed, you can use Jest to test your code. Jest can be run from the command line or integrated with your build tools.


Writing Tests


Jest is designed to be easy to use and write tests for. Tests are written in JavaScript, and they can be run from the command line or in a browser. Jest tests are organized into test suites and test cases. A test suite is a group of related tests, while a test case is a specific test that verifies a particular behavior.

Here's an example of a test suite that tests a simple function:


describe('sum', () => {

test('adds 1 + 2 to equal 3', () => {

expect(sum(1, 2)).toBe(3);




In this example, the describe function is used to create a test suite named 'sum'. Inside the test suite, the test function is used to create a test case that verifies the sum function adds two numbers correctly.

The expect the function is used to make assertions about the behavior of the code being tested. In this example, we expect the sum function to return the value 3 when called with the arguments 1 and 2. If the sum the function doesn't return 3, the test will fail.




Jest provides a variety of matchers that can be used to make assertions about the behavior of the code being tested. Some of the most commonly used matchers include:

  • toBe - checks if two values are strictly equal (===).
  • toEqual - checks if two values are equal.
  • toMatch - checks if a string matches a regular expression.
  • toThrow - checks if a function throws an error when called.

Here's an example that uses the toEqual matcher:


test('the data is peanut butter', () => {

expect(data).toEqual('peanut butter');



In this example, the expect the function is used to assert that the data the variable is equal to the string 'peanut butter'.




Jest provides a powerful mocking system that makes it easy to test code that depends on other modules or APIs. Mocking allows you to replace parts of your code with mock objects or functions, which can simulate the behavior of the real code in a controlled environment.

Here's an example that uses Jest's mocking system to mock a function:


test('mock function', () => {

const mockFn = jest.fn();





In this example, the jest.fn the function is used to create a mock function. The mockFn the function is then called, and the expect the function is used to assert that the mockFn the function was called.




Jest is a powerful and easy-to-use testing framework for JavaScript. It provides a wide range of features that make it easy to write and run tests for your code. Whether you're testing React components, Node.js applications, or any other JavaScript code, Jest can help you write high-quality unit test cases.



About Author

Rajat Soni

Rajat Soni is working as a Frontend Developer with approx 2+ years of experience and holding certification in the domain. He is skilled in AWS services ( EC2, S3 bucket, Open search), HTML/CSS, ReactJS, Client Management, Solution Architect and many more related technologies. He has been a part many successful projects namely Konfer project, where he has migrated the project from Angular js to Angular 12 , Virgin Media Project, I-Infinity project & many more along with leading the team to handling the project end to end.

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